Chapter 16

Question 1

Describe the phases of the eukaryotic cell cycle

Answer 1

Interphase is further divided into three stages:

1. G1 Phase (First Gap): The cell grows physically larger, copies organelles, and makes the molecular building blocks it will need in later steps
2. S Phase (Synthesis of DNA): The cell synthesizes a complete copy of the DNA in its nucleus
3. G2 Phase (Second Gap): The cell grows more, makes proteins and organelles, and begins to reorganize its contents in preparation for mitosis

Mitotic Phase involves the division of the cell:
1. Mitosis: Nuclear division during which duplicated chromosomes are segregated and distributed into daughter nuclei
2. Cytokinesis: The cell will divide after mitosis in a process where the cytoplasm is divided and two daughter cells are formed

Question 2

Explain how cyclins and cyclin dependent kinases (cdks) work together to advance a cell through the eukaryotic cell cycle

Answer 2

Cyclins are a group of related proteins that are associated with specific phases or transitions in the cell cycle and help drive the events of that phase. The levels of different cyclins vary across the cell cycle. A typical cyclin is present at low levels for most of the cycle, but increases strongly at the stage where it’s needed

Cyclin-dependent kinases (Cdks) are a family of enzymes that regulate cell cycle progression in combination with cyclins. A lone Cdk is inactive, but the binding of a cyclin activates it, making it a functional enzyme and allowing it to modify target proteins

Here’s how they work together:
1. Activation: Cyclins are synthesized and bind to Cdks, forming a cyclin-Cdk complex. This complex acts as a signal to the cell to pass to the next cell cycle phase

2. Regulation: The cyclin-Cdk complex modifies target proteins inside the cell. Cdks are kinases, enzymes that phosphorylate (attach phosphate groups to) specific target proteins. The attached phosphate group acts like a switch, making the target protein more or less active
3. Deactivation: Eventually, the cyclin degrades, deactivating the Cdk, thus signaling exit from a particular phase

Question 3

Discuss the events that occur during each of the cell cycle checkpoints

Answer 3

These checkpoints monitor the order, integrity, and fidelity of the major events of the cell cycle. These checkpoints ensure that cells don’t divide under unfavorable conditions, such as when their DNA is damaged, or when there isn’t room for more cells in a tissue or organ2. They are crucial for maintaining the health and functionality of cells. There’s three of them:
1. G1 Checkpoint (G1/S transition): This is the main decision point for a cell, the primary point at which it must choose whether or not to divide. Here, a cell checks whether internal and external conditions are right for division. Factors assessed include cell size, nutrient availability, molecular signals, and DNA integrity. If a cell doesn’t get the go-ahead cues it needs at the G1 checkpoint, it may leave the cell cycle and enter a resting state called G0 phase

2. G2 Checkpoint (G2/M transition): This checkpoint ensures all of the chromosomes have been accurately replicated and that the replicated chromosome is not damaged before the cell enters mitosis. It acts as an additional checkpoint before M phase to make sure that cell division goes smoothly
3. M Checkpoint (Spindle Checkpoint): This checkpoint occurs at the end of the metaphase of mitosis. It ensures that all chromosomes are properly attached to the spindle microtubules, allowing for equal distribution of chromosomes during cell division

Question 4

Describe how the replication of eukaryotic chromosomes produces sister chromatids

Answer 4

The replication of eukaryotic chromosomes occurs during the S phase (synthesis phase) of the cell cycle. Here’s how it produces sister chromatids:

1. DNA Replication: During the S phase, each chromosome is replicated to produce two identical copies. This process involves unwinding the DNA double helix and using each strand as a template to synthesize a new complementary strand
2. Formation of Sister Chromatids: The result of this replication is two identical DNA molecules, which are called sister chromatids. These sister chromatids are held together at a region called the centromere
3. Cohesion: Cohesin proteins hold the sister chromatids together at the centromere, ensuring they stay attached until they are separated during cell division

So, in essence, the replication of eukaryotic chromosomes during the S phase results in the formation of sister chromatids, which are two identical copies of the original chromosome

Question 5

Explain the structure and function of the mitotic spindle

Answer 5

Structure:
- The mitotic spindle is made of long fibers called microtubules. Hundreds or even thousands of these microtubules form the mitotic spindle
- Microtubules are rope-like components of the cytoskeleton. They act as a rope that lengthens and shortens to move duplicated chromosomes from the parent cell into the daughter cells
- The spindle apparatus is vaguely ellipsoid in cross section and tapers at the ends. In the wide middle portion, known as the spindle midzone, antiparallel microtubules are bundled by kinesins
- At the pointed ends, known as spindle poles, microtubules are nucleated by the centrosomes in most animal cells

Function:
- The mitotic spindle is necessary to equally divide the chromosomes in a parental cell into two daughter cells during both types of nuclear division: mitosis and meiosis
- During interphase, which occurs before mitosis, a parent cell’s chromosomes and DNA are replicated. During prophase, the mitotic spindle forms
- The duplicated chromosomes have a narrow midsection, called the centromere, which is the attachment point of the two chromosomes. Some of these attach to the central region of the sister chromatids, called the kinetochore
- The dynamic lengthening and shortening of spindle microtubules, through a process known as dynamic instability, determines to a large extent the shape of the mitotic spindle and promotes the proper alignment of chromosomes at the spindle midzone
- Once every chromosome is bi-oriented, anaphase commences and cohesin, which couples sister chromatids, is severed, permitting the transit of the sister chromatids to opposite poles

Question 6

Outline the key events that occur during the phases of mitosis

Answer 6

Mitosis is a type of cell division that results in two daughter cells each having the same number and kind of chromosomes as the parent nucleus. It consists of four basic phases:

1. Prophase: The chromatin condenses into discrete chromosomes. The nuclear envelope breaks down and spindles form at opposite poles of the cell. Chromatin fibers become coiled into chromosomes, with each chromosome having two chromatids joined at a centromere. The mitotic spindle, composed of microtubules and proteins, forms in the cytoplasm
2. Metaphase: The spindle reaches maturity and the chromosomes align at the metaphase plate, a plane that is equally distant from the two spindle poles. Chromosomes move randomly until they attach (at their kinetochores) to polar fibers from both sides of their centromeres
3. Anaphase: Chromatids break apart at the centromere and move to opposite poles. This is facilitated by the shortening of the microtubules
4. Telophase: Two nuclei are formed after nuclear envelopes reform around each group of chromosomes. The chromosomes begin to decondense, returning to their stringy form

Question 7

Describe the processes of synapsis and crossing over

Answer 7

Synapsis: This is the fusion of chromosome pairs at the start of meiosis. It mainly occurs during prophase I of meiosis I. A protein complex called the synaptonemal complex connects the homologues. The genes on the chromatids of the homologous chromosomes are aligned precisely with each other. The tight pairing of the homologous chromosomes is called synapsis. This helps to ensure that each cell divides a full set of chromosomes.
Crossing Over: This is the exchange of genetic material between homologous chromosomes after the tetrad forms. When synapsis happens, the homologous chromosomes end up next to each other. One of the sister chromatids from each chromosome will have the chance to exchange genetic material with a sister chromatid from the homologous chromosome. The points where homologs crossover and exchange genetic material are known as a chiasmata. These are chosen more or less at random, and they will be different in each cell that goes through meiosis. This repetition produces a wide variety of recombinant chromosomes, chromosomes where fragments of DNA exchanged between homologs

Question 8

Outline the key events that occur during the phases of meiosis

Answer 8

Meiosis is a type of cell division that results in four daughter cells each with half the number of chromosomes of the parent cell. It consists of two rounds of division, meiosis I and meiosis II, and each round has four stages: prophase, metaphase, anaphase, and telophase

Meiosis I:
Prophase I: Chromosomes condense and homologous chromosomes pair up. The nuclear envelope breaks down and spindles form. Crossing over, the exchange of genetic material between homologous chromosomes, occurs

Metaphase I: Homologous pairs of chromosomes align at the metaphase plate.

