| Mitotic Events | |||
| Process | Genomic | Cellular | |
| Prophase | The genome condenses. Visible chromosomes form. Each chromosome has 2 sister chromatids bound at the centromere by cohesin. |
Spindle fibers (aka spindle poles) emanate from the centromere. The two centrosomes form G2 sprout microtubules by polymerizing free-floating tubulin. The microtubules repel each other, pushing the centrosomes to opposite ends of the cell. This microtubular network is the start of the mitotic spindle. The nuclear double-membrane begins to reabsorb into the endoplasmic reticulum; when this finishes in prometaphase, the cell no longer has a nucleus. | |
| Prometaphase | Each sister chromatid forms a kinetochore at its centromere as a place for microtubules to latch onto the chromosome (thus, there are two kinetochores per centromere). With the nuclear envelope gone, microtubules invade the nuclear space and (once the centromere’s spindles are sufficiently long) clasp onto the kinetochore. The kinetochore has an ATP-dependent motor that is activated by the microtubules. Kinetochore motors push each chromosome along the microtubules toward the metaphase plate. The metaphase plate is an imaginary plane that is perpendicular to the plane between the two centrosomes. Microtubules not bound to a kinetochore find and bind other “free” microtubules from the opposite centrosome; this forms the mitotic spindle. | ||
| Metaphase | The chromosomes have aligned such that their centromeres convene on metaphase plate. This precise alignment is due to opposing kinetochores functioning like equally strong people in a tug of war. Some species’ chromosomes do not align, but instead move randomly back and forth between the poles before roughly lining up at the metaphase plate. The mitotic spindle checkpoint refers to an arresting signal emitted by any kinetchores still unattached to microtubules. This is important because proper chromosomes separation in anaphase requires attachment of every kinetochore to many microtubules. Anaphase commences only once every kinetochore has attached to a cluster of microtubules, with every chromosome thus lined up along the metaphase plate. | ||
| Anaphase |
Sister chromatids separate, resulting in two distinct populations of genetic material (one at each centrosome) that are identical. This occurs in two steps sometimes labeled early anaphase and late anaphase. Also, cytokinesis begins in anaphase and ends in telophase.
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| Telophase | The nonkinetochore microtubules continue to lengthen, elongating the cell even more. Sister chromosomes attach to opposite ends of the cell. Fragments of the parental nuclear envelope attach around each set of separated sister chromatids. Each set of nuclei, now surrounded by a new nuclear envelope, unfold back into chromatin. The cleavage furrow forms. | ||
| Cytokinesis | Often, (mistakenly) thought to be the same process as telophase cytokinesis, if slated to occur, is usually well under way by this time. In animal cells, a cleavage furrow develops where the metaphase plate used to be, pinching off the separated nuclei. In plant cells, vesicles derived from the Golgi apparatus move along microtubules to the middle of the cell, coalescing into a cell plate that develops into a cell wall, separating the two nuclei. Each daughter cell has a complete copy of the genome of its parent cell. Mitosis is complete. | ||
| Growth 1 | Each chromosome is copied during S phase, forming two identical sister chromatids that then separate into the future daughter cells during anaphase. Thus, each generation has exactly as much DNA as its predecessor. | Upon completion of cytokinesis, the two daughter cells are identical to their predecessor and enter G1, an interphase. Most cells spend the majority of the cell cycle in G1; some animal cells never undergo mitosis. | |
This is called open mitosis, and it occurs in most multicellular organisms. Some protists, such as algae, undergo a variation called closed mitosis where the microtubules are able to penetrate an intact nuclear envelope.
| Step | Overview |
|---|---|
| Growth 1 |
In a diploid eukaryotic cell, there are two versions of each chromosome, one from the mother and another from the father. The two corresponding chromosomes are called homologous chromosomes. Homologous chromosomes need not be genetically identical. During growth 1 (G1), an interphase, phase is the normal growth phase. Chromosomes are highly decondensed in most regions, allowing access of regulatory proteins to the DNA. Within the nucleus, individual chromosomes are found within diffuse but non-overlapping domains. |
| S Phase | Synthesis (S) DNA synthesis occurs. Results in 4n chromosomes in diploid organisms (like vertebrates); 2n chromosomes in haploid organisms (like yeast). n = number of distinct types of chromosomes. During the S phase of the cell cycle chromosomes are replicated to produce two complete copies of each. DNA replication results in an identical copy of each chromosome. These copies are called sister chromatids. Together, these chromatids are considered one chromosome. When separated, though, each sister chromatid is a chromosome. The 2 copies of the original chromosomes are called sister chromosomes. |
| Growth 2 | During Growth 2 (G2), another interphase, the cell doubles in size. Centromeres (Microtubule Organizing Centers–MTOCs) form. |
| Mitosis | When conditions are good, the cell will replicate. |
An obvious advantage of proteolysis for controlling passage through these critical points in the cell cycle is that protein degradation is an irreversible process, ensuring that cells proceed irreversibly in one direction through the cycle.
