Chromosomes and DNA: The Instruction Book Inside Your Cells
Extension topic: this whole chapter is Extension content in the 2025 curriculum.
Why do you have your mum's eyes or your dad's smile? The answer is hidden in a tiny molecule called DNA. In this chapter you will learn where DNA is kept, how many chromosomes humans have, how a baby's sex is decided, what DNA looks like and how its "code" works. You will also build a DNA model and see how to pull real DNA out of fruit!
How sperm and ovum meet is covered in Human reproduction; how traits pass down a family is covered in Heredity and variation.
1. Where Is the Genetic Material? Chromosomes in the Nucleus
From cell to DNA: zooming in
Almost every cell in your body has a nucleus, the control centre of the cell. Inside the nucleus are thread-like structures called chromosomes. Each chromosome contains a very long molecule of DNA (short for deoxyribonucleic acid), tightly coiled up together with some proteins.
DNA is the genetic material: it carries information from parents to children.
Big to small: cell → nucleus → chromosome → DNA
Memory aid: "Cute Nemo Can Dive".
Analogy: The nucleus is a library, the chromosomes are the books, and DNA is the writing inside. A short section of DNA that carries one instruction is called a gene, like one recipe in a recipe book.
Chromosomes are usually long and thin and hard to see. When a cell is about to divide, they coil up and become short and thick, so we can see them under a microscope.
Key takeaway: Chromosomes are found in the nucleus, and chromosomes contain DNA, the genetic material.
2. DNA Carries the Instructions for Traits
A trait is a feature of a living thing, such as eye colour, natural hair colour, blood group or the colour of a flower. DNA carries the instructions that determine these different traits. It also carries instructions that control the activities of the cell.
Nearly all the cells of one person carry the same DNA. A skin cell and a muscle cell have the same instructions, but each cell only "reads" the parts it needs for its job.
You get your DNA from both parents, so you share some traits with each of them but are not an exact copy of either.
Key takeaway: DNA carries the instructions that determine the different traits of an organism.
3. Counting Chromosomes: 23 and 46
Body cells: chromosomes come in pairs
A normal human body cell has 46 chromosomes. They come in 23 pairs. In each pair, one chromosome came from the father and one came from the mother. So a body cell carries two sets of chromosomes.
Sex cells: one set of 23
The sex cells (sperm and ovum) are special. Each sex cell carries only one set of chromosomes, which is 23 chromosomes, one from each pair.
The zygote: two sets of 46
At fertilisation, a sperm fuses with an ovum to form a zygote. The zygote gets one set from the sperm and one set from the ovum, so it carries two sets, or 46 chromosomes.
\(23 \text{ (sperm)} + 23 \text{ (ovum)} = 46 \text{ (zygote)}\)
Why only one set? If sperm and ovum each carried 46, the baby would have 92, and the number would double every generation. With 23 in each sex cell, the number stays at 46.
Did you know? Different species have different chromosome numbers: a chimpanzee has 48 and a dog has 78. More chromosomes does not mean more "advanced"!
Key takeaway: Human sex cells carry one set (23 chromosomes); the zygote carries two sets (46 chromosomes).
4. Boy or Girl? The Sex Chromosomes
XX and XY
Of the 23 pairs, 22 pairs are the same in males and females. The last pair is the sex chromosomes, which decide a person's sex.
Female: two X chromosomes (XX)
Male: one X and one Y chromosome (XY)
The Y chromosome is much smaller than the X, so it is easy to spot.
How the sex of the zygote is decided
Every ovum carries one X chromosome. A sperm carries either an X or a Y, about half of each.
Ovum (X) + sperm carrying X → zygote XX → girl
Ovum (X) + sperm carrying Y → zygote XY → boy
So the sex of the zygote is decided at fertilisation by its sex chromosomes, depending on which kind of sperm fuses with the ovum. The chance of a boy or a girl is about 50:50.
Skill: reading a photomicrograph of chromosomes
A photomicrograph is a photo taken through a microscope. Scientists photograph the chromosomes of one dividing cell and arrange them into pairs by size, shape and band pattern, from the largest (pair 1) to the smallest (pair 22), with the sex chromosomes last. This picture of a complete set is called a karyotype.
Step 1: Count the chromosomes. A normal human cell has 46.
Step 2: Find the sex chromosomes (the last pair, or the pair that does not match).
Step 3: Two X chromosomes of the same size → female. One large X and one small Y → male.
Exam tip: In a male karyotype, the sex chromosomes are the only pair that look clearly different.
Key takeaway: Sex chromosomes decide sex (XX female, XY male). They are the last pair in a photomicrograph of a complete set.
5. The Structure of DNA: A Twisted Ladder
Four bases: A, T, C and G
DNA is built from small units, each containing one base. There are only four kinds of bases:
A = adenine, T = thymine, C = cytosine, G = guanine
The double helix
A DNA molecule has two strands twisted around each other, like a twisted ladder or spiral staircase. This shape is called a double helix. The sides of the ladder are the strands' "backbones"; the rungs are pairs of bases, one from each strand, joined in the middle.
