Chapter 2: Mitosis and Meiosis Flashcards
(134 cards)
1
Q
What is a sister chromatid?
A
During interphase, the cell’s DNA replicates, producing an identical copy of each DNA molecule. By the end of mitosis, one copy of each DNA molecule has been moved to each daughter cell.
2
Q
The terminology associated with DNA replication can be confusing.
A
Before DNA replication, each individual DNA molecule and its associated proteins is considered a single, unreplicated chromosome.
The product of DNA replication is still considered a single chromosome (a replicated chromosome) even though it contains two identical DNA molecules, or sister chromatids.
During mitosis, the sister chromatids separate, at which point each chromatid is considered an individual (unreplicated) chromosome.
Key structures involved in mitosis: the two sister chromatids of each duplicated chromosome begin to attach to the mitotic spindle by means of their kinetochores. The centrosomes anchor the mitotic spindle at opposite ends of the cell.
3
Q
The cell cycle can be divided into 2 principle stages:
A
interphase and the mitotic (M) phase
Interphase consists of 3 subphases: G1, S, and G2. During interphase, the cell grows and copies its chromosomes in preparation for cell division.
The mitotic phase includes mitosis (division of the nucleus, which itself is divided into further subphases) and cytokinesis (division of the cytoplasm).
4
Q
A number of cellular structures are involved in physically separating the duplicated chromosomes into two daughter nuclei during mitosis.
Which of the following statements does not correctly describe the role of a cellular structure in mitosis?
Every chromosome is attached to the mitotic spindle by two sets of microtubules, one extending from each pole of the cell.
The kinetochore is the structure that holds the sister chromatids together.
The centrosomes are the organizing centers for the formation of the mitotic spindle in animal cells.
During mitosis, the attachment of the sister chromatids to each other at the centromere is broken, permitting the chromatids to separate.
A
The kinetochore is NOT the structure that holds the sister chromatids together.
In animal cells, the assembly of spindle microtubules starts at the centrosomes.
The mitotic spindle is composed of microtubules that are organized by the centrosomes as they migrate to the poles of the cell early in mitosis.
The 2 sister chromatids of a replicated chromosome are most closely attached at their centromeres.
During prometaphase, a protein structure called the kinetochore appears on each sister chromatid at the position of the centromere. The spindle microtubules attach to the kinetochore, which is the motor that moves the chromosome along the spindle microtubules.
5
Q
Prokaryotic DNA Replication
A
Need to replicate DNA with each cell division, eg. E. coli.
NO nuclear membrane.
Generally circular DNA molecule.
6
Q
2 major processes of Eukaryotic DNA replication
A
Mitosis: produces 2 identical daughter cells
Meiosis: reduces genetic material content & number of chromosomes by exactly one half. Essential for sexual reproduction.
7
Q
Kinetochore
A
Kinetochore proteins bind to centromeric proteins
Allows attachment to spindle fibers (made of alpha, beta, gamma tubulin), which facilitates transport to poles
8
Q
centromere
A
The specialized heterochromatic chromosomal region at which sister chromatids remain attached after replication,and the site to which spindle fibers attach to the chromosome during cell division. Location of the centromere determines the shape of the chromosome during the anaphase portion of cell division. Also known as the primary constriction.
The shape of the chromosome during anaphase of mitosis is determined by the position of the centromere.
9
Q
cytokinesis
A
The division or separation of the cytoplasm during mitosis or meiosis.
10
Q
chromatin
A
The complex of DNA, RNA, histones, and nonhistone proteins that make up uncoiled chromosomes, characteristic of the eukaryotic interphase nucleus.
11
Q
centrosome
A
Region of the cytoplasm containing a pair of centrioles.
12
Q
chromatin
A
The complex of DNA, RNA, histones, and nonhistone proteins that make up uncoiled chromosomes, characteristic of the eukaryotic interphase nucleus.
Chromatin is the combination or complex of DNA and proteins that make up the contents of the nucleus of a cell
13
Q
centrosome
A
Region of the cytoplasm containing a pair of centrioles.
the centrosome is the main microtubule organizing center of the animal cell as well as a regulator of cell-cycle progression.
14
Q
Sister chromatids
A
A sister chromatid refers to either of the two identical copies formed by the replication of a single chromosome, with both copies joined together by a common centromere.
15
Q
unreplicated chromosome
A
Before DNA replication, each individual DNA molecule and its associated proteins is considered a single, unreplicated chromosome.
