Difficulty: Introductory | Prerequisites: Basic cell structure, DNA structure
The cell cycle is the sequence of events a cell goes through from its formation to the point where it divides into two daughter cells. It sits at the heart of introductory biology because nearly every topic that follows (genetics, cancer biology, development) depends on understanding how cells copy themselves and what happens when that process goes wrong.
You should already be comfortable with basic cell structure (organelles, nucleus, chromosomes as structures inside the nucleus) and the idea that DNA carries genetic information. If those feel shaky, revisit your cell structure notes first.
Cells grow, copy their DNA, and divide through a tightly regulated cycle with built-in checkpoints. Mitosis produces two genetically identical diploid daughter cells; meiosis produces four genetically unique haploid cells (gametes). When checkpoint controls fail, uncontrolled division can lead to cancer.
Cell cycle
The ordered sequence of events (G1, S, G2, M) through which a cell duplicates its contents and divides. Think of it as the cell's life timeline from birth to division.
Interphase
The portion of the cell cycle between one cell division and the next, comprising G1, S, and G2. In simple terms, this is the long stretch where the cell grows and copies its DNA, before the drama of division begins.
G1 phase (Gap 1)
The first growth phase of interphase. The cell increases in size, produces new organelles, and carries out its normal functions. Think of it as the cell getting ready for the work ahead.
S phase (Synthesis)
The phase of interphase during which all of the cell's DNA is replicated. Each chromosome goes from one DNA molecule to two identical sister chromatids joined at the centromere.
G2 phase (Gap 2)
The second growth phase, after DNA replication and before mitosis. The cell checks its copied DNA for errors and stockpiles proteins needed for division.
G0 phase
A quiescent (resting) state in which a cell has exited the active cell cycle and is no longer preparing to divide. In simple terms, the cell has "retired" from dividing. Central nervous system neurons spend most of their lives here.
M phase (Mitotic phase)
The phase of the cell cycle that includes mitosis (nuclear division) and cytokinesis (cytoplasmic division). This is the shortest phase and involves the least cellular growth and synthesis.
Mitosis
Nuclear division that produces two genetically identical daughter nuclei. It occurs in somatic (body) cells.
Meiosis
A specialised form of cell division that produces four haploid gametes from one diploid cell. It occurs in germ cells and introduces genetic variation.
Chromosome
A structure of tightly coiled DNA and histone proteins found in the nucleus. Human somatic cells contain 46 chromosomes (23 pairs).
Chromatid / Sister chromatids
After DNA replication, each chromosome consists of two identical copies (sister chromatids) joined at the centromere. Think of sister chromatids as photocopies of the same page, stapled together in the middle.
Centromere
The region of a chromosome where sister chromatids remain attached to one another until anaphase. It is also the attachment point for spindle fibres via kinetochores.
Spindle fibres (mitotic spindle)
Protein structures (microtubules) responsible for moving chromosomes during mitosis and meiosis. They attach to kinetochores at the centromere and pull chromatids apart.
Kinetochore
A protein complex assembled on the centromere that serves as the attachment site for spindle fibres.
Apoptosis
Programmed cell death. When a cell's DNA is damaged beyond repair, the cell self-destructs in an orderly way rather than continuing to divide. Think of it as the cell's emergency shutdown.
Cytokinesis
The physical division of the cytoplasm into two daughter cells, occurring at the end of M phase.
Oncogene
A mutated or overexpressed version of a normal gene (proto-oncogene) that promotes uncontrolled cell division. In simple terms, it is a broken accelerator pedal stuck in the "on" position.
Proto-oncogene
A normal gene that codes for proteins that promote cell growth and division. It only becomes problematic when mutated into an oncogene.
Tumour suppressor gene
A gene whose protein product inhibits the cell cycle to prevent the development of tumours. Think of it as the braking system. When tumour suppressors are lost or inactivated, cells can divide without restraint.
G1 phase
The cell grows in size and produces new organelles.
Normal metabolic functions are active.
If a cell skips G1, it will be abnormally small because it never had time to grow.
The G1 checkpoint (restriction point) decides whether the cell proceeds to S phase or exits to G0.
S phase
All nuclear DNA is replicated.