Anaphase I: Homologous pairs separate and move to opposite poles.

Telophase I: Two new cells form, each with a haploid number of chromosomes.

Meiosis II:
Prophase II: The nuclear envelope breaks down and new spindles form.

Metaphase II: Chromosomes align at the metaphase plate.

Anaphase II: Sister chromatids separate and move to opposite poles.

Telophase II: Four new haploid cells form

Question 9

Compare and contrast mitosis and meiosis

Answer 9

Mitosis:
Purpose: Mitosis is how new body cells are produced for growth and repair
Divisions: It involves one round of cell division
Daughter Cells: Produces two daughter cells
Chromosome Number: The daughter cells are diploid (2n), meaning they have the same number of chromosomes as the parent cell
Genetic Variation: The daughter cells are genetically identical to the parent cell
Crossing Over: No crossing-over occurs

Meiosis:
Purpose: Meiosis is used to produce gametes (i.e., sperm and egg cells) for sexual reproduction
Divisions: It involves two rounds of cell division
Daughter Cells: Produces four daughter cells
Chromosome Number: The daughter cells are haploid (n), meaning they have half the number of chromosomes as the parent cell
Genetic Variation: The daughter cells are genetically unique due to crossing over and independent assortment

Question 10

Describe an advantage of sexual reproduction

Answer 10

Genetic Diversity: Sexual reproduction creates genetic variation in the offspring. This diversity is due to the combination of genetic material from two parents, which results in offspring that are genetically unique

Adaptation: The genetic variation produced by sexual reproduction allows species to adapt to new environments. This gives them a survival advantage as they can evolve and cope with environmental changes

Disease Resistance: A greater level of genetic diversity allows for higher levels of natural disease resistance within a species. This is because pathogens are unable to adapt to one specific genetic profile

Evolutionary Advancements: Genetic variation can lead to evolutionary advancements. This is similar to the “survival of the fittest” principles that Charles Darwin first introduced

Question 11

Distinguish among the life cycles of diploid-dominant species, haploid-dominant species, and species that exhibit an alternation of generations

Answer 11

Diploid-Dominant Life Cycle: In this type of life cycle, the multicellular diploid stage is the most obvious life stage. Nearly all animals, including humans, employ a diploid-dominant life cycle strategy where the only haploid cells produced by the organism are the gametes. Early in the development of the embryo, specialized diploid cells, called germ cells, are produced within the gonads. Germ cells are capable of mitosis to perpetuate the cell line and meiosis to produce gametes. Once the haploid gametes are formed, they lose the ability to divide again. There is no multicellular haploid life stage

Haploid-Dominant Life Cycle: In this type of life cycle, the multicellular haploid stage is the most obvious life stage. All fungi and some algae have a haploid-dominant life cycle. The single-celled zygote is the only diploid cell. In this type of life cycle, the multicellular (or sometimes unicellular) haploid stage is often multicellular

Alternation of Generations: In this type of life cycle, both the haploid and the diploid stages are multicellular. This is seen in plants and some algae. Both the haploid and the diploid stages are apparent to different degrees depending on the group

Question 12

Binary Fission

Answer 12

Binary fission is a relatively simple process, compared to mitosis, because binary fission does not involve reproducing organelles or complex chromosomes. It’s the process that bacteria use to carry out cell division. It’s actually how bacteria reproduce, or add more bacteria to the population

Binary fission is a type of asexual reproduction where a parent cell divides, resulting in two identical cells, each having the potential to grow to the size of the original cell. It’s the process through which asexual reproduction happens in bacteria. Here’s how it works:

1. DNA Replication: The process starts with the replication of the DNA within the cell. Mitochondria must also replicate their DNA before binary fission, though other organelles have no DNA
2. Separation of DNA: Then, the DNA is separated into alternate ends of the single cell
3. Cell Division: The plasma membrane pinches the cell apart, and one cell becomes two. With a fully-functioning DNA molecule, each cell is then capable of all the functions of life
4. Formation of Independent Organisms: Therefore, the cells become independent organisms. Organelles, though they are not independent organisms, separate in this way as well

Question 13

M-phase

Answer 13

The M-phase, also known as the mitotic phase, is a stage in the cell cycle where cell division takes place. It is composed of two distinct processes: mitosis and cytokinesis

Mitosis is a form of asexual cell reproduction in eukaryotes, equivalent in most respects to binary fission in prokaryotes. It includes prophase, prometaphase, metaphase, anaphase, and telophase, and it relies on the mitotic spindle at each cell pole

Cytokinesis is the process where the cell divides its cytoplasm to make two new cells

During the M-phase, the cell divides its copied DNA and cytoplasm to make two new, identical daughter cells. The M-phase is the most busy and dramatic part of the cell cycle, but the cell actually spends most of its time in interphase. The end of the G2 phase is signaled by a protein, marking what is called a G2 checkpoint. A similar G2 checkpoint marks the start of the M phase

Question 14

Interphase

Answer 14

Interphase is the phase of the cell cycle in which a typical cell spends most of its life. It is the longest stage in the eukaryote cell cycle. During interphase, the cell acquires nutrients, creates and uses proteins and other molecules, and starts the process of cell division by replicating the DNA

1. Gap 1 (G1): The cell performs its normal functions, and grows in size. The cell replicates organelles as necessary
2. Synthesis (S): During synthesis, the cell pauses its normal functioning. All resources are dedicated to replicating the DNA. This process starts with the two entwined stands of DNA being “unzipped” by various proteins. Other proteins, known as polymerase enzymes, start creating new strands to pair with each half of the DNA
3. Gap 2 (G2): The cell synthesizes proteins and continues to increase in size

The purpose of interphase in all cell types is to prepare for cell division, which happens in a different stage of the cell cycle. Depending on which species of organism is dividing, the functions of the cell during interphase can vary widely

Question 15

G0,G1,S,G2,M

Answer 15

1. G0 Phase: Cells in the G0 phase are not actively preparing to divide. The cell is in a quiescent (inactive) stage that occurs when cells exit the cell cycle
2. G1 Phase (First Gap): During G1 phase, the cell grows physically larger, copies organelles, and makes the molecular building blocks it will need in later steps
3. S Phase (Synthesis of DNA): In S phase, the cell synthesizes a complete copy of the DNA in its nucleus
4. G2 Phase (Second Gap): During the second gap phase, or G2 phase, the cell grows more, makes proteins and organelles, and begins to reorganize its contents in preparation for mitosis
5. M Phase (Mitotic Phase): During the mitotic (M) phase, the cell separates its DNA into two sets and divides its cytoplasm, forming two new cells

