Eukaryotic Cell Cycle Overview

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Questions and Answers

Which of the following is a consequence of excessive cyclin production?

Activation of p53

Induction of apoptosis

Uncontrolled cell division (correct)

Inhibition of DNA replication

What is the role of p21 in the cell cycle?

Replicates DNA

Activates cyclins

Inhibits CDK/cyclin complexes (correct)

Facilitates cell cycle progression

In cells with mutated p53, what is the likely outcome after exposure to high levels of radiation?

Enhanced DNA repair mechanisms

Increased apoptosis

Normal cell cycle progression

Failure to undergo apoptosis (correct)

What triggers programmed cell death when DNA damage is excessive?

Activation of p53-dependent genes Signup and view all the answers

What happens if defects in DNA replication are detected in the cell cycle?

Cell is arrested in the current phase Signup and view all the answers

What purpose does replication licensing serve during the cell cycle?

To prevent further DNA replication until the next mitosis. Signup and view all the answers

Which phase of the cell cycle is characterized by an increase in DNA polymerase levels?

S phase Signup and view all the answers

What triggers the transition from G2 phase to the mitotic (M) phase?

The completion of DNA replication Signup and view all the answers

Which of the following statements regarding cyclins is true?

Cyclins help regulate the cell cycle by varying in concentration. Signup and view all the answers

What is the role of checkpoints in the cell cycle?

To ensure prior phases have been adequately completed. Signup and view all the answers

In which phase do cells permanently exit the cell cycle?

G0 phase Signup and view all the answers

What primarily controls the progression through the various phases of the cell cycle?

Cyclins and cyclin-dependent kinases (Cdks) Signup and view all the answers

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

To replicate DNA Signup and view all the answers

What is the main function of CDK enzymes in the cell cycle?

To control progression through cell cycle checkpoints Signup and view all the answers

Which phase of the cell cycle is primarily associated with DNA synthesis?

S phase Signup and view all the answers

What happens to the retinoblastoma (Rb) protein when it is phosphorylated by CDK4 or CDK6?

It releases E2F, allowing gene transcription to occur Signup and view all the answers

During which checkpoint is the damage to DNA assessed before cell division can proceed?

G2 Checkpoint Signup and view all the answers

What is the role of cyclins in the activation of CDK complexes?

They bind and activate CDKs to promote cell cycle progression Signup and view all the answers

What occurs at the M checkpoint during the cell cycle?

Spindle assembly is assessed to ensure proper chromosome separation Signup and view all the answers

Which of the following can disrupt cell cycle control, potentially leading to cancer?

Disruption caused by oncoviruses and oncogenes Signup and view all the answers

What is characteristic of chromosomes during metaphase compared to interphase?

Chromosomes are nearly completely transcriptionally inactive Signup and view all the answers

Study Notes

Cell Cycle Overview

Eukaryotic cells have a cell cycle divided into interphase and the mitotic (M) phase.
Interphase is where the cell grows and copies its DNA.
The mitotic (M) phase is where the cell separates its DNA into two sets and divides its cytoplasm, forming two new cells.

Eukaryotic Cell Cycle Stages

G1 phase (Gap 1): The period before DNA replication.
S phase (Synthesis): DNA replication occurs.
G2 phase (Gap 2): Occurs before mitosis (M).
G0 phase: Cells that have stopped dividing, like mature T lymphocytes, are out of the cell cycle. Quiescent cells can re-enter the G1 phase to resume division.

DNA Synthesis in S Phase

DNA synthesis only occurs once per cell cycle and only during the S phase.
Eukaryotic cells have control mechanisms for this, gene products governing transitions between phases.
Cyclins are proteins changing in concentration at specific times. This cycle aspect gives them their name.
Cyclin-dependent kinases (Cdks) are important proteins for regulating progression through the cell cycle.
During S phase, nuclear DNA is completely replicated once. Chromatin is marked to prevent further replication until mitosis.
Replication licensing is a process that ensures DNA is replicated only once per cycle.

Cell Cycle Checkpoints

Cell cycle is controlled by checkpoints ensuring each phase completes correctly before the next.
Key protein classes involved include cyclins and cyclin-dependent kinases (Cdks).
G1 checkpoint: Checks for cell size, nutrients, growth factors, and DNA damage.
S checkpoint: Checks for DNA synthesis being complete.
G2 checkpoint: Checks for cell size, DNA replication, and damage.
M checkpoint: Checks for chromosome spindle attachment.

CDK Enzymes and Checkpoints

Kinases are a group of enzymes that move the cell to the next phase of the cell cycle.
Kinases bind to cyclins to form a CDK complex.
This complex phosphorylates a checkpoint protein that activates the protein and allows the cell cycle to continue.
Checkpoint proteins are broken down after their role is done.
Example: The D cyclins activate CDK4 and CDK6. These kinases are synthesized during G1 in actively dividing cells.

Retinoblastoma (Rb) Protein

Rb is a substrate for CDK4 or CDK6.
Rb binds to and inactivates a transcription factor (E2F).
Phosphorylation of Rb by CDK4/6 releases E2F, activating gene transcription, and progression to S phase.

Cyclins and Their Corresponding Kinases

Specific cyclins (D, E, A, B) regulate cell cycle progression by combining with specific kinases (CDK4/6, CDK2, CDK1) to drive the cycle.
These complexes have specific tasks at particular points in the cell cycle (i.e., entry into S phase).

Cell Cycle and Cancer

Cancer develops if cell cycle checkpoints are damaged, leading to uncontrolled cell division.
Oncoviruses and oncogenes can disrupt normal cell cycle regulation.
Excessive cyclin production, loss of CDK inhibitor, or a cyclin/CDK activation at the wrong time can cause uncontrolled cell division.
Some cancer drugs are inhibitors for CDK complexes.

p53 and DNA Damage

p53 activates transcription of genes that inhibit the cell cycle.
p21 is a protein that efficiently inhibits the action of CDKs, which stops the cell cycle.
If DNA damage is too extensive to repair, p53 triggers apoptosis.
Cells lacking functional p53 are more susceptible to developing cancer. This is because p53 acts as a tumor suppressor.

DNA, Chromosome, Chromosome Segregation

DNA, chromosome, and chromosome segregation integrity is monitored constantly in the cell cycle.
If DNA damage, incomplete DNA replication or defective chromosome segregation machinery is detected, the cell cycle stops in the phase where defects exist.
In some cases, irreparable damage can lead to programmed cell death (apoptosis).

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Description

This quiz covers the key components of the eukaryotic cell cycle, including interphase and the mitotic phase. You'll explore the stages of the cell cycle, including G1, S, G2, and G0 phases, as well as the mechanisms of DNA synthesis. Test your knowledge on cyclins and control mechanisms governing cell division.