B3 - Cell division

• There are many advantages of being multicellular as humans are. It allows an organism to become larger and more complex. It also allows different cells to take on different jobs. This is called cell differentiation.
• However, when an organism becomes multicellular it needs to have a system that can:

- allow communication between all cells in the body

- supply all the cells with enough nutrients

- control exchanges with the environment such as heat and gases.

• The process that produces new cells for growth is called mitosis.
• The cells that are made by mitosis are genetically identical. Before cells divide DNA replication must take place. This is so each cell produce still has two copies of each chromosome.
• Body cells in mammals have two copies of each chromosome, so they are called diploid cells.
• 'Before mitosis happens DNA is replicated' this involves:

- The two strands of the DNA molecule unzipping to form single strands.

- new double strands forming by DNA bases lining up in complementary pairings.

• Then mitosis occurs. The chromosomes line up along the centre of the cell and divide. The copies then move to opposite poles (ends) of the cell.

• 1 cell resting
• 2 each chromosome is copied, the single strand forms double-stranded 'x' shape.
• 3 spindle forms, chromosomes arranged along eqautor.

• 4 chromosomes single cell move to poles of cell

• 5 two genetically identically cells are produced

• The type of cell division that produces gametes is called meiosis.
• Gametes are haploid cells because they contain only one chromosome from each pair. This means that the zygote gets one copy of gene from one parent and another copy from the other parent. This produces genetic variation.
• The structure of a sperm cell is adapted to it's function, it has:

- many mitochondria to provide the energy for swimming to the egg.

- an acrosome that release enzymes to digest the egg membrane.

• In meiosis there are two divisions. First, the single starnds are sopied to make 'x' shaped chromosomes and chromosomes with the same genes pair up. Then:

- In the first division, one chromosome from each pair moves to opposite poles of the cell.

- In the second division, the copies of each chromosomes come apart and move to opposite sides of the pole.