During mitosis, the sister chromatids separate, at which point each chromatid is considered an individual (unreplicated) chromosome.
16
Q
single chromosome (a replicated chromosome)
A
The product of DNA replication is still considered a single chromosome (a replicated chromosome) even though it contains two identical DNA molecules, i.e., sister chromatids.
17
Q
The cell cycle can be divided into two principle stages:
A
interphase and the mitotic (M) phase.
18
Q
Every chromosome is attached to the mitotic spindle by….
A
two sets of microtubules, one extending from each pole of the cell.
19
Q
centrosomes
A
The centrosomes are the organizing centers for the formation of the mitotic spindle in animal cells.
20
Q
During mitosis, the attachment of the sister chromatids to each other at the …..
A
….centromere is broken, permitting the chromatids to separate.
21
Q
Role of checkpoints in the cell cycle
A
Checkpoints are control points in the cell cycle where “stop and go” signals regulate whether or not a cell continues to the next part of the cycle.
For example, cells that pass through the G1 checkpoint usually complete the cell cycle and divide. If a cell does not pass through the G1 checkpoint, it exits the cell cycle and enters a nondividing state called the G0 phase.
22
Q
What distinguishes the G2 phase from the S phase?
A
Once the cell passes through the G1 checkpoint, it enters the S phase, followed by the G2 phase.
23
Q
G1 checkpoint
A
A point in the G1 phase of the cell cycle when a cell becomes committed to initiating DNA synthesis and continuing the cycle or withdraws into the G0 resting stage.
24
Q
G0
A
A nondividing but metabolically active state that cells may enter from the G1 phase of the cell cycle
25
Which two statements correctly describe the processes that occur in the S and G2 phases?
DNA replication occurs in the S (synthesis) phase. The S phase is followed by the G2 phase, which is when the centrosome replicates.
26
Which three of the following processes occur during the M phase of the cell cycle?
formation of the mitotic spindle
separation of sister chromatids
cytokinesis
The M phase of the cell cycle consists of both mitosis (the division of the nucleus) and cytokinesis (the division of the cell’s cytoplasm). DNA replication and replication of the centrosome both occur before mitosis begins.
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checkpoints
The cell cycle is controlled by 3 internal checkpoints that evaluate the condition of the genetic information.
A checkpoint is one of several points in the eukaryotic cell cycle at which the progression of a cell to the next stage in the cycle can be halted until conditions are favorable.
Damage to DNA and other external factors are evaluated at the G1 checkpoint; if conditions are inadequate, the cell will not be allowed to continue to the S phase of interphase.
The G2 checkpoint ensures all of the chromosomes have been replicated and that the replicated DNA is not damaged before cell enters mitosis.
The M checkpoint determines whether all the sister chromatids are correctly attached to the spindle microtubules before the cell enters the irreversible anaphase stage.
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Phases of the cell cycle
Many organisms contain cells that do not normally divide. These cells exit the cell cycle before the G1 checkpoint.
Once a cell passes the G1 checkpoint, it usually completes the cell cycle--that is, it divides.
The first step in preparing for division is to replicate the cell’s DNA in the S phase.
In the G2 phase, the centrosome replicates.
In early M phase, the centrosomes move away from each other toward the poles of the cell, in the process organizing the formation of the mitotic spindle.
At the end of the M phase when mitosis is complete, the cell divides (cytokinesis), forming two genetically identical daughter cells.
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When are sister chromatids present?
Sister chromatids play an essential role in ensuring that each daughter cell receives genetic material that is identical to that which was present in the original parent cell.
Sister chromatids form when the DNA molecule in each chromosome is duplicated during the S phase. Until the sister chromatids separate in early anaphase, they are joined at one or more regions along their length. Once the sister chromatids separate, each chromatid becomes a full-fledged chromosome.
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Changes in DNA content during the cell cycle
Once a cell passes the G1 checkpoint, its DNA is replicated during the S phase of interphase. Replication means that an exact copy of the DNA in each chromosome is made, thus doubling the cell’s DNA content.
Only once cytokinesis is completed at the end of telophase does the cell’s DNA content return to the level found in G1 cells.
Sister chromatids form when DNA replicates in the S phase. The sister chromatids become individual chromosomes once they separate in early anaphase.
Similarly, the cellular DNA content doubles in the S phase when the DNA replicates. However, the cell’s DNA content does not return to its normal (undoubled) levels until after cytokinesis is complete and two daughter cells have formed.
