# DNA, Chromosomes and Cell Division

> The structure of DNA, how DNA is packed into chromosomes, DNA replication, mitosis, meiosis and how meiosis makes variation.

From HSC Biology on WhizzNotes, free study notes for NSW students: https://whizznotes.com/biology/dna-and-cell-replication/. Updated 3 November 2026.
Year 11 and Year 12 boxes go deeper than the Year 10 text around them.

### Story time: Two metres in every cell
If you took the DNA out of one of your cells and stretched it end to end, it would be about 2 metres long. It fits into a nucleus about 6 micrometres across. That is like packing 40 kilometres of fine thread into a tennis ball, and still being able to find and read any part of it in seconds.

Every time a cell divides, all of that DNA has to be copied, almost without a mistake, and shared out perfectly. Your body does this millions of times every second.

**In short:** DNA is a double helix of nucleotides in which the base A pairs with T and C pairs with G. Before a cell divides, DNA copies itself, then mitosis makes two identical cells for growth and repair, and meiosis makes four different gametes with half the chromosomes.

## Starting point: what you already know

- DNA carries genetic information and is found in the nucleus.
- Genes are sections of DNA. Humans have 46 chromosomes in body cells (Year 10 science).
- New cells come from existing cells (cell theory).

## The structure of DNA

**DNA** (deoxyribonucleic acid) is a **double helix**: two strands twisted like a spiral ladder. Each strand is a chain of **nucleotides**. Each nucleotide has three parts:

1. a **phosphate** group,
2. a **sugar** called deoxyribose, and
3. a **nitrogen base**: adenine (A), thymine (T), cytosine (C) or guanine (G).

The sugars and phosphates make the sides of the ladder. Pairs of bases make the rungs, joined by weak **hydrogen bonds**.

### Key idea: Complementary base pairing
**A** always pairs with **T**. **C** always pairs with **G**.

So if one strand reads **A T G C C A**, the other reads **T A C G G T**.

*Diagram: A section of DNA drawn as a ladder: two sugar-phosphate backbones as the sides, and base pairs A with T and C with G as the rungs, joined by hydrogen bonds*
*DNA untwisted into a ladder. Sugar and phosphate make the grey sides. Each rung is a base pair: A with T, C with G, held by weak hydrogen bonds (dotted).*

The two strands run in opposite directions (**antiparallel**). James Watson and Francis Crick built the model in 1953, using X-ray images made by **Rosalind Franklin** and Maurice Wilkins.

## From DNA to chromosomes

In eukaryotes, DNA wraps around proteins called **histones**, then coils and folds again and again into a **chromosome**. Before a cell divides, each chromosome is copied, so it looks like an X: two identical **sister chromatids** joined at a **centromere**.

Human body cells have **46 chromosomes** in **23 pairs**. One of each pair comes from each parent. Pairs that carry the same genes are **homologous chromosomes**. One pair are the **sex chromosomes**: XX in females, XY in males.

- **Diploid** cells have two sets of chromosomes, written $2n$. Body cells are diploid. In humans $2n = 46$.
- **Haploid** cells have one set, written $n$. Gametes are haploid. In humans $n = 23$.

In prokaryotes, the DNA is a single **circular chromosome**, not wrapped in histones, plus small loops called **plasmids**.

## DNA replication

Before a cell divides, its DNA must be copied. Replication is **semi-conservative**: each new DNA molecule keeps one old strand and gets one new strand.

1. **Helicase** unzips the double helix by breaking the hydrogen bonds between bases.
2. Free nucleotides pair with the exposed bases (A with T, C with G).
3. **DNA polymerase** joins the new nucleotides into a strand and checks for mistakes.
4. Two identical DNA molecules result.

## Mitosis and meiosis

| | Mitosis | Meiosis |
|---|---|---|
| Purpose | Growth, repair, asexual reproduction | Making gametes for sexual reproduction |
| Divisions | One | Two |
| Cells made | 2 | 4 |
| Chromosome number | Stays the same: diploid to diploid | Halves: diploid to haploid |
| Daughter cells | Genetically **identical** | Genetically **different** |
| Where in humans | All body cells | Testes and ovaries |

Mitosis stages: **prophase** (chromosomes condense), **metaphase** (they line up in the middle), **anaphase** (sister chromatids pulled apart), **telophase** (two nuclei form). Then the cytoplasm divides.

### Worked example: Counting chromosomes
A dog's body cell has 78 chromosomes. How many are in a dog's sperm cell, and in a cell made by mitosis of a skin cell?