| Step | Initiation | Overview | |
|---|---|---|---|
| Early G1 | DNA prepreplication complexes assemble at origins. However, they are not activated. Mitotic cyclin-CDKs activate early steps in mitosis. | ||
| G0 | Most cells pause midway at a so-called G0 interphase to carry out their functions for most or all of their existence. | ||
| Mid G1 | G1 cyclin-CDK inactivates human Cdh2 and human inhibitors of cyclin-dependent kinase 4 and 6 (INK4s), important tumor suppressors that inhibit passage through G1 by inhibiting the mid G1 cyclin-CDKs. Both genes encoding INK4a are mutated in many human tumors, as they are less able to inhibit passage into G1. | E2F is a critical transcription factor that is bound by its inhibitor, Rb. Mid G1 Cyclin-CDKs phosphorylate Rb, thus releasing it from E2F. This activated E2F stimulates expression of late G1 cyclin and CDK, S phase cyclin, and factors needed for DNA synthesis. | |
| Late G1 | E2F stimulates its own transcription, and the Late-G1-cyclin+CDK can phosphorylate Rb; this forms a positive feedback loop whereby mid-G1 cyclin-CDKs are no longer needed to enter S-phase. | Late-G1-cyclin+CDK activates expression of S-cyclin+CDK subunits. However, S-cyclin+CDK is promptly bound by its inhibitor Sic1. | |
| S Phase | Late-G1-cyclin+CDK phosphorylates the S-cyclin+CDK inhibitor (Sic1). This is a checkpoint: once the inhibitor is phosphorylated, G1-cyclin+CDKs are no longer needed for entry into S-phase. SCF degrades phosphorylated Sic1. | After SCF degrades Sic1 and thereby de-represses S-cyclin+CDK, S-cyclin+CDK is free to activate pre-replication complexes at the DNA origins. Active S-cyclin+CDK phosphorylates and activates proteins that initiate DNA synthesis at origins of replication. However, S-cylin+CDK is inhibited by Sic1 while S-phase cyclin and S-phase is being produced. The inhibitor is then precipitously degraded by Late-G1-Cyclin+CDK. S-cyclin+CDK is suddenly active en masse as a sudden event, rather than a slow rise in activity that would have occurred without the repressor. This permits the sudden activation of large numbers of DNA replication complexes. During the S phase, newly synthesized Cyclin B from maternal mRNA leads to formation of a new functional MPF heterodimer. | |
| G2 | Cdc25 phosphatase activates mitotic-cyclin+CDK (aka MPF). | ||
| Prophase | MPF activates early mitotic events. | Synthesis of mitotic cyclin leads to high MPF activity. | |
| Spindle Assembly Checkpoint | |||
| Metaphase | MPF drives cells into metaphase. | There are high levels of mitotic cyclin during metaphase, which results in high levels of MPF (mitotic-cyclin+CDK) activity. | |
| Chromsome Segregation Checkpoint | |||
| Anaphase | APC-Cdc20 degrades phosphorylated securin, an inhibitor bound to separase. Separase then digests cohesin’s Kleisin subunit, allowing sister chromatids to separate. | ||
| Telophase | APC-Cdh2 degrades mitotic cyclins | When sister chromosomes have moved apart sufficiently to ensure their complete separation into the two daughter cells, Cdc14 phosphatase activates CdcA phosphatase, which activates APC/C-Cdh2 to degrade mitotic cyclins. Thus, APC and Cdc14 phosphatase induce late steps in mitosis, telophase and cytokinesis. This results in low levels of mitotic cyclin and thus low MPF activity. | |
| Cytokinesis | |||
| Checkpoint | Overview |
|---|---|
| DNA Damage A | Midway through G1, ATM/R activates p53, which activates p21CIP, which blocks Mid-G1-Cyclin+CDK (Cyclin-D+CDK4 & CDK6) if DNA damage is detected. |
| DNA Damage B | At the start of S-phase, ATM/R activates: p53, which activates p21CIP, which blocks the late G1 cyclin (Cyclin E) and the S-Phase cyclin (Cyclin A) if DNA damage is detected; and Chk1/2, which blocks Cdc25A if DNA damage is detected. Cdc25A would otherwise activate the CDK2, which binds the late G1 cyclin (Cyclin E) and the S-phase cyclin (Cyclin A). Cyclin-E+CDK2 and Cyclin-A+CDK2 are needed to initiate S-phase. |
