Difficulty: Introductory | Prerequisites: None, though a basic grasp of cell structure (nucleus, cytoplasm, organelles) will help.
Every living organism needs to make new cells, whether to grow, repair damage, or reproduce. The cell cycle is the ordered sequence of events a cell goes through from the moment it forms to the moment it divides into two daughter cells. This topic sits at the foundation of genetics and development: you cannot understand how organisms grow, how cancer arises, or how gametes are produced without first understanding how a normal cell copies and partitions its DNA. You should already be comfortable with the idea that DNA is packaged into chromosomes inside the nucleus.
The cell cycle has four phases: G1 (growth), S (DNA replication), G2 (preparation), and M (mitosis + cytokinesis). Interphase (G1 + S + G2) is where the cell spends most of its life, doing its job and copying its DNA. M phase splits first the nucleus (mitosis) and then the cytoplasm (cytokinesis) to produce two genetically identical daughter cells.
Cell cycle
The ordered sequence of events in which a cell grows, replicates its DNA, and divides to produce two daughter cells. Think of it as the full life story of a cell from birth to division.
Interphase
The portion of the cell cycle between one M phase and the next, comprising G1, S, and G2. In simple terms, this is the long stretch where the cell is living its normal life and preparing to divide.
G1 phase (first gap)
The period after cell division when the cell carries out its normal functions and grows. This is also when the cell "decides" whether to commit to another round of division.
S phase (synthesis)
The phase in which the cell replicates (copies) all of its DNA. After S phase, every chromosome consists of two identical sister chromatids joined at the centromere.
G2 phase (second gap)
A preparatory period after DNA replication and before mitosis. The cell continues to grow, produces proteins needed for division, and checks for replication errors.
M phase
The division phase of the cell cycle, consisting of mitosis (nuclear division) and cytokinesis (cytoplasmic division).
Mitosis
Nuclear division that separates the replicated chromosomes into two identical sets, one for each daughter nucleus. Think of it as the precise choreography that ensures each new cell gets a complete copy of the genome.
Cytokinesis
The physical division of the cytoplasm that follows mitosis, producing two separate daughter cells.
Sister chromatids
The two identical copies of a single chromosome that are joined together after DNA replication in S phase. They separate during mitosis so each daughter cell gets one copy.
Prophase
First stage of mitosis. Chromatin condenses into visible chromosomes, and the mitotic spindle begins to form.
Prometaphase
Chromosomes attach to spindle fibres via their kinetochores. The nuclear envelope breaks down.
Metaphase
Chromosomes line up along the middle (metaphase plate) of the cell. Think of it as the moment everything is lined up and ready to be pulled apart.
Anaphase
Sister chromatids are pulled to opposite poles of the cell by spindle fibres.
Telophase
Chromosomes arrive at opposite poles, de-condense, and new nuclear envelopes form around each set.
The cell cycle runs in a fixed order: G1 → S → G2 → M.
G1, S, and G2 together make up interphase, the non-dividing portion of the cycle.
M phase includes both mitosis (nuclear division) and cytokinesis (cytoplasmic division).
In multicellular organisms, cells spend most of their time in G1, carrying out their specialised functions.
G1 (first gap): The cell grows, performs its normal metabolic roles, and produces the proteins and organelles it will need. G1 is also the decision point: the cell commits to replication and transitions to S phase, or it exits the cycle.
S phase (synthesis): The cell copies all of its chromosomal DNA. Each chromosome goes from being a single DNA molecule to a pair of identical sister chromatids joined at the centromere.
G2 (second gap): The cell continues growing, replicates remaining organelles, and checks the copied DNA for errors before entering mitosis.
By the end of G2, a human cell that started with 46 unreplicated chromosomes now has 46 replicated chromosomes, each made of two sister chromatids.
Mitosis divides the nucleus. The replicated chromosomes condense, line up, and are pulled apart so that each daughter nucleus receives one complete set.
Cytokinesis divides the cytoplasm. It typically begins during late mitosis and produces two physically separate daughter cells.
The result: two cells, each genetically identical to the original parent cell.
Biologists break mitosis into five stages based on the events occurring at each point:
Prophase: Chromatin condenses into distinct chromosomes. The mitotic spindle starts to assemble.
Prometaphase: The nuclear envelope breaks apart. Spindle fibres attach to the kinetochores on each chromosome.
