106 The Process of Meiosis
Learning Objectives
By the end of this section, you will be able to do the following:
- Describe the behavior of chromosomes during meiosis, and the differences between the first and second meiotic divisions
- Describe the cellular events that take place during meiosis
- Explain the differences between meiosis and mitosis
- Explain the mechanisms within the meiotic process that produce genetic variation among the haploid gametes
Sexual reproduction requires the union of two specialized cells, called gametes, each of which contains one set of chromosomes. When gametes unite, they form a zygote, or a fertilized egg that contains two sets of chromosomes. (Note: Cells that contain one set of chromosomes are called haploid; cells containing two sets of chromosomes are called diploid). If the reproductive cycle is to continue for any sexually reproducing species, then the diploid cell must somehow maintain their two sets of chromosomes and do so by reducing its number of chromosome sets to produce haploid gametes (reduction division); otherwise, the number of chromosome sets will double with every future round of fertilization.
Therefore, sexual reproduction requires a nuclear division that reduces the number of chromosome sets by half.
Link to Learning
Watch this video on Meiosis: The Reduction Division
In humans, 46 chromosomes can be found in each somatic cell (n= 23, haploid and 2n= 46, diploid). These chromosomes condense sufficiently to be seen under a compound light microscope. This makes them distinguishable from each other by size, the centromere position and the pattern of chromatin-binding stained colored bands is demonstrated as the karyotype; mitosis reveals there are two chromosomes each of 23 types, each of these pairs have the same length, centromere position and chromatin-staining pattern and are called homologous chromosomes (or homologs). These chromosome pairs carry information controlling the same or similar hereditary characters giving rise to traits. Generally, in gender determination, in human somatic cells, the two chromosomes referred to as X and Y do not follow this homologous rule; human females are chromosomally homologous (XX), while males have two sex chromosomes, the X and Y, which are non-homologous.
In summary, animals and plants and many unicellular organisms are diploid and therefore have two sets of chromosomes. In each somatic cell of the organism (all cells of a multicellular organism except the gametes or reproductive cells), the nucleus contains two copies of each chromosome, called homologous chromosomes. Homologous chromosomes are matched pairs containing the same genetic information in identical locations along their lengths. Diploid organisms inherit one copy of each homologous chromosome from each parent.
Mitosis vs. Meiosis
Meiosis is the nuclear division that forms haploid cells from diploid cells, and it employs many of the same cellular mechanisms as mitosis. However, as you h