Understanding Meiosis: The Process of Genetic Diversity and Reproduction

The Significance of Meiosis

Why Meiosis?

 Meiosis generates diversity by ensuring that the gametes it gives rise to will differ genetically from one another.

 Meiosis is unlike regular cell division, or mitosis, which produces daughter cells that are exact genetic copies of parent cells.

 Evolution is spurred on by differences among offspring, and meiosis and sexual reproduction ensure such differences.

 By contrast, asexual reproduction, as is seen in bacteria and other organisms, produces organisms that are exact genetic copies, or clones, of the parental organism.

 The genetic diversity brought about by Meiosis and sexual reproduction is responsible, to a significant extent, for the great diversity of lifeforms seen in the living world today. Diversity is a consequence of adaptation to the Environment.

Meiosis Generates Diversity

Meiosis generates genetic diversity in two ways:

1. First, in Prophase I, homologous chromosomes pair with each other. In the process called Crossing Over or Recombination, they exchange reciprocal chromosomal segments with one another.
2. Second, in Metaphase I, there is a random alignment or independent assortment of maternal and paternal chromosomes on either side of the metaphase plate. This random alignment determines which daughter cell each chromosome will end up in.

Meiosis in Human Reproduction: Sperm Formation

Mitosis

 Spermatogenesis is the process in which spermatozoa are produced from spermatogonial stem cells by way of mitosis and meiosis.

 Spermatogonial stem cells are diploid cells that to a set of specialized cells—the diploid Primary Spermatocytes.

Meiosis I and II

 Primary Spermatocytes go through meiosis, producing haploid secondary spermatocytes.

 These in turn give rise to the spermatids that develop into mature sperm cells.

 The entire process of spermatogenesis is variously estimated as taking 74-120 days.

Ovule Formation

In females, there are no counterparts to the male spermatogonia process.

 No cells function as reproductive stem cells.

 Female gamete formation begins with cells called oogonia.

 Most or all of which are produced, prior to the birth, in the ovary of the female embryo.

Mitosis (Early fetal life)

 Oogonia are formed in large numbers by mitosis early in female fetal development from primordial germ cells. They Oogonia become Primary oocytes (both are diploid cells) completed by the end of the 3^rd month of fetal development.

Meiosis I (Birth to pre-puberty)

 During the rest of fetal life, Primary oocytes begin the process of Meiosis but are halted at birth in Prophase I:

 A newborn baby girl has in her ovaries about 2,000,000 Primary oocytes.

 No new Primary oocytes will be produced during a female's entire life.

Meiosis I (post-puberty)

Most Primary Oocytes will die or be reabsorbed by the body so that, from the initial two million, only about 400,000 (0.2%) remain at the beginning of puberty (normally between ages 11–14).

• From Puberty, beginning with the first menstrual cycle, under the effect of sex hormones, Meiosis resumes, from Prophase I each 28 days (in humans), in one oocyte at the time.

Meiosis II

• During follicle evolution and ovulation, Meiosis II follows at once but will be arrested in the Metaphase II and will so remain until fertilization.
• Only those ovulated oocytes that are fertilized by sperm, complete Meiosis II.
• Only 400–500 (0.0002%) oocytes (at most) will be released during a woman's entire reproductive years.