1. Cell Biology Cell Division
Alberts, Bruce. Essential Cell Biology. 4th ed. New York, NY: Garland Science Pub., Print.
Copyright © Garland Science 2013

2. The Cell Division Cycle
A cell reproduces by carrying out an orderly sequence of events:
duplicates its contents and divides in two.
This cycle of duplication and division, known as the cell cycle, is the essential mechanism by which all living things reproduce.
Each cell cycle produces two genetically identical daughter cells.

3. The cell cycle is divided into M, G1, S, and G2 phases
The cell grows continuously in interphase, which consists of three phases: G1, S, and G2.
DNA replication is confined to S phase (S = synthesis)
G1 is the gap between M phase and S phase (G = gap).
G2 is the gap between S phase and M phase.
During M phase, the nucleus divides first, in a process called mitosis; then the cytoplasm divides, in a process called cytokinesis.
The period between one M phase and the next is called interphase.

4. Checkpoints in the cell-cycle control system ensure that key processes in the cycle occur in the proper sequence
Eucaryotic cells possess a cell-cycle control system that can stop the cycle at various checkpoints.
Three checkpoints :
The checkpoint in G1 determines whether the cell proceeds to S phase;
The one in G2 determines whether the cell proceeds to mitosis;
The one in M phase determines whether the cell is ready to pull the duplicated chromosomes apart and segregate them into two new daughter cells.

5. Progression through the cell cycle depends on cyclin-dependent protein kinases (Cdks)
A Cdk must bind a regulatory protein called a cyclin before it can become enzymatically active.
The active cyclin–Cdk complex phosphorylates key proteins in the cell that are required to initiate cell cycle.
The cyclin also helps direct the Cdk to the target proteins that the Cdk phosphorylates.

6. The accumulation of cyclins regulates the activity of Cdks
The formation of active cyclin–Cdk complexes drives various cell-cycle events, including entry into S phase or M phase.
The increase in the cyclin concentration helps form the active cyclin–Cdk complex that drives entry into M phase.
Although the enzymatic activity of the cyclin–Cdk complex rises and falls during the course of the cell cycle, the concentration of the Cdk component does not.

7. For a Cdk to be active, it must be phosphorylated at one site and dephosphorylated at two other sites
When it first forms, the cyclin-Cdk complex is not phosphorylated and is inactive.
Subsequently, the Cdk is phosphorylated at a site that is required for its activity
and at two other (overriding) sites that inhibit its activity.
This phosphorylated complex remains inactive until it is finally activated by a protein phosphatase that removes the two inhibitory phosphate groups.
For simplicity, only one inhibitory phosphate group is shown here.

8. A checkpoint in G1 offers the cell a crossroad
The cell can commit to completing another cell cycle by entering to S phase, pause temporarily until conditions are right, or withdraw from the cell cycle altogether and enter G0.
In some cases, cells in G0 can re-enter the cell cycle when conditions improve.
But many cell types permanently withdraw from the cell cycle when they differentiate, persisting in G0 for the lifetime of the animal.

9. The cell-cycle control system can arrest the cycle at various checkpoints
The red “T”s represent points in the cycle where the control system can apply molecular brakes (such as Cdk inhibitor proteins) to stop progression in response to DNA damage, intracellular processes that have not been completed, or an unfavorable extracellular environment.
The checkpoint indicated in M phase ensures that all of the chromosomes are appropriately attached to the mitotic spindle before the duplicated chromosomes are pulled apart.

10. S-Cdk triggers DNA replication and ensures that DNA replication is initiated only once per cell cycle
The origins of replication recruit the origin recognition complex (ORC), which remain associated with origins of replication throughout the cell cycle.
In early G1, the regulatory protein Cdc6 associates with the ORC.
Aided by Cdc6, additional proteins bind to the adjacent DNA, resulting in the formation of a pre-replicative complex (proteins and DNA).
S-Cdk triggers origin firing by causing the assembly of the protein complexes that initiate DNA synthesis.
S-Cdk also helps block re-replication by helping to phosphorylate Cdc6, which dissociates from the origin and is degraded.

