Meiosis is preceded by an interphase consisting of the G 1, S, and G 2 phases, which are nearly identical to the phases preceding mitosis. Meiosis II, in which the second round of meiotic division takes place, includes prophase II, prometaphase II, and so on. However, because there are two rounds of division, the major process and the stages are designated with a “I” or a “II.” Thus, meiosis I is the first round of meiotic division and consists of prophase I, prometaphase I, and so on. Because the events that occur during each of the division stages are analogous to the events of mitosis, the same stage names are assigned. To achieve this reduction in chromosome number, meiosis consists of one round of chromosome duplication and two rounds of nuclear division. However, the starting nucleus is always diploid and the nuclei that result at the end of a meiotic cell division are haploid. Meiosis employs many of the same mechanisms as mitosis. In mitosis, both the parent and the daughter nuclei are at the same ploidy level-diploid for most plants and animals. As you have learned, mitosis is the part of a cell reproduction cycle that results in identical daughter nuclei that are also genetically identical to the original parent nucleus. The nuclear division that forms haploid cells, which is called meiosis, is related to mitosis. Some plants and all fungi produce spores. All animals and most plants produce eggs and sperm, or gametes. Spores are haploid cells that can produce a haploid organism or can fuse with another spore to form a diploid cell. Haploid cells, containing a single copy of each homologous chromosome, are found only within structures that give rise to either gametes or spores.
During prophase full#
Diploid organisms inherit one copy of each homologous chromosome from each parent all together, they are considered a full set of chromosomes. Homologous chromosomes are matched pairs containing the same genes in identical locations along their length. Somatic cells are sometimes referred to as “body” cells. 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. Most animals and plants are diploid, containing two sets of chromosomes.
So, in addition to fertilization, sexual reproduction includes a nuclear division that reduces the number of chromosome sets. If the reproductive cycle is to continue, then the diploid cell must somehow reduce its number of chromosome sets before fertilization can occur again, or there will be a continual doubling in the number of chromosome sets in every generation. The number of sets of chromosomes in a cell is called its ploidy level. Cells containing two sets of chromosomes are called diploid. Haploid cells contain one set of chromosomes.
If those two cells each contain one set of chromosomes, then the resulting cell contains two sets of chromosomes. Sexual reproduction requires fertilization, the union of two cells from two individual organisms.
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