As mentioned in previous pages, the genetic information found in DNA is essential in creating all the characteristics of an organism. This remains the case when passing genetic information to offspring, that can occur via a process called meiosis where four haploid cells are created from their diploid parent cell.
For a species to survive, and genetic information to be preserved and passed on, reproduction must occur. This can be done by passing on the information found in the chromosomes via the gametes that are created in meiosis.
Humans are diploid creatures, meaning that each of the chromosomes in our body are paired up with another.
Haploid cells possess only one set of a chromosome. For example, a diploid human cell possesses 46 chromosomes and a gamete created by a human is haploid possesses 23 chromosomes.
Tetraploid organisms possess more than 3 sets of a particular chromosome.
Reproduction occurs in humans with the fusion of two haploid cells (gametes) that create a zygote. The nuclei of both these cells fuse, bringing together half the genetic information from the parents into one new cell, that is now genetically different from both its parents.
This increases genetic diversity, as half of the genetic content from each of the parents brings about unique offspring, which possesses a unique genome presenting unique characteristics. Meiosis as a process can increase genetic variation in many ways, explained soon.
The Process of Meiosis
The process of meiosis essentially involves two cycles of division, involving a gamete mother cell (diploid cell) dividing and then dividing again to form 4 haploid cells. These can be subdivided into four distinct phases which are a continuous process
- Prophase - Homologous chromosomes in the nucleus begin to pair up with one another and then split into chromatids (one half of a chromosome) where crossing over can occur. Crossing offer can increase genetic variation.
- Metaphase - Chromosomes line up at the equator of the cell, where the sequence of the chromosomes lined up is at random, through chance, increasing genetic variation via independent assortment.
- Anaphase - The homologous chromosomes move to opposing poles from the equator
- Telophase - A new nuclei forms near each pole alongside its new chromosome compliment.
At this stage two haploid cells have been created from the original diploid cell of the parent.
- Prophase II - The nuclear membrane disappears and the second meiotic division is initiated.
- Metaphase II - Pairs of chromatids line up at the equator
- Anaphase II - Each of these chromatid pairs move away from the equator to the poles via spindle fibres
- Telophase II - Four new haploid gametes are created that will fuse with the gametes of the opposite sex to create a zygote.
Overall, this process of meiosis creates gametes to pass genetic information from parents to offspring, continuing the family tree and the species as a whole. Each of these gametes possess unique genetic information due to situations in meiosis where genetic diversity is increased, all of which is elaborated upon on the next page.