BIOLOGY REVISION [ GCSE ]

Cell Biology - Chromosomes and Mitosis

    Most cells in your body have a nucleus. The nucleus contains your genetic material in the form of chromosomes. Chromosomes are coiled up lengths of DNA molecules.

    Each chromosome carries a large number of genes. Different genes control the development of different characteristics (e.g. hair colour). Body cells normally have two copies of each chromosome, one from its 'mother' and one from its 'father'.

THE CELL CYCLE:

    The cell cycle makes cells for growth, development and repair.

    Body cells in multicellular organisms divide to produce new cells as part of a series of stages called the cell cycle. The stage of the cell cycle where the cell divides is called mitosis.

    There are two major stages of the cell cycle:

GROWTH AND DNA REPLICATION:
- in a cell that's not dividing, the DNA is all spread out in long strings.
- before it divides, the cell has to grow and increase the amount of subcellular structures such as mitochondria and ribosomes.
- it then duplicates its DNA so there's one copy for each new cell.  The DNA is copied and forms x-shaped chromosomes.  Each 'arm' of the chromosome is an exact duplicate of the other.

MITOSIS:
Once it's contents and DNA have been copied, the cell is ready for mitosis.
- the chromosomes line up in the centre of the cell and cell fibres pull them apart.  The two 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 the two new cells.  The nucleus has divided.
- lastly, the cytoplasm and cell membrane divide.
- the cell has now produced two new daughter cells.  The daughter cells contain exactly the same DNA — they're identical.  Their DNA is also identical to the parent cell.