Mitosis & The Cell Cycle (AQA GCSE Combined Science: Synergy: Life & Environmental Sciences): Revision Note

Chromosomes

• In eukaryotic cells, the nucleus contains thread-like structures called chromosomes

• Chromosomes are made from highly coiled strands of relatively long DNA. Each chromosome is made from one DNA molecule

• In the body cells of diploid organisms, chromosomes are normally found in pairs

  One chromosome from each pair is inherited from the mother, the other from the father

• Different species of organisms have different numbers of chromosomes in their nuclei

  • Humans have 46 chromosomes in the nucleus of all their body cells, found in 23 pairs

  • Horses have 64 chromosomes in 32 pairs

  • Red blood cells are an exception – they lose their nuclei and therefore have no chromosomes

• Before a cell can divide, its genetic material needs to be doubled. This results in the characteristic ‘X’ shaped chromosomes we see in micrographs of cells preparing to divide

Chromosomes are usually uncoiled, when a cell prepares to divide they fold up into ‘worm-like’ structures that we recognise

The Cell Cycle

• Cell division is needed for growth in multicellular organisms and for repair of damaged tissues

• Dividing cells go through a series of stages called the cell cycle

• During the cell cycle:

  • The genetic material (DNA) is copied

  • The cell then divides to form two new cells

• The two new cells are genetically identical to each other and to the original cell

The Process of Mitosis

• Before a cell divides, it must:

  • grow physically larger

  • make copies of organelles such as mitochondria and ribosomes

  • replicate the chromosomes in the nucleus

• The cell then divides by mitosis

• During mitosis:

  • the two complete sets of chromosomes are pulled to opposite sides of the cell

  • two new nuclei form

• The cell then splits into two, forming two genetically identical daughter cells