DNA and meiosis and meitosis

Okay, so DNA is the information inside each cell that gives it instructions. DNA is separated into chromosomes, humans have 23 pairs of chromosomes (each pair contains two identical chromosomes). Genes are sections of DNA on chromosomes that code for something. So you have a gene for your eye colour for example (really its multiple genes but for the point of this you can simplify it to being one gene). Genes can come in different forms called alleles. As you have two copies of each chromosome, you have two copies of each gene. So for the eye colour, there is one version of the gene that codes for brown eyes and one for blue. The gene for brown eyes is dominant so we represent it with B. The gene for blue eyes is recessive, represented by a b. capital letter for dominant, lower case for recessive. As we have two copies of each chromosome, we have two copies of each gene. Dominant genes are more 'powerful' so if you have one or two of them, you get that trait (so if you have two dominants BB, or a dominant and a recessive Bb, you will get brown eyes, as the brown beats the blue). To get the recessive trait, blue eyes, you must have two copies of the recessive gene, bb.

Mitosis is what happens to the nucleus of the cell before the cell divides. In mitosis, two new nuclei are formed, each with identical chromosomes. They both have 23 pairs of chromosomes, the full amount, as the DNA had already replicated so each chromosome looked like an X instead of a stick/I, meaning it had doubled its genetic material. One side of each X goes to each new nucleus, so each nucleus has a full set of chromosomes. This is known as being diploid. Diploid is when the cell has two copies of each chromosome and is how cells normally exist. Cells formed by mitosis are genetically identical.

Meiosis is the process by which gametes (eggs and sperm) are produced. The cell first splits so that two nuclei form, but in this case each one only has one copy of each chromosome instead of a pair. Then, the X shaped chromosomes in each of those two new nuclei split again, forming 4 cells, each with one copy of each chromosome instead of two. This is known as haploid (i like the remember it as it starts with a H so it Half the normal amount of chromosomes). The cells formed are genetically different to each other (you don't need to know how that happens at GCSE). The gametes need to be haploid so that when an egg and sperm fuse, the resulting zygote has a full set of DNA and is diploid. If the gametes were diploid, the zygote would have 4 copies of each chromosome and wouldn't survive.

I hope that helps, it's been a couple of years since i did GCSE so i can't remember how much detail you need. If anythings still unclear then just ask