A comprehensive review of the cell cycle — key terms, phase summaries, checkpoint overview, and review questions to prepare for your exam.
The cell cycle is the highly regulated sequence of events through which a eukaryotic cell duplicates its genome and divides into two genetically identical daughter cells. It is broadly divided into interphase — the extended growth period comprising G1, S, and G2 phases — and the mitotic (M) phase, which includes mitosis (nuclear division) and cytokinesis (cytoplasmic division).
Progression through the cycle is governed by internal checkpoints that integrate signals about cell size, DNA integrity, and growth factor availability. Cyclin–CDK complexes act as the master switches at each transition, while tumour suppressors such as p53 and Rb function as brakes. When these regulatory mechanisms fail — through oncogene activation or tumour suppressor loss — cells may proliferate uncontrollably, leading to cancer.
Cell Cycle
The ordered sequence of events by which a cell duplicates its contents and divides into two daughter cells. It consists of interphase and the mitotic (M) phase.
Interphase
The longest stage of the cell cycle, comprising G1, S, and G2 phases, during which the cell grows, replicates its DNA, and prepares for division.
G1 Phase
The first gap phase of interphase in which the cell grows, synthesises proteins and organelles, and monitors environmental conditions before committing to DNA replication.
S Phase
The synthesis phase in which each chromosome is replicated to produce two identical sister chromatids joined at the centromere.
G2 Phase
The second gap phase in which the cell continues to grow, synthesises proteins needed for mitosis (e.g. tubulin), and checks that DNA replication is complete and error-free.
G0 Phase
A quiescent state in which a cell exits the active cell cycle. Some cells (e.g. neurons, muscle cells) remain in G0 permanently; others can re-enter G1 when stimulated.
Mitosis
The division of the nucleus into two genetically identical daughter nuclei. It comprises prophase, prometaphase, metaphase, anaphase, and telophase.
Prophase
The first stage of mitosis in which chromatin condenses into visible chromosomes, the mitotic spindle begins to form, and centrosomes migrate toward opposite poles.
Prometaphase
The stage following prophase in which the nuclear envelope fragments and spindle microtubules attach to kinetochores on the centromeres of chromosomes.
Metaphase
The stage of mitosis in which all chromosomes are aligned at the metaphase plate (cell equator), with each sister chromatid attached to spindle fibres from opposite poles.
Anaphase
The stage in which cohesin proteins holding sister chromatids together are cleaved by separase, allowing chromatids to be pulled to opposite poles by shortening spindle microtubules.
Telophase
The final stage of mitosis in which chromosomes decondense, nuclear envelopes reassemble around each set of chromosomes, and the nucleolus reappears.
Cytokinesis
The physical division of the cytoplasm following mitosis. In animal cells, a contractile ring of actin and myosin forms a cleavage furrow; in plant cells, a cell plate forms from Golgi-derived vesicles.
Cleavage Furrow
The indentation of the cell membrane in animal cells caused by contraction of the actin–myosin ring during cytokinesis, pinching the cell in two.
Cell Plate
A structure that forms at the centre of a dividing plant cell from fused Golgi vesicles, eventually becoming the new cell wall separating two daughter cells.
Checkpoint
A control mechanism in the cell cycle where the cell evaluates internal and external conditions (DNA integrity, cell size, growth signals) before proceeding to the next phase.
Cyclin
A family of regulatory proteins whose concentrations rise and fall throughout the cell cycle, activating cyclin-dependent kinases (CDKs) to drive phase transitions.
CDK (Cyclin-Dependent Kinase)
A family of protein kinases that, when bound to their cyclin partner, phosphorylate target proteins to trigger specific cell cycle events such as DNA replication or mitotic entry.
p53
A tumour suppressor protein that acts as a transcription factor, halting the cell cycle at G1 in response to DNA damage and triggering repair or apoptosis. Mutated in over 50 % of human cancers.
Rb (Retinoblastoma Protein)
A tumour suppressor that binds and inhibits E2F transcription factors, preventing progression from G1 to S phase until Rb is inactivated by CDK-mediated phosphorylation.
Oncogene
A mutated form of a proto-oncogene that is constitutively active, promoting uncontrolled cell division. Examples include Ras, Myc, and HER2.
Tumor Suppressor
A gene whose protein product inhibits cell proliferation or promotes apoptosis. Loss-of-function mutations in tumour suppressors (e.g. p53, Rb) remove brakes on the cell cycle.
Sister Chromatids
Two identical copies of a single replicated chromosome, joined at the centromere by cohesin proteins. They are separated during anaphase of mitosis.
Centromere
The constricted region of a chromosome where sister chromatids are most closely attached and where the kinetochore assembles for spindle fibre attachment.
