I. Mitosis (Figure 9-1)
· Mitosis is the process by which a cell with the diploid number of chromosomes, which in humans is 46, passes on the diploid number of chromosomes to daughter cells.
· The term diploid is classically used to refer to a cell containing 46 chromosomes.
· The term “haploid” is classically used to refer to a cell containing 23 chromosomes.
· Mitosis ensures that the diploid number of 46 chromosomes is maintained in cells.
· Mitosis occurs at the end of a cell cycle. Phases of the cell cycle are:
A. G0 (Gap) Phase
The G0 phase is the resting phase of the cell. The amount of time a cell spends in G0 is variable and depends on how actively a cell is dividing.
B. G1 Phase
The G1 phase is the gap of time between mitosis (M phase) and DNA synthesis (S phase). The G1 phase is the phase where RNA, protein, and organelle synthesis occurs. The G1 phase lasts about 5 hours in a typical mammalian cell with a 16-hour cell cycle.
C. G1 Checkpoint
Cdk2-cyclin D and Cdk2-cyclin E mediate the G1 →S phase transition at the G1 checkpoint.
D. S (Synthesis) Phase
The S phase is the phase where DNA synthesis occurs. The S phase lasts about 7 hours in a typical mammalian cell with a 16-hour cell cycle.
E. G2 Phase
The G2 phase is the gap of time between DNA synthesis (S phase) and mitosis (M phase). The G2 phase is the phase where high levels of ATP synthesis occur. The G2 phase lasts about 3 hours in a typical mammalian cell with a 16-hour cell cycle.
F. G2 Checkpoint
Cdk1-cyclin A and Cdk1-cyclin B mediate the G2 → M phase transition at the G2 checkpoint.
G. M (Mitosis) Phase
The M phase is the phase where cell division occurs. The M phase is divided into six stages called prophase, prometaphase, metaphase, anaphase, telophase, and cytokinesis. The M phase lasts about 1 hour in a typical mammalian cell with a 16-hour cell cycle.
1. Prophase. The chromatin condenses to form well-defined chromosomes. Each chromosome has been duplicated during the S phase and has a specific DNA sequence called the centromere that is required for proper segregation. The centrosome complex, which is the microtubule organizing center (MTOC), splits into two and each half begins to move to opposite poles of the cell. The mitotic spindle (microtubules) forms between the centrosomes.
2. Prometaphase. The nuclear envelope is disrupted, which allows the microtubules access to the chromosomes. The nucleolus disappears. The kinetochores (protein complexes) assemble at each centromere on the chromosomes. Certain microtubules of the mitotic spindle bind to the kinetochores and are called kinetochore microtubules. Other microtubules of the mitotic spindle are now called polar microtubules and astral microtubules.
3. Metaphase. The chromosomes align at the metaphase plate. The cells can be arrested in this stage by microtubule inhibitors (e.g., colchicine). The cells arrested in this stage can be used for karyotype analysis.
4. Anaphase. The centromeres split, the kinetochores separate, and the chromosomes move to opposite poles. The kinetochore microtubules shorten. The polar microtubules lengthen.
5. Telophase. The chromosomes begin to decondense to form chromatin. The nuclear envelope re-forms. The nucleolus reappears. The kinetochore microtubules disappear. The polar microtubules continue to lengthen.
6. Cytokinesis. The cytoplasm divides by a process called cleavage. A cleavage furrow forms around the middle of the cell. A contractile ring consisting of actin and myosin filaments is found at the cleavage furrow.
A. The checkpoints in the cell cycle are specialized signaling mechanisms that regulate and coordinate the cell response to DNA damage and replication fork blockage.
· When the extent of DNA damage or replication fork blockage is beyond the steady-state threshold of DNA repair pathways, a checkpoint signal is produced and a checkpoint is activated.
· The activation of a checkpoint slows down the cell cycle so that DNA repair may occur and/or blocked replication forks can be recovered.
B. The two main protein families that control the cell cycle are cyclins and the cyclin-dependent protein kinases (Cdks).
· A cyclin is a protein that regulates the activity of Cdks and is named because cyclins undergo a cycle of synthesis and degradation during the cell cycle.
· The cyclins and Cdks form complexes called Cdk-cyclin complexes.
· The ability of Cdks to phosphorylate target proteins is dependent on the particular cyclin that complexes with it.
III. Meiosis (Figure 9-2)
· Meiosis is the process of germ cell division (contrasted with mitosis which is somatic cell division) that occurs only in the production of the germ cells (i.e., sperm in the testes and oocyte in the ovary).
· In general, meiosis consists of two cell divisions (meiosis I and meiosis II) but only one round of DNA replication that results in the formation of four gametes, each containing half the number of chromosomes (23 chromosomes) and half the amount of DNA (1N) found in normal somatic cells (46 chromosomes, 2N).
· The various aspects of meiosis compared to mitosis are given in Table 9-1.
