How does meiosis contribute to human genetic diversity

How Does Meiosis Contribute to Human Genetic Diversity Genetic variability among humans is essential for survival. Reproduction playsa

significant role to create genetic variability in the offspring. This genetic variability

helps to cope up human beings to changing environment and therefore for evolution.

There are three major sources of such variations, namely, the independent assortment of

chromosomes during meiosis, reciprocal recombination of the linked genes on

chromosomes by crossing over in prophase I of meiosis, and random fertilization of

gametes, thereby raising possibility of innumerable combinations.

Sexual reproduction usually comprises of genetic contributions from the two

parental organisms. The key to sexual reproduction in eukaryotic human cell is meiosis.

The word meiosis comes from the Greek word meaning "to diminish", and that is what

meiosis does: it reduces the number of chromosomes by half, that is, if a cell contains 23

pairs of chromosomes before meiosis, after meiosis, the number of chromosomes reduces

to 23.
During meiosis, a diploid cell (containing 2n number of chromosomes found in

the body cells) undergoes two specialized cell divisions, meiosis I and meiosis II, to

produce four haploid (containing n number of chromosomes) daughter cells.

Meiosis I consists of four stages, prophase I, metaphase I, anaphase I, and

telophase I. During prophase I, chromosomes thicken and condense. Homologous or

similar chromosomes come together in pairs, and chiasmata occur as chromatids of

homologues exchange parts. These similar groups of chromosomes pair up side by side.

Strands of protein "zip" the adjacent homologous chromosomes together to result in a

synaptic complex. Within the nucleus where this division is taking place, enzymes are

secreted. They snip the chromatids and reattach them. This leads to formation of
chiasmata, which are formed when one end of a chromatid of a paternal chromosome is

attached to the other end of a chromatid of a maternal chromosome in the pattern of

English alphabet X. Thus the maternal and paternal chromosomes have intertwined

forming crosses or chiasmata. Chiasmata are the sites where DNAs from father and

mother exchange producing chromosomes that consists of part of father chromosome and

part of mother chromosome and differs from that of either parents. This process is known

as cross-over. Thus two homologous chromosomes arising from individual parents as a

result of sexual reproduction give rise to two entirely different chromosomes with an

entirely new DNA structure.

In the next stage or metaphase I, these paired homologues move together to the

equator of the cell with the two members of each pair facing opposite ends of the cell

respectively. Then, the homologous chromosomes separate during anaphase I and cluster

in the opposing poles of the cell, ultimately to form two nuclei.

Finally, fusion of two genetically unique and different gametes or sex cells add

further genetic variability to the offspring. The possibility is immense. Mathematically

fusion of gametes from just two different people could produce 64 trillion genetically

different children. Another interesting fact is that the meiotic cell division is perfectly

suitable for sexual reproduction in human beings. Since there is reduction in number of

chromosomes, it creates provision for union of two haploid (cell with half chromosome

numbers) cells to produce a diploid (two halves combine to one full) offspring cell which

in the course of meiotic cell division has acquired chromosomal components from both

the parents resulting into an entirely new chromosome with unique genetic property, and

unique genetic property multiplied over generations leads to diversity.

Reference: http://www.aw-bc.com/campbell/
Biology, Life on Earth by Audesirk, T and Audesirk, G, 4th Ed., Prentice Hall International Inc., 1996