Each chromosome is now composed of two sister chromatids joined at the centromere.
Chromosome number does not change during S phase, but the amount of DNA doubles.
G2 phase
The cell continues to grow and synthesises proteins needed for mitosis.
DNA is checked for replication errors.
A human somatic cell in G2 still has 46 chromosomes, but now each chromosome has two chromatids, giving 92 chromatids total.
Counting rule for G2
If a cell has N chromosomes at the end of mitosis, it still has N chromosomes in G2 of the next cycle (chromosomes are counted per centromere, not per chromatid). It will have 2N chromatids. For a cell ending mitosis with 24 chromosomes: G2 has 24 chromosomes and 48 chromatids.
Prophase
Chromosomes condense and become visible under a microscope.
The nuclear envelope begins to break down.
The mitotic spindle starts to form from centrosomes.
Prometaphase
The nuclear envelope fully disintegrates.
Spindle fibres attach to chromosomes at their centromeres via kinetochore proteins.
Metaphase
Chromosomes align at the cell's equatorial plane (the metaphase plate).
Spindle fibres are fully attached to all kinetochores.
The M checkpoint (spindle assembly checkpoint) operates here, verifying that every chromosome is properly attached before the cell proceeds.
Anaphase
Sister chromatids are pulled apart toward opposite poles of the cell as spindle fibres shorten.
Once separated, each chromatid is now considered an individual chromosome.
Telophase and cytokinesis
Chromatids (now chromosomes) arrive at the poles.
The nuclear envelope re-forms around each set.
Chromosomes begin to decondense.
Cytokinesis divides the cytoplasm, producing two genetically identical daughter cells.
G1 checkpoint (restriction point)
Assesses whether the cell is large enough and has sufficient nutrients and growth signals to proceed.
If conditions are not met, the cell exits the cycle into G0.
If this checkpoint fails entirely, cells may divide without adequate growth, producing abnormally small daughter cells.
G2 checkpoint
Checks that DNA replication is complete and undamaged.
If it fails, the daughters may inherit damaged DNA.
M checkpoint (spindle assembly checkpoint)
Verifies that all sister chromatids are properly attached to spindle fibres from both poles.
If this checkpoint fails, the cell may undergo nondisjunction: chromosomes do not separate correctly, and daughter cells receive the wrong number of chromosomes.
Cancer and the cell cycle
Proto-oncogenes are normal genes that promote cell growth. When mutated or overexpressed, they become oncogenes, which drive uncontrolled division.
Tumour suppressor genes inhibit the cell cycle. When they are inactivated (by mutation or deletion), the braking mechanism is lost and cells can proliferate unchecked.
Drugs that target the cell cycle (for example, drugs that arrest cells in S phase) can halt tumour growth. Arrested cells may eventually exit the cycle and undergo apoptosis.
Mitosis
Occurs in somatic cells.
Produces two diploid daughter cells, genetically identical to the parent.
One round of division.
Meiosis
Occurs in germ cells (cells that give rise to gametes).
Produces four haploid daughter cells, each genetically unique.
Two rounds of division: meiosis I separates homologous chromosomes; meiosis II separates sister chromatids.
Prophase I is the longest phase of meiosis.
Genetic variation in meiosis
Crossing over: during prophase I, homologous chromosomes pair up and exchange segments of DNA, creating recombinant chromosomes.
Independent assortment: the orientation of homologous pairs at the metaphase plate during metaphase I is random, so each gamete receives a different combination of maternal and paternal chromosomes.
Chromosome and chromatid counting quick reference
Human somatic cells: 46 chromosomes (2n = 46).
Human haploid cells (gametes): 23 chromosomes (n = 23).
After S phase / in G2: the chromosome count stays the same; the chromatid count doubles (each chromosome = 2 chromatids).
Number of centromeres = number of chromosomes (one centromere per chromosome, regardless of whether it has been replicated).
If a cell has 20 chromatids (post-replication), it has 10 centromeres and 10 chromosomes.
Students often think the chromosome number doubles during S phase. It does not. The number of chromosomes stays the same; each chromosome simply goes from one chromatid to two. Chromosomes are counted by centromeres.
