S4.2.3 Chapter 3

Meiosis and sex cells

A baby has 46 chromosomes in its cells. But a sperm and an egg joined to make it. How is that possible if each parent has 46?

Your guess

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

Your guess

A Each sex cell has all 46, and the extra ones are thrown away at fertilisation.
You picked A. Read on. At the end, the chapter comes back to your guess.
B Each sex cell has 23 chromosomes, half the usual number, so the two make 46.
You picked B. Read on. At the end, the chapter comes back to your guess.
C The baby takes all 46 from the mother, and the father gives none.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Explain why sex cells have half the chromosomes of body cells
Compare mitosis and meiosis

A new human begins as one cell. The cell forms when a sperm cell from the father joins an egg cell from the mother. Sperm and eggs are the sex cells.

Most body cells have 46 chromosomes. Suppose a sperm and an egg each had 46. The first cell of a baby would have 92. The next generation would have 184. That cannot go on. So each sex cell has only half the usual number, which is 23.

A sex cell gets one chromosome from each of the 23 pairs. When the sperm and the egg join, the cell has 23 + 23 = 46 chromosomes. They are in 23 pairs again. This joining is called fertilisation.

Fertilisation restores the number of chromosomes.

In a body cell, each pair has one chromosome from the mother and one from the father. The picture shows the mother's chromosomes in white and the father's in black.

Six of the 23 pairs. One chromosome of every pair came from each parent.

Sex cells are made by a special kind of cell division called meiosis. It takes place only in the sex organs: the ovaries, which make eggs, and the testes, which make sperm.

You know mitosis already. Mitosis makes two cells that are the same as the first. They have the same number of chromosomes and the same genes. Meiosis is different. One cell makes four cells, and each has half the chromosomes. In a man all four become sperm. In a woman only one becomes an egg, and the other three are tiny and die away.

One parent cell with four chromosomes. Mitosis keeps all four in each of two cells. Meiosis shares them out to four cells, and some parts swap first.

Two things make every sex cell different. The first is random sorting. In each pair, which chromosome goes into the sex cell is a matter of chance. The second is crossing over. Before the cell divides, the two chromosomes of a pair swap parts. A chromosome then holds some of the mother's DNA and some of the father's.

Worked example

A cell has 3 pairs of chromosomes. How many different sex cells can random sorting make?

For each pair, a sex cell gets one of 2 chromosomes.
So there are 2 choices for pair 1, 2 for pair 2 and 2 for pair 3.
The number of kinds of sex cell is 2 × 2 × 2 = 8.

Answer: 8 different sex cells

Worked example

A human cell has 23 pairs of chromosomes. How many different sex cells can random sorting alone make?

Each pair gives 2 choices, and there are 23 pairs.
With 10 pairs we would have 2¹⁰ = 1 024 kinds.
With 23 pairs we multiply 23 twos: 2²³ = 8 388 608.

Answer: 8 388 608 different sex cells

Each extra pair doubles the number. Crossing over adds even more kinds.

So one person can make more than 8 million different sex cells, before crossing over is counted. This is why brothers and sisters from the same parents are not the same.

For a human cell. Cells is how many cells one division makes. Each has is the number of chromosomes in each new cell. Mitosis is for growth and repair. Meiosis makes the sex cells, in the ovaries and testes.

Try it

Sort chromosomes by chance

You need: an old sheet of paper, a pencil, two cups or bowls.

Tear four small pieces of paper. Write L and M on the first, and L and F on the second. Write S and M on the third, and S and F on the fourth.
L stands for a long chromosome and S for a short one. M is from the mother and F is from the father.
Put the two L pieces face down in one cup. Put the two S pieces face down in the other cup.
Shake both cups. Pick one piece from each, without looking. Write down the two, such as LM and SF. This is one sex cell. Put the pieces back.
Do this 12 times. Count how many times each of the four sets of two came up.

What you will see: Usually all four sets come up, but none comes up every time. Chance decides which chromosome of each pair goes into a sex cell. With only 2 pairs there are 2 × 2 = 4 kinds. With 23 pairs there are millions.

If you cannot try it, this is what happens.

A wrong idea many people have

Brothers and sisters get exactly the same chromosomes from their mother.

Why it feels right: They have the same mother and the same father, so it seems they should get the same DNA.

The test that breaks it: Every egg is made by meiosis. Random sorting and crossing over mean each egg differs. Two children get different sex cells from the same parent.

What is true: Each sex cell has its own mix. Brothers and sisters share many genes, but not all. The exception is identical twins, who grow from one fertilised egg.

Key words

sex cells: egg cells and sperm cells, which have half the chromosomes of a body cell.
fertilisation: the joining of a sperm cell and an egg cell to make one new cell.
meiosis: cell division that makes four different cells, each with half the chromosomes.
mitosis: cell division that makes two identical cells with the same number of chromosomes.
crossing over: the swap of parts between the two chromosomes of a pair.

Back to your guess

The answer is B: Each sex cell has 23 chromosomes, half the usual number, so the two make 46.

A sex cell has only half the chromosomes of a body cell: 23. When a sperm and an egg join, the 23 and the 23 make 46 again.

Keep it

Copy this into your exercise book.

Sex cells have 23 chromosomes, half of 46. Fertilisation makes 46 again. Meiosis: 4 different cells. Mitosis: 2 identical cells. Random sorting and crossing over make the difference.

Draw: One parent cell with two arrows: mitosis to 2 cells, meiosis to 4 cells.