| DNA Damage C | Midway through S-phase, ATM/R activates: p53, which activates p21CIP, which blocks Cyclin A (S-Phase cyclin) if DNA damage is detected; and Chk1/2, which blocks Cdc25A if DNA damage is detected. Cdc25A would otherwise activate the CDK2, which binds Cyclin A. Cyclin-A+CDK2 is needed during S-phase. |
| Intra-S-Phase | At the cusp of G2 and M phase, ATR activates Chk1, which inactivates Cdc25C. Cdc25C would otherwise activate mitotic cyclins (Cyclin A and Cyclin B). |
| DNA Damage D | At the cusp of G2 and M phase, ATM/R activates p53, which activates p21CIP, which inactivates mitotic cyclins (Cyclin A & Cyclin B) if DNA damage is detected. Arrest in G2 allows DNA double-stranded breaks to be repaired before mitosis. |
| Spindle Assembly |
In the spindle assembly checkpoint (aka metaphase checkpoint), mitotic arrest deficient 2 (aka Mad2) blocks metaphase until every single kinetochore has properly attached to spindle microtubules. Mad2 exists in an open conformation (Mad2O) and a closed conformation (Mad2C).
Just a few Mad1-Mad2C tetramers bound to kinetochores can generate enough Mad2C-Cdc20 to overcome p31 activity. Once all kinetochores have attached to microtubules (thus releasing all Mad1-Mad2C tetramers), p31 activity predominates and active Cdc20 is released from Mad2C. The active Cdc20 binds APC/C, and the APC/C-Cdc20 degrades securin, the inhibitor of separase. Free separase digests the Kleisin subunit of cohesin, breaking open the Smc1-Smc3-Kleisin ring and allowing sister chromatids to separate. Cohesin is a Smc1/Smc3/Kleisin heterotrimer that holds together sister chromatids, with Kleisin acting like a clasp. |
| Spindle Position |
The spindle position checkpoint (aka the chromosome segregation checkpoint) blocks the onset of telophase by inactivation of Cdc14. Cdc14 would otherwise activate Sic1 and the degradation of Cyclin B’s by APC/C-Cdh2. |
Mitosis is the division of a parent cell into two daughter cells that are identical to itself. A cell does this to increase population size and to increase genetic diversity. The parent cell duplicates each of its chromosomes before mitosis; these duplicate chromosomes are called sister chromatids, and are attached at their centromere. In animals and plants, the nuclear envelope degrades during mitosis. Microtubules emanate from opposite ends of the cell to latch onto the sister chromatids, then shorten and pull the sister chromatids apart into sister chromosomes. The cell elongates and the nuclear envelope reforms. By now, cytokinesis is already underway, and by the time it finishes the parent cell has been split in half into two daughter cells that are identical to it. This is similar to binary fission in prokaryotes, but is different in that prokaryotes lack a nucleus and have a single chromosome without a centromere.
| Step | Events |
|---|---|
| S Phase | Chromosomes duplicate and condense. Sister chromatids are bound by cohesin. Centrosomes form and emanate microtubules. |
| Prophase | Chromosomes are fully condensed. Microtubules from different centrosomes repel each other, pushing centrosomes to opposite sides. The endoplasmic reticulum reabsorbs the nuclear envelope. |
| Prometaphase | The nuclear envelope finishes its reabsorption into the endoplasmic reticulum. Microtubules bind and bi-orient chromosomes, then pull them toward the equator between the two centrosomes. |
| Metaphase | Chromosomes aligned at the equator, forming an imaginary bisecting plane called the metaphase plate |
| Anaphase | APC/C activated, degrading cohesin binding sister chromatids. Chromosomes separate to the poles of the cell. |
| Telophase | The nuclear envelope reassembles, the cell is pinched at the metaphase plate, forming a contractile ring. |
| Cytokinesis | The interphase microtubule array reforms. The contractile ring forms the cleavage furrow. |
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