A. Meiosis I
Events that occur during meiosis I include:
1. DNA replication
2. Synapsis. Synapsis refers to the pairing of each duplicated chromosome with its homologue, which occurs only in meiosis I (not meiosis II or mitosis).
a. In female meiosis, each chromosome has a homologous partner so the two X chromosomes synapse and crossover just like the other pairs of homologous chromosomes.
b. In male meiosis, there is a problem because the X and Y chromosomes are very different. However, the X and Y chromosomes do pair and crossover. The pairing of the X and Y chromosomes is in an end-to-end fashion (rather than along the whole length as for all the other chromosomes), which is made possible by a 2.6 Mb region of sequence homology between the X and Y chromosomes at the tips of their p arms where crossover occurs. This region of homology is called the pseudoautosomal region.
c. Although the X and Y chromosomes are not homologs, they are functionally homologous in meiosis so there are 23 homologous pairs of the 46 duplicated chromosomes in the cell at this point.
3. Crossover. Crossover refers to the equal exchange of large segments of DNA between the maternal chromatid and paternal chromatid (i.e., nonsister chromatids) at the chiasma, which occurs during prophase (pachytene stage) of meiosis I. Chiasma is the location where crossover occurs forming an X-shaped chromosome and named for the Greek letter chi, which also is X-shaped.
a. Crossover introduces one level of genetic variability among the gametes.
b. During crossover, two other events (i.e., unequal crossover and unequal sister chromatid exchange) may occur, which introduces variable number tandem repeat (VNTR) polymorphisms, duplications, or deletions into the human nuclear genome.
4. Alignment. Alignment refers to the process whereby homologous duplicated chromosomes align at the metaphase plate. At this stage, there are still 23 pairs of the 46 chromosomes in the cell.
5. Disjunction. Disjunction refers to the separation of the 46 maternal and paternal duplicated chromosomes in the 23 homologous pairs from each other into separate secondary gametocytes (Note: the centromeres do not split).
a. The choice of which maternal or paternal homologous duplicated chromosomes enters the secondary gametocyte is a random distribution.
b. There are 223 (or 8.4 million) possible ways the maternal and paternal homologous duplicated chromosomes can be combined. This random distribution of maternal and paternal homologous duplicated chromosomes introduces another level of genetic variability among the gametes.
6. Cell division. Meiosis I is often called the reduction division, because the number of chromosomes is reduced by half, to the haploid (23 duplicated chromosomes, 2N DNA content) number in the two secondary gametocytes that are formed.
B. Meiosis II
Events that occur during meiosis II include:
1. Synapsis: absent.
2. Crossover: absent.
3. Alignment: 23 duplicated chromosomes align at the metaphase plate.
4. Disjunction: 23 duplicated chromosomes separate to form 23 single chromosomes when the centromeres split.
5. Cell division: gametes (23 single chromosomes, 1N) are formed.
IV. Oogenesis: Female Gametogenesis
A. Primordial germ cells (46,2N) from the wall of the yolk sac arrive in the ovary at week 4 and differentiate into oogonia (46,2N) which populate the ovary through mitotic division.
B. Oogonia enter meiosis I and undergo DNA replication to form primary oocytes (46,4N). All primary oocytes are formed by the month 5 of fetal life. No oogonia are present at birth. Primary oocytes remain dormant in prophase (diplotene) of meiosis I from month 5 of fetal life until puberty at ≈12 years of age (or ovulation at ≈50 years of age, given that some primary oocytes will remain dormant until menopause).
C. After puberty, 5 to 15 primary oocytes will begin maturation with each ovarian cycle, with usually only one reaching full maturity in each cycle.
D. During the ovarian cycle, a primary oocyte completes meiosis I to form two daughter cells: the secondary oocyte (23 chromosomes, 2N amount of DNA) and the first polar body, which degenerates.
E. The secondary oocyte promptly begins meiosis II but is arrested in metaphase of meiosis II about 3 hours before ovulation. The secondary oocyte remains arrested in metaphase of meiosis II until fertilization occurs. F. At fertilization, the secondary oocyte will complete meiosis II to form a one mature oocyte (23,1N) and a second polar body.
V. Spermatogenesis: Male Gametogenesis is classically divided into 3 phases
Primordial germ cells (46,2N) form the wall of the yolk sac, arrive in the testes at week 4, and remain dormant until puberty. At puberty, primordial germ cells differentiate into Type A spermatogonia (46,2N). Type A spermatogonia undergo mitosis to provide a continuous supply of stem cells throughout the reproductive life of the male. Some Type A spermatogonia differentiate into Type B spermatogonia (46,2N).
Type B spermatogonia enter meiosis I and undergo DNA replication to form primary spermatocytes (46,4N). Primary spermatocytes complete meiosis I to form secondary spermatocytes (23,2N). Secondary spermatocytes complete meiosis II to form four spermatids (23,1N).
Spermatids undergo a postmeiotic series of morphological changes to form sperm (23,1N). These changes include formation of the acrosome; condensation of the nucleus; and formation of head, neck, and tail.
1. The total time of sperm formation (from spermatogonia to spermatozoa) is about 64 days.
2. Newly ejaculated sperm are incapable of fertilization until they undergo capacitation, which occurs in the female reproductive tract and involves the unmasking of sperm glycosyltransferases and removal of proteins coating the surface of the sperm. Capacitation occurs before the acrosome reaction.