Kinetochore
A multi-protein complex that assembles on the centromere and serves as the attachment site for spindle microtubules, enabling chromosome movement during cell division.
Cohesin
A ring-shaped protein complex that holds sister chromatids together from S phase until anaphase, when it is cleaved by the protease separase.
Separase
A cysteine protease that cleaves the cohesin complex at the onset of anaphase, allowing sister chromatids to separate and move to opposite poles.
Spindle Assembly Checkpoint (SAC)
A surveillance mechanism during metaphase that delays anaphase onset until all kinetochores are properly attached to spindle microtubules, preventing chromosome mis-segregation.
Metaphase Plate
The imaginary plane at the cell equator where chromosomes align during metaphase, equidistant from both spindle poles.
Phragmoplast
A plant-cell-specific structure composed of microtubules and actin filaments that guides Golgi-derived vesicles to the centre of the dividing cell to form the cell plate.
| Phase | What Happens | Duration | Key Molecules |
|---|---|---|---|
| G1 | Cell growth, organelle duplication, protein synthesis; cell monitors growth factor signals | ~11 hrs | Cyclin D, CDK4/6 |
| S | DNA replication; each chromosome duplicated into two sister chromatids | ~8 hrs | DNA polymerase, Cyclin E–CDK2 |
| G2 | Continued growth, protein synthesis (tubulin, condensin), preparation for mitosis | ~4 hrs | Cyclin A–CDK2, Cyclin B–CDK1 |
| Prophase | Chromatin condenses into chromosomes; mitotic spindle begins to form; centrosomes separate | Minutes | Condensin, Aurora kinases |
| Prometaphase | Nuclear envelope breaks down; kinetochore microtubules attach to chromosomes | Minutes | Kinetochore proteins, Ran-GTP |
| Metaphase | Chromosomes align at the metaphase plate; spindle assembly checkpoint verifies attachment | Minutes | Spindle fibres, SAC (Mad1/2, BubR1) |
| Anaphase | Cohesin cleaved; sister chromatids pulled to opposite poles by shortening microtubules | Minutes | Separase, APC/C |
| Telophase | Chromosomes decondense; nuclear envelopes reassemble; nucleoli reappear | Minutes | Nuclear lamins, phosphatases |
| Cytokinesis | Cytoplasm divides — cleavage furrow (animal) or cell plate (plant) | Minutes | Actin/Myosin (animal), Golgi vesicles (plant) |
| Checkpoint | Location | What It Checks | Key Molecules |
|---|---|---|---|
| G1 (Restriction Point) | End of G1 | Cell size, nutrient availability, DNA damage, external growth signals | p53, Rb, Cyclin D–CDK4/6 |
| G2 Checkpoint | End of G2 | DNA replication complete and error-free, DNA damage repair | ATM/ATR kinases, Chk1/2, p53 |
| Spindle Assembly (M) | Metaphase | All kinetochores properly attached to spindle microtubules from opposite poles | Mad1/2, BubR1, APC/C |
Q1.What is the difference between mitosis and cytokinesis?
A: Mitosis is the division of the nucleus, producing two genetically identical nuclei, while cytokinesis is the division of the cytoplasm that physically separates the cell into two daughter cells. Mitosis can occur without cytokinesis, resulting in a binucleate cell.
Q2.Why is p53 called the ‘guardian of the genome’?
A: p53 monitors DNA integrity and, when damage is detected, halts the cell cycle at the G1 checkpoint to allow time for repair. If the damage is irreparable, p53 triggers apoptosis (programmed cell death), preventing cells with dangerous mutations from proliferating.
Q3.How do cyclins regulate the cell cycle?
A: Cyclins are regulatory proteins whose concentrations oscillate throughout the cell cycle. They bind to and activate cyclin-dependent kinases (CDKs), which phosphorylate downstream targets to drive the cell from one phase to the next. Timely degradation of cyclins by the ubiquitin–proteasome pathway ensures that each phase transition occurs only once per cycle.
Q4.What distinguishes anaphase I of meiosis from anaphase of mitosis?
A: In anaphase of mitosis, sister chromatids separate and move to opposite poles. In anaphase I of meiosis, homologous chromosome pairs (each still consisting of two sister chromatids) are pulled apart. This halves the chromosome number and is the basis of genetic reduction.
Q5.How does cancer relate to cell cycle regulation?
A: Cancer arises when mutations disable cell cycle checkpoints — typically through gain-of-function mutations in proto-oncogenes (creating oncogenes that promote division) and loss-of-function mutations in tumour suppressor genes (removing brakes on proliferation). The result is uncontrolled cell division, genomic instability, and tumour formation.
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