Molecular and Cell Biology: The Cell Cycle

Questions and Answers

What is the primary function of the retinoblastoma protein (pRB) in the G1 phase of the cell cycle?

Activate cyclin-dependent kinases

Prevent progression into S phase (correct)

Enhance mitogen signaling

Initiate DNA replication

Which of the following statements accurately describes the role of cyclins in the cell cycle?

Cyclins regulate cell division by activating cdks. (correct)

Cyclins are always active without cdks.

Cyclins prevent the phosphorylation of pRB.

Cyclins are synthesized continuously throughout the cycle.

How do activated cyclin-dependent kinases (cdks) influence the cell cycle?

By deactivating the E2F transcription factor

By phosphorylating the retinoblastoma protein (pRB) (correct)

By preventing DNA replication during the G2 phase

By inhibiting mitogen signaling

Which of the following best describes the regulatory mechanism of cdks during the G1 phase?

cdks must bind to cyclins to become enzymatically active Signup and view all the answers

What role does topoisomerase II play in the G2 phase of the cell cycle?

Untangles chromosomes to facilitate replication Signup and view all the answers

What is the primary function of the G1 phase in the cell cycle?

Cells grow and prepare for DNA synthesis. Signup and view all the answers

Which proteins are primarily involved in regulating cell cycle checkpoints?

Cyclins and CDKs (Cyclin-dependent kinases) Signup and view all the answers

What role do ATM/ATR kinases play in the cell cycle?

They are responsible for DNA repair mechanisms. Signup and view all the answers

What is the significance of the restriction point in the G1 phase?

It signifies the commitment to enter the S phase based on favorable signals. Signup and view all the answers

Which statement correctly describes intercellular signals' role in the cell cycle?

They are necessary for the stimulation of the G1 phase. Signup and view all the answers

What initiates the activation of cdk1 during the G2/M checkpoint?

Activation by cdc25 phosphatase Signup and view all the answers

What is true about the M phase of the cell cycle?

It includes both mitosis and cytokinesis. Signup and view all the answers

What role do checkpoints play in the cell cycle?

They serve as regulatory mechanisms to prevent uncontrolled growth. Signup and view all the answers

What are the consequences if no checkpoints function correctly in the cell cycle?

Cells will undergo hyperplasia and neoplasia. Signup and view all the answers

Which statement about the G0 phase is accurate?

Most cells in G0 are terminally differentiated and cannot re-enter the cell cycle. Signup and view all the answers

Which complex is responsible for initial phosphorylation of pRB in cells re-entering the cell cycle from G0?

cdk4/6-cyclin D Signup and view all the answers

What ultimately happens during telophase and cytokinesis?

Separation of daughter cells occurs. Signup and view all the answers

How many pairs of chromosomes are present in a human cell after mitosis?

46 Signup and view all the answers

Which protein complexes are crucial in controlling the transition from G1 to S phase?

Cyclin D/cdk4/6 Signup and view all the answers

What role does dephosphorylation of lamins play in the cell cycle?

It leads to the reformation of the nuclear membrane. Signup and view all the answers

Study Notes

Overview of the Cell Cycle

The cell cycle consists of phases: G0, G1, S, G2, and M.
It regulates cell division to maintain proper cellular function and integrity.

Phases of the Cell Cycle

G1 Phase (Growth phase):
- Allows daughter cells to grow post-cytokinesis.
- Monitors environmental signals for favorable growth (e.g., growth factors).
- Contains a restriction point, determining commitment to enter S phase.
- Activated by cyclin-dependent kinases (cdk) regulated by cyclins D and E.
S Phase (DNA Synthesis):
- DNA replication occurs, regulated by cdk2/cyclin A.
G2 Phase (Growth phase):
- Cells prepare for mitosis by building cell contents and organizing chromatin.
- Monitors DNA for damage and overall cell conditions, regulated by the same cdk2/cyclin A complex.
M Phase (Mitosis):
- Involves nuclear division, chromosome separation, and cytokinesis to produce two daughter cells with 23 chromosome pairs.

Cell Cycle Regulation

Cyclins and cyclin-dependent kinases (cdks) play critical roles in cell cycle progression.
pRB (Retinoblastoma Protein):
- Functions as a tumor suppressor.
- When hypophosphorylated, it prevents cell cycle progression by binding E2F transcription factor.
- Phosphorylation by cdk4/6/cyclin D and cdk2/cyclin E releases E2F to initiate DNA replication.

Checkpoints and Control Mechanisms

Cell cycle checkpoints enforce control and ensure DNA integrity.
G1 Checkpoint:
- Ensures favorable conditions for DNA synthesis.
- If conditions are unfavorable, pRB remains bound to E2F, halting progression.
G2/M Checkpoint:
- Monitors for DNA damage before mitosis.
- Regulated by cdk1/cyclin B activity; influenced by the inactivation of cdc25 and activation of Wee1 kinase.
Metaphase-Anaphase Checkpoint:
- Prevents chromosome abnormalities by ensuring chromosomes are correctly attached to the spindle apparatus.

Role of ATM/ATR in DNA Repair

ATM/ATR kinases act as sensors for DNA damage.
Upon detecting damage, they induce cell cycle arrest and activate pathways leading to repair.
They stimulate the production of p53, a key tumor suppressor, which then promotes p21 to inhibit cdks and prevent further cycle progression.

Implications for Cancer

Disruption of the cell cycle can lead to uncontrolled growth, hyperplasia (reversible) and cancer (irreversible).
Elevated cdks or depressed checkpoint functions are linked to cancer progression and are potential targets for therapeutic interventions.

Summary

The cell cycle is tightly regulated by cyclins, cdks, and tumor suppressors like p53 and pRB.
Checkpoints ensure the integrity of the genome before cell division.
Understanding these processes is crucial for insights into cell proliferation, differentiation, and cancer development.

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Description

This quiz focuses on the cell cycle, its phases, and regulatory proteins involved in cell division. Participants will identify various checkpoints and distinguish between the restriction point and checkpoints, along with understanding the role of ATM/ATR kinases in DNA repair during the cell cycle.