OCR Specification focus:
‘Describe interphase, mitosis and cytokinesis; outline checkpoint control of the cycle.’
The cell cycle is the sequence of events that a cell undergoes to grow, replicate its DNA, and divide into two genetically identical daughter cells. Its regulation ensures accuracy and prevents uncontrolled division.
The Cell Cycle Overview
The cell cycle consists of interphase and the mitotic (M) phase. Interphase occupies the majority of the cycle, during which the cell grows and prepares for division. The mitotic phase includes mitosis and cytokinesis, resulting in two daughter cells
Cell Cycle: The ordered sequence of events that leads to cell growth, DNA replication, and division into two daughter cells.
Cells that are not actively dividing enter a resting phase known as G₀, where they perform normal functions but do not progress through the cycle.
Interphase
Interphase is the period of cell growth and preparation for division, subdivided into G₁, S, and G₂ phases.
G₁ Phase (First Growth Phase)
The cell increases in size and synthesises new organelles, membranes, and proteins.
Normal metabolic activities occur, and the cell prepares for DNA replication.
Checkpoint control ensures that the environment and cell are suitable for DNA synthesis.
Checkpoint: A control mechanism that verifies whether the processes at each phase of the cell cycle have been accurately completed before progression.
If the conditions are unfavourable, the cell may enter the G₀ phase, a non-dividing state.
S Phase (Synthesis Phase)
DNA replication occurs, producing identical sister chromatids for each chromosome.
The amount of DNA doubles, but the chromosome number remains constant.
Histone proteins are synthesised to package the new DNA.
G₂ Phase (Second Growth Phase)
Further growth occurs, and energy stores are replenished.
Proteins required for mitosis, such as microtubule components, are produced.
A checkpoint ensures accurate DNA replication and adequate cell size before mitosis.
The Mitotic Phase
The mitotic phase (M phase) includes mitosis and cytokinesis. Mitosis involves the precise separation of duplicated chromosomes, while cytokinesis divides the cytoplasm and cell membrane.
Mitosis: The process of nuclear division producing two genetically identical nuclei from one parent nucleus.
Mitosis is essential for growth, tissue repair, and asexual reproduction in eukaryotic organisms.
A labelled sequence depicting prophase, metaphase, anaphase, and telophase, with cytokinesis indicated. Chromosomes, spindle fibres, and nuclear envelope changes are shown schematically to emphasise accurate chromosomal segregation. The layout is minimal and well annotated for study use. Source.
Stages of Mitosis
1. Prophase
Chromatin condenses into visible chromosomes, each composed of two sister chromatids joined at a centromere.
The nuclear envelope begins to disintegrate.
Centrioles (in animal cells) migrate to opposite poles, forming the spindle apparatus.
2. Metaphase
Chromosomes align along the metaphase plate.
Spindle fibres attach to the centromeres via kinetochores.
The metaphase checkpoint ensures chromosomes are correctly attached to spindle fibres before separation.
3. Anaphase
Spindle fibres shorten, pulling sister chromatids apart toward opposite poles.
Each chromatid becomes an independent chromosome.
4. Telophase
Chromosomes uncoil back into chromatin.
Nuclear envelopes reform around each set of chromosomes, producing two nuclei.
Cytokinesis
Cytokinesis completes cell division by separating the cytoplasm.
Side-by-side panels compare animal cell cytokinesis via a contractile ring/cleavage furrow and plant cell cytokinesis via Golgi-derived vesicles forming a cell plate that becomes the new cell wall. Labels are minimal and focused on structures named in the notes. No extraneous detail beyond OCR requirements is included. Source.
In animal cells, a cleavage furrow forms as the cell membrane constricts to split the cell into two.
In plant cells, vesicles from the Golgi apparatus assemble at the cell’s equator to form a cell plate, which develops into a new cell wall.
Cytokinesis: The division of the cytoplasm following mitosis, resulting in two separate daughter cells.
Cell Cycle Checkpoints and Regulation
The cell cycle is tightly regulated by a network of checkpoints and regulatory proteins to ensure accuracy and maintain genetic stability.
Key Checkpoints
G₁ Checkpoint (Restriction Point)
Ensures the cell is large enough, has sufficient nutrients, and that DNA is undamaged. If conditions are unsuitable, the cell may enter G₀.G₂ Checkpoint
Verifies that DNA replication has been completed accurately without damage before mitosis begins.Metaphase Checkpoint (Spindle Assembly Checkpoint)
Ensures that all chromosomes are properly aligned and spindle fibres are correctly attached to centromeres.
