Cell cycle

1. What do G1 and G2 phases ensure?

• DNA is synthesized correctly

• That conditions are suitable and preparations are complete

• Cell divides quickly

• Cell skips mitosis

The transition from G1 to S phase is crucial because:

• Initiates mitosis and cell division

• This regulates signals from other cells that control cell proliferation.

• Inactivates essential proteins for cell cycle regulation.

• Prepares the mitotic spindle for chromosome segregation.

If the attachment of duplicated chromosomes to the mitotic spindle is not properly regulated, what could be the most immediate consequence for the daughter cells?

• The unequal chromosome distribution, leading to genetic disorders.

• Incorrect DNA replication, causing mutations in the genetic material.

• Complete cessation of cell division due to mitotic spindle collapse.

• Skipping of the M phase, resulting in early entry into interphase.

Why cyclins important for regulating the cell cycle, despite having no enzymatic activity themselves?

• Can provide structural support to the cell-cycle kinases, enabling them to function.

• Activate cell-cycle kinases, allowing them to become enzymatically active and trigger cell-cycle events.

• Serve as the primary drivers of cell division by directly regulating DNA replication

• Inhibit the action of protein kinases, preventing premature cell-cycle progression.

Which mechanism would most likely halt the cell cycle if DNA damage is detected during G1?

• Activation of phosphatases

• Increased levels of Cdk inhibitors

• Degradation of cyclins

• Activation of apoptosis signaling during mitosis

A researcher claims that cells in G0 can re-enter the cell cycle under certain conditions. Which observation would best support this claim?

• Increased levels of M-Cdk in G0 cells

• Expression of cyclins and Cdks in response to growth signals

• Complete bypass of S phase in G0 cells

• Chromosome alignment during G0 phase

Come up with designing a sequence of events that a cell must follow to transition from G1 to S phase. Which step is essential to initiate DNA synthesis?

• Mitogen activation → G1/S cyclin buildup → Rb protein inactivation → DNA synthesis

• Cyclin elimination → Mitotic spindle formation → Rb protein activation → DNA synthesis

• Phosphorylation of Cdk → Chromosome alignment → Rb protein inactivation → S phase

• Cyclin elimination → Chromosome duplication → Rb protein activation → DNA synthesis

Explain the difference in cell division rates between gut epithelial cells and liver cells:

• Gut epithelial cells are permanently arrested in G0, while liver cells cycle frequently.

• Liver cells spend most of their time in G0, while gut epithelial cells rapidly cycle due to short G1 phases.

• Gut epithelial cells divide more slowly than liver cells due to longer G1 phases.

• Gut epithelial cells rely more heavily on p53 than liver cells.

Which mechanism ensures that DNA replication occurs only once per cell cycle, preventing re-replication?

• S-Cdk inactivates the origin recognition complex (ORC) and Cdc6 through phosphorylation.

• DNA damage response inhibits Cdc25, delaying M-phase entry.

• The pre-replicative complex is activated by the removal of inhibitory phosphates by Cdc25

• Cdc6 accumulates throughout S phase to prevent re-initiation of DNA replication.

Why is the precise regulation of cyclin concentrations critical during the cell cycle?

• It ensures cyclins accumulate only when needed and are degraded at the correct time, enabling proper phase transitions.

• They allows cyclins to remain active at all times, speeding up the cell cycle

• Prevents cyclins from being synthesized during mitosis, ensuring mitotic proteins are activated instead.

• Guarantees the continuous activation of cyclins, ensuring uninterrupted progression through the cycle.