The condensation state of the DNA is not related to the presence or absence of sister chromatids. The DNA condenses in prophase and remains condensed until after the sister chromatids separate and the new daughter cells begin to form. In late telophase/cytokinesis, the emphasis shifts to cell growth and DNA replication for the next cell cycle. For these processes to occur, the DNA needs to be de-condensed so it is accessible to the cellular machinery involved in transcription.
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How can you distinguish between phases?
As micrographs demonstrate, cellular events observable by light microscopy can be used to define the 6 stages of mitosis and cytokinesis. However, deciphering which stage is which in real cells can be much more challenging than in the drawings of idealized cells you see in your textbook. Thus, it is important to carefully observe the completeness of the mitotic spindle and the location of the chromosomes, as well as how condensed the chromosomes are.
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Chromosome position in metaphase
Metaphase is characterized by the alignment of the chromosomes along the metaphase plate, a plane equidistant from the poles of the spindle.
Although metaphase is often illustrated with all the chromosomes neatly aligned along the metaphase plate, this is not always the case. In real cells, the arms of each chromosome may extend some distance away from the metaphase plate, and only the centromere of each chromosome may be aligned on the metaphase plate.
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Metaphase: dividing mitosis into 2 groups of cellular events
In metaphase, the centromeres of the duplicated chromosomes align along the metaphase plate, midway between the poles of the cell.
To help you sort the processes, consider that many of the cellular events that precede metaphase are reversed after metaphase. For example, prior to metaphase, the DNA condenses in preparation for chromosome separation. After metaphase, the DNA de-condenses once chromosome separation is complete.
34
During mitosis, what is the sequence of events involving kinetochores?
Kinetochores are assemblies of proteins that function in attaching the chromosomes to the mitotic spindle. By the end of prometaphase, each sister chromatid of a duplicated chromosome has a kinetochore located at its centromere.
In some types of cells, the kinetochore contains motor proteins that are responsible for moving chromosomes along the spindle microtubules (see diagram below).
In other types of cells, the chromosomes are pulled toward the poles by motor proteins associated with the centrosomes. In these cells, the kinetochore only attaches the chromosome to the mitotic spindle.
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cohesin
A protein complex that holds sister chromatids together during mitosis and meiosis and facilitates attachments of spindle fibers to kinetochores.
36
telophase
the kinetochore microtubules of the spindle disassemble. As the chromosomes reach the poles of the cell, the nuclear envelopes of the two new daughter nuclei form.
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prophase
the microtubules of the spindle apparatus begin to assemble from individual tubulin subunits. As the identical chromatids of each pair of sister chromatids condense during this stage, they are held together by cohesin proteins.
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Prometaphase
fragmentation of the nuclear envelope, expansion of the spindle into the nuclear region, and attachment of some spindle fibers to the chromosomes via the kinetochores.
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Metaphase
the alignment of chromsomes along the metaphase plate, is brought about by kinetochores aligning and then remaining motionless relative to the poles of the cell.
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anaphase
the cohesin proteins are cleaved, and the kinetochores move toward the poles of the cell, separating the sister chromatids.
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cohesin
A protein complex that holds sister chromatids together during mitosis and meiosis and facilitates attachments of spindle fibers to kinetochores.
42
How do kinetochore and nonkinetochore microtubules differ?
Kinetochore microtubules extend from the spindle poles toward the center of the cell and attach to the chromatids at a structure called the kinetochore. The nonkinetochore microtubules also extend from the spindle toward the center of the cell, but they do not attach to the chromatids. Instead they overlap at the center of the spindle.
The distinction between kinetochore and nonkinetochore microtubules becomes evident during prometaphase as the mitotic spindle is completed.
Some of the spindle microtubules attach to the sister chromatids, becoming kinetochore microtubules.
The remaining microtubules, which overlap with each other near the metaphase plate but do not attach to the chromosomes, are the nonkinetochore microtubules.
The kinetochore microtubules arrange the sister chromatids at the metaphase plate and move the chromosomes to the poles of the cell in anaphase. At the same time, the nonkinetochore microtubules push past each other, in part causing the cell to elongate.
In animal cells, while the chromosomes move toward the poles of the cell during anaphase, the nonkinetochore microtubules cause the cell to elongate. The nonkinetochore microtubules form pairs (one from each centrosome), which overlap and are in contact with each other near the metaphase plate.
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Part C - The mitotic spindle
The mitotic spindle is the machinery that guides the separation of chromosomes in anaphase.
Prior to metaphase, the mitotic spindle is constructed by lengthening microtubules that extend from each centrosome.
