Chapter 1 of 8

All 8 chapters
  1. 1DNA, genes and chromosomes, readunderstood
  2. 2The structure of DNA, readunderstood
  3. 3Meiosis and sex cells, readunderstood
  4. 4Boy or girl: X and Y, readunderstood
  5. 5Dominant and recessive, readunderstood
  6. 6Family trees and genetic disorders, readunderstood
  7. 7Variation, readunderstood
  8. 8Mutation and genetic engineering, readunderstood

S4.2.1 Chapter 1

DNA, genes and chromosomes

Children look like their parents. What exactly is handed down from one to the other?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Put DNA, gene, chromosome, nucleus and cell in order of size
  • Explain that a gene is a code for making a protein

Look at a family photo. Children often look like their parents. They are not copies of either one. Something is passed from parents to children, and it is in every cell.

Most of your cells have a nucleus. Inside the nucleus is DNA, a very long molecule. It holds the instructions for building and running a living thing.

The DNA is not loose. It is wound up into thread-like bundles called chromosomes. One chromosome is one long molecule of DNA, coiled up tightly.

Try it

Coil a long thread

You need: a piece of string about 1 m long, a pencil or a thin stick.

  1. Hold the string straight. See how long it is.
  2. Tie one end to the pencil.
  3. Turn the pencil to wind the string round it, close and tight.
  4. Look at the pencil. Where is the string now?

What you will see: One metre of string now takes up the room of a short bundle on the pencil. That is how a chromosome holds a very long molecule of DNA in a tiny space.

If you cannot try it, this is what happens.
Three boxes joined by arrows, from left to right: long string, wind it round a pencil, short coiled bundle.longstringwind itround apencilshortcoiledbundle

The DNA of one human cell is about 2 m long if you stretch it out. The nucleus that holds it is less than 10 µm across. Coiling into chromosomes makes this possible.

Along each chromosome lie many genes. A gene is a section of the DNA that holds the code for making one protein. Proteins do much of the work in your body. They build muscle, carry oxygen in the blood and speed up reactions in cells.

Some genes code for other molecules, but every gene in this book codes for a protein. Think of DNA as a very long recipe book. A chromosome is one volume, and a gene is one recipe.

A chain of panels, largest at the top. Each panel zooms in on a part of the one above. They show the cell, then the nucleus inside it, then a chromosome inside the nucleus, then a gene on the chromosome, then the bases of the gene, drawn as a short ladder of lettered boxes in pairs. The panels are named cell, nucleus, chromosome, gene and DNA bases.ATTAGCCGcellnucleuschromosomegeneDNA bases
Zoom in from the cell to the DNA bases. The panels are not to one scale. Each one is a close up of a part of the panel above.

So the order of size, from the largest, is cell, nucleus, chromosome, gene and one DNA base. A whole chromosome is one DNA molecule, so DNA is not smaller than a gene. The bases that build DNA are smaller.

Most human body cells have 46 chromosomes. They come in 23 pairs. You get one chromosome of each pair from your mother and the other from your father. Your 46 chromosomes hold about 20 000 genes.

The 46 chromosomes of a human body cell, drawn as 23 pairs in order of length. Pairs 1 to 22 are numbered, and the last pair, XY, is labelled sex chromosomes.12345678910111213141516171819202122XYsex chromosomes
One body cell of a man: 23 pairs, which is 46 chromosomes. Pair 1 is the longest.

Worked example

A body cell of a pea plant has 7 pairs of chromosomes. How many chromosomes is that?

  1. Each pair is made of 2 chromosomes.
  2. Chromosomes = pairs × 2.
  3. Chromosomes = 7 × 2 = 14.

Answer: 14 chromosomes

A table with the columns Living thing and Chromosomes. Human 46. Pea plant 14. Fruit fly 8. Dog 78.Living thingChromosomeshuman46pea plant14fruit fly8dog78
Chromosomes in one body cell. A dog has more than a human, so the number says nothing about how clever or complex a living thing is.

Zoom in on the DNA itself and it looks like a ladder twisted round and round. Chapter 2 shows why it has this shape.

A sketch of DNA: two strands twisted round each other, with rungs between them. The backbone and a base pair are named, and the words two strands twisted sit under the picture.backbonebase pairtwo strands twisted
A short piece of DNA. It is far too small to see, even with a light microscope.

A wrong idea many people have

One gene builds one body part, such as an eye.

Why it feels right: News stories speak of "the gene for" blue eyes or for height, as if one gene did the whole job.

The test that breaks it: Think of an eye. It holds many kinds of cell, and each kind needs many proteins. Each protein has its own gene.

What is true: A gene is the code for one protein. Many genes work together to build any body part.

Key words

DNA
the long molecule in cells that holds the instructions for a living thing.
chromosome
one long molecule of DNA, coiled up tightly.
gene
a section of DNA with the code for making one protein.
protein
a molecule that does work in the body, such as a muscle protein or an enzyme.

Back to your guess

The answer is C: Instructions written in a molecule called DNA, copied into every cell.

Parents pass on instructions written in DNA. Almost every cell of the baby gets a copy. Blood does not pass from parent to child, and habits are learned, not inherited.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. Which list puts these in order of size, largest first?

    Tap your answer.

    1. Not this one. A gene is a section of a chromosome, so a chromosome is larger than a gene. Try another.

    2. Yes, that is right. A cell holds a nucleus. The nucleus holds chromosomes. A chromosome holds many genes. A gene is made of many DNA bases.