11. Cohesins tie together the two adjacent sister chromatids in each replicated chromosome
After the chromosomes have been duplicated in S phase, the two copies of each replicated chromosome remain tightly bound together as identical sister chromatids.
The sister chromatids are held together by protein complexes called cohesins, which assemble along the length of each sister chromatid as the DNA is replicated in S phase.
Cohesins form large protein rings that surround the sister chromatids, preventing them from coming apart, until the rings are broken late in mitosis.

12. DNA damage can arrest the cell cycle at G1
When DNA is damaged, specific protein kinases respond by activating the p53 protein and halting its normal rapid degradation.
Activated p53 protein then accumulates and binds to DNA.
There it stimulates the transcription of the gene that encodes the Cdk inhibitor protein, p21.
The p21 binds to G1/S- Cdk and S-Cdk and inactivates them, so that cell cycle arrests in G1.

13. For M-Cdk to be active, it must be phosphorylated at one site and dephosphorylated at other sites
The M cyclin–Cdk complex is enzymatically inactive when first formed.
Subsequently, the Cdk is phosphorylated at one site that is required for its activity by an enzyme called Cdk-activating kinase, Cak.
It is also phosphorylated at two other, overriding sites that inhibit its activity (by an enzyme called Wee1); for simplicity, only one inhibitory phosphate group is shown.

14. Activated M-Cdk indirectly activates more M-Cdk, creating a positive feedback loop
Once activated, M-Cdk phosphorylates, and thereby activates Cdk- activating phosphatase (Cdc25).
The phosphatase can now activate more M-Cdk
by removing the inhibitory phosphate groups from the Cdk subunit.

15. Condensins help to coil the mitotic chromatids into smaller, more compact structures for segregation during mitosis
Condensin proteins might compact a single chromatid by coiling up long loops of DNA.
A scanning electron micrograph of a condensed human mitotic chromosome, consisting of two sister chromatids joined along their length.
The constricted region (arrow) is the centromere, where each chromatid will attach to the mitotic spindle to pulls the sister chromatids apart toward the end of mitosis.

16. Two transient cytoskeletal structures mediate M phase in animal cells
The mitotic spindle assembles first to separate the replicated chromosomes.
Then, the contractile ring assembles to divide the cell in two.
Whereas the mitotic spindle is based on microtubules, the contractile ring is based on actin and myosin filaments.

17. Cell division occurs in the M phase of the cell cycle.
M phase consists of nuclear division, or mitosis, and cytoplasmic division, or cytokinesis.
Mitosis is itself divided into five stages together with cytokinesis.
During interphase, the cell increases in size. The DNA of the chromosomes is replicated, and the centrosome is duplicated.

18. Prophase and prometaphase
At prophase, the replicated chromosomes, each consisting of two closely associated sister chromatids, condense.
Outside the nucleus, the mitotic spindle assembles between the two centrosomes, which have begun to move apart.
Prometaphase starts abruptly with the breakdown of the nuclear envelope.
Chromosomes can now attach to spindle microtubules via their kinetochores and undergo active movement.

19. Metaphase and anaphase
At metaphase, chromosomes are aligned at the equator of the spindle, midway between the spindle poles.
The paired kinetochore microtubules on each chromosome attach to opposite poles of the spindle.
At anaphase, the sister chromatids synchronously separate, and each is pulled slowly toward the spindle pole it is attached to.
The kinetochore microtubules get shorter, and the spindle poles also move apart, both contributing to chromosome segregation.

20. Telophase and cytokinesis
During telophase, the two sets of chromosomes arrive at the poles of the spindle.
A new nuclear envelope reassembles around each set, completing the formation of two nuclei and marking the end of mitosis.
During cytokinesis of an animal cell, the cytoplasm is divided in two by a contractile ring of actin and myosin filaments, which pinches in the cell to create two daughters, each with one nucleus.
The division of the cytoplasm begins with the assembly of the contractile ring.