In metaphase, the kinetochore microtubules have attached each pair of sister chromatids, and the nonkinetochore microtubules overlap extensively at the metaphase plate.
During anaphase, the kinetochore microtubules shorten as the chromosomes move toward the poles of the cell. At the same time, the nonkinetochore microtubules lengthen and push past each other, elongating the cell.
By the end of telophase, all the microtubules associated with the mitotic spindle have disassembled.
44
Consider an animal cell in which motor proteins in the kinetochores normally pull the chromosomes along the kinetochore microtubules during mitosis.
Suppose, however, that during metaphase, this cell was treated with an inhibitor that blocks the function of the motor proteins in the kinetochore, but allows the kinetochore to remain attached to the spindle. The inhibitor has no effect on any other mitotic process, including the function of the nonkinetochore microtubules.
Consider these 3 questions concerning this animal cell that has been treated with the inhibitor:
Will this cell elongate during mitosis? (yes or no?)
Will the sister chromatids separate from each other? (yes or no?)
Will the chromosomes move to the poles of the cell? (yes or no?)
Will the cell elongate? Part C of this tutorial reviewed the processes that lead to cell elongation during mitosis in animal cells. The cellular structures most closely associated with cell elongation during mitosis are the nonkinetochore microtubules. Does the inhibitor affect the function of the nonkinetochore microtubules?
Will the sister chromatids separate?
The cellular structures that are most important in separation of the sister chromatids (after they have become disconnected from each other) are the kinetochore microtubules. Read carefully about the effect of the inhibitor and think through the following questions:
Does the inhibitor block the function of the kinetochores in chromosome movement?
Does the inhibitor cause the chromosomes to disconnect from the kinetochore microtubules?
Are the chromosomes connected to the centrosomes by the kinetochore microtubules?
Are there other processes going on in the cell that are causing the centrosomes to move farther apart?
Will these processes also affect the position of the chromosomes?
Will the chromosomes move to the poles of the cell?
In this type of cell, motor proteins in the kinetochore are responsible for moving the chromosomes along the kinetochore microtubules. Is any component of this machinery affected by the inhibitor treatment? If so, will the chromosomes be able to move to the poles of the cell?
The inhibitor does not affect the cleavage of cohesins (the proteins that hold the sister chromatids together), the attachment of the chromosomes to the kinetochore microtubules, or the elongation of the cell due to the nonkinetochore microtubules. The inhibitor only affects the motor protein that pulls the chromosome along the kinetochore microtubule in anaphase.
Thus, in the treated cell, the sister chromatids can still separate at the beginning of anaphase due to the fact that the cell is elongating (the centrosomes at the poles of the cell are moving farther apart) and the kinetochore microtubules still connect the chromosomes to the centrosomes. However, because the chromosomes cannot move along the kinetochore microtubules, they will never reach the poles of the cell.
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During which stage can cells either exit the cell cycle or become committed to completing the cell cycle?
G1
Cells can exit the cell cycle and enter G0 or be committed to initiate DNA synthesis late in G1.
46
The longest stage of interphase is....
The longest stage of interphase is S; cells typically spend about 7 hours in this stage. The shortest stage of interphase is G2 (3 hours), although the shortest stage of the entire cell cycle is mitosis (1 hour).
47
Which condition is evaluated at the G2/M checkpoint?
Precise replication of DNA
A cell checks for precise replication of DNA at the G2/M checkpoint.
48
Which list of steps in the eukaryotic cell cycle is given in the correct order? (step order listed left to right)
G1 - S - G2 - M - G1 . . .
The S phase is both preceded and followed by a period of growth (G1 and G2, respectively). After the M phase, the cell re-enters the G1 phase.
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Do Haploid cells undergo mitosis?
Both haploid and diploid cells can undergo mitosis.
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Each of the following events occurs during mitosis except_______.
microtubules assemble between centrioles
chromosomes condense
polar microtubules contract, pulling attached chromosomes toward the poles
nuclear membrane breaks down
polar microtubules contract, pulling attached chromosomes toward the poles
This describes the role of kinetochore microtubules during anaphase. Polar microtubules lengthen during anaphase, causing the cell to become elliptical.
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The cell cycle does consist of 2 phases, but mitosis is the shorter phase.
Interphase is composed of the G1, S, and G2 stages, which together are much longer than mitosis.
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In animal cells, this structure is found in the spindle-organizing area.
Centriole
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Which structure is not found in all mitotic cells?
Centriole
Centrioles are thought to organize the spindle fibers in animal cells, but they are not found in many plant, fungal, or algal cells.