Students frequently confuse chromatids with chromosomes. Two sister chromatids joined at a centromere still count as one chromosome until they are separated at anaphase.
"M phase has the most cellular activity" is a common assumption. In terms of growth and synthesis, M phase has the least. The heavy lifting (growth, DNA replication, protein production) happens during interphase.
Students sometimes think meiosis I and meiosis II both separate the same things. Meiosis I separates homologous chromosomes; meiosis II separates sister chromatids. These are distinct events.
⚠️ Expect questions that ask you to count chromosomes and chromatids at different stages of the cell cycle. Know the rule: chromosomes = centromeres; chromatids = DNA copies.
⚠️ Be able to identify the stages of mitosis from a microscope image (onion root tip is a classic). Know the key visual feature of each stage.
⚠️ Know which checkpoint does what and what goes wrong when each one fails (especially the M checkpoint and nondisjunction).
⚠️ Understand the roles of oncogenes, proto-oncogenes, and tumour suppressors, and how mutations in each contribute to cancer.
⚠️ Be able to interpret flow cytometry data showing cell populations in G1, S, and G2/M, and explain what a shift in distribution implies about a drug's effect on the cell cycle.
True or false: The S phase is when the cell grows and makes new organelles. (False, that is G1. S phase is DNA replication.)
Fill in the blank: Meiosis I separates ________; meiosis II separates ________. (homologous chromosomes; sister chromatids)
True or false: A human somatic cell in G2 has 92 chromosomes. (False. It has 46 chromosomes and 92 chromatids.)
Fill in the blank: When a cell's DNA is damaged beyond repair, it undergoes ________. (apoptosis)
True or false: Oncogenes are normal genes that have not been mutated. (False. Oncogenes are mutated or overexpressed forms of proto-oncogenes, which are the normal versions.)
Q: A cell has 24 chromosomes at the end of mitosis. How many chromosomes does it have in G2 of its next cell cycle?
A: 24. The chromosome count does not change between the end of mitosis and G2. DNA replication during S phase doubles the number of chromatids, not chromosomes.
Q: If there are 20 chromatids in a cell, how many centromeres are there?
A: 10. Each pair of sister chromatids shares one centromere, so the number of centromeres is half the number of chromatids.
Q: What would happen if a cell skipped G1?
A: The cell would be abnormally small. G1 is the phase in which the cell grows in size and produces organelles. Without it, the cell moves directly to DNA replication without adequate growth.
Q: What does the M checkpoint verify, and what happens if it fails?
A: It checks that all sister chromatids are properly attached to spindle fibres. If it fails, the cell may undergo nondisjunction, where chromosomes are distributed unevenly between daughter cells.
Q: A cancer drug causes cells to accumulate in S phase (visible on flow cytometry as a large S-phase peak). What is the drug likely doing, and what would happen to those cells over time?
A: The drug arrests cancer cells during DNA replication, preventing them from proceeding through the cell cycle. Over time, these arrested cells would likely exit the cycle and undergo apoptosis, slowing or stopping tumour growth.
Q: Central nervous system nerve cells spend most of their lives in which phase?
A: G0. These cells have exited the active cell cycle and are in a quiescent, non-dividing state.
This material connects directly to genetics: meiosis is the mechanism that produces the gametes whose inheritance patterns Mendel described. Understanding crossing over and independent assortment here will make dihybrid crosses and gene linkage far more intuitive.
It also connects to cancer biology. The checkpoints, oncogenes, and tumour suppressors covered here are the foundation for understanding how cancers develop and how chemotherapy drugs work by targeting specific phases of the cell cycle.
cell cycle phases, interphase, G1 S G2, mitosis stages, prophase prometaphase metaphase anaphase telophase, cytokinesis, meiosis I meiosis II, crossing over, independent assortment, homologous chromosomes, sister chromatids, centromere, kinetochore, spindle fibres, mitotic spindle, apoptosis, programmed cell death, G0 quiescent, cell cycle checkpoints, restriction point, spindle assembly checkpoint, nondisjunction, oncogene, proto-oncogene, tumour suppressor, flow cytometry cell cycle, chromosome counting, chromatid counting, diploid, haploid, 2n, n, germ cells, somatic cells