Topic 4.5 Notes – Cell Cycle

1. The Cell Cycle

The cell cycle is the ordered series of events that allows a cell to:

• Grow
• Copy its DNA
• Divide into two genetically identical daughter cells

This matters for:

• Growth and development (from embryo to adult)
• Tissue repair (replacing damaged cells)
• Asexual reproduction (in many eukaryotes)

The most important idea is this: DNA is copied once and only once, then separated accurately.
If chromosomes are not copied or separated correctly, daughter cells will be missing genes or have extras.

The cycle alternates between:

• Interphase (preparation: G1, S, G2)
• Division (mitosis + cytokinesis)

Interphase is where most of the cell’s life happens.

2. The Stages of the Cell Cycle

The five main stages are G1, S, G2, mitosis, and cytokinesis.

G1 Phase (Gap 1)

This is a period of intense activity.

• Cell grows larger
• Organelles and cytosolic components are duplicated
• High metabolic activity
• The cell performs its normal function

There is a major G1 checkpoint here. The cell asks:

• Is the DNA damaged?
• Is there enough nutrients and energy?
• Is the cell big enough?

If conditions are not right, the cell does not move forward.

Some cells leave the cycle here and enter G0.

S Phase (Synthesis)

This is where DNA replication occurs.

• DNA is in loose chromatin form
• Each chromosome is copied
• After replication, each chromosome consists of:
  • Two sister chromatids
  • Joined at a centromere
• Centrosome duplication begins

This is huge for the AP exam:
A chromosome after S phase is duplicated but still counts as one chromosome until sister chromatids separate.

G2 Phase (Gap 2)

Final preparation for division.

• More cell growth
• Protein synthesis, especially proteins for mitosis
• High ATP production
• Centrosomes finish replicating

A G2 checkpoint ensures DNA was replicated correctly. If replication errors are detected, the cycle pauses.

Mitosis

Mitosis is nuclear division. The goal is to separate sister chromatids so each new nucleus gets a complete genome.

Cytokinesis

This is division of the cytoplasm.

After cytokinesis, you have two separate daughter cells.

3. G0 and Cell Cycle Regulation

G0 Phase

Some cells exit the cycle and enter G0, a non-dividing state.

• They perform normal functions.
• Some (like neurons) stay permanently.
• Others (like liver cells) can reenter the cycle if signaled.

On exams, they like asking which cells are most likely in G0. Think specialized cells that rarely divide.

Checkpoints and Control

The cycle is controlled by cyclins and cyclin-dependent kinases (CDKs).

• Cyclins bind to CDKs.
• The cyclin-CDK complex activates progression to the next phase.

Major checkpoints:

• G1 checkpoint (restriction point)
• G2 checkpoint
• M checkpoint (ensures chromosomes are properly attached to spindle)

If regulation fails:

• Uncontrolled division → cancer
• Severe damage → apoptosis (programmed cell death)

FRQs often give mutations in cyclins or CDKs and ask you to predict the outcome. Loss of control usually means excessive division.

4. The Phases of Mitosis

Here is the standard sequence of mitosis. The diagram below shows the full cycle, including interphase leading into mitosis and cytokinesis following telophase.

Overview of the stages of mitosis

Prophase

• Chromatin condenses into visible chromosomes
• Each chromosome = two sister chromatids
• Mitotic spindle forms
• Centrosomes move to opposite poles

Metaphase

• Spindle fibers attach to kinetochores at centromeres
• Chromosomes line up at the metaphase plate

This alignment ensures equal separation.

Anaphase

• Sister chromatids separate
• Spindle fibers shorten
• Each chromatid is now an independent chromosome

Students often forget this shift in terminology. Once separated, they are chromosomes.

Telophase

• Chromosomes arrive at poles
• Nuclear envelopes reform
• Spindle breaks down
• Chromosomes decondense into chromatin

Mitosis maintains genetic continuity because each daughter nucleus receives one complete set.

5. Cytokinesis and Genetic Continuity

Cytokinesis in Animal vs Plant Cells

Plants cannot pinch because of their rigid cell wall.

Why Mitosis Matters

Mitosis:

• Produces two genetically identical daughter cells
• Maintains chromosome number
• Enables growth, repair, and asexual reproduction

The flow to remember:

S phase copies DNA →
Mitosis separates identical sister chromatids →
Cytokinesis divides the cell →
Each daughter cell gets a complete, identical genome.

That is transmission of genetic information from one cell generation to the next.