Difficulty: Introductory | Prerequisites: Basic cell biology, DNA structure
The cell cycle is the sequence of events a cell goes through from one division to the next. It underpins growth, tissue repair and reproduction in all living organisms. If you understand DNA structure and the difference between prokaryotes and eukaryotes, you have what you need. This topic sits at the heart of introductory biology and connects forward to meiosis, genetics and cancer biology.
Cells grow and copy their DNA during interphase (G1, S and G2), then divide during the mitotic phase (mitosis + cytokinesis). Three checkpoints (G1, G2 and M) act as quality-control gates, and cells that will never divide again exit into a resting state called G0. The whole process produces two genetically identical daughter cells.
Cell cycle
The ordered sequence of events from one cell division to the next, comprising interphase and the mitotic phase. Think of it as the full life loop a cell completes before it splits in two.
Interphase
The longest part of the cell cycle, during which the cell grows, carries out normal functions and replicates its DNA. In simple terms, this is everything the cell does between one division and the next.
G1 phase (Gap 1)
The first gap phase of interphase. The cell grows, produces proteins and organelles, and prepares for DNA synthesis. Think of it as the "get ready" stage.
S phase (Synthesis)
The stage of interphase in which the cell's entire DNA is replicated, producing sister chromatids joined at the centromere. This is when the chromosome count effectively doubles in DNA content.
G2 phase (Gap 2)
The second gap phase, after DNA replication and before mitosis. The cell continues to grow, the centrosome replicates, and final preparations for division are made.
Mitotic phase (M phase)
The portion of the cell cycle that includes both mitosis (nuclear division) and cytokinesis (cytoplasmic division). This is the actual "splitting" part.
Mitosis
The division of the nucleus into two genetically identical daughter nuclei. It includes prophase, prometaphase, metaphase, anaphase and telophase.
Cytokinesis
The division of the cytoplasm, which usually follows mitosis and physically separates the two daughter cells. It often accompanies telophase but does not always occur.
Chromatin
The dispersed, loosely organised complex of DNA and histone proteins that fills the nucleus during interphase. In simple terms, it is the "unwound" form of chromosomes, accessible for transcription.
Sister chromatids
Two identical copies of a single chromosome, produced during S phase, joined at the centromere. They separate during anaphase to become individual chromosomes.
Centromere
The region where the two sister chromatids are most tightly attached to each other after DNA replication. Think of it as the pinch-point holding two copies together.
Kinetochore
A protein structure on the centromere where spindle microtubules attach during mitosis. It is the connection point between the chromosome and the machinery that pulls it.
Mitotic spindle
A structure made of microtubules that forms during early mitosis and guides chromosome separation. It includes kinetochore microtubules (attached to chromosomes) and nonkinetochore microtubules (which push the cell's poles apart).
Centrosome
The microtubule-organising centre of the cell. It replicates during G2, and the two centrosomes move to opposite poles to anchor the mitotic spindle.
G0 phase
A resting or quiescent state in which a cell has exited the cell cycle and is not preparing to divide. Neurons and many other specialised cells remain in G0 permanently.
The cell cycle divides into interphase and the mitotic phase. Interphase is by far the longer part, typically occupying about 90% of the cycle. During interphase the cell grows, carries out its normal functions, and replicates its DNA and organelles. The mitotic phase covers nuclear division (mitosis) and cytoplasmic division (cytokinesis).
G1 (Gap 1): The cell grows in size, produces RNA and synthesises proteins. Organelles are duplicated. Nucleoli are present and visible. This is the main growth phase before DNA synthesis.
S (Synthesis): DNA replication occurs. Each chromosome is copied, producing two sister chromatids joined at the centromere. After S phase, the cell has twice as much DNA as it did in G1, though the number of centromeres stays the same (one per duplicated chromosome).
G2 (Gap 2): The cell continues to grow and prepares for mitosis. The centrosome replicates, giving the cell two centrosomes that will later anchor the mitotic spindle at opposite poles.
G1 checkpoint: The most important decision point. The cell assesses whether conditions are favourable for division (sufficient size, nutrients, growth signals, DNA integrity). Once a cell passes G1, it usually completes the rest of the cycle. Cells that will not divide exit here into G0.
G2 checkpoint: Verifies that DNA replication is complete and checks for DNA damage before the cell enters mitosis. Two centrosomes should have formed by this point.
M checkpoint (spindle checkpoint): Occurs during metaphase. Confirms that all chromosomes are properly attached to spindle microtubules at their kinetochores before allowing anaphase to proceed.