These phases occur in strict sequential order, and cytokinesis - the process of dividing the cell contents to make two new cells - starts in anaphase or telophase. The goal of the cell cycle is to ensure that each daughter cell gets a perfect, full set of chromosomes

Question 16

DNA Polymerase

Answer 16

DNA Polymerase plays a crucial role in both mitosis and meiosis, as it is responsible for replicating the DNA in a cell before it divides

Mitosis: During the interphase of the mitotic cell cycle, specifically in the S phase, DNA Polymerase helps in duplicating the cell’s DNA. This replication is necessary so that when the cell divides during mitosis, each new daughter cell will have a complete set of DNA

Meiosis: Similar to mitosis, DNA Polymerase also plays a key role in the DNA replication that occurs during the S phase of meiosis. This replication results in the formation of sister chromatids, which are then separated during the two divisions of meiosis. This ensures that the resulting gametes have the correct number of chromosomes

In both processes, DNA Polymerase adds nucleotides to the 3’ end of the template, synthesizing a new strand of DNA that is complementary to the template strand. This enzyme is essential for the accurate replication of the genome and thus for the production of new cells in both mitosis and meiosis

Question 17

Histones

Answer 17

Histones are highly basic proteins found in eukaryotic cell nuclei and in most Archaeal phyla. They act as spools around which DNA winds to create structural units called nucleosomes. Nucleosomes in turn are wrapped into 30-nanometer fibers that form tightly packed chromatin

The tight wrapping of DNA around histones is largely due to the electrostatic attraction between the positively charged histones and the negatively charged phosphate backbone of DNA

Histones play important roles in gene regulation and DNA replication. They can be chemically modified through the action of enzymes to regulate gene transcription. The most common modifications are the methylation of arginine or lysine residues or the acetylation of lysine. Methylation can affect how other proteins such as transcription factors interact with the nucleosomes. Lysine acetylation eliminates a positive charge on lysine, thereby weakening the electrostatic attraction between histone and DNA, resulting in partial unwinding of the DNA, making it more accessible for gene expression

Question 18

Chromatid vs. Sister Chromatid

Answer 18

In summary, a chromatid refers to one half of a duplicated chromosome, while sister chromatids refer to the paired identical copies of a chromosome

A chromatid is one of two strands of a copied chromosome. When a chromosome is replicated, it splits longitudinally, creating two identical copies. These copies are called chromatids

Sister chromatids are two identical copies of the same chromosome formed by DNA replication, attached to each other by a structure called the centromere. During cell division, they are separated from each other, and each daughter cell receives one copy of the chromosome. Sister chromatids are formed in both the cellular division processes of mitosis and meiosis

Question 19

Homolog/Homologous

Answer 19

In biology, a homolog refers to a structure, sequence, gene, or chromosome that is similar in different organisms, usually indicating a common ancestry. Homologs can be proteins, DNA sequences, or anatomical structures. Homology between protein or DNA sequences is defined in terms of shared ancestry

Homologous is an adjective form of the term “homolog”. It describes traits, structures, or sequences that are similar because of shared ancestry. For example, the forelimbs of humans and the wings of bats are homologous structures. They may serve different functions, but they share a common ancestral origin

In genetics, the term “homolog” is used both to refer to a homologous protein and to the gene (DNA sequence) encoding it. Homologous sequences are also called conserved. This is not to be confused with conservation in amino acid sequences in which the amino acid at a specific position has been substituted with a different one with functionally equivalent physicochemical properties. One can, however, refer to partial homology where a fraction of the sequences compared (are presumed to) share descent, while the rest does not. For example, partial homology may result from a gene fusion event

Question 20

Chromosome

Answer 20

A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes, the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells, the most important of these proteins are the histones. These proteins, aided by chaperone proteins, bind to and condense the DNA molecule to maintain its integrity

Chromosomes are normally visible under a light microscope only during the metaphase of cell division (where all chromosomes are aligned in the center of the cell in their condensed form). Before this happens, each chromosome is duplicated (S phase), and both copies are joined by a centromere, resulting either in an X-shaped structure (pictured above), if the centromere is located equatorially, or a two-arm structure, if the centromere is located distally. The joined copies are now called sister chromatids

Chromosomes play a significant role in genetic diversity. If these structures are manipulated incorrectly, through processes known as chromosomal instability and translocation, the cell may undergo mitotic catastrophe. Usually, this will make the cell initiate apoptosis leading to its own death, but sometimes mutations in the cell hamper this process and thus cause progression of cancer. Some use the term chromosome in a wider sense, to refer to the individualized portions of chromatin in cells, either visible or not under light microscopy

Question 21

Centrosome vs Centromere

Answer 21

Centrosome: A centrosome is an organelle made up of microtubules. It nucleates all the microtubules inside a cell to form the spindle apparatus during the prophase of cell division. Centrosomes are only found in animal cells, where they organize the microtubules and monitor the cell cycle. The centrosome is usually attached to the plasma membrane. During the prophase of the cell division, the centrosome duplicates to form two centrosomes, and these two centrosomes move to the opposite poles of the cell

Centromere: A centromere is a highly constricted region of DNA found in the middle of the chromosome. It holds the two sister chromatids together during cell division. The prime activity of the centromere is to provide a location in the center of a chromosome for microtubule binding through kinetochores. Centromeres occur in all eukaryotic cells and are responsible for the movement of chromosomes at the time of mitosis.

In summary, a centrosome is an organelle that organizes microtubules and forms the spindle apparatus for cell division, while a centromere is a region of DNA that holds sister chromatids together during cell division

Question 22

Kinetochore

Answer 22

A kinetochore is a disc-shaped protein structure associated with duplicated chromatids in eukaryotic cells. It’s where the spindle fibers attach during cell division to pull sister chromatids apart. The kinetochore assembles on the centromere and links the chromosome to microtubule polymers from the mitotic spindle during mitosis and meiosis

The kinetochore contains two regions: an inner kinetochore, which is tightly associated with the centromere DNA and assembled in a specialized form of chromatin that persists throughout the cell cycle; and an outer kinetochore, which interacts with microtubules. The outer kinetochore is a very dynamic structure with many identical components, which are assembled and functional only during cell division

Kinetochores start, control, and supervise the striking movements of chromosomes during cell division. During mitosis, two sister chromatids are held together by a centromere. Each chromatid has its own kinetochore, which face in opposite directions and attach to opposite poles of the mitotic spindle apparatus. Following the transition from metaphase to anaphase, the sister chromatids separate from each other, and the individual kinetochores on each chromatid drive their movement to the spindle poles that will define the two new daughter cells

Question 23

Microtubule

Answer 23

Microtubules play a crucial role in both mitosis and meiosis, as they are involved in the separation and movement of chromosomes during these processes

Mitosis: During mitosis, microtubules form the mitotic spindle, a structure that separates replicated chromosomes to opposite sides, creating two daughter cells. The choreography of microtubules, centrosomes, and chromosomes during mitosis is beautifully designed by nature. Finely regulated and synchronized movements of these super-macromolecular complexes against the entropic forces within a dividing cell ensure the fidelity of the genetic material in both daughter cells

Meiosis: Similar to mitosis, microtubules also play a key role in meiosis. They contribute to the formation of the meiotic spindle, which is used to pull apart homologous chromosomes during meiosis I and sister chromatids during meiosis II

