Meiosis is a type of cell division in sexually reproducing organisms
that reduces the number of chromosomes in gametes (the sex
cells, or egg and sperm). In humans, body (or somatic) cells are
diploid, containing two sets of chromosomes (one from each
parent). To maintain this state, the egg and sperm that unite
during fertilization must be haploid, with a single set of
chromosomes. During meiosis, each diploid cell undergoes two
rounds of division to yield four haploid daughter cells — the
gametes.
Meiosis. Humans have 46 chromosomes in almost every cell, 23
that came from one of our parents and 23 very similar
chromosomes that came from the other one of our parents. It is
really important to have the right number of chromosomes in a
cell. If a cell has extra chromosomes or is missing a chromosome,
that can have very substantial impacts on how it functions. We
can think of meiosis as a way cells very carefully count and divide
their chromosomes so that each gamete, each egg or sperm, has
exactly 23 chromosomes. Then when an egg with its 23
chromosomes is fertilized by a sperm with its 23 chromosomes,
the resulting fertilized egg has exactly 46 chromosomes. And a
new human that grows from that fertilized egg will have 46
chromosomes in all of its cells.
