4.4 - 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

• The cell cycle is the repeating sequence a cell goes through: grow, copy its DNA, divide, and round again.
• A cell that isn't dividing has its DNA spread out in long strings.
• Before a cell can divide, it must prepare so there's enough of everything to share between the two new cells.

Preparing to divide

1. The cell grows and increases the number of its sub-cellular structures, such as mitochondria and ribosomes, so there's enough for both new cells.
2. It then duplicates its DNA, making one full copy for each new cell.
3. The copied DNA forms X-shaped chromosomes, where each half is an exact copy of the other.
4. Once its contents and DNA have been copied, the cell is ready for mitosis.

Interphase takes up most of the cycle

• The long growth-and-preparation period is called interphase.
• A cell spends most of its time in interphase, with mitosis only a small part of the cycle.

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.

Cancer - Uncontrolled mitosis

• The rate at which cells divide by mitosis is normally controlled by the genes (chemical instructions) in the cell's DNA - so the body only makes new cells when it needs them.
• If one of the genes that controls cell division changes, the cell may start dividing uncontrollably, far faster than the body needs.
• This produces a mass of abnormal cells called a tumour.
• If the tumour invades and destroys the surrounding tissue, it is called cancer.

Key points about the change

• A random change in a gene like this is called a mutation.
• Cancer is a non-communicable disease - it cannot be passed from one person to another.