B3 - Meiosis, Gametes and Fertilisation

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Meiosis is another types of cell division - it creates gametes

Gametes are formed by meiosis in the ovaries and testes. Gametes are the sex cells - eggs and sperm

The body cells of mammals are diploid. This means that each cell has two copies of each chromosome in its nucleus - one from its mum, one from its dad

But gametes are haploid - they only have one copy of each chromosome, this is so that when the egg and sperm combine, they'll form a cell with the diploid number of chromosomes

Meiosis involves two divisions

Meiosis starts in exactly the same way as mitosis does - the DNA replicates itself to form double armed chromosomes

After this the chromosomes arrange themselves into pairs. Humans have 23 pairs of chromosomes, so 46 all together. Both chromosomes in pairs contain information about similar features.One chromosome comes from your dad, the other your mum

In the first division, the pairs split up, the pairs in each chromosome move to opposite sides of the pole, in each of the two new cells, there are no pairs at all. Each new cell ends up with a mixture of your mum's and your dad's chromosomes but only half, so there is only 23

The second division of meiosis is like mitosis, each chromosome splits in half and one arm is in each new cell

You end up with four new cells, the first two from the first division and the other two from when the first two split again. These cells are genetically different from each other because all of the chromosomes get shuffled up during meiosis and each gamete only gets half of them - at random

Fertilisation creates genetic variation

At fertilisation, male and female gametes combineto form a diploid cell. This cells is called zygote

The characteristics of this cell are controlled by the combination of genes on its chromosomes

Since the zygote will inherit chromosomes from two parents, it will show features of both parents but won't be exactly like either of them

Sperm cells are adapted for their function

A sperm's function is to transport the male's DNA to the female's egg

Sperm are small and have long tails so they can swim to the egg

Sperm have lots of mitochondria to provide the energy needed to swim the distance

Sperm have acrosome at the front of the head, which release enzymes they need to digest their way through the membrane of the egg cell

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