Sexual reproduction requires both meiosis and fertilization.
In sexual reproduction, two parents give birth to an offspring with an unique gene combination derived from both of them--each parent gives 1/2 of his/her genes to the offspring.
A gene is a discrete unit of information on the DNA that codes for one protein, perhaps one of the many enzymes needed by our bodies.
Somatic cells (soma = body) have two sets of chromosomes; one set from each parent. For example, in humans 1 set = 23 chromosomes, so our somatic cells have 46 chromosomes arranged in 23 pairs.
The two chromosomes in each pair are referred to as being homologous chromosomes, so we could say that humans have 23 pairs of homologous chromosomes. (homo = same, like, alike) .
The 2 chromosomes of each pair carry genes for the same trait (for example, eye color) at the same location, but not necessarily the same form of that gene.
HAPLOID VS. DIPLOID
Somatic cells have two sets of autosomes and one pair of sex chromosomes. Somatic cells are said to be diploid or 2n cells (diplo = double, two).
Meiosis is a special type of cell division that produces gametes with half as many chromosomes (meio = less; -sis = the act of). Meiosis involves reduction/division. The cells that result from meiosis are haploid or 1n.
The opposite process would be syngamy or fertilization, which is the union of the egg and sperm to restore the 2n number (syn = with, together; -gamy = marriage, reproduction).
This results in a zygote, the first cell formed by fertilization (zygo = yoke), a completely new and different organism with unique genetic information different from either parent. The zygote divides and grows to form an embryo which developes into a young organism, then an adult.
Life cycles of all sexually-reproducing organisms follow this pattern of alternation of generations.
The 2n adult produces 1n gametes by the process of meiosis. These unite in the process of syngamy to produce a new 2n generation.
Thus, the life cycles alternate between 1n and 2n stages, and between the processes of meiosis and fertilization.
The steps in meiosis are similar to mitosis and even have the same names. However, there is a significant difference in how the chromosomes line up initially.
In mitosis, chromosomes line up individually, while in meiosis, the homologous pairs line up next to each other. This pairing process is called synapsis (syn = with, together, apsis = a juncture), and the resulting homologous pair is called a bivalent in reference to the two chromosomes or a tetrad in reference to the four sister chromatids involved.
In prophase I, notice the difference in how the homologous chromosomes behave. They come together and match up (synapsis) in pairs (tetrads or bivalents--bi = two; valent = strength; tetra = four). In human females, this stage happens prior to birth when the ovaries are forming, and then stops.
A baby girl is born with all the precursor egg cells she will ever have in a sort-of "suspended animation" until puberty (hence one of the dangers of x-rays, and hence the greater likelihood that a 40-yr-old mother will have a baby with Down Syndrome as compared to a 20-yr-old mother.
Two cellular divisions - in the first division the chromosome number is reduced by half. In the second division the sister chromatids are separated.
1. Meiosis can occur only in cells with the diploid (or polyploid) number of chromosomes. Chromosomes must have a partner.
2. During meiosis the diploid nucleus divides twice producing four nuclei in all. The chromosomes replicate only once prior to the first nuclear division.
3. In meiosis each of the four nuclei produced contains half of the chromosomes in the original cell.
4. The haploid nucleus contains new combinations of chromosomes.
Homologous pairs of chromosomes are assorted randomly. Moreover, because of crossing over, these chromosomes are not identical to the chromosomes at the beginning of meiosis.
1. For sexually reproducing organisms it maintains a constancy in the number of chromosomes for the 2n individual resulting from the union of the two gametes (fertilization).
2. It generates in the individual an astronomical number of combinations of chromosomes in the production of gametes. From the random assortment of the twenty three pair of homologous chromosomes there are 223 possible combinations that can occur when these pairs are separated.
3. Because of the crossing over that occurs between the members of a homologous pair of chromosomes during prophase I, genetic material is shuffled between the maternal and pateranl members of the homologous pair. This creates new combinations of genes on all of the chromosomes. This contributes to the variation that is expressed in any population for any given trait.
Non disjunction - When cromosomes don't separate during anaphase.
One of the resulting nuclei will have one more chromosome than the original cell, and one of the nuclei will have one less chromosome than the original cell. This will have dramatic consequences for the developing embryo. Usually this is lethal. When the embryo survives, this is expressed as a genetic disorder or syndrome.
A syndrome is a condition where not one, but many systems are affected by the defect. Down syndrome arises when an individual has three not two copies of chromosome 21.
The frequency of births of infants with down syndrome arises dramatically as the age of the mother exceeds 30 - 40 years of age.
Translocation - Sometimes a segment of one chromosome is transfered to and becomes part of another non homologous chromosome. The deleted portion can be lethal, or the addition of the transferred material can have the same consequences as the presence of an extra chromosome.
Meiosis occurs in the reproductive organs, the testes of males, the ovaries of females.
Males: Spermatogonium enlarge to become primary spermatocyes. Each primary spermatocyte divides into two haploid secondary spermatocytes.
The secondary spermatocytes divide again to form four spermatids (1n) that can now mature into the gametes (four sperm cells). The normal ejaculate of a male produces 300 - 400 million sperm cells.
Females: Oogonium give rise to primary oocytes (2n) that undergo the first division to produce a secondary oocyte and a smaller polar body. In the second meiotic division the seconary oocyte divides again to form the Ovum (or egg) and a polar body. The first polar body divides again to form two more polar bodies. The result from the two divisions is a large ovum or egg and three smaller polar bodies.
The first meiotic division begins at about three months in the embryo and isn't completed until ovulation. The second meiotic division does't occur until after fertilization. http://www.sus.mcgill.ca/bio202/suter/lecture3/sld024.htm