Regulatory Molecules
Cyclins and cyclin-dependent kinases (CDKs) control progression through checkpoints.
Cyclin: A regulatory protein whose concentration fluctuates cyclically, controlling the activity of CDKs during the cell cycle.
Cyclin-Dependent Kinase (CDK): An enzyme activated by binding to cyclin, which phosphorylates target proteins to drive the cell through the cycle.
Cyclin levels rise and fall at specific points:
When cyclins bind to CDKs, they form cyclin-CDK complexes that phosphorylate target proteins, activating them for the next phase.
After checkpoint completion, cyclins are degraded, inactivating CDKs and preventing premature progression.
Control of the Cell Cycle and Cancer
If checkpoint regulation fails, mutations may accumulate, leading to uncontrolled cell division.
Mutations in genes coding for regulatory proteins (e.g. proto-oncogenes or tumour suppressor genes) can result in continuous cell cycle progression.
This uncontrolled proliferation forms tumours, which may become malignant, invading surrounding tissues.
Proto-oncogene: A normal gene that promotes cell division; when mutated, it becomes an oncogene, causing uncontrolled cell growth.
Tumour Suppressor Gene: A gene that normally inhibits cell division or promotes apoptosis; when inactivated, it allows uncontrolled proliferation.
Maintaining the fidelity of the cell cycle through accurate regulation is therefore vital for tissue integrity, organismal development, and the prevention of cancer.
FAQ
Some cells, such as nerve and muscle cells, permanently enter the G₀ phase, becoming terminally differentiated and losing the ability to divide.
Other cells, like liver cells, may remain in G₀ temporarily and can re-enter the cycle in response to injury or specific growth factors.
This resting state helps conserve energy and maintain tissue stability by preventing unnecessary cell division.
Progression through checkpoints is influenced by extracellular growth factors and hormones.
Mitogens activate signalling pathways that increase cyclin synthesis, driving the cell past the G₁ checkpoint.
Contact inhibition prevents cell division when cells are densely packed, stopping progression beyond G₁.
Nutrient availability and oxygen concentration also determine whether a cell continues or pauses in the cycle.
The G₂ checkpoint ensures that DNA has been accurately replicated before mitosis begins.
If errors or incomplete replication are detected, enzymes such as p53 halt the cycle, allowing time for DNA repair mechanisms to act.
Failure at this checkpoint can result in aneuploidy or mutations, contributing to cancer development or cell death.
These genes act as opposing controls on cell division:
Proto-oncogenes stimulate progression through the cycle when appropriate.
Tumour suppressor genes inhibit division or trigger apoptosis when errors are detected.
A balance between these ensures normal growth. Mutations converting proto-oncogenes into oncogenes, or inactivating tumour suppressors, can remove this control and promote uncontrolled proliferation.
Before entering the S phase, specialised sensor proteins (e.g. ATM and ATR kinases) scan DNA for breaks or mismatches.
If damage is found, they activate checkpoint proteins like p53, which:
Halt the cycle to allow repair, or
Induce apoptosis if damage is irreparable.
This system prevents replication of faulty DNA, preserving genomic integrity across generations of cells.
Practice Questions
Question 1 (2 marks)
Describe the role of the G₁ checkpoint in the cell cycle.
Mark scheme:
1 mark for identifying that the G₁ checkpoint ensures the cell is large enough and has sufficient nutrients to proceed to DNA replication.
1 mark for stating that it checks for DNA damage before allowing entry into the S phase (or halts the cycle/initiates repair if damage is detected).
Question 2 (5 marks)
Explain how cyclins and cyclin-dependent kinases (CDKs) regulate the progression of a cell through the cell cycle.
Mark scheme:
1 mark for stating that cyclins are regulatory proteins whose concentrations fluctuate during the cell cycle.
1 mark for describing that cyclins bind to specific CDKs, activating them to form cyclin-CDK complexes.
1 mark for explaining that the activated CDKs phosphorylate target proteins involved in progression to the next phase.
1 mark for noting that different cyclin-CDK complexes operate at different checkpoints (e.g. G₁/S, G₂/M).
1 mark for stating that after each phase, cyclins are broken down, inactivating the CDKs and preventing premature or uncontrolled progression.
