WEEK 3: Mitosis & Meiosis (LAB)

Question 1

DNA

Answer 1

• where the bulk of information required to generate any individual for more organisms
• the amount of DNA depends on the species

Question 2

Chromosome

Answer 2

A chromosome is a single, linear molecule of DNA that is packaged with histone proteins into a compact structure and serves as the genetic material of an organism. A structure consisting of DNA and associated proteins that carry and transmit genetic information.
- count chromosomes by counting the centromere

Chromosomes are the packaging system used by eukaryotes; a chromosome consists of a molecule of DNA wrapped around and associated with various proteins. There are two different types of packaging:
- loose packaging
- tight packaging

Question 3

Loose Packaging

Answer 3

DNA is wrapped around histone proteins; both DNA replication and transcription require loose packaging.
- Looks like “beads on a string” when we look at DNA wound on nucleosomes

Question 4

Nucleosomes

Answer 4

The basic repeating unit of chromatin, consists of a core of eight histone proteins (two each of H2A, H2B, H3, and H4) and about 146 bp of DNA that wraps around the core about two times.

Question 5

Chromatin

Answer 5

The material found in the eukaryotic nucleus; consists of DNA and histone proteins.

Question 6

Histone

Answer 6

Low-molecular-weight proteins found in eukaryotes that associate closely with DNA to form chromosomes.

Question 7

Tight Packaging

Answer 7

Highly condensed chromosome; the transmission of DNA into new cells (cell division) requires a high level of packaging.

Question 8

Telomere

Answer 8

Stable end of a eukaryotic chromosome.

Question 9

Centromere

Answer 9

Constricted region on a chromosome that stains less strongly than the rest of the chromosome; serves as the attachment point for spindle microtubules.
- A constricted region of a chromosome where spindle fibres attach
- Moderately repetitive DNA in heterochromatin form
- Chromosome fragments that lack centromeres are lost in mitosis

Question 10

Metacentric Chromosome

Answer 10

Chromosomes in which the two chromosome arms are approximately the same length.

Question 11

Acrocentric Chromosomes

Answer 11

Chromosome in which the centromere in near one end, producing a long arm at one end and a knob, or satellite, at the other end.

Question 12

Telocentric Chromosomes

Answer 12

Chromosome in which the centromere is at or very near one end.

Question 13

Gametes

Answer 13

In animals, gametes are the eggs and sperm cells.
- Gametes are haploid (n) in terms of chromosome number because these cells contain half the number of chromosomes as the respective somatic cells.
- Human gametes contain 23 chromosomes and are designed as n =23 (each of these chromosomes are different)

Question 14

Somatic Cells

Answer 14

Somatic cells are referred to as diploid (2n) because these cells have twice the number of chromosomes as the respective gametes.
- Human somatic cells (e.g., neurons, chondrocytes, white blood cells) are diploid and described 2n = 46.
- Each somatic cell contains two copies of chromosome 1, two copies of chromosome 2 and so on. We refer to the pairs of the same chromosome in diploid cells as homologous chromosomes.

Question 15

Homologous Chromosome / Homologous Pair

Answer 15

Homologous chromosomes are pairs of chromosomes that contain the same set of genes but may have different versions of those genes (alleles).

A pair of chromosomes that are alike in structure and size and that carry the genetic information for the same set of hereditary characteristics. One chromosome of a homologous pair is inherited from the male parent, and the other is inherited from the female parent.
- Each somatic cell contains two copies of chromosome 1, two copies of chromosome 2 and so on. We refer to the pairs of the same chromosome in diploid cells as homologous chromosomes.

Question 16

Homolog

Answer 16

A homolog refers to one member of a pair of homologous chromosomes.

Question 17

Haploid (n)

Answer 17

Haploid cells carry one set of genetic information. i.e. possessing a single set of chromosomes (one genome).
- Gametes are haploid (n) in terms of chromosome number because these cells contain half the number of chromosomes as the respective somatic cells.

Question 18

Diploid (2n)

Answer 18

Diploid calls carry two sets of genetic information; i.e. possessing two sets of chromosomes (two genomes).
- Human somatic cells (e.g., neurons, chondrocytes, white blood cells) are diploid and described 2n = 46.