21. The centrosome in an interphase cell duplicates to form the two mitotic spindle poles
In most animal cells in interphase, a centriole pair (dark green bars) is associated with the centrosome matrix (light green) that nucleates microtubule outgrowth.
Centrosome duplication begins at the start of S phase and is complete by the end of G2.
The two centrosomes remain together initially but separate in early M phase with its own aster.
The microtubules that interact between the two asters elongate preferentially to form a bipolar mitotic spindle.
When the nuclear envelope breaks down, the spindle microtubules are able to interact with the chromosomes.

22. A bipolar mitotic spindle is formed by the selective stabilization of interacting microtubules.
New microtubules grow out in random directions from the two centrosomes.
The minus end of a microtubule is anchored in the centrosome, whereas the free plus ends are dynamically unstable and switch suddenly from uniform growth to rapid shrinkage.
When two microtubules from opposite centrosomes interact in an overlap zone, motor proteins and other microtubule- associated proteins cross-link the microtubules together (black dots).
The cross-link stabilizes the plus ends by decreasing their depolymerization.

23. Kinetochores attach chromosomes to the mitotic spindle
(A) A fluorescence micrograph of a replicated mitotic chromosome. The DNA is stained with a fluorescent dye, and the kinetochores are stained red with fluorescent antibodies that recognize kinetochore proteins.
The spindle microtubules are attached to the chromosomes through specialized protein complexes called kinetochores, which assemble on the centcondensed chromosomes during late prophase
(B) Schematic drawing of a mitotic chromosome:
Its two sister chromatids are attached to kinetochore microtubules.
Each kinetochore forms a plaque on the surface of the centromere.

24. Three classes of microtubules make up the mitotic spindle
(A) A spindle with chromosomes attached, showing the three types of spindle microtubules: astral microtubules, kinetochore microtubules, and interpolar microtubules.
(B) Fluorescence micrograph of chromosomes at the metaphase plate of a real mitotic spindle. In this image, kinetochores are labeled in red, microtubules in green, and chromosomes in blue.

25. Motor proteins and chromosomes can direct the assembly of a functional bipolar spindle in the absence of centrosomes
In these fluorescence micrographs, the microtubules are stained green and the
chromosomes red.
The top micrograph shows a normal spindle formed with centrosomes in a normally fertilized embryo.
The bottom micrograph shows a spindle formed without centrosomes in an embryo that initiated development without fertilization and thus lacks the centrosome normally provided by the sperm when it fertilizes the egg.
Note that the spindle with centrosomes has an aster at each pole, whereas the spindle formed without centrosomes does not. Both types of spindles are able to segregate the daughter chromosomes.

26. During metaphase, chromosomes gather halfway between the two spindle poles
This fluorescence micrograph shows multiple mitotic spindles at metaphase in a fruit fly (Drosophila) embryo.
The microtubules are stained red, and the chromosomes are stained green.
At this stage of Drosophila development, there are multiple nuclei in one large cytoplasmic compartment, and all of the nuclei divide synchronously at the same stage of the cell cycle: metaphase.
When metaphase spindles are viewed in three dimensions, the chromosomes are seen to be gathered at a platelike region at the equator of the spindle - the metaphase plate.

27. At the beginning of anaphase, each pair of sister chromatids separates
During mitosis, the cohesin proteins are cleaved and the sister chromatids are pulled to opposite poles of the cell by the mitotic spindle.
Metaphase
Anaphase

28. The APC triggers the separation of sister chromatids by promoting the destruction of cohesins
Activated APC (anaphase- promoting complex) indirectly triggers the cleavage of the cohesins that hold sister chromatids together.
APC catalyzes the ubiquitylation and destruction of an inhibitory protein called securin.
Securin inhibits the activity of a proteolytic enzyme called separase; when freed from securin, separase cleaves the cohesin complexes, allowing the mitotic spindle to pull the sister chromatids apart.
The APC also targets M cyclin for destruction, thus rendering the M-Cdk complex inactive, which helps to initiate the exit from mitosis.

29. Two processes segregate daughter chromosomes at anaphase
In anaphase A, the daughter chromosomes are pulled toward opposite poles as the kinetochore microtubules depolymerize at the kinetochore. The force driving this movement is generated mainly at the kinetochore.
In anaphase B, two spindle poles move apart as the result of two separate forces:
(1) the elongation and sliding of the interpolar microtubules past one another pushes the two poles apart, and (2) forces exerted by outward-pointing astral microtubules at each spindle pole pull the poles away from each other, toward the cell cortex.

