4.5Cell Communication and Cell Cycle

Cell Cycle — AP BIOLOGY Revision Notes & Topical Questions

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4.5 Cell Cycle — AP Biology notes: Describe the phases of the cell cycle and the events of mitosis and cytokinesis.Cell cycle = the highly regulated sequence of events between one cell division and the next.Interphase — the cell grows, carries out normal functions and replicates …

G1 decision
Prophase detail
Metaphase detail
Anaphase detail
Telophase detail
Cohesin and separase

Section 1 of 10Phases of the cell cycle

Cell cycle

Cell cycle the highly regulated sequence of events between one cell division and the next.

Interphase

Interphase the cell grows, carries out normal functions and replicates DNA.
G1
- cell grows, prepares for DNA replication and makes new organelles.
S — DNA is replicated; each chromosome becomes two sister chromatids joined at the centromere.
G2
- further growth and preparation for mitosis, e.g. centrosome duplication.

Mitosis

Mitosis nuclear division.

Cytokinesis

Cytokinesis division of the cytoplasm and organelles between the daughter cells.

Timing

Timing in a typical human cell lasting about 24 hours, interphase takes about 90% of the time and mitosis about an hour

G1 decision

G1 decision the cell decides in G1 whether to commit to division

S phase

DNA is replicated semi-conservatively, the amount of DNA doubles but the chromosome number stays the same, and centrosomes are duplicated

G2 the cell synthesises proteins for mitosis such as tubulin and checks that replication is complete and undamaged

Section 2 of 10Non-dividing cells (G0)

G0 a non-dividing state; cells (e.g. mature neurons) leave the cycle, which can happen at any stage.

Reasons

Reasons cells are mature or conditions for growth are poor.

Re-entry

Re-entry some cells re-enter the cycle in response to chemical signals.

Re-entry

Re-entry some cells, such as liver cells, can re-enter G1

Section 3 of 10Stages of mitosis

Prophase: chromatin condenses into visible chromosomes and the spindle begins to form.

Prometaphase: the nuclear envelope breaks down and kinetochore microtubules attach to the centromeres.

Metaphase: chromosomes line up on the metaphase plate at the equator.

Anaphase: spindle fibres shorten and pull sister chromatids to opposite poles, each now a chromosome.

Telophase: nuclear envelopes re-form around the two sets of chromosomes, which decondense.

1Prophase detail

Prophase detail the nucleolus disappears and the spindle forms from centrosomes moving to opposite poles

2Metaphase detail

Metaphase detail sister kinetochores are attached to opposite poles

3Anaphase detail

Anaphase detail the cell elongates as kinetochore microtubules shorten

4Telophase detail

Telophase detail the spindle breaks down and nucleoli re-form

5Cohesin and separase

Cohesin and separase cohesin holds sister chromatids together until anaphase,
- when separase cleaves it,
- so each daughter gets one copy of every chromosome

Section 4 of 10Cytokinesis

Animal cells

Animal cells an actin ring forms a cleavage furrow that pinches the cell in two.

Plant cells

Plant cells a cell plate forms from Golgi vesicles.

Timing

Timing cytokinesis usually begins in late anaphase or telophase

Contractile ring

Contractile ring in animal cells it is made of actin and myosin

Plant cell plate

Golgi vesicles move to the equator and fuse into a plate that grows outwards to form the new wall and membranes

Prokaryotes

Prokaryotes they divide by binary fission with no spindle, the circular chromosome is copied and the cell splits after membrane and wall growth

Section 5 of 10Outcome and importance of mitosis

Result

Result two genetically identical diploid daughter cells, each with a complete genome.

Uses

Uses growth, tissue repair and asexual reproduction.

DNA content

DNA content if G1 = x, then after S and in G2 it is 2x, and each daughter returns to x.

Mitotic index

Mitotic index cells in mitosis ÷ total cells counted (× 100 for a percentage).

Importance

Importance mitosis produces genetically identical daughter cells for growth, repair, cell replacement and asexual reproduction

Section 6 of 10Phases of mitosis compared

Phase	Chromosomes	Spindle	Nuclear envelope	Key event
Prophase	Condense	Forms	Breaks down	Chromosomes visible
Metaphase	Line up at plate	Attached at kinetochores	Gone	Alignment
Anaphase	Chromatids separate	Shortens	Gone	Move to poles
Telophase	Decondense	Disassembles	Re-forms	Two nuclei
Cytokinesis	—	—	—	Cleavage furrow or cell plate

Section 7 of 10Interpreting cell-cycle data

1Mitotic index

× 100, found from root-tip squashes stained with acetic orcein or toluidine blue
- (cells in mitosis ÷ total cells counted)
It shows how actively tissue is dividing, and tumours usually have a high mitotic index.

2Stage duration

Stage duration the proportion of cells in a stage is proportional to the time spent in it,
so a stage share multiplied by the cycle length gives its duration, and interphase is longest while mitosis (about 4%) is very short
- (about 90% of cells)

3DNA graph

DNA doubles during S, stays high through G2 and mitosis, then halves at cytokinesis

Section 8 of 10More from the specification

Cyclins

Chemical signals that trigger movement between phases of the cell cycle.

Checkpoint

Control point that can halt the cycle

Apoptosis

Programmed cell death

Mutation

A random change in genetic material.

Cancer

Disease in which cells divide uncontrollably and spread.

Section 9 of 10Key terms

Term	Meaning
Cell cycle	G1, S, G2 (interphase) and the mitotic phase
Interphase	growth and DNA replication
Mitosis	division of the nucleus
Cytokinesis	division of the cytoplasm
G0	non-dividing state
Sister chromatids	identical copies joined at the centromere
Mitotic index	Proportion of cells in mitosis.
Kinetochore	Protein structure at the centromere where spindle microtubules attach.
Cohesin	Protein holding sister chromatids together.
Cell plate	New wall formed by Golgi vesicles in plant cytokinesis.

Section 10 of 10Exam technique

Saying DNA replicates in mitosis — it replicates in S phase of interphase.

Saying sister chromatids separate in metaphase — separation is in anaphase.

Treating cytokinesis as part of mitosis — mitosis is nuclear division only.

Saying interphase is a resting stage — the cell grows and replicates DNA.

Saying chromosome number doubles in S phase — only the DNA amount doubles.

Last updated: 14 October 2026