MITOSIS AND MEIOSIS
The cell has to copy everything it contains so that when it splits in half the two new cells will contain the right amount of material. This is done by the chromosomes being copied:
1. Molecule of DNA splits
2. Bases on free floating nucleotides pair up with matching bases on the DNA.
3. Cross-links form between the bases and the 'old' DNA strands and the nucleotides are joined together.
4. Two molecules identical to the original one are formed
Then it splits into two by mitosis...
1. The cell has two copies of its DNA all spread out
2. Before the parent cell divides, the DNA forms X - shaped chromosomes. (Each 'arm' of the chromosome is an exact duplicate of the other)
3. The chromosomes then line up at the centre and the cell fibres pull them apart so the two 'arms' of each chromososme go to opposite ends of the cell
4. Cell Membranes form around each of the sets of chromosomes (these become the nuclei of the two new cells.
5. The Cytoplasm divides
6. There are now two new daughter cells which are genetically identical to each other and the parent cell
Mitosis is done so that our bodies can produce more cells so that we can grow and repair damaged cells
MEIOSIS produces cells which have HALF the normal number of chromosomes and ONLY occurs in REPRODUCTIVE ORGANS
Gametes = Egg or Sperm cells
Zygote = Fertilied Egg
During sexual reproduction, an egg and a sperm cell combine to form a new cell (a zygote)
All human body cells have two copies of 23 chromosomes (46 in total) . But gametes only have one copy of each chromosome
So when the egg and sperm combine the zygote will contain 46 chromosomes - one set of 23 from each parent
Meiosis only happens in the reproductive organs (Ovaries and Testes)
1. The cell starts to divide as it duplicates its DNA - one 'arm' of each chromosome is an exact copy of the other 'arm'
2. FIRST DIVISION - The chromosome pairs line up in the centre of the cell and are then pulled apart so each new cell has one copy of each chromosome from each parent
3. SECOND DIVISION - The chromosomes line up again in the centre of the cell and the 'arms' of the chromosome are pulled apart
4. There are now 4 gametes with ONLY a SINGLE SET of chromosomes in it