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2.3.1Cell Structure

The Cell Cycle and Interphase — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 8 parts Lesson 16 of 47 1 quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
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2.3.1 The Cell Cycle and Interphase — AQA A Level Biology 7402 notes: Describe the stages of the eukaryotic cell cycle, the events of interphase and the checkpoints that control progression.The cell cycle is the ordered sequence of events through which a eukaryotic cell grows, co…

  • mitotic (M) phase
  • G1 checkpoint
  • G2 checkpoint
  • Spindle (metaphase) checkpoint
  • cyclin-dependent kinases (CDKs)
  • asexual reproduction

Section 1 of 8The cell cycle

The cell cycle is the ordered sequence of events through which a eukaryotic cell grows, copies its DNA and divides to form two genetically identical daughter cells.

1Interphase
It consists of interphase (the period between divisions, when the cell is not dividing) and the mitotic (M) phase, which includes mitosis (division of the nucleus) and cytokinesis (division of the cytoplasm).
2
In a typical cultured human cell the whole cycle takes about 24 hours, with interphase taking most of that time.
3
Some cells, such as neurones, leave the cycle in a resting state (G0) and do not divide again.

Section 2 of 8Stages of interphase

StageEvents
G1 (first gap)Cell grows; organelles are made and duplicated; protein synthesis is rapid; the cell checks that conditions are suitable for division.
S (synthesis)DNA is replicated semi-conservatively, so each chromosome now consists of two identical sister chromatids joined by a centromere.
G2 (second gap)Cell continues to grow; the proteins needed for mitosis (such as tubulin for the spindle) are made; ATP is stored; the DNA is checked for errors and repaired.

The cell grows throughout interphase, and its DNA content doubles during S phase. At the end of G2 the chromosomes are not yet visible because the DNA is uncoiled as chromatin, which allows access for replication and transcription enzymes.

Section 3 of 8Control by checkpoints

The cell cycle is controlled at checkpoints, where the cell either proceeds or is held back:

1G1 checkpoint
G1 checkpoint checks that the cell is big enough, nutrients are available, and the DNA is undamaged. If conditions are not suitable the cell may enter G0.
2G2 checkpoint
G2 checkpoint checks that DNA replication is complete and undamaged.
3Spindle (metaphase) checkpoint
Spindle (metaphase) checkpoint checks that all chromosomes are attached to spindle fibres at the equator before the chromatids are separated.

The checkpoints are controlled by proteins called cyclins and cyclin-dependent kinases (CDKs).

A cyclin binds a CDK and activates it, and the active complex phosphorylates target proteins that drive the cell to the next stage.
Cyclin levels rise and fall through the cycle.
P53
Genes such as p53 (a tumour suppressor) halt the cycle if DNA is damaged or trigger apoptosis if the damage cannot be repaired; faults in these genes lead to cancer (see 2.3.6).

Section 4 of 8The importance of the cell cycle

The cycle makes possible growth of multicellular organisms, repair and replacement of damaged cells, and asexual reproduction.

The importance of the cell cycle
Because DNA is copied exactly in S phase and shared equally in mitosis, the daughter cells are genetically identical to the parent cell.
Interphase is not a resting phase; it is the time of greatest metabolic activity, with cells making proteins, replicating DNA and growing.

Section 5 of 8Interpreting data about the cycle

Graphs often show the DNA content per cell against time.

Interpreting data about the cycle
DNA content stays constant in G1, doubles gradually during S phase, remains doubled in G2 and the first stages of mitosis, then halves in anaphase/telophase and cytokinesis when the cell divides.
Questions may give the proportion of cells in each stage in a sample and ask for the duration of each stage: the time taken for a stage is the proportion of cells in that stage multiplied by the total cycle time.
For example, if 20% of cells are in G2 and the cycle takes 24 hours, G2 lasts 4.8 hours.

Linkmitosis follows in 2.3.2, and cell division in cancer in 2.3.6.

Remember the vocabulary for chromosomesbefore S phase each chromosome is a single DNA molecule, and after S phase each is made of two sister chromatids.

Interpreting data about the cycle
The number of chromosomes does not change when DNA is replicated, only the amount of DNA.
A diploid human cell has 46 chromosomes in G1, still 46 chromosomes in G2 (each with two chromatids, so 92 chromatids), and 46 in each daughter cell after division.
Students often confuse the number of chromosomes with the number of chromatids or the quantity of DNA, so state which you mean.

Cell cycle length differs between cell types: epithelial cells of the gut divide every day or so, liver cells only occasionally, and neurones not at all.

Interpreting data about the cycle
Embryonic cells may complete a cycle in a few hours, with very short gap phases.
When an exam question gives a table of times for each stage, check that your answers add up to the full cycle duration.

Section 6 of 8Exam-style questions

  1. Describe what happens in the S phase of the cell cycle and explain its importance. [3]3 marks

  2. In a sample of 400 cells, 280 are in interphase, 60 in prophase, 20 in metaphase, 16 in anaphase and 24 in telophase. Calculate the mitotic index. [2]2 marks

  3. Explain the importance of the G2 checkpoint. [2]2 marks

Section 7 of 8Key terms

TermMeaning
Cell cycleThe sequence of events from one cell division to the next.
InterphaseThe part of the cycle between divisions, including G1, S and G2.
CheckpointA control point at which the cell cycle may be halted.
CyclinA protein whose concentration changes during the cycle and regulates CDKs.
Sister chromatidsTwo identical copies of a chromosome joined at the centromere.
G0A resting stage outside the cell cycle.
Mitotic indexProportion of cells in a tissue that are undergoing mitosis.

Section 8 of 8Exam technique

1
Describing interphase as a resting stage. It is highly active.
2
Saying that chromosomes are visible in interphase.
3
Saying DNA replication occurs in G1 or in mitosis. It occurs in S phase.
4
Forgetting that the cycle includes cytokinesis.
5
Saying the checkpoints are only at the end of interphase.

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