Module 2: Eukaryotic Cell Cycle
These free OCR A Level Biology Eukaryotic Cell Cycle revision notes have been written for specification points 2.1.6(a) and 2.1.6(b).
The Eukaryotic Cell Cycle
The cell cycle is the distinct phases a cell goes through in preparation for cell division.
The cell cycle in eukaryotes is outlined in the table below:
| Stage | Description |
|---|---|
| Interphase | Cellular growth and DNA replication; broken into G₁, S and G₂ and (sometimes) G₀ phase. |
| Mitosis (M) | Division of the nucleus to produce two genetically identical nuclei. |
| Cytokinesis | Division of the cytoplasm, producing two genetically identical cells. |
The three phases (G₁, S and G₂) are separated by restriction points; checkpoints that reduce the risk of a cell failing to divide and mutating.
The table below gives an overview of the phases of the cell cycle, and some of the mechanisms in place to reduce the risk of mutation:
| Phase | Events | Anti-mutation mechanisms |
|---|---|---|
| G₁ |
– The cell grows in size – Protein synthesis (transcription and translation) – Organelles duplicate |
– p53 (a tumour suppression gene), CDKs (cyclin-dependent kinases) and cyclins (proteins) regulate this phase. – G₁/S restriction point prevents uncontrolled division and repairs damaged DNA. |
| S | – DNA is replicated, producing sister chromatids | – CDKs and cyclins regulate this phase. |
| G₂ | – Proteins responsible for forming the spindle and condensing chromosomes are made |
– CDKs and cyclins regulate this phase. – G₂/M restriction point prevents uncontrolled division and repairs damaged DNA |
| M | – Mitosis occurs, the nucleus’ contents divide | – Spindle restriction point (in metaphase) ensures that the right number of chromosomes end up in each daughter cell |
| Cytokinesis | – Cytoplasmic division | n/a |
| G₀ |
– The cell cycle pauses – Differentiation – Senescence (ageing) – Apoptosis | n/a: p53 can cause cells to enter G₀ as a result of mutation and undergo apoptosis. |
Cancer cells typically arise from mutations in the genes responsible for regulating the cell cycle directly (p53) or indirectly (cyclins and CDKs that control the restriction points).
If a restriction point is unable to repair damaged DNA, or fix a nutrient deficiency or lack of organelles, then the cell may undergo apoptosis (programmed cell death); this eliminates a potentially cancerous cell to protect the entire organism.