Question 19

Genome

Answer 19

Complete set of genetic instructions for any organism.
- All the genes present in a gamete are known as a genome.

Question 20

C-value (c)

Answer 20

Amount of DNA per cell in an organism; the nu,ber of DNA molecules per cell equals the number of chromosomes when the chromosomes are unreplicated (no sister chromatids are present) and twice the number of chromosomes when sister chromatids are present.
- The measure of the amount of DNA in a genome is termed the C-value (c) and is expressed in units of mass or length (base pairs = bp).

Question 21

Karyogram

Answer 21

Karyograms are the study of a whole set of chromosomes arranged in pairs by size and position of the centromere.
- A karyogram is an image that displays the chromosomes within a cell.
- The chromosomes used for a karyogram are specifically metaphase chromosomes and are customarily arranged by size and type.
- All chromosomes of the same size and with the same centromere position are sorted and grouped together with homologous chromosomes placed directly beside each other.

Question 22

Cell Cycle

Answer 22

Stages through which a cell passes from one cell division to the next; the process by which cells alternate between a growth phase (interphase) and a dividing phase (mitosis and cytokinesis) to produce two new daughter cells.

Question 23

Interphase (I)

Answer 23

Is the time during the cell cycle when cells perform designated functions, maintain structural and metabolic functions, and finally prepare to divide.
Is made up of three sub-phases/sub-stages:
- gap 1 (G1) phase,
- S-phase (S)
- Gap 2 (G2) phase

During G1 phase, the cell grows and carries out normal metabolic processes. During S phase, DNA replication occurs, in preparation for cell division. During G2 phase, the cell checks for DNA damage and makes final preparations for division.

Time spent in each stage can vary considerably between species and cell types.
- It is safe to assume that most cells spend the majority of time in interphase.

Question 24

Gap 1 (G1) Phase

Answer 24

Stage of interphase in the cell cycle in which the cell grows and develops; begins when the previous round of cell division ends.
- Predominant G1 cellular functions include performing cell-specific duties and building new cellular components.
- DNA is not made during G1

Question 25

S (Synthesis) Phase

Answer 25

Stage of interphase in the cell cycle. In the S-phase, DNA replicates. Starts when DNA is packaged as chromosomes) replication begins.
- At the beginning of S-phase each chromosome consists of a single piece of DNA.
- By the end of the S-phase, each chromosome becomes a replicated chromosome that consists of two identical DNA molecules termed sister chromatids.

Question 26

Sister Chromatids

Answer 26

Two copies of a chromosome are held together at the centromere. Each sister chromatid consists of a single molecule of DNA.
- These sister chromatids are held together by proteins located at the centromere throughout interphase and mot of cell division.

Question 27

Chromatid

Answer 27

A chromatid is one-half of a duplicated chromosome. During cell division, chromosomes are replicated, producing two identical copies of each chromosome, called sister chromatids, which are joined at a central point called the centromere.

Question 28

Gap 2 (G2) Phase

Answer 28

Stage of interphase in the cell cycle that follows DNA replication. In G2, the cell prepares for division. The cell carries on with its cell-specific duties and the building of new cellular components. The cell also prepares for cell division.
- Biochemical preparation for cell division

Question 29

Mitosis (M-Phase)

Answer 29

The process by which the nucleus of a eukaryotic cell divides. Cell division is achieved collectively through the process of mitosis - a division of the nucleus, and cytokinesis - the division of the cytoplasm.
- Mitosis consists of all four stages, all of which contribute to the equal division of the nucleus of one cell into two genetically identical daughter cells.
- Mitosis is followed by cytokinesis. The conclusion of cytokinesis signifies the end of cell division and the beginning of another cell cycle in both daughter cells.
- M phase can be divided into several sub-stages, including prophase, metaphase, anaphase, and telophase.

Question 30

Cytokinesis

Answer 30

The process by which the cytoplasm of a cell divides.
- Mitosis is followed by cytokinesis. The conclusion of cytokinesis signifies the end of cell division and the beginning of another cell cycle in both daughter cells.

Question 31

Prophase

Answer 31

Stage of mitosis in which the chromosomes contract and become visible the cytoskeleton breaks down, and the mitotic spindle begins to form. This stage marks the beginning of mitosis. Key features include:
- the progressive condensation of replicated chromosomes into highly compact structures and the dismantling of the nuclear envelope.