Base pairing: A with T, C with G
The bases always pair up in the same way:
A pairs with T
C pairs with G
This is called base pairing, and the double helix structure depends on it. A-T and C-G pairs fit neatly, so the two strands stay the same distance apart, like a ladder with rungs of equal length.
Memory aid: "Apple in the Tree, Car in the Garage."
Worked example: One strand reads A T T C G A. What is the partner strand?
A→T, T→A, T→A, C→G, G→C, A→T, so the partner strand reads T A A G C T.
Because of base pairing, in any piece of DNA the number of A equals the number of T, and the number of C equals the number of G.
Did you know? The double helix model of DNA was published in 1953 by James Watson and Francis Crick. Their work used X-ray photos of DNA made by Rosalind Franklin.
Key takeaway: DNA has four bases (A, T, C, G). Its double helix depends on base pairing: A with T, C with G.
6. The Code: Base Sequence and Proteins
The order of the bases along a DNA strand is called the base sequence. The instructions in DNA depend on this sequence, just like the meaning of a word depends on the order of its letters. "TEA" and "EAT" use the same letters but mean different things!
The base sequence tells the cell which proteins to make. Proteins do many jobs: keratin builds hair and nails, haemoglobin carries oxygen in red blood cells, and enzymes speed up reactions such as digestion.
The proteins a cell makes affect your traits. For example, eye colour depends on how much brown pigment is made, which depends on the proteins that make it.
If the base sequence changes, the protein made may change too, and so the trait may change.
Did you know? One set of human chromosomes contains about 3 billion base pairs. If you could stretch out all the DNA from just one of your cells, it would be about 2 metres long!
Key takeaway: The base sequence of DNA is the code. It decides which proteins are made, and proteins affect traits.
7. Hands-on Science: Making and Seeing DNA
Key Practical Task: building a DNA model
Materials: jelly sweets or paper squares in 4 colours (the bases), 2 straws or paper strips (the backbones), toothpicks, tape.
1. Choose a colour code, e.g. red = A, yellow = T, green = C, blue = G.
2. Write a base sequence for strand 1, e.g. A C G T T A G C.
3. Work out the partner strand: T G C A A T C G.
4. Join each base pair with a toothpick to make a "rung".
5. Fix the ends of each rung to the two backbones.
6. Twist gently to make a double helix.
7. Check every rung: only A-T and C-G pairs.
What the model shows: two strands, a double helix and correct base pairing. Limits: real DNA is far smaller and longer, and its bases have no colours. A model is a simplified explanation.
Safety: Never eat sweets used in the laboratory.
Suggested activity: extracting DNA from fruit
You can extract DNA from fruit such as strawberries, bananas or kiwis.
1. Mash the fruit in a sealed plastic bag – this breaks the cells apart.
2. Add extraction solution (water, washing-up liquid, a little salt) and mix gently. The washing-up liquid breaks the cell and nuclear membranes to release the DNA; the salt helps DNA clump together.
3. Filter through a coffee filter to remove solid bits.
4. Slowly pour ice-cold alcohol down the side of the tube to form a top layer. DNA does not dissolve in cold alcohol, so it comes out.
5. Look for white, stringy material between the layers. That is DNA! Lift it out with a glass rod.
Safety: Alcohol is flammable. Keep it away from flames and wear safety goggles.
Did you know? Strawberries are great for this activity because each of their cells has 8 sets of chromosomes, so there is a lot of DNA to collect.
Key takeaway: A DNA model shows the double helix and base pairing; DNA extraction lets you see a large clump of real DNA.
Common Mistakes
Common mistake 1: "Sex cells have 46 chromosomes." No! Sperm and ovum each have 23 (one set). The zygote and body cells have 46.
Common mistake 2: "The mother decides the baby's sex." No! Every ovum carries X. The sperm (X or Y) makes the difference.
Common mistake 3: Pairing A with C or G with T. Remember: only A-T and C-G.
Common mistake 4: Thinking every X-shaped chromosome in a photo is an X chromosome. Most chromosomes look X-shaped because they have copied themselves before the cell divides.
Common mistake 5: Thinking the white strands from fruit show a double helix. They are millions of DNA molecules clumped together; the helix is far too small to see.
Quick Review
1. Chromosomes in the nucleus contain DNA, the genetic material.
2. DNA carries the instructions that determine traits.
3. Sex cells: one set (23). Zygote: two sets (46).
4. XX = female, XY = male.
5. In a karyotype, the sex chromosomes are the last pair.
6. DNA has four bases: A, T, C and G.
7. Double helix held by base pairing: A-T, C-G.
8. The base sequence decides which proteins are made.
Great work! Keep practising the pairing rules and the 23/46 numbers, and you will be ready for any question on this topic.