    3. Not this one. The nucleus is inside the cell, so the cell is larger. Try another.

    4. Not this one. That list is right but turned round. It starts with the smallest, and the question asks for the largest first. Try another.

    See the answer

    cell, nucleus, chromosome, gene, DNA base

    A cell holds a nucleus. The nucleus holds chromosomes. A chromosome holds many genes. A gene is made of many DNA bases.

    • If you chose A: A gene is a section of a chromosome, so a chromosome is larger than a gene.
    • If you chose C: The nucleus is inside the cell, so the cell is larger.
    • If you chose D: That list is right but turned round. It starts with the smallest, and the question asks for the largest first.

  2. What is a gene?

    Tap your answer.

    1. Not this one. A gene is not a protein. It is the code for one. The protein is made from the code. Try another.

    2. Not this one. A chromosome holds many genes, about 20 000 in a whole human cell. Try another.

    3. Yes, that is right. A gene is a short section of a DNA molecule. Its order of bases is the code for one protein.

    See the answer

    A section of DNA with the code for making one protein

    A gene is a short section of a DNA molecule. Its order of bases is the code for one protein.

    • If you chose A: A gene is not a protein. It is the code for one. The protein is made from the code.
    • If you chose B: A chromosome holds many genes, about 20 000 in a whole human cell.

  3. How many chromosomes are there in most human body cells?

    Tap your answer.

    1. Yes, that is right. There are 23 pairs of chromosomes and each pair has 2. So 23 × 2 = 46.

    2. Not this one. There are 23 pairs. Each pair has 2 chromosomes, so you need to double. Try another.

    3. Not this one. About 20 000 is the number of genes, not chromosomes. A chromosome holds many genes. Try another.

    See the answer

    46 chromosomes

    There are 23 pairs of chromosomes and each pair has 2. So 23 × 2 = 46.

    • If you chose B: There are 23 pairs. Each pair has 2 chromosomes, so you need to double.
    • If you chose C: About 20 000 is the number of genes, not chromosomes. A chromosome holds many genes.

  4. Why can about 2 m of DNA fit inside a nucleus under 10 µm across?

    Tap your answer.

    1. Yes, that is right. A long thread coils into a small bundle, like the string on the pencil. A chromosome is one DNA molecule coiled up.

    2. Not this one. The nucleus stays tiny. The DNA is what changes shape: it is coiled. Try another.

    3. Not this one. The DNA is kept in the nucleus, in 46 long pieces called chromosomes, each one coiled up. Try another.

    See the answer

    It is coiled up tightly into chromosomes.

    A long thread coils into a small bundle, like the string on the pencil. A chromosome is one DNA molecule coiled up.

    • If you chose B: The nucleus stays tiny. The DNA is what changes shape: it is coiled.
    • If you chose C: The DNA is kept in the nucleus, in 46 long pieces called chromosomes, each one coiled up.

  5. What does a gene give the code for?

    Tap your answer.

    1. Not this one. Body parts are built from many proteins, so many genes work together on one part. Try another.

    2. Not this one. Chromosomes are long DNA molecules. They are not made from the code in one gene. Try another.

    3. Yes, that is right. The order of bases in a gene tells the cell how to make one protein. Many genes together build a body part.

    4. Not this one. Each gene has its own job. Its code makes one protein. Try another.

    See the answer

    One protein

    The order of bases in a gene tells the cell how to make one protein. Many genes together build a body part.

    • If you chose A: Body parts are built from many proteins, so many genes work together on one part.
    • If you chose B: Chromosomes are long DNA molecules. They are not made from the code in one gene.
    • If you chose D: Each gene has its own job. Its code makes one protein.

  6. Where are the chromosomes in a human body cell?

    Tap your answer.

    1. Not this one. The membrane is the thin skin of the cell. It holds no chromosomes. Try another.

    2. Yes, that is right. The nucleus is the control centre of the cell. It holds the chromosomes, and the chromosomes hold the DNA.

    3. Not this one. In a human cell the cytoplasm holds the other parts. The chromosomes stay in the nucleus. Go to S3.1.4. Try another.

    See the answer

    In the nucleus

    The nucleus is the control centre of the cell. It holds the chromosomes, and the chromosomes hold the DNA.

    • If you chose A: The membrane is the thin skin of the cell. It holds no chromosomes.
    • If you chose C: In a human cell the cytoplasm holds the other parts. The chromosomes stay in the nucleus. Go to S3.1.4.

  7. How many chromosomes are drawn in the picture of three pairs?

    Three pairs of chromosomes from a cell, numbered 1, 2 and 3.123
    See the answer

    6 chromosomes

    There are 3 pairs. Each pair has 2 chromosomes, so 3 × 2 = 6.

  8. A body cell of a dog has 39 pairs of chromosomes. How many chromosomes is that?

    See the answer

    78 chromosomes

    Each pair has 2 chromosomes. 39 × 2 = 78.

  9. A body cell of a fruit fly has 8 chromosomes. How many pairs is that?

    See the answer

    4 pairs

    Chromosomes come in pairs of 2. 8 ÷ 2 = 4.

Keep it

Copy this into your exercise book.

A cell has a nucleus. The nucleus holds chromosomes. A chromosome is one coiled DNA molecule. A gene is a section of DNA that codes for one protein. Humans: 46 chromosomes, 23 pairs.

Draw: The five panels in order of size: cell, nucleus, chromosome, gene, DNA base.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

No internet tomorrow? Save the whole book as one file (873 KB). It opens on any phone.