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What is the arrangement of chromosomes during metaphase?
Sister chromatids are aligned along the equatorial plane of the cell.
During metaphase, sister chromatids are aligned along the equatorial plane, or metaphase plate, of the cell.
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During which stage of prophase I does crossing over take place?
Crossing over occurs during pachynema when bivalents are closely paired.
Crossing over is thought to occur at chiasmata, which are visible during diplonema.
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Tetrads contain two pairs of sister chromatids.
A tetrad is composed of one pair of homologous chromosomes at synapsis of prophase I.
Chromosomes are duplicated during interphase; at synapsis of prophase I, one chromosome (with two chromatids) in a tetrad is paternally inherited while the other is maternally inherited.
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Homologous chromosome
A homologous chromosomes is a set of one maternal chromosome and one paternal chromosome that pair up with each other inside a cell during meiosis. These copies have the same genes in the same locations, or loci.
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When do sister chromatids separate during meiosis?
Sister chromatids from each dyad separate during anaphase II.
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Review of changes to chromosomes during cell division in eukaryotes
In eukaryotic cells, replication of each chromosome is accompanied by complex changes in the structure of the chromosomes. These changes begin during S phase of interphase and result in the formation of a pair of fully condensed sister chromatids in prometaphase of mitosis.
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What structures in bacterial cells are likely involved in chromosome separation?
the origin of replication (where DNA replication starts on the chromosome) and the plasma membrane.
A bacterial cell contains a single, circular chromosome. As in a eukaryotic cell, after replication of the DNA, the chromosomes move to opposite ends of the bacterial cell prior to cell division by a mechanism that is not fully understood. In order for the chromosomes to separate, they must attach to something in the cell.
The mechanism of chromosome separation in bacteria is not entirely understood. However, the origin of replication (the point at which replication of the DNA in the chromosome begins) probably plays a key role via proteins that interact with it. Once duplication of the chromosome begins, the two copies of the origin become attached to the plasma membrane. By the time duplication is complete, the two origins have been moved to opposite ends of the cell. Elongation of the cell prior to division contributes to, but is not the only mechanism involved in, separating the two duplicated chromosomes.
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How do structural properties of bacterial chromosomes influence what happens to them during cell division?
Although all chromosomes consist of DNA, bacterial chromosomes are structurally distinct from their eukaryotic cousins. How do these differences affect what happens to bacterial chromosomes during cell division?
Following replication of the DNA, bacterial chromosomes remain physically associated with each other until just before cell division occurs.
A bacterial chromosome is highly folded and coiled, but lacks the fully condensed structure of a eukaryotic chromosome during cell division.
Although bacteria contain only a single, circular chromosome that is typically much shorter than the chromosomes of eukaryotic cells, cell division in bacteria is still complex. Throughout the bacterial cell cycle, the chromosome remains highly folded, but it does not condense as eukaryotic chromosomes do. After DNA replication (which occurs in all cells before cell division), the two copies of the duplicated bacterial chromosome remain closely associated with each other until the cell divides. However, this association is not as apparent as the attachment of the two sister chromatids of a duplicated eukaryotic chromosome.
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In all cells, separation of replicated chromosomes is a prerequisite for cell division. However, the mechanism of chromosome separation in bacteria is distinct from that in eukaryotes in several ways.
bacteria only: chromosome separation begins at the origin of replication on DNA
eukaryotes only: before separation duplicated chromosomes condense, nuclear envelope fragments permitting chromosome separation, two copies of the duplicated chromosome are attached at their centromeres before separating
both bacteria and eukaryotes: chromosomes replicate before cell division, replicated chromosomes separate by attaching to some other structural feature of the cell
Although the processes of chromosome separation in bacteria and eukaryotes have a common evolutionary origin, the actual mechanisms are different. Structurally, bacterial cells contain a single chromosome that is much shorter than those in eukaryotic cells, and bacterial cells lack a mitotic spindle. The bacterial chromosome does not fully condense before separation. However, the physical separation of the replicated bacterial chromosomes still involves attachment to some structure in the cell: possibly the plasma membrane at the origins of replication.
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Formation of the cell plate
During cytokinesis in a plant cell, the cell plate forms as vesicles derived from the Golgi apparatus fuse along the plane of cell division. As more vesicles fuse with the cell plate, it grows outward until it fuses with the plasma membrane of the parent cell.
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What role, if any, do either microtubules or microfilaments play in cytokinesis in plant cells?