Question 32

Metaphase

Answer 32

Stage of mitosis in which chromosomes align in the center of the cell.

Microtubules connect to each replicated chromosome at the chromosome’s centromere and move those chromosomes to the middle of the cell. The movement of each replicated chromosome occurs independently of all other replicated chromosomes and ends when the centromeres of each chromosome are on the metaphase (equatorial) plate.

Question 33

Metaphase Plate

Answer 33

The metaphase plate is the location between the poles o the dividing cells at which the replicated chromosomes align.

Question 34

Anaphase

Answer 34

Stage of mitosis in which sister chromatids separate and move toward opposite spindle poles.

Microtubules contract and pull the sister chromatids of each replicated chromosome to opposite poles. Chromatids gain the status of chromosomes as soon as they segregate form their sister.

Question 35

Telophase

Answer 35

Stage of mitosis in which the chromosomes arrive at the spindle poles, the nuclear membrane re-forms, and the chromosomes relax and lengthen.

A nuclear membrane reforms around the chromosomes at each pole nd those chromosomes become less condensed (become loosely packaged). The result is two genetically equal nuclei in that they contain the same number and types of chromosomes. These two nuclei become a part of their own cells at the conclusion of cytokine.

Question 36

Meiosis

Answer 36

The process by which the chromosomes of a eukaryotic cell divide to give rise to haploid reproductive cells (2n to n). Consists of two divisions:
- Meiosis I
- Meiosis II

Meiosis occurs at some stage in the life cycle of all sexually reproducing eukaryotes. It occurs in specialized diploid cells called meiocytes.

The complete process involves two successful nuclear divisions to produce four cells (meiotic products = gametes) that are haploid (n).

Meiosis does two things:
- reduces the chromosome number of its products (gametes) to half of that of any somatic cell of the same species (ensures that chromosome number remains constant in species from generation to generation).
- creates genetic diversity

Question 37

Meiocyte

Answer 37

A meiocyte is a type of cell that differentiates into a gamete through the process of meiosis. Through meiosis, the diploid meiocyte divides into four genetically different haploid gametes. The control of the meiocyte through the meiotic cell cycle varies between different groups of organisms.

Only a very select few cells (meiocytes), found in the organs called gonads, are capable of undergoing meiosis.

The haploid meiotic products mature into gametes that can subsequently fuse at fertilization to give rise to a zygote.

Question 38

Primary Meiocyte

Answer 38

Any cell in which meiosis is initiated is called a primary meiocyte.

The primary meiocyte then enters the first nuclear division (meiosis I) of the process, which comprises four stages that are arbitrarily designated as prophase I, metaphase I, and telophase I.

Question 39

Meiosis I

Answer 39

The first phase of meiosis. In meiosis I, the chromosome number is reduced by half.

Meiosis I differs from mitosis in that homologous chromosomes segregate, not centromeres.

At the end of meiosis I, each chromosome retains its replicated structure, however, the genetic make-up of each chromatid is no longer identical because of crossing over.

Question 40

Prophase I

Answer 40

Stage of meiosis I in which chromosomes condense and pair, crossing over takes place, the nuclear membrane breaks down, and the mitotic spindle forms; Begins when replicated chromosomes start to condense, which is similar to that of prophase during mitosis.

Prophase I in meiosis can be distinguished from prophase of mitosis and prophase II in meiosis by two physical events:
- the pairing of homologous chromosomes
- crossing over

Prophase I takes longer to complete than the other prophases because of the extra time needed to pair and cross over. During this process, the replicated chromosomes continue to condense until they appear as short and thick structures.

At this stage, each chromosome is comprised of two sister chromatids.

The end of prophase I is marked by the complete degradation of the nuclear membrane.

Question 41

The pairing of Homologous Chromosomes

Answer 41

The pairing of homologous chromosomes is an essential feature of meiosis, acting to promote high levels of recombination and to ensure the segregation of homologs.
- Pairing of homologous chromosomes in prophase I is one critical feature that enables meiosis to be reductional in nature.