In both processes, microtubules, as part of the spindle apparatus, attach to the chromosomes at the kinetochores, which are protein structures located at the centromeres. This attachment allows the microtubules to move the chromosomes to the correct locations during cell division

Question 24

Spindle Fiber

Answer 24

Spindle fibers are aggregates of microtubules that move chromosomes during cell division. They are found in eukaryotic cells and are a component of the cytoskeleton. The spindle apparatus of a cell, which includes spindle fibers, motor proteins, chromosomes, and in some animal cells, microtubule arrays called asters, ensures even chromosome distribution between daughter cells during mitosis and meiosis

In mitosis, spindle fibers are highly active. They migrate throughout the cell and direct chromosomes to go where they need to go. The process involves several stages:

Prophase: Spindle fibers form at opposite poles of the cell

Metaphase: Spindle fibers called polar fibers extend from cell poles toward the midpoint of the cell known as the metaphase plate

Anaphase: Spindle fibers pull apart the sister chromatids to opposite poles of the cell

In meiosis, spindle fibers function similarly, where four daughter cells are formed instead of two, by pulling homologous chromosomes apart after they have been duplicated to prepare for division

Question 25

Karyotype

Answer 25

A karyotype is the complete set of chromosomes of an individua. It describes the chromosome count of an organism and what these chromosomes look like under a light microscope. Attention is paid to their length, the position of the centromeres, banding pattern, any differences between the sex chromosomes, and any other physical characteristics

In both mitosis and meiosis, karyotypes can be used to identify the chromosome number and any abnormalities. During these processes, cells are often arrested during metaphase when chromosomes are most condensed. Metaphase chromosomes are then photographed, and a karyotype is produced

In mitosis, which occurs outside of the reproductive organs, a normal diploid organism will have autosomal chromosomes present in two copies. For example, a diploid human nucleus has 23 pairs of chromosomes (2n=46)

In meiosis, which occurs in the reproductive organs, the chromosome number in the germ-line (the sex cells) is n (humans: n = 23). This process leads to the formation of four daughter cells, each with a haploid set of chromosomes

Overall, karyotyping is a powerful tool in cytogenetics, allowing for the detection of chromosomal abnormalities and providing valuable information about cellular function and past evolutionary events

Question 26

Prophase and Prometaphase

Answer 26

In prophase:
- The chromosomes condense and become visible
- The nuclear envelope remains intact
- The centrosomes, which are the organizing centers of microtubules, begin to separate towards opposite poles of the cell
- The mitotic spindle, an arrangement of microtubules responsible for aligning duplicated chromosomes in later phases, begins to form

In prometaphase:
- The nuclear envelope breaks down, releasing the chromosomes
- The mitotic spindle grows more, and some of the microtubules start to capture chromosomes
- The kinetochores, which are the attachment points for the spindle microtubules, become fully matured on the centromeres of the chromosomes
- The disruption of the nuclear envelope allows for the mitotic spindles to gain access to the mature kinetochores
- Once they have captured chromosomes, the kinetochore microtubules begin to exert force on the chromosomes, moving them

Question 27

Metaphase

Answer 27

In metaphase:
- The chromosomes align along the spindle’s center
- A spindle assembly checkpoint occurs during metaphase in both mitosis and meiosis. If these checkpoints are skipped, or do not function properly, the cell will begin anaphase before the chromosomes are properly attached to microtubules and aligned on the metaphase plate

In anaphase:
- The phase separates duplicate genetic materials that are carried in the nucleus of the parent cell, into two identical daughter cells
- During anaphase, each pair of chromosomes separates into two identical but independent chromosomes
- Separation occurs simultaneously at the centromere and each separated chromosome gets pulled by the spindles to the opposite poles of the cell
- The function of anaphase is to ensure that each daughter cell receives identical sets of chromosomes before the final phase of the cell cycle, which is telophase

Question 28

Telophase and Cytokinesis

Answer 28

In telophase:
- The nuclear envelopes reform around the new nuclei in each half of the dividing cell
- The nucleolus, or ribosome producing portions of the nucleus return
- The chromosomes release from their tightly bound structure back into loose chromatin
- The main parts of the spindle apparatus fall apart

Cytokinesis is the process in which the cell actually divides into two. With the two nuclei already at opposite poles of the cell, the cell cytoplasm separates, and the cell pinches in the middle, ultimately leading to cleavage. The first signs of this puckering are usually visible sometime during anaphase. The disassembled cytoskeletal filaments are used in a different way during cytokinesis. Cleavage occurs by the contraction of a thin ring of actin filaments that form the contractile ring. The contractile ring defines the cleavage line for the cell. If the ring is not positioned at the center of the cell, an asymmetrical division takes place. The ring contracts and eventually pinches the cell until it separates into two independent daughter cells

Question 29

Meiosis

Answer 29

Meiosis I:
Prophase I: The chromosomes condense and move towards the middle of the cell
Metaphase I: Homologous chromosomes line up at the equator
Anaphase I: Homologous chromosomes separate and are pulled to opposite poles
Telophase I and Cytokinesis: The cell splits into two cell
Meiosis II:
Prophase II: The chromosomes condense
Metaphase II: Chromosomes line up at the equator
Anaphase II: Sister chromatids separate and move to opposite poles
Telophase II and Cytokinesis: The cells split again resulting in four haploid daughter cells

This process ensures that offspring have the same number of chromosomes as their parents. If the number of alleles of each gene is not reduced to 1 in the gametes that produce the zygote, there will be 4 copies of each gene in the offspring. In many animals, this would lead to many developmental defects. However, if the organism cannot survive if they are polyploidy, meiosis must occur before reproduction

Question 30

Mitosis

Answer 30

Mitosis is a type of cell division in which one cell (the mother) divides to produce two new cells (the daughters) that are genetically identical to itself. In the context of the cell cycle, mitosis is the part of the division process in which the DNA of the cell’s nucleus is split into two equal sets of chromosomes

The main goal of mitosis is to ensure that each daughter cell gets a perfect, full set of chromosomes. Cells with too few or too many chromosomes usually don’t function well: they may not survive, or they may even cause cancer. So, when cells undergo mitosis, they don’t just divide their DNA at random and toss it into piles for the two daughter cells. Instead, they split up their duplicated chromosomes in a carefully organized series of steps

Prophase: The chromosomes condense and move towards the middle of the cell

Metaphase: Chromosomes line up at the equator

Telophase: The nuclear enveloped reform around the new nuclei in each half of the dividing cell

Cytokinesis: The cell splits into two cells

Question 31

Daughter Cell vs. Parent Cell

Answer 31

In mitosis, a type of cell division that occurs in all cells except for sex cells, the parent cell divides to form two daughter cells. These daughter cells are genetically identical to the parent cell and each other, and they contain the same number of chromosomes as the parent cell

In meiosis, which occurs only in sex cells, the parent cell divides twice to produce four daughter cells. These daughter cells have half the number of chromosomes as the parent cell, making them haploid. This reduction in chromosome number is crucial for sexual reproduction, as it allows the fusion of two sex cells (each haploid) to produce a diploid offspring with the correct number of chromosomes

Overall, the key difference between a parent cell and a daughter cell is that the parent cell is the original cell that undergoes division, while the daughter cells are the new cells that result from this division. The exact characteristics of the daughter cells (such as their number and genetic composition) depend on the type of cell division (mitosis or meiosis) that produced them

