Fertilisation

Sperm Contains half the chromosomes needed to make a body ‘Tail’ region: this helps the sperm swim towards the egg ‘Head’ region Acrosome: contains chemicals which break down the wall of the egg

Nucleus: contains the other half of the chromosomes needed to build a body Cytoplasm: contains a large food store to feed the developing new organism in its very early life Outer layer: to protect the egg. A sperm has to penetrate this. Cell membrane This is a very large cell due to its food store Egg

Sperm and eggs are called GAMETES When a sperm and an egg combine to begin a new life it is called FERTILISATION

Where are sperm and eggs made? Fallopian tube: fertilisation happens here Ovary: eggs develop here. One is released each month Uterus: embryo develops here Testis: sperm are made here continuously Urethra: sperm pass through here during sexual intercourse

Sexual Reproduction Parent cell in ovary or testis: DIPLOID Two copies of each chromosome: DIPLOID HAPLOID Sperm/ Eggs (gametes): each has just one copy of each chromosome: HAPLOID Meiosis cell division

What happens to the sets during fertilisation? haploid One set of chromosomes in each gamete: haploid diploid Two sets of chromosomes in the fertilised egg: diploid This cell then simply copies itself, including all its chromosomes, to build a whole organism. This type of cell division is called mitosis

Which all leads to….. Waaaaaah!

So………… We have two copies of each chromosome. One copy we inherit from our father and one copy we inherit from our mother.

Is each pair of chromosomes identical? Eye colour Blue allele Eye colour Brown allele The chromosomes in a pair do contain the same genes. But sometimes in different versions: ALLELES

Key questions Why is the chromosome number halved during the production of gametes? A normal diploid cell in a human has 46 chromosomes. How many will their be in each haploid gamete?

Genetic reasons why sexually reproduced organisms vary: 1.Genes exist in different versions called ______ 2.Gametes only contain ___ of the two copies of each gene that the parent has. Different gametes therefore may contain different alleles for the same gene. 3.Two gametes at random fuse together in _________. These come from ____ different parents so may well contain different alleles.