Question 42

Bivalent

Answer 42

A homologous pair of synapsed chromosomes consisting of four chromatids; also called a tetrad; as prophase I, progresses the pairing becomes tighter and tighter until a bivalent is formed - two replicated homologous chromosomes coupled by a protein structure known as the synaptonemal complex.
- the pairs of homologs connected with each other
- the result of synapsis;
- The tetrad represents a single unit of genetic information and is essential for ensuring that each daughter cell receives one copy of each chromosome, leading to the proper reduction of the chromosome number and the formation of haploid cells.

Question 43

Crossing-over

Answer 43

Exchange of genetic material between homologous but non-sister chromatids.

The pairing of homologous chromosomes facilitates specific molecular events that exchange genetic material between the non-sister chromatids in each bivalent. These events are called crossovers.

Question 44

Non-sister Chromatids

Answer 44

A non-sister chromatid refers to either one of the two chromatids of paired homologous chromosomes. During prophase, I of meiosis I, the non-sister chromatids of (homologous chromosomes) form chiasma(ta) to exchange genetic material.

Question 45

Chiasmata

Answer 45

Chiasmata (singular: chiasma) are the physical manifestations of crossing over, a process that occurs during meiosis in eukaryotic cells. Chiasmata are structures that form where homologous chromosomes exchange genetic material as they synapse during meiosis I.

Crossing over results in the exchange of genetic material between homologous chromosomes, leading to the creation of new combinations of genes. The chiasmata represent the physical locations where the chromosomes have broken and then rejoined after exchanging genetic material.

Question 46

Recombination

Answer 46

The process that produces new combinations of alleles. Any shuffling of genetic material is known as recombination.

Question 47

Intrachromosomal Recombination

Answer 47

Intrachromosomal recombination occurs through crossovers between loci on the same chromosomes. The shuffling of genetic mateiral between homolouga chromosomes, which occurs during crossing-over, is known specifically as Intrachromosomal Recombination.

Question 48

Metaphase I

Answer 48

Stage of meiosis in which homologous pairs of chromosomes align in the center of the cell along the metaphase plate. The alignment of each bivalent within a meiocyte is independent and random (this creates a genetic difference between gametes).
- tetrads line up on the metaphase plate

Question 49

Independent Assortment / Principle of Independent Assortment (Mendel’s Second Law)

Answer 49

The principle of heredity discovered by Mendel that states that genes encoding different characteristics (genes at different loci) separate independently; applies to genes located only on different chromosomes or to genes far apart on the same chromosome.
- The independent and random orientation of bivalent in each meiocyte gives rise to independent assortment.

Question 50

Interchromosomal Recombination

Answer 50

Any genetic variation caused by independent assortment.

Interchromosomal recombination is a type of genetic recombination where the sequences of nucleotides are exchanged between two identical molecules of DNA. Intrachromosomal recombination results due to crossing over between two linked gene pairs of two non-homologous chromosomes.

Question 51

Anaphase I

Answer 51

Stage of meiosis I in which homologous chromosomes separate and move toward spindle poles; microtubules attach to each replicated chromosome in each bivalent. HOWEVER, the centromeres of chromosomes do not segregate.
- chromosomes segregate independently; this allows chromosomes to shuffle (i.e., genetic diversity)
- this is the SECOND PLACE in meiosis where there is a generation of genetic diversity

Instead, bivalents are pulled apart as each homologous chromosome is pulled towards opposite ends of the cell. Segregation is the key process to reducing the chromosome number of the primary meiocyte.

At the end of this phase, each pole receives one half of the original complement of replicated chromosomes with the primary meiocyte and will differ genetically.

Question 52

Telophase I

Answer 52

Stage of meiosis I in which chromosomes arrive at the spindle poles. In telophase I, two haploid cells, secondary meiocytes, are formed when a nuclear membrane reforms around each set of replicated chromosomes and the primary meiocyte is divided into two via cytokinesis.

Question 53

Meiosis II

Answer 53

The second phase of meiosis. Events in meiosis II are similar to those in mitosis.
- Begins with 2 haploid cells, and results in 4 haploid cells.
- In meiosis II, the sister chromatids separate, making haploid cells with non-duplicated chromosomes.

