Division of cells involves division of nucleus and division of cell as a whole.
- Mitosis = produces 2 daughter cells, same number of chromosomes as parent cell and as each other.
- Meiosis = produces 4 daughter cells, each with half the number of chromosomes as parent cell.
Why is meiosis necessary?
In sexual reproduction, two gametes fuse to give new offspring.
If each gamete had diploid number of chromosomes, cell they produce has double that number.
To maintain a constant number of chromosomes in the adult of species, number of chromosomes must be halved at some stage - during meiosis.
Every diploid cell = two sets of chromosomes, one from each parent.
Chromosome pairs separate during meiosis - one chromosome from each parent enters each gamete. - known as the haploid number of chromosome.
Diploid number is restored at fertilisation.
Process of Meiosis
1) First nuclear division = homologous chromosomes pair up - chromatids wrap around each other. By the end of stage homologous pairs separated = one chromosome from each pair going into two daughter cells.
2) Second nuclear divison = Chromatids move apart, daughter cells divide to produce 4 daughter cells overall.
Meiosis also produces genetic variation, allowing offspring to adapt and survive, in two ways:
- Independant segregation of homologous chromosomes.
- Recombination of homologous chromosomes by crossing over.