Regulation of Cell Cycle - AP Biology

Which option best distinguishes cytokinesis from mitosis?

Cytokinesis divides cytoplasm; mitosis divides the nucleus. Two distinct processes that together complete cell division into daughter cells.
What event characterizes the anaphase of mitosis?

Separation of sister chromatids. Cohesin cleavage allows chromosome movement to poles.
What is the most direct cell-cycle effect of p53 activation after DNA damage?

It induces p21 to inhibit CDKs and arrest the cycle (often at $G_1$ ). p21 binds and inhibits CDK complexes needed for S-phase progression.
Identify the immediate consequence if the M checkpoint fails during mitosis.

Chromosome mis-segregation leading to aneuploid daughter cells. Unequal chromosome distribution creates cells with abnormal chromosome numbers.
Which protein is crucial for the G₁ checkpoint control?

p53 protein. Guardian of the genome; halts cycle for DNA repair.
Which option best predicts the effect of a nonfunctional APC/C on mitosis?

Failure to enter anaphase due to securin and cyclin persistence. Unable to degrade key proteins required for anaphase progression and mitotic exit.
What is the main characteristic of the S phase?

DNA replication occurs. Chromosome duplication creates sister chromatids.
What is the primary signal for mitotic entry?

Activation of cyclin B/CDK1 complex. Active kinase triggers nuclear envelope breakdown.
What is the role of growth factors in animal cell-cycle control?

They provide external signals that stimulate passage through $G_1$ . Signal adequate nutrients and conditions for cell division to proceed.
Which checkpoint confirms all kinetochores are attached to spindle microtubules before anaphase?

The M checkpoint (spindle assembly checkpoint). Prevents chromosome mis-segregation by ensuring proper spindle attachment.
Name the protein complex that degrades cyclins to regulate the cell cycle.

Anaphase-promoting complex (APC). Ubiquitin ligase targets cyclins for destruction.
What is the normal function of the Rb (retinoblastoma) protein in $G_1$ ?

It inhibits E2F, preventing transcription of S-phase genes. Hypophosphorylated Rb sequesters E2F; phosphorylation releases E2F to activate S-genes.
What is anchorage dependence in animal cells?

Cells must attach to a surface (ECM) to progress through the cell cycle. Prevents division of cells that have lost proper tissue contact.
What is the role of cyclins in the cell cycle?

Regulate the progression of the cell cycle. Oscillating proteins that activate CDKs at checkpoints.
What does the term "checkpoint" mean in the context of the cell cycle?

A control point that can pause progression until conditions are correct. Ensures proper order and prevents errors by monitoring completion of previous events.
What is the primary role of cyclins in regulating CDK activity?

They bind CDKs and activate them at specific cell-cycle stages. Cyclins provide timing and specificity by accumulating at precise cell-cycle phases.
Which checkpoint is most directly activated by unreplicated DNA in eukaryotic cells?

The $G_2$ checkpoint. Detects incomplete DNA synthesis and halts progression until replication finishes.
Which phase of mitosis involves chromatin condensation?

Prophase. First phase organizes chromosomes for division.
What protein complex triggers anaphase by ubiquitinating securin and cyclins?

APC/C (anaphase-promoting complex/cyclosome). Ubiquitin ligase that targets securin and M-cyclins for proteasomal degradation.
What is the function of the protein kinase Wee1?

Inhibits CDKs to regulate cell cycle entry. Phosphorylates CDK1 to keep it inactive until appropriate.
Which checkpoint primarily ensures DNA replication is complete and undamaged before mitosis?

The $G_2$ checkpoint. Prevents mitosis when DNA replication errors or damage are detected.
Identify the phase in which the cell prepares for mitosis after DNA replication.

$G_2$ phase. Cell synthesizes proteins required for chromosome condensation and spindle formation.
Which protein regulates the G₂/M transition by inhibiting CDK1?

Wee1 kinase. Checkpoint kinase maintains $G_2$ arrest when needed.
What ensures the fidelity of chromosome segregation during mitosis?

Spindle assembly checkpoint. Monitors attachment status before allowing progression.
What is density-dependent inhibition in animal cells?

Crowding reduces division via cell-cell contact and signaling. Contact inhibition prevents overgrowth when cell density becomes too high.
Which phase follows the completion of mitosis?