The chromosomes in each product are not the same, which would be the case in mitosis, and that is due to crossing over. And because of independent assortment, any gamete produced is genetically different, in terms of combinations of allele it contains, from any other gamete.

Question 54

Prophase II

Answer 54

Stage of meiosis in which events of interkinesis are reversed; chromosomes recondense, the spindle reforms, and the nuclear membrane breaks down. Some cells skip this stage.

Question 55

Metaphase II

Answer 55

Stage of meiosis II in which individual chromosomes align in the center of the cell.

Question 56

Anaphase II

Answer 56

Stage of meiosis II in which sister chromatids separate and the chromatids are pulled to opposite poles.

Question 57

Telophase II

Answer 57

Stage of meiosis II in which chromosomes arrive at the spindle poles.

Question 58

N-value

Answer 58

We use “n” to represent the number of complete sets of chromosomes, the number of chromosomes.

The number of chromosomes per cell equals the number of functional centromeres.

Question 59

Reductional Division

Answer 59

Reductional division, also known as meiosis I, is a type of cell division that results in the reduction of the number of chromosomes in a cell by half. This is important for sexual reproduction, as it allows for the formation of gametes, such as sperm and egg cells, that carry only one set of chromosomes (haploid) rather than the two sets (diploid) found in most cells of the body.

During meiosis I, duplicated chromosomes (consisting of two identical chromatids joined at a central point called the centromere) align and separate at the metaphase plate, forming two identical daughter cells. Each of these daughter cells will then go on to undergo meiosis II, in which the chromatids are separated, resulting in four haploid cells, each with one chromatid from each of the original homologous chromosomes.

Question 60

Equational Division

Answer 60

Equational division, also known as mitosis, is a type of cell division that results in the replication and distribution of an exact copy of the genetic material (DNA) in a cell to two identical daughter cells. Unlike reductional division (meiosis), equational division results in daughter cells with the same number of chromosomes as the parent cell, and therefore the same number of genetic copies (diploid).

Mitosis occurs in many types of cells in the body and is responsible for growth and repair. During mitosis, duplicated chromosomes (consisting of two identical chromatids joined at a central point called the centromere) align and separate at the metaphase plate, forming two identical daughter cells. Each daughter cell receives an exact copy of the genetic material, including one chromatid from each of the original chromosomes.

Question 61

How do the c-value and n-value change in the reductional division?

Answer 61

To summarize, the c-value remains the same during reductional division, while the n-value is reduced by half. During reductional division, the c-value and n-value change in the following way:

The c-value remains the same. Reductional division, also known as meiosis, involves the replication of the DNA in a cell and the separation of duplicated chromosomes into daughter cells. However, the total amount of DNA in the cell remains the same, so the c-value does not change.

The n-value is reduced by half. During reductional division, the number of chromosomes in the cell is reduced from diploid (2n) to haploid (n). This occurs because each duplicated chromosome, consisting of two identical chromatids, separates into two daughter cells, resulting in half the number of chromosomes in each cell compared to the parent cell. For example, in a typical human cell with 46 chromosomes (23 pairs of homologous chromosomes), the reductional division would result in daughter cells with 23 chromosomes each.

Question 62

How do the c-value and n-value change in equational division?

Answer 62

To summarize, during equational division, the c-value and n-value both remain the same. The daughter cells receive the same number of chromosomes as the parent cell, and each chromosome consists of an identical copy of the DNA. During equational division, also known as mitosis, the c-value and n-value change in the following way:

The c-value remains the same. Equational division involves the replication of the DNA in a cell and the separation of duplicated chromosomes into two identical daughter cells. However, the total amount of DNA in the cell remains the same, so the c-value does not change.

The n-value remains the same. During equational division, the number of chromosomes in the cell is replicated exactly, resulting in two daughter cells with the same number of chromosomes as the parent cell. For example, in a typical human cell with 46 chromosomes (23 pairs of homologous chromosomes), equational division would result in two daughter cells, each with 46 chromosomes.

Question 63

DNA Synthesis

Answer 63

DNA synthesis, also known as replication, is the process by which a cell makes an exact copy of its genetic material in preparation for cell division. This process occurs before cell division and is critical for ensuring that each daughter cell receives an identical copy of the genetic material.