Question 32

Synapsis

Answer 32

Synapsis is an event that occurs during meiosis in which homologous chromosomes pair with their counterparts and remain bound due to the exchange of genetic information1. This process mainly occurs during prophase I of meiosis I. A protein complex called the synaptonemal complex connects the homologues. The chromatids intertwine, breaking apart and exchanging pieces with one other in a process called crossing-over. The cross-over site forms an “X” shape called a chiasma. Synapsis organizes homologues so they can be separated in meiosis I

Question 33

Bivalent

Answer 33

A bivalent, also known as a tetrad, is a structure formed during meiosis that consists of two homologous chromosomes paired together due to synapsis. Each of these chromosomes consists of two sister chromatids, so a bivalent has four chromatids in total. The formation of bivalents is crucial for the process of crossing-over, which leads to the exchange of genetic material between homologous chromosomes. This exchange increases genetic diversity in the resulting gametes

Question 34

Recombination and Crossing Over

Answer 34

Recombination and crossing over are two processes that lead to genetic variations among offspring. Both events occur during the prophase 1 of meiosis 1 in eukaryotes

Recombination is the production of offspring with different combinations of traits compared to their parents. It often occurs naturally and is a result of the separation of genes that occurs during gamete formation in meiosis, the random uniting of these genes at fertilization, and the transfer of genes that takes place between chromosome pairs. Recombination produces genetic variation among the offspring and also works as a repair mechanism for double-strand breaks during meiosis

Crossing over is the exchange of genetic material between non-sister chromatids. It occurs at points called chiasma, which are created between non-sister chromatids. Chiasma allows the exchange of DNA segments between non-sister chromatids. This exchange of DNA segments produces new combinations of alleles among the offspring, which is identified as genetic recombination. Synapsis, the pairing of homologous chromosomes during prophase 1, leads to crossing over

In summary, the main difference between recombination and crossing over is that recombination is the production of different combinations of alleles in the offspring whereas crossing over is the exchange of genetic material between non-sister chromatids, the event which produces recombination

Question 35

Chiasma/Chiasmata

Answer 35

A chiasma is a point where two non-sister chromatids of homologous chromosomes physically connect and can exchange genetic material, a process known as chromosomal crossover. This typically occurs during meiosis, contributing to genetic diversity in offspring. The number of chiasmata can vary based on the species and chromosome length. They play a crucial role in ensuring proper chromosomal segregation during meiosis, and their absence can lead to issues like aneuploidy. Chiasmata are essential for the correct separation of homologous chromosomes during meiosis I

Question 36

Haploid vs. Diploid

Answer 36

Haploid and diploid are terms that describe the number of sets of chromosomes in a cell

Haploid cells contain only one set of chromosomes (n). They are produced by meiosis and are genetically diverse. The most common type of haploid cells are gametes or sex cells. When the haploid cells from male and female fuse together during fertilization, it forms a diploid cell

Diploid cells contain two complete sets (2n) of chromosomes. They are produced by mitosis. Most mammals are diploid, i.e., they have two homologous copies of each chromosome in the cells. The somatic cells in humans are diploid cells

In summary, the main difference between haploid and diploid cells is the number of chromosome sets found in the nucleus. Haploid cells have only a single set of chromosomes while diploid cells have two sets of chromosomes

Question 37

Alternation of generations

Answer 37

Alternation of generations, also known as metagenesis or heterogenesis, is the type of life cycle predominantly found in plants and algae1. It involves the alternation of two phases: a haploid sexual phase (the gametophyte) and a diploid asexual phase (the sporophyte)1.

In this cycle:

• The mature sporophyte produces haploid spores by meiosis
• These spores germinate and grow into multicellular haploid gametophytes
• At maturity, a gametophyte produces gametes by mitosis
• Two haploid gametes fuse to produce a diploid zygote, which develops into a multicellular diploid sporophyte

Question 38

Cyclin/CDK/Complex

Answer 38

A Cyclin-CDK complex is a protein complex formed by the association of an inactive catalytic subunit of a protein kinase, cyclin-dependent kinase (CDK), with a regulatory subunit, cyclin. Once cyclin-dependent kinases bind to cyclin, the formed complex is in an activated state

Cyclins drive the events of the cell cycle by partnering with a family of enzymes called the cyclin-dependent kinases (Cdks). A lone Cdk is inactive, but the binding of a cyclin activates it, making it a functional enzyme and allowing it to modify target proteins. Cdks are kinases, enzymes that phosphorylate (attach phosphate groups to) specific target proteins. The attached phosphate group acts like a switch, making the target protein more or less active

When a cyclin attaches to a Cdk, it has two important effects: it activates the Cdk as a kinase, but it also directs the Cdk to a specific set of target proteins, ones appropriate to the cell cycle period controlled by the cyclin. The structure of CDKs in complex with a cyclin subunits (CDKC) has long been a goal of structural and cellular biologists

Question 39

Briefly describe the cell cycle and the key events that occur at each stage

Answer 39

Interphase is the period of the cell cycle during which the cell is not dividing. It is divided into three stages:

• G1 phase: The cell grows physically larger, copies organelles, and makes the molecular building blocks it will need in later steps
• S phase: The cell synthesizes a complete copy of the DNA in its nucleus and duplicates a microtubule-organizing structure called the centrosome
• G2 Phase: The cell grows more, makes proteins and organelles, and begins to reorganize its contents in preparation for mitosis

The mitotic phase follows interphase. During this phase, the cell separates its DNA into two sets and divides its cytoplasm, forming two new cells. The mitotic phase is further divided into several stages:
Prophase: The chromosomes condense and move towards the middle of the cell

Metaphase: Chromosomes line up at the equator

Anaphase: Sister chromatids separate and move to opposite poles

Telophase: The nuclear envelopes reform around the new nuclei in each half of the dividing cell

Cytokinesis: The cell splits into two cells

In summary, the cell cycle is a repeating series of events that include growth, DNA synthesis, and cell division that produces two new daughter cells. Cells on the path to cell division proceed through a series of precisely timed and carefully regulated stages of growth, DNA replication, and division that produce two genetically identical cells

Question 40

Why is it important for organisms to go through mitotic cell division?

Answer 40

Mitotic cell division is crucial for organisms for several reasons:

1. Growth: Mitosis allows organisms to grow and form larger tissues and structures
2. Cell Replacement: Mitosis replaces old, worn-out cells with new ones, ensuring the continuous renewal of cells
3. Healing and Repair: Mitosis is essential for wound healing where cells outside the wound divide and fill the space from the injury
4. Reproduction: In some organisms, mitotic cell division can be used for reproduction, or the creation of new organisms. For example, single-celled eukaryotic organisms, such as amoebas, reproduce using mitosis
   In summary, mitotic cell division is a fundamental process that enables growth, repair, and reproduction in organisms

Question 41

What occurs in G1, S and G2 phases?

