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Cell Cycle Phases
and Regulation

Master the stages of cell division, checkpoint control, and what happens when regulation fails: the molecular basis of cancer and growth signaling.

MCAT Biological Foundations

What Is the Cell Cycle?

The cell cycle is the ordered series of events by which a cell duplicates its contents and divides into two daughter cells. It ensures faithful transmission of genetic information from one generation to the next.

Interphase
Cell grows, replicates DNA, and prepares for division (~90% of cycle time)
M Phase
Mitosis (nuclear division) + Cytokinesis (cytoplasmic division)

The Three Stages of Interphase

1
\(G_1\) Phase (Gap 1): Cell grows in size, synthesizes proteins and organelles. The cell has \(2n\) chromosomes, each as a single chromatid.
2
S Phase (Synthesis): DNA replication occurs. Each chromosome is duplicated into two sister chromatids joined at the centromere. While the chromosome number stays at \(2n\), the DNA content doubles to a state represented as \(4n\).
3
\(G_2\) Phase (Gap 2): Final preparations for mitosis: centriole replication, additional protein synthesis, and error checking of replicated DNA.

Mitosis: Five Phases of Nuclear Division

Prophase Chromatin condenses Prometa- phase Envelope breaks down Metaphase Align at metaphase plate Anaphase Sisters separate Telophase Nuclei reform

DNA Content Through the Cell Cycle

0 \(2n\) \(4n\) DNA content \(G_1\) S \(G_2\) M \(G_1\)

Cell Cycle Checkpoints

Checkpoints are surveillance mechanisms that ensure each phase is completed correctly before the cell proceeds. Three major checkpoints control the cell cycle:

\(G_1\)/S Checkpoint
The "Restriction Point": checks cell size, nutrients, growth factors, and DNA integrity before committing to S phase
\(G_2\)/M Checkpoint
Verifies DNA replication is complete and error-free before entering mitosis
Spindle Checkpoint
Ensures all chromosomes are properly attached to spindle fibers at the metaphase plate before anaphase begins

Cyclins and CDKs: The Molecular Engine

Cell cycle progression is driven by Cyclin-Dependent Kinases (CDKs) that are activated when bound to their regulatory partner proteins called cyclins. CDKs phosphorylate target proteins to push the cell through each phase.

CDK (inactive) + Cyclin CDK Cyclin Active Complex Phosphorylates targets

Cyclin Concentration Over Time

Concentration \(G_1\) S \(G_2\) M \(G_1\) Cyclin D Cyclin E Cyclin A Cyclin B

Tumor Suppressors vs Oncogenes

Tumor Suppressors Oncogenes
Normal Function Inhibit cell division Promote cell division
Mutation Effect Loss-of-function (both alleles) Gain-of-function (one allele)
Inheritance Recessive (two hits needed) Dominant (one hit sufficient)
Key Examples p53, Rb, BRCA1/2 Ras, Myc, HER2
Cancer Analogy "Broken brake pedal" "Stuck gas pedal"

The Guardians: p53 and Rb

p53: "Guardian of the Genome"
Activated by DNA damage. Halts the cell cycle at \(G_1\)/S by inducing p21 (a CDK inhibitor). If damage is irreparable, p53 triggers apoptosis (programmed cell death).
Rb (Retinoblastoma Protein)
In its hypophosphorylated state, Rb binds and inhibits E2F transcription factors, blocking S phase entry. Cyclin D/CDK4 phosphorylates Rb, releasing E2F to activate S phase genes.
Rb binds E2F S phase OFF CDK4/6 p-Rb releases E2F E2F activates S phase ON

Growth Factor Signaling Cascade

1
Growth factor binds to a receptor tyrosine kinase (RTK) on the cell surface.
2
Receptor dimerization and autophosphorylation activates intracellular signaling.
3
Ras activation: GTP replaces GDP on Ras, launching the MAP kinase cascade (\( \text{Ras} \rightarrow \text{Raf} \rightarrow \text{MEK} \rightarrow \text{ERK} \)).
4
Transcription factors (e.g., Myc, Fos, Jun) are activated, promoting expression of cyclins and genes for cell division.

