Chromosomes and Mitosis

Chromosomes

• Chromosomes are coiled up bundles of DNA molecules
• Each chromosome has a large number of genes
• Different genes control different characteristics like eye/hair colour
• body cells usually have 2 copies of each chromosome - one from the father and one from the mother
• In total you have 23 pairs (so 46 chromosomes) in each human cell

The cell cycle

• Body cells in multi-cellular organisms split or divide into new cells to increase their numbers
• This is called mitosis (when the cell divides into 2)
• Organisms with multiple cells use mitosis to grow or replace damaged cells
• These new cells are identical to the original cell as they have the same amount of chromosomes

Why the cell cycle is important

• The cell cycle is important because otherwise you would not be able to replace damaged cells
• Your body closely controls which cells split and when they do
• If this control was to go wrong. A cancerous cell could form leading to cancer (a malignant tumour)

Cell Growth

• In a cell that is not dividing, the DNA is all spread out in long strings
• Before a cell divides, it has to grow, and increase the amount of organelles eg. mitochondria and ribosomes
• It then makes a copy of its DNA, so one set for the new cell it will produce, and one set for itself
• When the DNA is copied, it forms x shaped chromosomes, and each 'arm' is an exact duplicate of the other

Mitosis

• Once the cell's contents (organelles) and DNA has been copied, it is ready to undergo mitosis
• The chromosomes group in the middle of the cell, then cell fibres pulls them apart
• The 2 'arms' of each chromosome go to opposite ends of the cell
• Membranes form around each of the sets of chromosomes, these become the nuclei of both cells
• Lastly, the cytoplasm and cell membranes divide to make 2 identical cells