Answer 41

The G1, S, and G2 phases are part of interphase, which is the period of the cell cycle during which the cell is not dividing. Here’s what happens in each phase:

1. G1 Phase: Also known as the first gap phase, the cell grows physically larger, copies organelles, and makes the molecular building blocks it will need in later steps
2. S Phase: The cell synthesizes a complete copy of the DNA in its nucleus. It also duplicates a microtubule-organizing structure called the centrosome
3. G2 Phase: During the second gap phase, the cell grows more, makes proteins and organelles, and begins to reorganize its contents in preparation for mitosis

Question 42

Explain why mitosis leads to exact (or as exact as possible) diploid copies of the parent cells, while meiosis leads to unique haploid cells

Answer 42

Mitosis is used for growth, repair, and asexual reproduction. In mitosis, a single cell (the parent) divides to produce two new cells (the daughters) that are genetically identical to itself. This is because the DNA in the parent cell is precisely duplicated during the S phase of the cell cycle, and then the copies are carefully separated into the two daughter cells during mitosis. As a result, the daughter cells have exact copies of their parent cell’s DNA, so no genetic diversity is created through mitosis in normal healthy cells. Mitosis cell division creates two genetically identical daughter diploid cells

On the other hand, meiosis is a special type of cell division that produces gametes (sperm and egg cells) for sexual reproduction. During meiosis, one diploid cell undergoes two rounds of cell division but only one round of DNA replication, resulting in four haploid daughter cells. These daughter cells are not genetically identical to the parent cell or to each other. This is due to the process of crossing over (exchange of genetic material between homologous chromosomes) and independent assortment (random alignment of homologous pairs at the cell’s equator), both of which occur during meiosis I. These processes increase genetic variation among the gametes and, therefore, genetic diversity in the population

In summary, mitosis leads to exact diploid copies of the parent cells because its purpose is to produce identical cells for growth, repair, and asexual reproduction. In contrast, meiosis leads to unique haploid cells because its purpose is to generate genetic diversity for sexual reproduction

Question 43

Why is sexual reproduction so useful with regard to evolution and adaptation? How does this occur?

Answer 43

Genetic Variation: Sexual reproduction results in offspring with genetic combinations different from their parents. This genetic variation within a population can be acted upon by natural selection, allowing for adaptation to changing environments

Combining Beneficial Mutations: The process of sexual reproduction can bring together different beneficial mutations into the same genome, potentially leading to faster rates of adaptation

Elimination of Deleterious Mutations: Sexual reproduction can also help in the elimination of deleterious mutations from the population, contributing to the overall fitness of the species

Question 44

How can you tell how many chromosomes are in a cell? Explain regarding homologous chromosomes and sister chromatids

Answer 44

The number of chromosomes in a cell can be determined by examining the cell during the metaphase stage of cell division, when the chromosomes are most visible. This process is often done through a technique called karyotyping. In most sexually reproducing organisms, somatic cells (body cells) are diploid, meaning they contain two copies of each chromosome, while the sex cells (sperm and egg) are haploid, having one copy of each chromosome. For example, human somatic cells have 23 pairs of chromosomes

1. Homologous Chromosomes: These are pairs of chromosomes, one from each parent, that are similar in shape and size and carry the same type of genetic information. They pair up during meiosis, a type of cell division involved in sexual reproduction. Each chromosome in the pair is referred to as a homologue. While they carry the same genes, the specific versions of these genes (alleles) may differ
2. Sister Chromatids: These are identical copies of a single chromosome that are produced during the DNA replication phase of cell division. They are joined together at a region called the centromere. Unlike homologous chromosomes, sister chromatids are exact copies of each other

Question 45

Why is DNA not condensed during Interphase?

Answer 45

During interphase, the DNA in a cell is not condensed. This is because the cell needs to access its genetic information for regular functioning, which includes processes like transcription and DNA replication

In its uncondensed state, the DNA is organized into a structure called chromatin, which is a complex of DNA and proteins. This structure allows the cell to fit its large amount of DNA into the small space of the nucleus, while still permitting access to the genetic information

When the cell enters the division phase (mitosis), the chromatin condenses into tightly packed chromosomes to ensure accurate and efficient segregation of genetic material to the daughter cells. This condensed form is much more compact than the uncondensed chromatin present during interphase

So, the state of DNA (condensed or uncondensed) in a cell depends on the cell’s stage in the cell cycle and the cellular processes that need to occur during that stage

Question 46

What are centrosomes? When are they duplicated? What do they do?

Answer 46

Centrosomes are organelles that serve as the main microtubule-organizing centers for animal cells. They are made up of two barrel-shaped clusters of microtubules called “centrioles” and a complex of proteins that help additional microtubules to form. This complex is also known as the microtubule-organizing center (MTOC), as it helps organize the spindle fibers during mitosis

Centrosomes assist with several important functions, including:
- Organizing changes to the shape of the cell membrane that allow the membrane to “pinch” in two during cell division
- Ensuring that chromosomes are properly distributed to daughter cells by creating and shortening mitotic spindle fibers
- Overseeing other important changes to cell membrane shape, such as those seen in phagocytosis

Centrosomes duplicate before cell division. The duplication process starts just before, or at, the onset of S phase. After duplication, as division begins, the two centrosomes move to opposite ends of the cell. They then help to organize the microtubules and the cell division process. During cell division, when chromosomes are lined up and then pulled toward opposite ends of the cell, it is the centrosomes that are responsible. The centrosomes, which migrate to opposite “poles” of the cell as the cell prepares for division, direct the mitotic spindle fibers. These spindle fibers pull the sister chromatids apart and ensure that one copy of each chromosome ends up in each daughter cell

Question 47

Is metaphase in Mitosis more like metaphase I or metaphase II in meiosis? Explain your reasoning

Answer 47

Metaphase in Mitosis is more similar to Metaphase II in Meiosis. Here’s why:

1. Alignment of Chromosomes: In both Metaphase of Mitosis and Metaphase II of Meiosis, individual chromosomes line up at the metaphase plate. This is different from Metaphase I of Meiosis, where homologous pairs of chromosomes line up at the metaphase plate
2. Nature of Chromosomes: In Metaphase of Mitosis and Metaphase II of Meiosis, the chromosomes that line up are single chromosomes with two sister chromatids each. In contrast, in Metaphase I of Meiosis, paired chromosomes (bivalents) with four chromatids in each pair line up
3. Separation of Chromosomes: In both Metaphase of Mitosis and Metaphase II of Meiosis, sister chromatids are separated during the subsequent anaphase. However, in Metaphase I of Meiosis, it is the homologous chromosomes that separate

So, while all stages of Metaphase involve chromosomes aligning at the metaphase plate, the nature of the chromosomes and the subsequent separation process make Metaphase in Mitosis more similar to Metaphase II in Meiosis

Question 48

How does the formation of a bivalent lead to reductive division

Answer 48

1. Formation of Bivalent: During the first phase of meiosis (Prophase I), homologous chromosomes pair up and form a structure called a bivalent. A bivalent consists of two homologous chromosomes, each consisting of two sister chromatids
2. Crossing Over: At this stage, an exchange of genetic material can occur between non-sister chromatids of the homologous chromosomes, a process known as crossing over. This results in new combinations of alleles in the daughter cells
3. Separation of Homologous Chromosomes: The proteins that hold the bivalent together are released when the cell enters Anaphase I, allowing the homologous chromosomes to be separated. This ensures that the two homologs do not stay together during the second phase of meiosis
4. Reductive Division: The separation of homologous chromosomes results in the reduction of the chromosome number by half, from diploid (two copies of each chromosome) to haploid (one copy of each chromosome). This is why meiosis is referred to as a reductive division

Question 49

What is cancer? Describe at least two ways that this can occur via errors in oncogenes and tumor suppressor genes.