Cytokinesis. Physical division of cytoplasm creates two cells.
Which protein can trigger apoptosis if DNA damage is irreparable?

p53 protein. Activates apoptotic pathway when damage is severe.
Name the phase where cells exit the cell cycle and stop dividing.

G₀ phase. Quiescent state for non-dividing differentiated cells.
What is the function of the proteasome in cell-cycle regulation?

It degrades ubiquitin-tagged regulatory proteins (for example, cyclins). Destroys cell-cycle regulators to ensure unidirectional progression through phases.
What is the consequence of a defective spindle assembly checkpoint?

Aneuploidy due to improper chromosome segregation. Unequal chromosome distribution causes genomic instability.
Identify the phase of mitosis where nuclear envelope re-forms.

Telophase. Final mitotic phase reverses nuclear breakdown.
Identify the phase where the cell grows and prepares for DNA replication.

G₁ phase. Gap phase for organelle synthesis and growth.
Which enzyme is activated by cyclins to progress the cell cycle?

Cyclin-dependent kinases (CDKs). Phosphorylate target proteins when bound to cyclins.
Identify the cell-cycle stage where sister chromatids are produced but still joined by cohesin.

After S phase (during $G_2$ and early M). Sister chromatids remain attached until anaphase when cohesin is cleaved.
Which enzyme degrades cohesin to allow anaphase progression?

Separase. Protease cleaves cohesin rings holding chromatids.
Identify the checkpoint ensuring all chromosomes are attached before anaphase.

Spindle assembly checkpoint. M checkpoint ensures proper chromosome attachment.
What is the effect of the overexpression of cyclin E?

Accelerated entry into S phase. Excessive cyclin E bypasses normal checkpoint control.
Which term best describes cancer cells ignoring density-dependent inhibition?

Loss of contact inhibition (reduced density-dependent inhibition). Cancer cells continue dividing despite crowded conditions that stop normal cells.
What is the primary function of cyclin-dependent kinases (CDKs) in cell-cycle regulation?

They phosphorylate target proteins to drive cell-cycle transitions. Phosphorylation activates substrates that control key transitions like S-phase entry or mitosis onset.
Identify the phase in which the cell primarily grows and performs normal functions before S phase.

$G_1$ phase. Cell accumulates materials and energy needed for DNA replication and division.
At which checkpoint is the cell cycle arrested if DNA damage is detected?

G₁/S checkpoint. Restriction point prevents replication of damaged DNA.
Which molecule is a key regulator of the G₁/S transition?

Cyclin D. Early cyclin promotes entry into S phase.
What is the restriction point in animal cells, and where is it located in the cell cycle?

A commitment point in late $G_1$ after which the cell proceeds to S phase. Once passed, cells are committed to completing the division cycle.
What is the name of the nondividing state that cells can enter from $G_1$ ?

$G_0$ phase. Quiescent state where cells exit the cycle but remain metabolically active.
Which tumor suppressor protein can halt the cell cycle when DNA is damaged?

p53. Guardian of the genome that responds to DNA damage by activating checkpoints.
What role does the protein p21 play in the cell cycle?

Inhibits CDKs, acting as a checkpoint regulator. CDK inhibitor enforces $G_1$ /S checkpoint arrest.
Which option best predicts the effect of a loss-of-function mutation in p53?

Reduced DNA-damage arrest and increased accumulation of mutations. Loss of p53 removes DNA damage surveillance and allows damaged cells to proliferate.
What molecular event inactivates Rb and promotes entry into S phase?

Phosphorylation of Rb by cyclin-CDK complexes. Phosphorylated Rb releases E2F transcription factor to promote S-phase entry.
Name the phase where spindle fibers attach to kinetochores.

Prometaphase. Transition phase where kinetochore assembly occurs.
Name the checkpoint that ensures DNA is undamaged before mitosis.

G₂/M checkpoint. Prevents mitosis with damaged or unreplicated DNA.
What is the key event that defines the transition from metaphase to anaphase?

Separase activation leading to cohesin cleavage and sister chromatid separation. Securin degradation releases separase to cleave cohesin rings holding sister chromatids.
What is ubiquitination, in the context of cell-cycle control?

