There are two types of cell division. Mitosis is used for growth and repair and produces diploid cells identical to each other and the parent cell.
Meiosis is used for sexual reproduction and produces haploid cells different to each other and the parent cell.
Humans are made of millions of cells. This has a number of benefits:
New cells are needed throughout life. These are for growth, to replace damaged cells and repair worn out tissues. Normal human body cells are diploid – they have two of each chromosome. When new cells are made, these 46 chromosomes (in other organisms the number is different) are copied exactly in a process called mitosis.
Mitosis is the type of cell division used for growth and repair. Mitosis occurs wherever new cells are needed. It produces two cells that are identical to each other, and the parent cell.
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.
Constant cell division ensures that cells never get too large. The larger the cell becomes, the smaller its surface area to volume ratio. Objects with a small surface area to volume ratio find it difficult to maintain exchange of materials with their environment. Large cells could run out of oxygen, and accumulate too much waste, such as carbon dioxide. For this reason it's more efficient for large organisms to be multicellular, rather than single-celled.
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