Question 64

Repetitive DNA (Non-coding)

Answer 64

Repetitive DNA, also known as non-coding DNA, refers to DNA sequences that are present in multiple copies in the genome of an organism. These sequences do not contain information for the production of proteins and are not involved in the coding of functional genes.
- tandemly repeated DNA sequences that are present in the centromeres, telomeres, and other regions of chromosomes. They are usually composed of simple sequence repeats and do not contain any functional genes.

Question 65

Telomeres

Answer 65

• Repetitive sequence at the end of linear chromosomes
• solves the end replicaion problem
• provides stability

A telomere is a repetitive DNA sequence located at the end of a chromosome that serves to protect the genetic information within the chromosome. Telomeres are important because they play a key role in preserving the stability of the genome and in regulating cell division.

Every time a cell divides, a portion of the telomere is lost. Over time, as cells continue to divide and the telomeres become shorter, the cell eventually reaches a point where it can no longer divide and undergoes senescence or apoptosis. This process helps to limit the number of cell divisions and prevent the accumulation of DNA damage.

Question 66

The pattern of base pairs in Telomeres

Answer 66

Typically a series of A/T + 3 G/Cs
- Telomeres consist of repeated DNA sequences made up of a specific pattern of base pairs. In humans, the most common sequence is TTAGGG, repeated hundreds to thousands of times in a tandem array. This sequence helps to protect the genetic information on the chromosome and prevent its loss during cell division.

Question 67

G-rich strand

Answer 67

In the context of telomeres, the g-rich strand refers to the strand of the telomere repeat sequence that is rich in guanine nucleotides. The g-rich strand is paired with a complementary c-rich strand, which is rich in cytosine nucleotides. The interaction between the g-rich and c-rich strands is thought to play a key role in the protection of the telomere and the stability of the chromosome.

In summary, the g-rich strand refers to a strand of DNA that is rich in guanine nucleotides. In the context of telomeres, it refers to the strand of the telomere repeat sequence that is rich in guanine and is paired with a complementary c-rich strand.

Question 68

T-loop

Answer 68

G-rich strand folds over and pairs with a short strength of DNA. The t-loop is a structural feature of telomeres that helps to protect the ends of chromosomes from damage. The t-loop is formed when the single-stranded 3’ end of the telomere invades the double-stranded telomere repeat sequence, forming a loop that covers the end of the chromosome and protects it from degradation and fusion with neighbouring chromosome

Question 69

G1/S Checkpoint

Answer 69

The G1/S checkpoint occurs at the transition from G1 phase to S phase and determines whether the cell is ready to enter S phase and replicate its DNA.

At the G1/S checkpoint, the cell assesses its internal and external environment, including factors such as growth factors, nutrients, and DNA damage. If the cell determines that conditions are favorable and that its DNA is undamaged, it will enter S phase and proceed with DNA replication. If conditions are not favorable or DNA damage is detected, the cell will halt in G1 phase until conditions improve or the damage is repaired.

The G1/S checkpoint is an important mechanism for ensuring that cells divide only under the right conditions, and that cells with damaged DNA do not proceed with replication and division, which could lead to the transmission of genetic mutations to daughter cells.

Question 70

G2/M Checkpoint

Answer 70

The G2/M checkpoint is a stage in the cell cycle that acts as a control point for cell division, occurring at the transition from G2 phase to M phase. The cell checks for DNA damage and ensures that DNA replication has been completed at the G2/M checkpoint to determine whether it is ready to undergo cell division.

Question 71

Spindle-assembly Checkpoint

Answer 71

The spindle-assembly checkpoint is a stage in the cell cycle that occurs during mitosis and ensures that the chromosomes are properly aligned and attached to the spindle fibers before the cell divides. The checkpoint plays a critical role in ensuring that each daughter cell receives a complete and accurate set of chromosomes after cell division.

Question 72

Mitosis is the separation of _____________________ while cytokinesis is the separation of ________.

Answer 72

Sister chromatids; cytoplasm.

Question 73

Summary of the Cell Cycle (Interphase & M Phase)

Answer 73

Question 74

What are the key differences between Meiosis II and Mitosis?

Answer 74

Chromosome number: Meiosis reduces the chromosome number by half, whereas mitosis results in daughter cells with the same number of chromosomes as the parent cell.