These are tested repeatedly, so it pays to have the logic down.
In G1, a diploid cell with 2n = 10 has 10 chromosomes, 10 centromeres and 1x DNA content.
After S phase (in G2), the same cell still has 10 centromeres (one per duplicated chromosome), but each chromosome now consists of two sister chromatids. DNA content is 2x.
At metaphase, if a cell contains 20 sister chromatids, that means 10 duplicated chromosomes. A G1 cell of the same organism would therefore have 10 chromosomes.
At anaphase, centromeres split and each sister chromatid becomes an independent chromosome. The chromosome count temporarily doubles.
Neurons, mature muscle cells and some other highly specialised cells exit the cell cycle before the G1 checkpoint and enter G0. They carry out their functions but do not prepare for or undergo division. Some cells in G0 can be stimulated to re-enter the cycle; others remain in G0 permanently.
Students often think that the number of centromeres doubles after S phase. It does not. Each duplicated chromosome still has one centromere holding its two sister chromatids together, so the centromere count stays the same from G1 through G2.
Students often confuse "chromosome" with "chromatid." After replication, one chromosome consists of two sister chromatids. They only become separate chromosomes when the centromere splits at anaphase.
Students sometimes believe mitosis is the whole cell cycle. Mitosis is only one part of the mitotic phase, which is itself a small fraction of the total cycle. Most of the cycle is interphase.
Students often assume cytokinesis always happens. In some cell types it does not follow mitosis, resulting in multinucleated cells.
Expect questions that give you a diploid number (2n) and ask how many centromeres, chromatids or chromosomes are present at a specific phase. Know the counting rules cold.
The G1 checkpoint is the most commonly tested checkpoint. Understand that passing it commits the cell to division, and that cells exit to G0 before this point.
Know what happens at each interphase sub-phase. A common question format is: "During ______, the cell grows and replicates both its organelles and its chromosomes." (Answer: interphase.)
Understand the difference between DNA content and chromosome number. DNA content doubles in S phase but does not return to 1x until after cytokinesis.
The nuclear envelope must disappear for microtubules to attach to kinetochores. This is a frequently tested cause-and-effect relationship.
True or false: DNA replication occurs during G2. (False. It occurs during S phase.)
Fill in the blank: Cells that do not divide exit the cell cycle into ______. (G0.)
True or false: After S phase, a cell with 2n = 6 has 12 centromeres. (False. It still has 6 centromeres, one per duplicated chromosome.)
Fill in the blank: Nucleoli are present during ______. (Interphase.)
True or false: Cytokinesis always accompanies mitosis. (False. It usually does, but not always.)
Q: In a diploid cell with 5 chromosome pairs (2n = 10), how many centromeres will be found in a nucleus at G2?
A: 10. There is one centromere per duplicated chromosome, and the number of chromosomes has not changed, only each chromosome now has two sister chromatids.
Q: During which part of the cell cycle does the cell grow and replicate both its organelles and its chromosomes?
A: Interphase (specifically across G1, S and G2).
Q: If a cell at metaphase of mitosis contains 20 sister chromatids, how many chromosomes will be present in a G1 cell?
A: 10. Twenty sister chromatids means 10 duplicated chromosomes. Each duplicated chromosome becomes one chromosome in a G1 daughter cell.
Q: Why must the nuclear envelope disappear during the cell cycle?
A: To permit microtubules from the centrosomes to attach to kinetochores on the chromosomes. The envelope is a physical barrier between the spindle and the chromosomes.
Q: Why do neurons and some other specialised cells divide infrequently?
A: They have exited the cell cycle and entered G0, a non-dividing resting state.
Cell cycle, cell division cycle, interphase, G1 phase, S phase, G2 phase, mitotic phase, M phase, mitosis, cytokinesis, chromatin, sister chromatids, centromere, kinetochore, centrosome, mitotic spindle, G0, quiescent cells, cell cycle checkpoints, G1 checkpoint, restriction point, G2 checkpoint, M checkpoint, spindle assembly checkpoint, diploid, 2n, chromosome number, DNA content, nucleoli
Connections to other topics: The cell cycle connects directly to meiosis (a modified cell division that produces haploid gametes), to genetics (errors in DNA replication during S phase are the basis of mutations), and to cancer biology (loss of checkpoint control leads to unregulated cell division). Understanding the mitotic spindle and centrosomes also feeds into cell biology topics on the cytoskeleton and motor proteins.