Answer 49

Cancer is a disease characterized by the development of abnormal cells that divide uncontrollably and have the ability to infiltrate and destroy normal body tissue

Oncogenes: Oncogenes are mutated genes that can contribute to the development of cancer. In their non-mutated state, everyone has genes which are referred to as proto-oncogenes. When proto-oncogenes are mutated or increased in numbers (amplification) due to DNA damage (such as exposure to carcinogens), the proteins produced by these genes can affect the growth, proliferation, and survival of the cell, and potentially result in the formation of a malignant tumor. The proteins produced by oncogenes lack normal regulatory functions. While the products (proteins) produced by proto-oncogenes are subject to the presence of growth factors and other signals to stimulate cell growth, the products of oncogenes may lead to cell growth even when these other signals are not present

Tumor Suppressor Genes: Tumor suppressor genes, or antioncogenes, tell healthy cells when to slow down growth, when to repair DNA, and when to destroy themselves (a process called apoptosis). If tumor suppressor genes get turned off due to mutations, cells are at risk of becoming cancerous and growing uncontrollably. Mutations in tumor suppressor genes can lead to tumor genesis, or the uncontrolled growth of cells6. You have two copies of most genes in your body, one from each of your parents. If one copy of a tumor suppressor gene is mutated, the cell can still function normally. But if the second copy is also mutated, the cell may become cancerous

Question 50

How is metaphase I different from metaphase II?

Answer 50

Metaphase I and Metaphase II are stages in meiosis, a type of cell division involved in sexual reproduction. Here’s how they differ:

Metaphase I: During Metaphase I, homologous pairs of chromosomes (tetrads) are attached to the meiotic spindle at the metaphase plate. Microtubules of one pole are attached to kinetochores of one of the two chromosomes facing to the same pole

Metaphase II: Metaphase II is very similar to metaphase in mitosis. During Metaphase II, individual chromosomes are arranged at the metaphase plate. Single chromosomes are attached to the microtubules of the meiotic spindle via kinetochores of each centromere. Sister chromatids of each chromosome are separated apart due to the pulling power created by microtubules

Question 51

What is the purpose of synapsis and genetic recombination?

Answer 51

Synapsis is the pairing of homologous chromosomes prior to their separation into daughter cells. It occurs during prophase I of meiosis I. In addition to stabilizing the homologous chromosomes so they separate correctly, synapsis facilitates the exchange of genetic material between the chromosomes. This exchange of genetic material is known as crossing-over. An X-shaped structure called a chiasma forms where the arms of chromosomes overlap. The DNA breaks at the chiasma and the genetic material from one homologue swaps with that from the other chromosome. The main functions of synapsis in humans are to organize homologous chromosomes so they can divide properly and ensure genetic variability in offspring

Genetic recombination is the exchange of genetic material between different organisms which leads to the production of offspring with combinations of traits that differ from those found in either parent. In eukaryotes, genetic recombination during meiosis can lead to a novel set of genetic information that can be further passed on from parents to offspring. Most recombination occurs naturally and can be classified into two types: interchromosomal recombination, occurring through independent assortment of alleles whose loci are on different but homologous chromosomes; and intrachromosomal recombination, occurring through crossing over

Question 52

How does nondisjunction affect the gametes’ chromosome numbers?

Answer 52

Nondisjunction is a process that occurs when chromosomes do not separate properly during cell division, resulting in cells with imbalanced chromosome numbers. This can significantly affect the chromosome numbers in gametes (sex cells) and can occur during either meiosis I or meiosis II

1. Nondisjunction in Meiosis I: If nondisjunction happens during anaphase I of meiosis, a pair of homologous chromosomes do not separate before being distributed into two daughter cells. This results in two daughter cells carrying an extra chromosome (n + 1), and two daughter cells with one less chromosome (n – 1). All four gametes that arise have altered number of chromosomes
2. Nondisjunction in Meiosis II: If nondisjunction occurs during anaphase II, separation of sister chromatids fails, resulting in an uneven distribution of the chromatids into the newly formed daughter cells. This can lead to two normal cells (n), one cell with an extra chromosome (n + 1), and one cell with one less chromosome (n - 1)

If these abnormal gametes combine with normal gametes during fertilization, the resulting zygote will have an abnormal number of chromosomes. This can lead to conditions such as monosomy (where a chromosome is missing) or trisomy (where there is an extra chromosome). These chromosomal abnormalities can give rise to a number of conditions

Question 53

Germ line cells undergo meiosis to form —
a. Gametes
b. Reproductive cells
c. Haploid cells
d. Sperm and egg
e. All of the above

Answer 53

e. All of the above

Explanation: Germ line cells undergo meiosis to form gametes, which are reproductive cells. These gametes are haploid cells, meaning they contain half the number of chromosomes as the parent cell. In humans and many other organisms, the gametes are the sperm and egg. So, all the options listed are correct.

Question 54

The union of gametes is termed
a. Meiosis
b. Mitosis
c. Fertilization
d. Alternation of generations
e. Inoculation

Answer 54

c. Fertilization

The correct answer is: c. Fertilization

Explanation: The union of gametes (sperm and egg) is termed fertilization. This process results in the formation of a zygote, which is the first cell of a new organism. The other terms listed refer to different biological processes. Meiosis and mitosis are types of cell division, alternation of generations refers to a life cycle that alternates between a sexual phase and an asexual phase, and inoculation is a term often used in microbiology or immunology to refer to the introduction of microorganisms or antigens into growth media or a host organism

Question 55

Genetic recombination occurs when —
a. Sister chromatids align gene for gene
b. Genes are exchanged between adjacent chromosomes
c. Homologous pairs line up across the metaphase plate
d. Chromosomes condense into heterochromatin

Answer 55

b. Genes are exchanged between adjacent chromosomes

Explanation: Genetic recombination occurs when genes are exchanged between homologous chromosomes during the process of crossing over, which happens in Prophase I of meiosis. This exchange of genetic material leads to new combinations of genes in the offspring, contributing to genetic diversity. The other options listed refer to different stages or processes in cell division

Question 56

Homologous pairs of chromosomes align across the middle of the cell during
a. Metaphase I
b. Anaphase I
c. Metaphase II
d. Anaphase II
e. Telophase III

Answer 56

a. Metaphase I

Explanation: During Metaphase I of meiosis, homologous pairs of chromosomes align across the middle of the cell at the metaphase plate. This is a crucial step before the chromosomes are separated and distributed into two daughter cells. The other options listed refer to different stages of meiosis where this specific alignment does not occur

Question 57

Homologous chromosomes contain the same genes but are inherited from different parents.
a. This is true
b. This is false

Answer 57

a. This is true

Explanation: Homologous chromosomes are a pair of chromosomes, one from each parent, that have the same genes at the same loci, but possibly different alleles. Each chromosome in the pair is similar in size, shape, and gene content. However, the specific versions of the genes (alleles) on each chromosome may be different, reflecting the genetic contribution of each parent. So, the statement is true

Question 58

The four phases of meiosis II are essentially identical to the phases of mitosis.
a. This is true
b. This is false