Chromosome distribution: In Meiosis II, the chromosomes are distributed randomly to the daughter cells, leading to genetic diversity, whereas in mitosis, the chromosomes are distributed evenly to the daughter cells.

Crossing over: During Meiosis, homologous chromosomes may exchange genetic material through a process called crossing over, which increases genetic diversity. This process does not occur in mitosis.

Question 75

Synopsis

Answer 75

Close pairing of homologous chromosomes; i.e., the process of the pairing.

Synapsis is a process that occurs during meiosis, specifically during the prophase I stage. It is the close pairing of homologous chromosomes, which are the pairs of chromosomes that contain the same genes but with different alleles.
- During synapsis, the homologous chromosomes align and physically interact with each other, often leading to the exchange of genetic material through a process called crossing over. This exchange of genetic material results in genetic diversity in the daughter cells produced by meiosis.
- Synapsis ensures that each daughter cell receives one copy of each chromosome, which is necessary for the proper reduction of the chromosome number and the formation of haploid cells.

Question 76

Crossing Over

Answer 76

Exchange of genetic material b/w homologous chromosomes.
- Occurs in prophase I
- During crossing over, homologous chromosomes break and exchange segments of DNA, leading to the formation of new combinations of genetic material. This exchange of genetic material results in genetic diversity in the daughter cells produced by meiosis.

Question 77

Independent Segregation

Answer 77

Question 78

Spindle Microtubules

Answer 78

Responsible for the separation of homologous chromosomes @ MI, and separation of sister chromatids @ MII (BOTH IN ANAPHASE).
- the single microtubules are composed of tubulin subunits.
- the tubules grow & shrink via polymerization/depolymerization; microtubules lengthen and shorten at both the (+) and the (-) ends.

Question 79

Cohesin

Answer 79

Cohesin is a complex of proteins that holds sister chromatids together after DNA replication.
- Cohesin is a ring-like heterotrimer (i.e., hetero = non-identical, tri = three, mer = protein; it is a protein made from three different polypeptides)

Question 80

Shugoshin

Answer 80

Shugoshin plays an important role in both anaphase I and anaphase II.

During anaphase I:
- Shugoshin protects cohesin at the centromeres from the action of separase.
- Centromeric cohesin remains intact.
- The sister chromatids remain together

In anaphase I of meiosis, shugoshin helps to ensure that sister chromatids separate correctly. Shugoshin does this by binding to cohesin, a complex of proteins that holds sister chromatids together after DNA replication. Shugoshin helps to prevent the premature separation of sister chromatids by protecting cohesin from degradation.

During Anaphase II:
- Shugoshin breaks down.
- Centromeric cohesin is cleaved by separase.
- The sister chromatids separate.

Question 81

Summary of Meiosis

Answer 81

Question 82

Independent Segregation Results

Answer 82

Genetic variation resulting from independent segregation of chromosomes.
- the cell has three homologous pairs of chromosomes
- one of each pair is maternal in origin, and the other is paternal
- there are four possible ways for the three pairs to align in metaphase I
Conclusion: eight different combinations of chromosomes in the gametes are possible, depending on how the chromosomes align and separate in meiosis I and II.

Question 83

Recombination via Crossing Over Results

Answer 83

Genetic variation resulting from recombination via crossing over.

Question 84

Genetic Consequences of Interphase & Mitosis (c-value, n-value)

Answer 84

Question 85

Genetic Consequences of Interphase & Meiosis (c-value, n-value)

Answer 85

Question 86

Compare Mitosis, Meiosis I, and Meiosis II:
- Cell division
- Reduction in chromosome number
- genetic variation produced
- crossing over
- random distribution of material and paternal chromosomes
- metaphase
- anaphase

Answer 86

Question 87

kinetochore

Answer 87

The kinetochore serves as the attachment site for spindle microtubules, which are responsible for the proper segregation of chromosomes during cell division. In the absence of a kinetochore, the chromosomes would not be properly attached to the spindle and would not be properly separated during cell division.

Without a kinetochore, it would result in an unequal distribution of chromosomes between the daughter cells, leading to aneuploidy (an abnormal number of chromosomes).