Sperm comes from meiosis, so it is haploid: $78 \div 2 = 39$. Mitosis keeps the number the same: 78.

**Mind map**

```text
DNA and cell division
  DNA
    Double helix
    Nucleotide: phosphate, sugar, base
    A with T, C with G
  Chromosomes
    DNA on histones
    Diploid 46, haploid 23
    Prokaryotes: circular DNA
  Replication
    Helicase unzips
    DNA polymerase builds
    Semi-conservative
  Mitosis
    2 identical cells
    Growth and repair
  Meiosis
    4 different gametes
    Halves the number
```

### Year 11: How meiosis makes variation
Meiosis makes every gamete different in two main ways.

1. **Crossing over**: in prophase I, homologous chromosomes pair up and swap matching pieces. Each chromosome ends up with a new mix of alleles from both parents.
2. **Independent assortment**: in metaphase I, each homologous pair lines up independently. With 23 pairs, there are $2^{23} \approx 8.4$ million possible combinations in each gamete, before crossing over is even counted.

Add **random fertilisation** (any sperm can meet any egg) and every child is unique, except identical twins.

**Meiosis I and II.** In meiosis I, **homologous chromosomes** separate (so the cells become haploid). In meiosis II, **sister chromatids** separate, as in mitosis.

**Counting DNA.** A cell doubles its DNA before dividing. If a body cell has 6 units just after division, it has 12 just before it divides, and a sperm cell (haploid, unreplicated) has 3.

**Why hybrids are sterile.** A zebra (2n = 44) and a donkey (2n = 62) give a hybrid with 22 + 31 = 53 chromosomes. They do not form matching pairs, so meiosis fails and few working gametes are made.

**Keeping the number steady.** Meiosis halves the chromosome number in gametes, fertilisation restores it, and mitosis keeps it the same in every body cell.

### Year 12: Modelling DNA replication and mistakes
HSC questions often ask you to **model** replication. Show the two strands separating, free nucleotides pairing by complementary base pairing, and two daughter molecules, each with one old and one new strand.

The **Meselson and Stahl** experiment (1958) proved replication is semi-conservative. Bacteria were grown with heavy nitrogen-15, then moved to normal nitrogen-14. After one round of replication, all the DNA was of medium density: one heavy strand and one light strand.

DNA polymerase makes about one uncorrected error per billion bases. Those rare errors are **mutations**, studied in Module 6.

After a **second** round in nitrogen-14, half the DNA was medium density and half was light.

**Base counting (Chargaff's rule).** Because A pairs with T and C with G, %A = %T and %C = %G. If DNA is 32% C, it is also 32% G, leaving 36% for A and T, so 18% A. A section with 40 C and 50 T also has 40 G and 50 A: 180 bases in total.

**Leading and lagging strands.** DNA polymerase can only build a new strand in one direction. One new strand (the **leading** strand) is made in one long piece. The other (the **lagging** strand) runs the opposite way, so it is made in short pieces (Okazaki fragments), which **DNA ligase** joins together.

**Why accuracy matters.** A copying mistake is passed on to every cell made from that cell, and can change a protein. In gametes it can be passed to offspring.

## Quick summary

- DNA is a double helix of nucleotides (phosphate, deoxyribose, base). A pairs with T, C with G.
- DNA wraps around histones to form chromosomes. Humans have 46 (diploid). Gametes have 23 (haploid).
- Replication is semi-conservative: helicase unzips, DNA polymerase builds the new strands.
- Mitosis makes 2 identical diploid cells for growth and repair.
- Meiosis makes 4 different haploid gametes. Crossing over and independent assortment create variation.

## Quiz

Try these questions from the chapter quiz.

1. Which base pairs with adenine in DNA?
   - A. Cytosine
   - B. Guanine
   - C. Thymine
   - D. Uracil
   - Answer: C. Thymine. A always pairs with T.

2. One DNA strand reads ATTGC. What is the complementary strand?
   - A. ATTGC
   - B. TAACG
   - C. GCCAT
   - D. UAACG
   - Answer: B. TAACG. Pair A with T and C with G.

3. A DNA nucleotide is made of
   - A. a phosphate, a deoxyribose sugar and a base
   - B. an amino acid and a sugar
   - C. a protein and a base
   - D. glucose and phosphate
   - Answer: A. a phosphate, a deoxyribose sugar and a base. Each nucleotide has three parts.

The whole chapter quiz is on the page, for signed-in students: https://whizznotes.com/biology/dna-and-cell-replication/#quiz