Cell Cycle and Cell Division

Syllabus Unit: Cell Structure and Function

Why it matters

Chapter overview

Every cell in the body arose from a pre-existing cell dividing, and how it divides — precisely or carelessly — is the difference between normal growth and cancer. This topic walks through the ordered phases of the cell cycle, the exact mechanics of mitosis (which produces two identical daughter cells for growth and repair) and meiosis (which halves chromosome number and shuffles genes for sexual reproduction), and closes with how a broken cell cycle checkpoint produces cancer — one of NEET's most diagram-heavy topics.

Biology Syllabus Cell Structure and Function 15% weightage

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1G1: cell growth (2C)

2S: DNA replication (2C→4C)

3G2: prepares for mitosis (4C)

4M: cell divides (back to 2C)

1The Cell Cycle & Its Phases

A cell's life is an ordered cycle of growth, DNA replication, and division — not a random process, but one governed by strict checkpoints.

Learning Objectives

Name the two main phases of the cell cycle: interphase and M phase (mitotic phase).
Sequence the three sub-phases of interphase: G1, S, G2, and state what happens in each.
State the DNA content (C value) at the start of G1, end of S, and end of M phase.
Explain the role of the G1/S and G2/M checkpoints in cell cycle control.

Key Terms

InterphaseThe 'resting' phase between two cell divisions, comprising G1, S and G2, during which the cell grows and replicates its DNA; makes up ~95% of the cell cycle's duration.

S PhaseThe Synthesis phase of interphase, when DNA replicates, doubling DNA content from 2C to 4C while chromosome number stays constant.

G1 Phase (Gap 1)The phase between the end of the previous M phase and the start of S phase, when the cell grows and synthesises proteins/organelles needed for DNA replication.

CheckpointA cell cycle control point (e.g. G1/S, G2/M) where the cell verifies conditions are correct (DNA undamaged, size adequate) before proceeding to the next phase.

The **cell cycle** is the ordered sequence of events by which a cell grows and divides into two daughter cells. It has two main phases: **interphase** (preparation) and the **M phase / mitotic phase** (actual division), and interphase itself is not resting — it is the busiest part of the cycle.

The three sub-phases of interphase

G1 (Gap 1) — the cell grows in size, synthesises RNA and proteins, and prepares the machinery for DNA replication; DNA content is 2C (diploid).
S (Synthesis) — DNA replicates; DNA content doubles from 2C to 4C, but chromosome number stays the same because each chromosome now consists of two identical sister chromatids joined at the centromere.
G2 (Gap 2) — the cell continues growing and synthesises proteins needed for mitosis (e.g. tubulin for the spindle); DNA content remains 4C.

G1 (2C) → S phase, DNA replicates → G2 (4C) → M phase, cell divides → two daughter cells (2C each)

Checkpoints: quality control

The cell cycle is tightly regulated at **checkpoints** — the G1/S checkpoint verifies DNA is undamaged and the cell is ready to replicate; the G2/M checkpoint verifies DNA replication is complete and undamaged before mitosis begins. Failure of these checkpoints (e.g. mutated p53) allows damaged DNA to be passed on, a key step toward cancer.

Key PointSome cells (e.g. neurons) exit the cycle into a G0 (quiescent) phase and do not divide further — this is why nerve cell damage is often permanent.