Chapter 17 - Cell Cycle

What are DNA damage checkpoints?

Surveillance mechanisms that detect damaged DNA and delay cell-cycle progression until damage is repaired or the cell is directed toward apoptosis.

What are their functions during cell-cycle progression?

They protect genome integrity by preventing damaged DNA from being replicated or passed to daughter cells. ATM and ATR initiate checkpoint signaling; checkpoint kinases inhibit Cdc25 so Cdks cannot drive progression.

What events activate maturation-promoting factor (MPF)?

Cyclin B is synthesized in G2 and binds Cdk1. Cdk1 receives activating phosphorylation at Thr161 and inhibitory phosphorylation at Thr14 and Tyr15. Cdc25 phosphatase removes the inhibitory phosphates at Thr14 and Tyr15 while Thr161 stays phosphorylated. Active cyclin B–Cdk1 (MPF) promotes entry into mitosis

What events terminate MPF activity?

APC/C ubiquitin ligase marks cyclin B for proteasomal degradation at the end of mitosis. Without cyclin B the Cdk1 complex loses activity; Cdk1 is also dephosphorylated.

Which enzymes and sites regulate MPF?

Kinases add phosphates: activating Thr161 phosphorylation and Wee1-mediated inhibitory Thr14/Tyr15 phosphorylation. Cdc25 is the phosphatase that removes inhibitory phosphates. APC/C is the ubiquitin ligase that promotes cyclin B destruction.

How does Cdk4/6–cyclin D drive G1 progression?

Growth factors activate the Ras/Raf/MEK/ERK signaling pathway and stimulate cyclin D synthesis. Cyclin D associates with Cdk4/6 and the complex phosphorylates Rb to permit progression past the G1 restriction point.

What are the major targets of Cdk4/6–cyclin D and their transcriptional roles?

Rb is the key target and is a tumor suppressor that normally restrains E2F. Phosphorylation of Rb releases E2F; E2F activates transcription of cyclin E and DNA synthesis proteins. Cyclin E–Cdk2 helps initiate S phase.

How is cyclin E–Cdk2 regulated?

The Cdk inhibitor p27 binds and inhibits cyclin E–Cdk2. Growth-factor signaling reduces p27 synthesis and promotes phosphorylation and degradation of p27; cyclin E accumulation and Cdk2 activation support S-phase entry.

Compare the two responses to DNA damage.

Repairable damage triggers a temporary checkpoint arrest so repair can occur. Severe or unrepaired damage can trigger p53-dependent apoptosis to prevent propagation of damaged cells.

What are checkpoint kinases and how do they arrest the cell cycle?

DNA damage activates ATM and ATR kinases. They activate checkpoint kinases Chk1/Chk2; these inhibit Cdc25 phosphatases needed to activate Cdk1 and Cdk2. Consequently the cell cycle arrests. ATM and Chk2 also promote p53 activation.

What does MDM2 do to p53 without DNA damage and after severe DNA damage?

Without DNA damage MDM2 keeps p53 levels low by promoting its degradation. In response to DNA damage phosphorylation of p53 reduces its interaction with MDM2; p53 accumulates and activates arrest or apoptotic programs

How does MDM2 modify p53?

MDM2 is an E3 ubiquitin ligase that ubiquitinates p53 and targets it for proteasomal degradation.

What does p53 do after DNA damage?

p53 is a tumor suppressor and transcription factor. It induces p21 (a Cdk inhibitor) to arrest the cycle; if damage persists it can induce pro-apoptotic gene expression.

What roles do APC/C ubiquitin ligase, securin and separase play in metaphase-to-anaphase transition?

Before proper spindle attachment the mitotic checkpoint complex (MCC) inhibits APC/C. Once all kinetochores are attached Cdc20 can activate APC/C. APC/C ubiquitinates securin and cyclin B for degradation. Destruction of securin releases separase which cleaves cohesin and allows sister chromatids to separate; cyclin B destruction promotes mitotic exit.

What are the roles of cohesins and condensins in chromatin condensation?

Cohesin complexes hold sister chromatids together after S phase. During mitosis most cohesin is removed from chromosome arms while centromeric cohesin is retained until anaphase. Activated condensin complexes organize chromatin into compact loops and promote chromosome condensation.

Which kinases regulate chromatin condensation?

Mitotic Cdk1–cyclin B (MPF) and its downstream mitotic kinases Aurora and Polo regulate phosphorylation of proteins involved in mitotic chromosome restructuring

What structural changes occur from G2 to prophase to metaphase?

G2: replicated DNA remains relatively diffuse and sister chromatids are held together. Prophase: chromosomes condense; centrosomes separate and the spindle forms. By metaphase: highly condensed chromosomes align at the metaphase plate with kinetochores attached to spindle microtubules.

How is the centromeric region maintained during condensation?

Most cohesin is removed from chromosome arms during early mitosis but cohesin near centromeres is protected until the metaphase-to-anaphase transition. This keeps sister chromatids paired as chromosomes condense.

How are cohesins replaced by condensins?

During entry into mitosis mitotic kinase-dependent changes remove most cohesin from chromosome arms and activate condensin to organize DNA loops. This is a coordinated remodeling of chromosome structure rather than literal exchange of identical binding sites.

What happens to the nuclear envelope during mitosis and which proteins and enzymes are involved?

MPF (Cdk1–cyclin B) phosphorylates nuclear lamins (intermediate filament proteins) causing nuclear lamina disassembly. Nuclear-envelope breakdown allows spindle access to chromosomes. At mitotic exit dephosphorylation permits lamina and nuclear envelope reassembly.

How do early embryonic cycles differ from typical eukaryotic cycles?

Early embryonic cleavage cycles alternate rapidly between S and M with little or no G1 and G2. Typical somatic cycles include G1 growth and metabolic activity; S DNA replication; G2 error checking and preparation; and M division.

What accounts for different cell-cycle durations?

Time spent in G1 and G2 and the extent of growth and checkpoint control vary. Early embryos skip or shorten gap phases and divide quickly using preexisting cellular resources; resulting daughter cells become progressively smaller.