VI. Comparison Table of Meiosis and Mitosis (Table 9-1)
Figure 9-1. Diagram of the stages of the M (mitosis) phase. Only one pair of homologous chromosomes (i.e., chromosome 18) is shown (white = maternal origin and black = paternal origin) for simplicity's sake.
Figure 9-2. Meiosis (A) A schematic diagram of chromosome 18 shown in its “single chromosome” state and “duplicated chromosome” state that is formed by DNA replication during meiosis I. It is important to understand that both the “single chromosome” state and “duplicated chromosome” state will be counted as one chromosome 18. As long as the additional DNA in the “duplicated chromosome” is bound at the centromere, the structure will be counted as one chromosome 18 even though it has twice the amount of DNA. The “duplicated chromosome” is often referred to as consisting of two sister chromatids (chromatid 1 and chromatid 2). (B) Schematic representation of meiosis I and meiosis II, emphasizing the changes in chromosome number and amount of DNA that occur during gametogenesis. Only one pair of homologous chromosomes (i.e., chromosome 18) is shown (white = maternal origin and black = paternal origin) for simplicity sake. The point at which DNA crosses over is called the chiasma. Segments of DNA are exchanged thereby introducing genetic variability to the gametes. In addition, various cell types along with their appropriate designation of number of chromosomes and amount of DNA is shown.
Table 9-1 Comparison of Meiosis and Mitosis
1. The X and Y chromosomes pair in meiosis at the pseudoautosomal regions. A nondisjunction of the X and Y chromosomes in a male during meiosis I would produce which of the following combinations of gametes?
(A) one sperm with two X's, and three sperm with Y's
(B) two sperm with two X's and two sperm with two Y's
(C) one sperm with no X's, and three sperm with an X and a Y
(D) a sperm with two X's, a sperm with two Y's, and two sperm with no sex chromosomes
1. The answer is (D). During meiosis I, the nondisjunction of the paired and doubled X and Y would cause them to go into one of the daughter cells with no X or Y chromosomes going to the other daughter cell. During meiosis II, the doubled X chromosome and the doubled Y chromosome would go to separate daughter cells and the cell with no sex chromosomes would give rise to two daughter cells with no sex chromosomes. Because the X and Y chromosomes are doubled, the daughter cell receiving the X chromosomes will have two copies and the daughter cell receiving the Y chromosomes will have two copies.
2. Which of the following describes the main difference between meiosis and mitosis?
(A) homologous chromosomes pair during meiosis
(B) the number of chromosomes is reduced by half during mitosis
(C) after meiosis is complete, there are 46 chromosomes in each cell
(D) after mitosis is complete there are 23 chromosomes in each cell.
2. The answer is (A). Homologous chromosomes pair during meiosis but not during mitosis. The number of chromosomes is reduced by half, from 46 to 23 during meiosis and the daughter cells are genetically different, but during mitosis, the chromosome number of 46 is maintained and the daughter cells are genetically identical.
3. Crossing over and random segregation produce much of the genetic variation in human populations. These events occur during which of the following?
3. The answer is (B). Crossing-over and random segregation of the maternal and paternal chromosomes occur during meiosis.
4. Tetraploid cells are the result of the failure of which one of the following processes?
(A) anaphase of mitosis
(B) S (synthesis) phase of the cell cycle
(C) cytokinesis of mitosis
(D) G1 phase of the cell cycle
4. The answer is (C). The 46 doubled chromosomes separate during anaphase, resulting in 92 chromosomes, and if cytokinesis (cell division) fails, one cell with a tetraploid number of chromosomes, 92, is the result instead of two daughter cells with the normal diploid number of 46 in each.
5. The “reduction division” in which the number of chromosomes in a germ cell is reduced from 46 to 23 chromosomes occurs during which of the following?
(B) meiosis I
(C) meiosis II
5. The answer is (B). During meiosis I, the 23 paired, doubled homologs randomly separate, resulting in two daughter cells with 23 chromosomes each.
6. At the completion of oogenesis, how mature oocytes are formed from each primary oocyte?
6. The answer is (A). Only one mature oocyte results from each primary oocyte. At each of the two cell divisions in meiosis, a polar body is formed, which usually degenerates.
7. Which one of the following has a haploid number of chromosomes?
(A) primary spermatocyte
(B) secondary spermatocyte
7. The answer is (B). A secondary spermatocyte results from meiosis I, the reduction division, in a primary spermatocyte and the chromosome number is reduced from 46 to 23.
8. Karyotype analysis can be conducted on cells that have entered which one of the following stages of cell division?
(A) meiosis I
(B) meiosis II
(C) metaphase of mitosis
(D) anaphase of mitosis
8. The answer is (C). Prometaphase and metaphase of mitosis is when the chromosomes are condensed enough to visualize for cytogenetic analysis.
9. One of the two places in the cell cycle where a response to DNA damage occurs is which one of the following?
(A) G0 phase
(C) m (synthesis) phase
(D) G2 checkpoint
9. The answer is (D). The other time in the cell cycle when there is a response to DNA damage is at the G1 checkpoint before the S (synthesis) phase begins.