Metaphase: All chromosomes are aligned at the metaphase plate (the cell's equator), with spindle fibres from opposite poles attached to sister chromatids.
Anaphase: The connection between sister chromatids is severed. Each chromatid (now called a daughter chromosome) is pulled toward an opposite pole of the cell.
Telophase: Chromosomes arrive at the poles, decondense back into chromatin, and new nuclear envelopes form around each set.
Cytokinesis overlaps with telophase, splitting the cell in two.
No quantitative formulas for this topic, but keep the following sequence diagram in mind:
G1 → S → G2 → Prophase → Prometaphase → Metaphase → Anaphase → Telophase → Cytokinesis → back to G1
Key visual to know for exams: a diagram showing a cell with unreplicated chromosomes in G1, replicated chromosomes (joined sister chromatids) after S phase, condensed chromosomes lining up in metaphase, and separated daughter chromosomes in anaphase.
Cancer is, at its core, a disease of uncontrolled cell division: mutations that disable cell-cycle checkpoints allow cells to keep dividing when they should stop. Many chemotherapy drugs work by targeting cells in specific phases of the cell cycle (for example, disrupting the mitotic spindle during M phase).
Wound healing relies on cells near the injury re-entering the cell cycle from G1 to produce new tissue.
Students often think mitosis and the cell cycle are the same thing. They are not. Mitosis is just the nuclear-division step within the larger cell cycle.
Students often confuse "chromosome" and "chromatid." After S phase, one chromosome consists of two sister chromatids. They are still counted as one chromosome until they separate in anaphase.
Students sometimes assume cytokinesis is part of mitosis. Cytokinesis is a separate process that divides the cytoplasm; mitosis only divides the nucleus.
Students often think cells spend most of their time dividing. In reality, most of the cell cycle is spent in interphase, doing normal cellular work.
⚠️ Be able to list the four cell cycle phases in order (G1, S, G2, M) and state what happens in each.
⚠️ Know the five stages of mitosis in order: prophase, prometaphase, metaphase, anaphase, telophase.
⚠️ Understand that daughter cells produced by mitosis are genetically identical to the parent cell.
⚠️ Be ready to distinguish between mitosis (nuclear division) and cytokinesis (cytoplasmic division).
⚠️ Know the difference between a replicated chromosome (two sister chromatids) and an unreplicated chromosome (one chromatid).
True or false: G2 is the phase where DNA replication occurs. (False, that is S phase.)
Fill in the blank: After mitosis is complete, ________ divides the cytoplasm to produce two separate cells. (Cytokinesis.)
True or false: A cell spends most of its life in M phase. (False, most time is spent in interphase.)
Fill in the blank: During ________, chromosomes line up at the cell's equator. (Metaphase.)
True or false: Sister chromatids separate during anaphase. (True.)
Q: Name the four phases of the cell cycle in order and briefly state the main event of each.
A: G1 (cell growth and normal function), S (DNA replication), G2 (preparation for division), M (mitosis and cytokinesis, producing two daughter cells).
Q: What is the difference between mitosis and cytokinesis?
A: Mitosis is the division of the nucleus, distributing replicated chromosomes equally to two daughter nuclei. Cytokinesis is the division of the cytoplasm that physically separates the cell into two daughter cells.
Q: A cell in G1 has 12 chromosomes. How many chromosomes does it have in G2? How many does each daughter cell have after mitosis?
A: In G2 it still has 12 chromosomes (each now consisting of two sister chromatids). After mitosis and cytokinesis, each daughter cell has 12 chromosomes.
Q: During which stage of mitosis do sister chromatids separate?
A: Anaphase.
Q: Why is the G1/S transition considered a critical control point in the cell cycle?
A: Because G1 is when the cell decides whether to commit to DNA replication. Once it passes into S phase, it is committed to dividing. This is why mutations at this checkpoint can contribute to uncontrolled cell division (cancer).
This connects directly to meiosis, which uses a modified version of the cell cycle to produce haploid gametes instead of diploid daughter cells. Understanding normal mitotic division is the prerequisite for understanding where meiosis diverges (two rounds of division, crossing over, independent assortment).
It also connects to genetics: errors in chromosome separation during mitosis or meiosis (nondisjunction) lead to conditions like Down syndrome. And it ties into molecular biology, since DNA replication in S phase relies on the same enzymatic machinery (DNA polymerase, helicase) covered in that unit.
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