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
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.
Biology, Life on Earth by Audesirk, T and Audesirk, G, 4th Ed., Prentice Hall International Inc., 1996