Cell division is the splitting of a cell into two daughter cells. This produces new cells. New cells need to be made for growth, repairing damaged tissue and for replacement of damaged cells.
Normal body cells are called diploid cells. When new cells are made, all 46 chromosomes from a diploid cell are copied exactly in a process called mitosis.
Mitosis is the type of cell division used for growth, repair and asexual reproduction. It produces two cells that are genetically identical to each other. It happens with diploid (body) cells.
In mitosis each chromosome is copied exactly. The new chromosomes are moved to opposite sides of the cell, before the cell divides leaving one complete set of 46 chromosomes in each of the two new cells:
1. Chromosomes in the nucleus of the parent cell are copied.
2. The chromosomes are pulled apart and seperate. They move to opposite sides of the cell dividing.
3. The cell divides into two genetically identical daughter diploid cells.
Controlling cell sizes
Mitosis helps to control the size of cells. This is because this cell division ensures that cells never become too large.
The larger the cell becomes, the smallers its surface-area-to-volume ratio. Objects with this small ratio do not exchange materials with their environment very well. Therefore, large cells could run out of oxygen and give out too much waste e.g. carbon dioxide.
So large organisms like humans are multicelluar so that they can have lots of little cells. Cell division helps to keep cells small enough to get enough oxygen.
Fertilisation is the joining together of a haploid gamete cell to another, each haploid cell from each parent, during sexual reproduction.
Each haploid gamete cell has half the normal number of chromosomes to make up a new cell with all 46 chromosomes containing both parents' genetic information. This new cell is a diploid zygote.
The cells from each parent that join to form a zygote are called gametes. In humans, these are sperm from the man and an egg from the woman. Human sperm contains 23 chromosomes and a human egg contains 23 chromosomes. A human zygote formed by the fertilisation of them contains 46 chromosomes. A zygote diploid cell grows to form an embryo in the womb.
Gametes contain different genetic information to each other and to the parent cell. Meiosis is responsible for genetic variation.
All gametes are haploid (have half the normal number of chromosomes). They have adaptions to increase the chances of fertilisation and successful development of the embryo:
- Tail to propel them so they can move to get to egg cell.
- Mitochondria provides them with energy to move.
- Front of sperm contains enzymes to digest egg membrane.
- Large food store to support developing zygote until it can feed from the placenta.
Meiosis is a different kind of cell division to mitosis. It is used to produce male and female gametes. Sometimes meiosis is called reductive division because gametes contain 23 chromosomes rather than 46.
In Meiosis, the following happens:
1. The parent cell's chromosomes make identical copies of themselves.
2. Similar chromosomes pair up.
3. Sections of DNA get swapped.
4. The pairs of chromosomes divide.
5. The chromosomes divide.
6. Four daughter haploid cells are produced, each with 23 chromsomes with different genetic information.