30. The nuclear envelope breaks down and re-forms during mitosis
The phosphorylation of nuclear pore proteins and lamins helps trigger the
disassembly of the nuclear envelope at prometaphase.
Dephosphorylation of pore proteins and lamins at telophase helps reverse the process.

31. The contractile ring divides the cell in two
(A) Scanning electron micrograph of an animal cell in cytokinesis.
(B) Diagram of the midregion of a similar cell showing the contractile ring beneath the plasma membrane and the remains of the two sets of interpolar microtubules.
(B)
(C) An electron micrograph of a dividing animal cell. Cleavage is almost complete, but the daughter cells remain attached by a thin strand of cytoplasm containing the remains of the overlapping interpolar microtubules of the central mitotic spindle.

32. Animal cells change shape during M phase
In these micrographs of a mouse fibroblast dividing in culture, the same cell was photographed at successive times.
Note how the cell rounds up as it enters mitosis; the two daughter cells then flatten out again after cytokinesis is complete.

33. Apoptosis is mediated by an intracellular proteolytic cascade
Each suicide protease (caspase) is made as an inactive proenzyme, a procaspase, which is itself often activated by proteolytic cleavage by another member of the same protease family.
Two cleaved fragments from each of two procaspase molecules associate to form an active caspase, which is formed from two small and two large subunits; the two prodomains are usually discarded.

34. Apoptosis is mediated by an intracellular proteolytic cascade
Each activated caspase molecule can then cleave many procaspase molecules, thereby activating them, and these can activate even more procaspase molecules.
An initial activation of a small number of protease molecules can lead, via an amplifying chain reaction (a cascade), to the explosive activation of a large number of protease molecules.
Some of the activated caspases then break down a number of key proteins in the cell, such as nuclear lamins, leading to the controlled death of the cell.

35. Bax and Bak can trigger apoptosis
Bax and Bak are death-promoting members of the Bcl2 family.
When Bak or Bax is activated by an apoptotic stimulus, it aggregates in the outer mitochondrial membrane, leading to the release of cytochrome c into the cytosol.
Cytochrome c then binds to an adaptor and assembles into a seven-armed complex.
This complex procaspase-9 to form a structure called an apoptosome.
The procaspase-9 become activated within the apoptosome and now activate different procaspases in the cytosol, leading to a caspase cascade and apoptosis.

36. Cell death helps adjust the number of developing nerve cells
Cell death helps adjust the number of developing nerve cells to the number of target cells they contact.
More nerve cells are produced than can be supported by the limited amount of survival factor released by the target cells.
Therefore, some cells receive insufficient amounts of survival factor to keep their suicide program suppressed and, as a consequence, undergo apoptosis.
This strategy of overproduction followed by culling ensures that all target cells are contacted by nerve cells and that the ‘extra’ nerve cells are automatically eliminated.

37. Survival factors suppress apoptosis by regulating Bcl2 family members
Survival factors usually act by binding to cell- surface receptors.
The activated receptor activates a transcription regulator in the cytosol.
This protein moves to the nucleus, where it activates the gene encoding Bcl2, a protein that inhibits apoptosis.

38. Mitogens stimulate cell proliferation by inhibiting the Rb protein
(A) In the absence of mitogens (secreted signal proteins), dephosphorylated Rb (Retinoblastoma) protein holds the transcription regulators to inhibit the transcription of their target genes needed for cell proliferation.
(B) Mitogens bind to cell-surface receptors and activate the signaling pathways that lead to activation of the G1-Cdk and G1/S-Cdk complexes, which phosphorylate and inactivate the Rb protein. This leads to the transcription of target genes for cell proliferation.

39. Extracellular growth factors and cell growth
Extracellular growth factors increase the synthesis and decrease the degradation of macromolecules.
This leads to a net increase in macromolecules and thereby cell growth.
Some extracellular signal proteins can act as both growth factors and mitogens, stimulating both cell growth and progression through the cell cycle.
Such proteins help ensure that cells maintain their appropriate size as they proliferate.