Both microtubules and microfilaments are parts of the cytoskeleton and are important in many cellular processes, including motion and determination of cell shape.
Microtubules guide Golgi-derived vesicles to the middle of the cell where they form the cell plate.
In all eukaryotic cells, the mitotic spindle is composed of microtubules. In plant cells, once the chromosomes have reached the poles of the cell, the microtubules of the spindle reorganize themselves to provide guides that bring vesicles from the Golgi apparatus to the plane of cell division. Here, these vesicles fuse and form the cell plate.
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Cytokinesis in plant cells
Cytokinesis in animal cells is accomplished by constriction of the cell along the plane of cell division (formation of a cleavage furrow). In plant cells, which have cell walls, a completely different mechanism of cytokinesis has evolved.
Vesicles from the Golgi apparatus move along microtubules, coalesce at the plane of cell division, and form a cell plate.
The cell plate consists of the plasma membrane and cell wall that will eventually separate the two daughter cells.
In plant cell division, after chromosome separation, the microtubules of the mitotic spindle reorganize into a network that guides vesicles derived from the Golgi apparatus to the plane of cell division. These vesicles begin to fuse, forming the cell plate. As more vesicles are added to the cell plate, it grows outward, eventually fusing with the parent cell plasma membrane. Membrane from the vesicles forms the new plasma membrane for each daughter cell. At the same time, materials that were enclosed in the vesicles form the new cell wall between the new plasma membranes of the daughter cells.
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Comparison of cell walls in plants and bacteria
Nearly all plant cells and many bacteria are surrounded by a cell wall. However, the roles these walls play in cell division differ dramatically. In plants, the wall is sufficiently rigid so that in most cells, constriction of a ring of proteins could not cause the wall to fold in and eventually pinch off. Rather, a new plant cell wall is produced between the daughter cells by the formation of a cell plate, which eventually joins with the existing cell wall. In contrast, in bacteria, the cell wall is sufficiently flexible to fold in during constriction of the FtsZ protein ring.
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What roles do microfilaments and microtubules or related proteins play in cell division in bacteria, plants, and animals?
Bacteria lack the distinct microfilaments and microtubules that dominate the cytoskeletons of plant and animal cells. However, bacteria contain proteins that are similar to actin (the building block of microfilaments) and tubulin (the building block of microtubules), and these proteins function in cell division.
Either proteins of the cytoskeleton (in eukaryotes) or cytoskeleton-like proteins (in bacteria) are involved in cytokinesis. In plant cells, microtubules that made up the mitotic spindle reorganize in late telophase to guide Golgi-derived vesicles to the cell plate, where a new cell wall is constructed. In bacterial cells, tubulin-like proteins (FtsZ) form a ring inside the plasma membrane in the region where the cell will divide; constriction of this ring causes cell division. Animal cells also divide by constriction, but the proteins involved are microfilaments to make the cleavage furrow, not microtubules.
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Comparison of cytokinesis in plant and animal cells and bacterial cell division
Proteins associated with the cytoskeleton are essential to cytokinesis in plants and animals. Similarly, cytoskeleton-like proteins are essential to cell division in bacteria. The bacterial origin of plants and animals might suggest that these proteins and mechanisms of division are all related; however, evolution is not always that straightforward. Bacterial and animal cells divide in a very similar manner but accomplish division using different proteins. In contrast, plants and bacteria divide in a very different manner but accomplish division using very similar proteins.
The physical division of one cell into two during cell division is common to all types of cells. In all cases, proteins related to the cytoskeleton play some critical role. However, the mechanism by which division occurs depends on whether a rigid cell wall is present. In bacteria and animal cells, which do not have a rigid cell wall, division occurs by constriction of a ring of proteins (microtubule-like proteins in bacteria and microfilaments in animal cells) that pinches the cell in two. In plants, which do have a rigid cell wall, microtubules guide the aggregation of Golgi-derived vesicles to form the cell plate, which eventually forms the new cell wall and plasma membrane between the daughter cells.
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Cells divide by constriction of a ring of protein:
Bacteria and Animals
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The presence of a cell wall prevents the cell from dividing by constriction:
Plants
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Tubulin subunits or tubulin-like molecules function in the division of the cell
Bacteria and Plants
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If two chromosomes of a species are the same length and have similar centromere placements and yet are not homologous, what is different about them?
Genetic content, the pattern and time of replication during S phase.
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What is the probability that, in an organism with a haploid number of 12, a sperm will be formed that contains all 12 chromosomes whose centromeres were derived from maternal homologs?