Covalent tagging of proteins for degradation by the proteasome. Small protein tag that marks regulatory proteins for destruction by proteasome.
Identify the phase in which DNA replication occurs in the eukaryotic cell cycle.

S phase. DNA synthesis creates sister chromatids joined at centromeres.
What is an oncogene?

A gain-of-function mutation that promotes uncontrolled cell division. Mutated proto-oncogene that drives excessive cell proliferation when overactive.
Which phase of the cell cycle is the longest?

Interphase. Includes $G_1$ , S, and $G_2$ phases combined.
What is the role of p21 in regulating the cell cycle?

It is a CDK inhibitor that blocks cyclin-CDK activity. Blocks S-phase gene transcription by preventing E2F transcription factor activity.
Which checkpoint primarily verifies cell size, nutrients, and DNA integrity before DNA synthesis?

The $G_1$ checkpoint (restriction point in animals). Prevents S-phase entry if growth conditions are inadequate or DNA is damaged.
Which checkpoint is most directly activated by DNA damage detected before replication begins?

The $G_1$ checkpoint. Responds to DNA breaks or lesions before they can be replicated.
Which molecule acts as a checkpoint inhibitor in the G₂/M transition?

Chk1 kinase. DNA damage checkpoint kinase prevents mitotic entry.
Name the process responsible for dividing the cytoplasm after mitosis.

Cytokinesis. Contractile ring pinches cell membrane in two.
Which option best predicts the effect of a mutation preventing Rb phosphorylation?

Cell-cycle arrest in $G_1$ due to continued inhibition of E2F. Rb remains active and blocks E2F, preventing S-phase gene expression.
Which option best describes how cyclin levels typically change during the cell cycle?

They rise and fall due to regulated synthesis and proteasomal degradation. Periodic destruction and synthesis creates oscillating levels that drive transitions.
What is a tumor suppressor gene?

A gene whose loss of function removes restraints on cell division. Normally acts as brake on cell division; loss promotes cancer development.
What is the primary function of the cell cycle?

To ensure accurate DNA replication and cell division. Prevents genetic defects by controlling division timing.
Identify the mechanism that makes cyclin-CDK control largely unidirectional.

Irreversible proteolysis of key regulators (for example, cyclins) via APC/C. Once cyclins are destroyed, the process cannot reverse until new synthesis occurs.
Which option best predicts the effect of constitutively active cyclin-CDK complexes?

Premature checkpoint bypass and increased uncontrolled proliferation. Removes normal cell-cycle brakes, leading to continuous proliferation signals.
Identify the checkpoint that prevents premature mitosis initiation.

G₂/M checkpoint. Ensures cell is ready for nuclear division.
What is the function of the protein cdc25?

Activates CDKs by dephosphorylation. Phosphatase removes inhibitory phosphates from CDKs.
Which term best describes cancer cells dividing without anchorage dependence?

Anchorage-independent growth. Cancer cells can divide without attachment to extracellular matrix or surfaces.
In which phase are sister chromatids aligned at the cell's equator?

Metaphase. Chromosomes align at metaphase plate before separation.
What is the role of the Rb protein in the cell cycle?

Inhibits the cell cycle; tumor suppressor. Retinoblastoma protein blocks $G_1$ /S transition.
What is checked at the spindle assembly checkpoint?

Chromosome attachment to spindle fibers. Verifies proper bipolar attachment before segregation.
What is the outcome of the G₁/S checkpoint activation?

Cell cycle arrest until DNA repair is complete. Halts progression until repair mechanisms finish.
Identify the phase of the cell cycle where DNA replication occurs.

S phase. Synthesis phase duplicates chromosomes for division.
What is a proto-oncogene?

A normal gene that can become an oncogene when mutated or overexpressed. Encodes proteins that normally promote controlled cell division and growth.
What event marks the beginning of prometaphase?

Nuclear envelope breakdown. Nuclear membrane dissolves for spindle access.
What is apoptosis, and how does it relate to cell-cycle regulation?

Programmed cell death that removes damaged or abnormal cells. Eliminates cells with irreparable damage that could bypass cell-cycle controls.
Which molecule is degraded to allow transition from metaphase to anaphase?

Securin. Inhibitor protein prevents premature chromatid separation.
Which protein complex ensures proper chromosome separation?

Cohesin complex. Holds sister chromatids together until anaphase.

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