2.1 - Mitosis

What mitosis is and why the body needs it

• Most body cells have a nucleus holding genetic material as chromosomes - coiled-up lengths of DNA.
• Each chromosome carries many genes, and different genes control the development of different characteristics.
• Body cells normally carry two copies of each chromosome, one from each parent, which makes them diploid.

What mitosis produces

• Mitosis is a type of cell division in which one cell divides to produce two new cells (daughter cells).
• The daughter cells are genetically identical to each other and to the original parent cell - each has exactly the same chromosomes, and so the same genes.
• The chromosome number is kept constant: because the body cell is diploid, each daughter cell is also diploid, with the full set, and no genetic information is lost.
• The full number of chromosomes, counting both copies, is the diploid chromosome complement.

Why the body needs mitosis

• Mitosis is the stage of a repeating sequence called the cell cycle at which the cell actually divides.
• Multicellular organisms use it to:
  • Grow - By increasing the number of body cells
  • Replace cells that have worn out or been damaged, repairing the tissue
• Mitosis is also how some organisms reproduce asexually, producing offspring genetically identical to the single parent.

The cell cycle

• In a cell that isn't dividing, the DNA is spread out in long strings.
• Before a cell can divide, it has to grow and copy everything it needs. This growth-and-preparation period is called interphase.
• After interphase comes mitosis (the M phase), and then the whole repeating sequence - the cell cycle - starts again.
• The cell spends most of its time in interphase; the M phase is only a small part of the cycle.

The three phases of interphase

1. Gap phase 1 (G₁) - The cell grows and makes the proteins and sub-cellular structures (such as mitochondria and ribosomes) needed for division.
2. Synthesis (S) - The cell duplicates its DNA, so there is one full copy for each new cell. The copied DNA forms X-shaped chromosomes, where each half is an exact copy of the other.
3. Gap phase 2 (G₂) - The cell grows again and makes the final structures and proteins it needs.

Once interphase is complete, the cell is ready for mitosis.

The stages of mitosis

• Before mitosis, the cell has grown and copied its DNA, so each chromosome is now an X shape made of two identical copies.
• Mitosis shares the doubled set of chromosomes out equally, so each new cell ends up with a complete, identical set.

What happens during mitosis

1. Chromosomes line up - The chromosomes line up across the centre of the cell.
2. Cell fibres pull them apart - Fibres attach to the chromosomes and pull them apart, dragging one of the two copies of each chromosome to each end of the cell.
3. Each end holds a complete set - Every end of the cell now has a complete set of chromosomes.
4. Two nuclei form - A membrane forms around each set, creating two new nuclei - the nucleus has divided.
5. The cell splits - The cytoplasm and cell membrane divide down the middle, producing two separate daughter cells.

The daughter cells

• The two daughter cells contain exactly the same chromosomes as each other and as the parent cell, so they are genetically identical.
• Each cell stays diploid, because the original chromosome number is maintained.