2.44×10−4
= (1/2)^12 = .00024
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Prokaryotic DNA Replication
Need to replicate DNA with each cell division, eg. E. coli
NO nuclear membrane
Generally circular DNA molecule
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Nucleolus
The nuclear site of ribosome biosynthesis and assembly; usually associated with or formed in association with the DNA comprising the nucleolar organizer region.
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at metaphase of mitosis, chromosomes are _____ and _____
at metaphase of mitosis when chromosomes are condensed and visible
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Diploids
Chromosomes exist as homologous pairs
Types of chromosomes based on position of centromere (distinctive region on chromosome)
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metacentric chromosome
A chromosome that has a centrally located centromere and therefore chromosome arms of equal lengths.
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submetacentric chromosome
having the centromere almost, but not quite, at the metacentric position.
pertaining to a chromosome in which the centromere is located approximately equidistant between the center and one end so that the arms of the chromosomes are not equal in length.
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acrocentric chromosome
Chromosome with the centromere located very close to one end.
Human chromosomes 13,14,15,21,and 22 are acrocentric.
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Telocentric
having the centromere at one end of the chromosome (terminal centromere) so that the chromosome has only one arm.
a chromosome with a terminal centromere; such chromosomes in humans are unstable and arise by misdivision or breakage near the centromere and are usually eliminated within a few cell divisions.
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Karyotype
Photograph of squash of chromosomes.
Picture taken at metaphase of cell cycle (following DNA synthesis) when sister chromatids are still attached to one another
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N vs. C
There are 2 ways to quantify genetic material in a cell
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N
N= # of chromosomes in a haploid set
eg, human sperm cell:
N = haploid = 23
2N = diploid,
eg, human liver cell is 2N=46
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C
C= amount of DNA (weight) in an N nucleus
human C=0.3pg
frog C=3pg
pg = 10^-12 grams
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Genome can also be expressed as # of base pairs
Human: 3 x 10^9
Drosophila (fly): 1.8 x 10^8
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Cell Cycle
2 general portions:
Interphase: “resting” phase
Mitosis: cell division
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Mitosis
Occurs when in life cycle?
- In embryo
- During organ development
- Pathogenic events
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Mitosis in adults:
| Places and times
Renewable tissue (have stem cells):
- Epidermis
- GI tract
- Mammary glands
- Lung
- Blood forming tissue
- Testis and Ovary
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G0
Cell differentiates, leaves cell cycle
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Mitosis Prophase
centriole migration
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Mitosis Metaphase
chromosomes lining up
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Mitosis Anaphase
chromosome migration to opposite poles
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Mitosis Telophase
daughter chromosomes at opposite ends, cell divides (cytokinesis)
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2N = 4
4 Chromosomes
8 Chromatids
8 Centromeres
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Centromeres
regions of the chromosome that bind proteins that result in transport of chromosomes to opposite poles of the cell.
Centromeric DNA is specific DNA (tandem repeated sequences) that allows binding by centromeric proteins
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Kinetochore proteins
Kinetochore proteins bind to centromeric proteins
this binding process allows attachment to spindle fibers (made of alpha, beta, gamma tubulin), which facilitates transport to poles
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2 pairs of centrioles =
microtubule organizing center (MOC)
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Cell cycle regulation and checkpoints
Cell cycle features are tightly regulated by numerous gene products
Cell cycle checkpoints are places in the cell cycle where DNA or the cell is checked for damage and if damage is detected, repair or self-destruction of damaged cell occurs.
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Cell cycle checkpoints
Specific times in cell cycle when damage to cell is assessed prior to proceeding to mitosis
Prevents damaged cells from replicating
Damaged cells moved towards apoptosis (cell death)
Checkpoints in cell cycle are part if the decision process for proceeding to division
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apoptosis
programmed cell death
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M Checkpoint
checks for incomplete spindle formation
103
G2 Checkpoint
checks for DNA damage
104
Cell division cycle (cdc) mutations
Many of these genes produce kinase enzymes (add phosphates to other proteins)
Specific kinases depend on binding to another protein (cyclins) to achieve active form. These are “Cyclin-dependent kinases” (Cdk)
Different mutations operate at various checkpoints of cell cycle
Eg. p53: recognizes DNA damage at G1/S checkpoint
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What are kinase enzymes??
