## Learning Outcomes

- Explain the stages of the cell cycle and what happens in interphase
- Describe the sequence of events in mitosis and cytokinesis using precise biological terminology
- Compare mitosis and meiosis in terms of chromosome behaviour, outcomes, and biological role
- Explain how meiosis creates genetic variation through crossing over and independent segregation
- Calculate and interpret mitotic index from root tip data
- Apply cell division knowledge to synoptic topics such as cancer and cell cycle control

## AQA A-Level Biology (7402) Syllabus

For the AQA A-Level Biology (7402), you are expected to understand cell division, with a focus on the following syllabus points:

- Know that the cell cycle consists of interphase, mitosis and cytokinesis
- Identify what happens in G₁, S and G₂ during interphase
- Recall the appearance and behaviour of chromosomes at each stage of mitosis
- Explain the role of spindle fibres and centromeres in the separation of chromatids
- Understand the differences between mitosis and meiosis, including chromosome number and genetic similarity
- Explain how meiosis generates genetic variation through crossing over and independent segregation
- Calculate mitotic index and interpret what a high or low value suggests
- Understand that failure of normal cell cycle control can lead to tumours and cancer

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. What happens during S phase of interphase?
2. What is the difference between homologous chromosomes and sister chromatids?
3. In which division of meiosis do homologous chromosomes separate, and in which do sister chromatids separate?
4. How does crossing over increase genetic variation?
5. How is mitotic index calculated?

## Introduction

Cell division is one of the most tested parts of cell biology because it links structure, genetics, growth, repair, reproduction and disease. On AQA Paper 3, it can also appear in synoptic questions that connect cells to inheritance, mutations, cancer or practical skills.

To do well, you need more than a list of stage names. You must know what each stage achieves, how chromosome behaviour changes, and why mitosis and meiosis give different outcomes. Precise wording matters. Examiners often separate students by whether they say **homologous chromosomes** when they mean homologous chromosomes, and **sister chromatids** when they mean sister chromatids.

> **Key Term: cell cycle**
> The sequence of events between one cell division and the next, consisting of interphase, mitosis and cytokinesis.

### The cell cycle and mitosis

Not all cells in a multicellular organism divide. Some, such as many neurones, usually do not re-enter the cycle. Cells that do divide follow the cell cycle, which is a controlled sequence rather than a continuous random process.

Most of the cycle is **interphase**, which is often described as a resting phase, but that label can mislead. The cell is highly active metabolically. It grows, makes proteins, replicates organelles, increases ATP supply and copies its DNA.

Interphase has three stages:

- **G₁ phase**: proteins are synthesised, organelles increase in number, and the cell grows
- **S phase**: DNA replication occurs
- **G₂ phase**: further growth occurs, ATP increases, and the cell prepares for division

The key event for chromosome behaviour is in S phase. Before replication, each chromosome contains one DNA molecule. After replication, each chromosome consists of **two sister chromatids** joined at a **centromere**. The chromosome number has not doubled, but the DNA content has.

> **Key Term: sister chromatids**
> Two genetically identical copies of a replicated chromosome, joined together at the centromere.
>
> **Key Term: centromere**
> The region joining sister chromatids, where spindle fibres attach during cell division.

Mitosis is nuclear division that produces two genetically identical nuclei. It is followed by **cytokinesis**, which divides the cytoplasm and usually forms two daughter cells.

The stages are best understood by tracking chromosomes.

In **prophase**, chromosomes condense and become visible as shorter, thicker structures. The nuclear envelope breaks down. In animal cells, centrioles help organise spindle formation, though plant cells can form a spindle without centrioles. Spindle fibres, made of microtubules, begin to interact with chromosomes.

In **metaphase**, chromosomes line up at the equator of the cell. This matters because equal alignment helps each daughter nucleus receive one copy of every chromosome.

In **anaphase**, the centromeres divide and spindle fibres pull sister chromatids to opposite poles. This is a common exam trap: in mitosis, it is **sister chromatids** that separate, not homologous chromosomes.

In **telophase**, chromatids reach the poles, uncoil, and new nuclear envelopes form around each group. Cytokinesis then completes division of the cell.

> **Key Term: mitosis**
> Nuclear division in a eukaryotic cell that produces two genetically identical daughter nuclei with the same chromosome number as the parent nucleus.