Apoptosis: Programmed Cell Death

Apoptosis is a controlled self-destruction pathway that eliminates damaged or unnecessary cells without triggering inflammation. It is essential for development, immune regulation, and cancer prevention.

Intrinsic Pathway
Triggered by internal signals (DNA damage, oxidative stress). Mitochondria release cytochrome c, activating caspase-9.
Extrinsic Pathway
Triggered by external death ligands (FasL, TNF) binding death receptors. Activates caspase-8 directly.

Mitosis vs Meiosis: Key Differences

Mitosis Meiosis
Divisions 1 2
Daughter Cells 2 diploid (\(2n\)) 4 haploid (\(n\))
Genetic Identity Identical to parent Genetically unique
Crossing Over No Yes (prophase I)
Function Growth, repair Gamete production
Homolog Pairing No Yes (bivalents in meiosis I)

How to Analyze a Cell Cycle Problem

1
Identify the phase: What stage is the cell in? Look for clues: condensed chromosomes (M), DNA synthesis (S), cell growth (\(G_1\)/\(G_2\)).
2
Determine DNA content: Is it \(2n\) or \(4n\)? Has S phase occurred? Have sister chromatids separated?
3
Check regulation: Which checkpoint applies? Is a tumor suppressor or oncogene involved? Consider p53, Rb, Ras, and cyclin/CDK pairs.
4
Predict the outcome: Will the cell proceed, arrest, or undergo apoptosis? What happens if a checkpoint fails?

Example: p53 Mutation Scenario

A cell sustains UV-induced DNA damage. In a normal cell, what happens at the \(G_1\)/S checkpoint?

1
DNA damage is detected by sensor kinases (ATM/ATR).
2
p53 is stabilized (normally degraded by MDM2; damage blocks degradation).
3
p53 activates transcription of p21, a CDK inhibitor.
4
p21 inhibits Cyclin E/CDK2, arresting the cell in \(G_1\) until DNA is repaired.

Example: Flow Cytometry Analysis

A flow cytometry experiment measures DNA content per cell. You observe two peaks: one at \(2n\) and one at \(4n\), with a bridge of cells between them.

DNA content per cell Cell count \(2n\) \(G_1\)/\(G_0\) S phase \(4n\) \(G_2\)/M

Common Mistakes

✕ Confusing chromosome number with DNA content
Chromosome number (\(2n\)) doesn't change until separation. DNA content doubles in S phase: a cell in \(G_2\) has \(2n\) chromosomes but \(4n\) DNA.
✕ Oncogenes are "mutant" genes
Proto-oncogenes are normal genes. They become oncogenes only after a gain-of-function mutation. The normal version promotes controlled growth.
✕ Thinking p53 is always active
In normal cells, p53 is constantly degraded by MDM2. It only accumulates and becomes active when DNA damage stabilizes it.

Phase Summary Table

Phase Key Events DNA Content Checkpoint
\(G_1\) Cell growth, organelle synthesis \(2n\) \(G_1\)/S (Restriction)
S DNA replication \(2n \to 4n\) Intra-S
\(G_2\) Protein synthesis, error check \(4n\) \(G_2\)/M
Mitosis Chromosome segregation \(4n \to 2n\) Spindle assembly
\(G_0\) Quiescence (non-dividing) \(2n\) N/A

Question 1 of 5

A cell in \(G_2\) of the cell cycle has how many copies of its genome?

Question 2 of 5

Which protein directly inhibits CDK activity to arrest the cell cycle at the \(G_1\)/S checkpoint after DNA damage?

Question 3 of 5

A researcher discovers a mutation that causes the Ras protein to be constitutively active. This mutation is best classified as:

Question 4 of 5

During which phase of mitosis are sister chromatids pulled to opposite poles of the cell?

Question 5 of 5

A cell treated with a drug that prevents phosphorylation of Rb would most likely arrest in which phase?