Answer 58

a. This is true

Explanation: The four phases of meiosis II (prophase II, metaphase II, anaphase II, and telophase II) are essentially identical to the phases of mitosis. In both processes, the chromosomes align at the metaphase plate, sister chromatids separate, and the cell divides. However, the key difference is that meiosis II occurs in a haploid cell (a cell with a single set of chromosomes), while mitosis occurs in a diploid cell (a cell with two sets of chromosomes). So, while the processes are similar, the genetic content of the cells undergoing these processes is different

Question 59

Homologous chromosomes are present in meiosis II.
a. This is true
b. This is false

Answer 59

b. This is false

Explanation: Homologous chromosomes are not present in meiosis II. They separate during anaphase I of meiosis I. In meiosis II, the sister chromatids of each chromosome separate. So, the statement is false

Question 60

Which phrase best describes the human karyotype
a. 42 pairs of autosomes
b. one pair of sex chromosomes and 23 pairs of autosomes
c. X chromosomes and 22 pairs of autosomes
d. One pair of sex chromosomes and 22 pairs of autosomes

Answer 60

d. One pair of sex chromosomes and 22 pairs of autosomes

Explanation: The human karyotype consists of 46 chromosomes: 22 pairs of autosomes (non-sex chromosomes) and one pair of sex chromosomes. The sex chromosomes determine the individual’s sex: females have two X chromosomes (XX), and males have one X and one Y chromosome (XY). So, the statement is true

Question 61

Sister chromatids are present after–
a. Prophase
b. Metaphase
c. G1 Phase
d. S Phase
e. Cytokinesis

Answer 61

The correct answer is: d. S Phase

Explanation: Sister chromatids are present after the S (synthesis) phase of the cell cycle. During the S phase, the DNA in the cell is replicated, resulting in two identical copies of each chromosome, known as sister chromatids. These sister chromatids remain attached to each other until they are separated during mitosis or meiosis. So, the statement is true

Question 62

This group of enzymes are involved in triggering events in the cell cycle –
a. Proteases
b. Transferases
c. Kinases
d. nucleases

Answer 62

c. Kinases

Explanation: Kinases are a group of enzymes that play a key role in the cell cycle. They function by adding a phosphate group to other molecules, a process known as phosphorylation. This can activate or deactivate the target molecule, triggering specific events in the cell cycle. Cyclin-dependent kinases (CDKs) are a type of kinase that are particularly important in regulating the cell cycle. So, the statement is true

Question 63

The order of mitosis is—.
a. Interphase, prophase, metaphase, anaphase, telophase
b. Telophase, anaphase, metaphase, prophase, interphase
c. Prophase, metaphase, anaphase, telophase
d. anaphase, metaphase, prophase, interphase

Answer 63

c. Prophase, metaphase, anaphase, telophase

Explanation: The stages of mitosis occur in a specific order: Prophase, Metaphase, Anaphase, and Telophase. During Prophase, the chromosomes condense and the mitotic spindle forms. In Metaphase, the chromosomes align at the metaphase plate. Anaphase is characterized by the separation of sister chromatids, which are then pulled to opposite poles of the cell. Finally, during Telophase, the chromosomes decondense, the nuclear envelope re-forms, and the cell prepares to divide into two daughter cells. So, the statement is true

Question 64

Gametes are produced by Mitosis in animals.
a. This is true
b. This is false

Question 65

After fertilization, the newly formed zygote grows by
a. Meiosis
b. Binary fission
c. Syngamy
d. Mitosis
e. Pachytene

Answer 65

b. This is false

Explanation: In animals, gametes (sex cells) are produced by a process called meiosis, not mitosis. Meiosis is a type of cell division that results in four daughter cells each with half the number of chromosomes of the parent cell1. These daughter cells are the gametes. Mitosis, on the other hand, is a type of cell division that results in two daughter cells each with the same number and kind of chromosomes as the parent nucleus, typical of ordinary tissue growth

Question 66

If a gorilla has somatic cells containing 48 chromosomes, how many chromosomes will be found in its
gametes?
a. 23
b. 24
c. 48
d. 96

Answer 66

b. 24

Explanation: Gametes are produced by a process called meiosis, which results in cells (gametes) that have half the number of chromosomes as the parent cell. So, if a gorilla’s somatic (body) cells contain 48 chromosomes, its gametes (sex cells) would contain half that number, or 24 chromosomes

Question 67

Prokaryotic cells divide by —
a. Mitosis
b. Cytokinesis
c. Binary fission
d. Replication
e. Conversion

Answer 67

c. Binary Fission

Explanation: Prokaryotic cells, such as bacteria, divide by a process called binary fission. This is a simple and rapid cell division process that results in two daughter cells that are genetically identical to the parent cell. The other options listed refer to different biological processes or stages of cell division in eukaryotic cells

Question 68

Which of the following phases of the cell cycle are included in interphase?
a. G1
b. S
c. G2
d. All of the above
e. None of the above

Answer 68

d. All of the above

Explanation: Interphase is a part of the cell cycle in which a cell grows, replicates its DNA, and prepares for division. It includes three phases:

G1 phase (Gap 1): The cell grows physically larger, copies organelles, and makes the molecular building blocks it will need in later steps.
S phase (Synthesis): The cell synthesizes a complete copy of the DNA in its nucleus.
G2 phase (Gap 2): The cell grows more, makes proteins and organelles, and begins to reorganize its contents in preparation for mitosis. So, all of the options listed are included in interphase.

Question 69

What happens during the S phase of the cell cycle?
a. Growth and maturation
b. Replication of nuclear DNA
c. Production of extra organelles
d. Separation of chromosomes into sister chromatids

Answer 69

b. Replication of nuclear DNA

Explanation: The S phase (Synthesis phase) of the cell cycle involves the replication of nuclear DNA. This is a crucial step to ensure that each new cell will receive an exact copy of the genetic material. The other options listed refer to different processes or stages of the cell cycle.

Question 70

Homologous chromosomes and sister chromatids are the same thing.
a. This is true
b. This is false

Answer 70

B. This is false

Homologous chromosomes are pairs of chromosomes, one from each parent, that have the same genes at the same loci, but possibly different alleles. Each chromosome in the pair is similar in size, shape, and gene content. However, the specific versions of the genes (alleles) on each chromosome may be different, reflecting the genetic contribution of each parent

Sister chromatids, on the other hand, are two identical copies of a single chromosome that are produced during the DNA replication phase of cell division. They are joined together at a region called the centromere. Unlike homologous chromosomes, sister chromatids are exact copies of each other

Question 71

If a researcher looked at a cell and noticed a straight line of sister chromatids, which phase would
they be looking at?
a. Prophase
b. Metaphase
c. Anaphase
d. Telophase
e. Interphase

Answer 71

b. Metaphase

Explanation: During Metaphase, the chromosomes (each consisting of two sister chromatids) align at the metaphase plate, forming a straight line. This alignment ensures that in the next phase, Anaphase, when the sister chromatids separate, each new nucleus will receive one copy of each chromosome. So, the statement is true

Question 72

DNA replication occurs during
a. Prophase
b. Metaphase
c. Anaphase
d. Telophase
e. None of the above

Answer 72

DNA replication occurs during the synthesis phase, or S phase, of the cell cycle, before the cell enters mitosis or meiosis. So, the correct answer is:

e. None of the above