Why does mitosis produce identical cells? Because DNA was copied in S phase, then one copy of each chromatid is pulled into each daughter nucleus. There is no pairing of homologous chromosomes and no exchange of genetic material, so the daughter cells keep the same genetic information as the parent cell, apart from rare mutations.

Mitosis is essential for:

- growth
- repair of damaged tissue
- replacement of worn-out cells
- asexual reproduction in some organisms
  > **Test Tip:** When identifying mitosis stages from images, focus on chromosome position. Chromosomes lined up across the middle indicate metaphase. Chromatids moving apart toward opposite poles indicate anaphase.

### Meiosis and the production of genetic variation

Meiosis is different from mitosis in both purpose and outcome. Its role is to reduce chromosome number and generate variation for sexual reproduction.

> **Key Term: meiosis**
> A type of cell division in which one diploid cell undergoes two divisions to produce four haploid cells that are genetically different.

If gametes were formed by mitosis, they would be diploid. Fertilisation would then double chromosome number every generation. Meiosis prevents this by halving chromosome number before fertilisation. In humans, diploid cells have 46 chromosomes, while gametes have 23.

> **Key Term: homologous chromosomes**
> A pair of chromosomes, one maternal and one paternal, that carry the same genes at the same loci but may contain different alleles.

Before meiosis begins, DNA replicates once during interphase. That means each chromosome again consists of two sister chromatids. After that, there are **two divisions**.

In **meiosis I**, homologous chromosomes pair up. This pairing is called synapsis, and each pair forms a **bivalent**. Because there are four chromatids in a bivalent, meiosis I creates the conditions for genetic variation.

> **Key Term: crossing over**
> The exchange of DNA between non-sister chromatids of homologous chromosomes during prophase I, producing new combinations of alleles.

During **prophase I**, homologous chromosomes pair and non-sister chromatids can cross over. The visible points where chromatids remain attached are called **chiasmata**. This does not create new genes; it creates new combinations of alleles on the chromatids.

In **metaphase I**, bivalents line up at the equator. Their orientation is random. A maternal chromosome may face one pole or the other independently of other pairs.

> **Key Term: independent segregation**
> The random orientation and separation of homologous chromosome pairs during meiosis I, leading to different combinations of chromosomes in gametes.

In **anaphase I**, homologous chromosomes separate and move to opposite poles. The centromeres do **not** divide here, so sister chromatids remain together. This is the reduction division because chromosome number is halved.

After telophase I and cytokinesis, two haploid cells are formed. However, each chromosome still consists of two sister chromatids.

**Meiosis II** is similar to mitosis. In metaphase II, chromosomes line up individually at the equator. In anaphase II, centromeres divide and sister chromatids separate. Telophase II and cytokinesis then produce four haploid daughter cells.

The final cells are genetically different for three reasons:

- crossing over in prophase I
- independent segregation in metaphase I and anaphase I
- random fertilisation between any gamete from one parent and any gamete from the other

A very common weak answer says meiosis causes variation because “DNA mixes”. That is too vague. The exam wants the mechanism. You should refer to **exchange of sections of DNA between non-sister chromatids**, and **random orientation of homologous pairs**.

> **Test Tip:** In meiosis questions, check whether the mark scheme wants “homologous chromosomes separate” or “sister chromatids separate”. Those are not interchangeable, and using the wrong one usually loses the mark.

### Mitosis and meiosis compared

Students often know both processes separately but still confuse them when comparison is needed. The cleanest way is to compare them point by point.

Mitosis

- one division
- produces two cells
- chromosome number stays the same
- daughter cells are genetically identical
- used in growth, repair and asexual reproduction
- no homologous pairing
- no crossing over

Meiosis

- two divisions
- produces four cells
- chromosome number is halved
- daughter cells are genetically different
- used in sexual reproduction
- homologous chromosomes pair in prophase I
- crossing over and independent segregation occur

The reason the outcomes differ is not just “one makes two and one makes four”. The deeper explanation is chromosome behaviour. Mitosis separates sister chromatids once. Meiosis first separates homologous chromosomes, then separates sister chromatids. That extra division changes chromosome number and allows variation.

Another common confusion is between **homologous chromosomes** and **sister chromatids**. Homologous chromosomes are a maternal and paternal pair carrying the same genes. Sister chromatids are identical copies of one chromosome made by replication.