- They modify other proteins by phosphorylating them (adding a phosphate)
- The addition of the phosphate can change target protein’s localization, activity, or association with other proteins
- Kinases are involved in cell signaling pathways and cell division pathways
- Mutated kinases are associated with disease phenotypes including cancer
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Kinase conserved regions
Kinase activation segment: domain frequently shown to harbor oncogenic mutations in known kinase cancer genes
P-loop: conserved structural region of kinases involved in ATP-binding
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Cyclin Proteins
Cyclin levels vary during cell cycle
Therefore, levels of activated kinase complexes vary throughout cell cycle
Result is variable phosphorylation by kinases and different results/effects depending on point in cell cycle
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p53
Different mutations operate at various checkpoints of cell cycle
p53: recognizes DNA damage at G1/S checkpoint
It targets damaged cell to apoptosis pathway
Mutant p53 permits proliferation of damaged cell---> tumor formation
Therefore, p53 is a tumor-suppressor gene (a gene family)
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p53 Tumor Suppressor Gene
Found in inactivated and activated forms
Under normal conditions inactivated form predominates
Many events can lead to increase in active form: chemical damage to DNA, dsDNA breaks, UV light damage
Leads to increased activated amounts of p53, (activation is caused by phosphorylation and acetylation)
Activated p53 form is a DNA transcription factor that “turns on” other genes
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p53 Tumor Suppressor Gene continued.....
p53 initiates 2 response to DNA damage
1. Arrest of cell cycle followed by DNA repair
2. Apoptosis and cell death if repair is not possible
p53 can act at other points in cell cycle: S, G2/M
So..cells lacking functional p53 unable to respond to damaged DNA---->malignant cells
p53: “Guardian of the Genome”
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“Guardian of the Genome”
p53
112
Meiosis
KEY difference:
Meiosis I: Reduction division 2N-->N
Meiosis II: stays the same, N--->N
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STUDY slide 46 & 51 of 1st lecture
The combination of chromosomes produced following telophase II is dependent on the random alignment of each tetrad and dyad on the equatorial plate during metaphase I and metaphase II, respectively.
Several other combinations can also be formed.
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Meiosis Prophase I Stages
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Leptonema
Zygonema
Pachynema
Diplonema
Diakinesis
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Bivalents
a pair of homologous chromosomes.
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# of bivalents = N
Each bivalent contains 4 chromatids
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116
Synaptonemal complex
intimate pairing between
| the two members of each bivalent
117
Diakinesis
Tetrads move towards equatorial plate
118
Dyads in anaphase 1 of meiosis
Dyad = one pair of sister chromatids
Dyads move to poles in what is called the REDUCTION STEP!
119
Disjunction
Separation of chromatids
120
independent assortment
The independent behavior of each pair of homologous chromosomes during their segregation in meiosis I. The random distribution of maternal and paternal homologs into gametes.
paired chromatids can go either direction
121
Germ cells
Subset of cells that do not give rise to any particular tissue
Later migrate to the gonad
Then undergo meiosis to form gametes (germ cells)
122
Germ cells vs. somatic cells
They differ only in their fate
- Meiosis I: 2N ----> N
- Fertilization: N -----> 2N
123
Human germ cells
Primordial germ cells in yolk sac move to developing gonad
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Human egg cells
All oocytes present at birth, resting at diakinesis stage of cell cycle
1st follicle cell matures, ~12yrs, oocyte at ovulation goes to metaphase II, lost unless fertilized
Oogonia develop, 1/month
Meiosis results in differentiated ovum (egg) and Polar bodies
If egg fertilized, oocyte no longer arrested in metaphase II
125
Human sperm cell
Stem cell (spermatogonium) population present
Spermatogonium (2N)
Undergo meiosis to spermatocytes (N)
Further differentiation into sperm
Meiosis results in 4 sperm cells
Continual process
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Father’s age and number of de novo mutations.
As women and men age, the quality of their eggs and sperm decrease
127
Nondisjunction
Separation of chromatids fails to occur
Can happen in meiosis I or meiosis II
Abnormal gametes produced (monosomic, trisomic)
128
Nondisjunction
Most nondisjuntions are lethal
Some viable, eg. trisomy 21
Probability increase with age
129
Down syndrome
Trisomy 21
130
Edwards syndrome
Trisomy 18
131
Nullisomy
Missing a pair of homologs
| Pre-implantation lethal
132
aneuploidy
A condition in which the chromosome number is not an exact multiple of the haploid set.
133
Monosomy
one chromosome missing
| Embryonic lethal
134
Trisomy
one extra chromosome
Usually lethal, trisomy 21 viable, trisomy 18 may survive to term
