Mitosis and the Cell Cycle

A chromosome is a long, coiled molecule of DNA found in the nucleus of a cell. Each chromosome carries a large number of genes — short sections of DNA that each code for a particular characteristic.

In human body cells the chromosomes are found in pairs. Humans have 23 pairs, giving 46 chromosomes in total. The two chromosomes in a pair carry the same genes in the same order, one copy inherited from each parent.

Term	What it is
DNA	The molecule chromosomes are made of
Chromosome	A long molecule of DNA carrying many genes
Gene	A short section of DNA coding for one characteristic
Pair	Two matching chromosomes carrying the same genes

A body cell with chromosomes in pairs is called diploid. In humans the diploid number is 46 (23 pairs).

Because the chromosomes come in pairs, a cell holds two copies of the genetic information. This matters for cell division: before a cell splits, every chromosome must be copied so each new cell receives a full set.

Cells do not divide instantly. They go through a repeating sequence called the cell cycle. For AQA you need the three overall stages — you do not need the named phases inside mitosis itself.

Stage	Name	What happens	Why it matters
1	Growth & DNA copying	Cell grows; sub-cellular structures (ribosomes, mitochondria) increase; DNA replicates so each chromosome becomes two copies	Builds the materials the two new cells will need
2	Mitosis	One set of chromosomes is pulled to each end of the cell; the nucleus divides	Separates an identical set of genes to each side
3	Cytokinesis	The cytoplasm and cell membranes divide	Splits one cell into two complete cells

The result is two genetically identical diploid daughter cells, each with a full set of 46 chromosomes — the same as the parent cell.

Stage 1 is the longest part of the cycle. Most of a cell's life is spent growing and copying its DNA, not dividing.

Before a cell can divide it must first grow and prepare. Two things happen in this longest stage.

First, the cell increases the number of its sub-cellular structures — it makes more ribosomes and more mitochondria so that each daughter cell will have enough to function.

Second, the DNA replicates. Each chromosome is copied to form two identical copies, joined together. So a cell that started with 46 single chromosomes now holds 46 doubled chromosomes — 92 copies of DNA in total, still organised as 46 pairs of copies.

Replication must finish before mitosis begins. If the DNA were not copied first, each daughter cell would receive only half a set of chromosomes.

This careful copying is what allows the two daughter cells to end up genetically identical to each other and to the original cell.

Mitosis is the stage where the nucleus divides. The copied chromosomes are separated so that each end of the cell receives one complete set.

One copy of each chromosome is pulled to each end of the cell. Because the chromosomes were copied in Stage 1, both ends receive a full, identical set of 46 chromosomes. The nucleus then divides into two.

Mitosis is followed by cytokinesis (Stage 3), when the cytoplasm and cell membranes pinch apart to form two separate cells. The outcome of the whole cycle is two genetically identical diploid daughter cells.

You do not need the named phases of mitosis for AQA 8461 — only that the chromosomes are pulled to each end and the nucleus divides.

Mitosis produces identical cells, and organisms rely on this in several ways. The same process serves four important purposes.

Purpose	Example
Growth	A baby grows into an adult as the number of cells increases
Development	Cells multiply and organise to build tissues and organs in an embryo
Repair / replacement	Damaged or worn-out cells, such as skin and blood cells, are replaced
Asexual reproduction	Some organisms, such as plants reproducing by runners or strawberry plants, make identical offspring by mitosis

A skill the exam tests is recognising mitosis in context. Whenever cells of a multicellular organism are increasing in number to grow, heal a wound, or replace old cells, mitosis is responsible.

Because mitosis makes identical copies, asexual reproduction produces offspring that are clones of the parent — genetically identical, with no variation.

Example 1 — Identify the cell-cycle stage.

A cell is described as having just finished copying its DNA, with each chromosome now present as two joined identical copies; the cell has not yet started to divide.

This is the end of Stage 1 (growth and DNA replication). Justification: DNA replication has happened (chromosomes are doubled), but the chromosomes have not yet been pulled apart and the nucleus has not divided — so mitosis (Stage 2) has not begun.

Example 2 — How many cells after $n$ divisions?

Each mitotic division turns one cell into two, so after $n$ divisions one starting cell becomes $2^n$ cells.

Find the number of cells after a single cell divides 4 times:

• After 1 division: $2^1=2$
• After 2 divisions: $2^2=4$
• After 3 divisions: $2^3=8$
• After 4 divisions: $2^4=16$

So one cell produces $2^4=16$ cells after 4 divisions. Check by doubling step by step: $1\to 2\to 4\to 8\to 16$. ✓

Every one of those 16 cells is genetically identical to the original — mitosis does not create variation.

1. Saying mitosis makes the cell divide before the DNA is copied

DNA replication happens in Stage 1, before mitosis. If you describe the chromosomes separating without first being copied, the daughter cells would not each get a full set.

2. Confusing mitosis with the whole cell cycle

Mitosis is only one of the three overall stages (it is the nuclear division). The full cycle also includes growth/DNA replication and the division of the cytoplasm and membranes.

3. Forgetting the daughter cells are genetically identical

Mitosis produces two genetically identical diploid daughter cells, each with the same number of chromosomes as the parent (46 in humans). Do not confuse this with meiosis, which makes non-identical sex cells.

4. Saying chromosomes are made of genes

It is the other way round: chromosomes are made of DNA, and a gene is a short section of that DNA. One chromosome carries many genes.

5. Not learning the three stages by name

You need all three overall stages: growth and DNA replication, mitosis (nucleus divides), then division of the cytoplasm and membranes. Listing only mitosis loses marks for an incomplete cycle.