> **Key Term: cytokinesis**
> Division of the cytoplasm following nuclear division, producing separate daughter cells.

You may also meet **binary fission** in synoptic comparisons. Prokaryotes do not divide by mitosis or meiosis. Their circular DNA replicates, attaches to the membrane, and the cell splits into two. This matters because examiners may ask you to compare cell division in prokaryotes and eukaryotes.

### Required practical, mitotic index and synoptic links to cancer

AQA can test cell division through practical work, especially the root tip squash.

Root tip meristems are used because they contain actively dividing cells. The meristem is a region of rapid mitosis just behind the root cap. That gives a high chance of seeing cells in different stages.

The usual practical method includes:

- cutting a small root tip
- treating it with hydrochloric acid
- rinsing it
- staining it
- macerating the tissue
- placing on a slide under a coverslip
- gently squashing it to spread the cells into a thin layer
- observing under an optical microscope

Hydrochloric acid softens the tissue and helps break down the middle lamella between plant cells, so the cells separate more easily. The stain improves contrast so chromosomes can be seen more clearly. Squashing reduces overlap.

> **Key Term: mitotic index**
> The proportion of cells in a sample that are in mitosis, calculated as number of cells in mitosis divided by total number of cells observed.

Worked example:

A student counts 200 cells in a root tip sample.  
Of these, 18 are in prophase, 6 in metaphase, 4 in anaphase and 2 in telophase.

Step 1: calculate cells in mitosis

Cells in mitosis = 18 + 6 + 4 + 2 = 30

Step 2: divide by total cells counted

Mitotic index = 30 ÷ 200 = 0.15

Step 3: convert to percentage if required

Mitotic index (%) = 0.15 × 100 = 15%

Interpretation: 15% of the observed cells were in mitosis at that moment. A higher mitotic index suggests a greater proportion of cells are actively dividing.

Common student errors:

- including interphase cells in the “cells in mitosis” value
- forgetting to divide by total number of observed cells
- missing the instruction to express the answer as a percentage

The practical also has limits. You only see a snapshot, not the full cycle over time. Stage boundaries can be hard to judge, some cells overlap, staining may be uneven, and small samples reduce reliability. Better reliability comes from larger sample sizes, repeated counts, several fields of view and consistent treatment conditions.

Cell division also links directly to cancer. The cell cycle is controlled by genes and regulatory proteins including **cyclins** and cyclin-dependent kinases. Checkpoints monitor whether DNA has replicated correctly, whether damage is present, and whether chromosomes are attached properly to the spindle.

If mutations affect genes controlling this system, cells may continue dividing when they should stop. Uncontrolled mitosis can produce a **tumour**. A malignant tumour invades surrounding tissues and can form cancer. Some cancer treatments aim to slow cell division, for example by interfering with DNA replication or spindle formation.

This is a good synoptic link because it connects cell division to mutation, gene control, apoptosis and disease.

> **Key Term: checkpoint**
> A control point in the cell cycle where progression only continues if conditions are suitable and key processes have been completed correctly.
>
> **Test Tip:** In longer synoptic answers, do not stop at “cancer is caused by uncontrolled mitosis”. Add the mechanism: mutation in genes controlling the cell cycle, loss of checkpoint control, continued division, tumour formation.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- The cell cycle consists of interphase, mitosis and cytokinesis
- Interphase includes G₁, S and G₂, and S phase is when DNA replication occurs
- After DNA replication, each chromosome consists of two sister chromatids joined by a centromere
- Mitosis produces two genetically identical daughter cells with the same chromosome number as the parent cell
- In mitosis, centromeres divide in anaphase and sister chromatids separate
- Meiosis produces four haploid daughter cells that are genetically different
- In meiosis I, homologous chromosomes pair and then separate; centromeres do not divide at anaphase I
- In meiosis II, centromeres divide and sister chromatids separate
- Genetic variation in meiosis is caused by crossing over, independent segregation and random fertilisation
- Root tip meristems are used in the mitosis practical because they contain many actively dividing cells
- Mitotic index is calculated as cells in mitosis divided by total cells observed
- Failure of cell cycle control can lead to tumours and cancer

## Key Terms and Concepts

- cell cycle
- sister chromatids
- centromere
- mitosis
- meiosis
- homologous chromosomes
- crossing over
- independent segregation
- cytokinesis
- mitotic index
- checkpoint


Cells - Cell division