Question

Why is meiosis necessary? Why can't organisms just use mitosis to produce every single cell?

How do independent assortment of chromosomes and crossing over (recombination) of homologous pairs contribute to genetic variation?

Answer 1

1) most of the organisms are diploid, mitosis is equational division, that is the number of chromosomes in both parental cells and daughter cells are the same. If all the cells in an organism divide by mitosis, all cells will be diploid. so progenies will be exactly similar to parents, and genetic diversity can not be generated, for natural selection to occur there should be genetic variation. so meiosis is necessary to produce haploid cells, haploid gametes fuse to form zygote which develops to diploid progeny with different combinations of alleles compared to parents, so genetic diversity arises in population. so Meiosis is necessary, germ-line cells undergo meiosis to produce haploid gametes.

2) before meiosis DNA replicates, so during prophase when chromatin condense each chromosome have two sister chromatids attached to each other at the centromere, then homologous chromosomes pair up and exchange genetic material between non-sister chromatids, so allele combination on the sister chromatids changes, then during metaphase I these paired homologous chromosomes align at the metaphase plate such that any one of the chromosomes can move to the daughter cell, this is called independent assortment of chromosomes, because these two processes the gametes produced are unique, Since homologous chromosomes are separated during meiosis I the daughter cells formed by meiosis I are haploid, then during meiosis II the sister chromatids are separated and sister chromatids ends up in gametes, but the gametes formed from a single cell via meiosis II are not same, because due to recombination in Prophase I allele combination in sister chromatids changes, so crossing over and independent assortment produces unique gametes.