We have examined mitosis as a type of cell division responsible for growth, repair of tissues and in some cases the formation of new organisms (lab 3) . The significant feature of mitosis was that the chromosome number did not change during the division so that the daughter cells carried the same number of chromosomes as the parent cell.
Meiosis
(Campbell 6th Ed. 239-244; 7th Ed. 243-249)
In contrast, meiosis results in daughter cells with half the chromosome number of the parent cell and is thus often referred to as a reduction division. The basic overall pattern is shown in the following illustration:
In this way, after completing meiosis, a 46 chromosome human cell has been reduced to four daughter cells, each with 23 chromosomes. In animals, meiosis is directly responsible for the production of sex cells or gametes. In plants, meiosis leads to a haploid stage, the spore, which later gives rise to a multicellular haploid gametophyte, which in turn produces gametes.
A general animal life cycle, including meiosis is shown below:
Eventually a mating process leads to the fusion of male and female gametes resulting in a cell known as a zygote. The zygote will have the 2n or diploid chromosome number in its nucleus. Development of the zygote through mitosis results in the formation of the mature organism.
Meiosis involves two division sequences in succession which ultimately results in the formation of four new cells, each of which is haploid. In order to appreciate the significance of the meiotic division you will have to understand the meaning of terms like somatic cell, gamete, diploid number, and homologous chromosomes. (See your lecture notes and text).
The first division sequence (meiosis I) results in a reduction in the number of chromosomes; the second division sequence resembles a mitotic one and converts the remaining double-stranded chromosomes to single-stranded.
