The G 1 phase (the “first gap phase”) is focused on cell growth. Meiosis is preceded by an interphase consisting of G 1, S, and G 2 phases, which are nearly identical to the phases preceding mitosis. Department of Biology, Texas A&M University. Credit: Rao, A., Tag, A, Fletcher, S., and Ryan, K. going from diploid cells to haploid gametes. During meiosis, two successive rounds of division reduces the number of chromosomes (ploidy) of the cell by half. Prior to meiosis, chromosomes are replicated in S-phase to ensure proper number of chromosomes in the resulting gametes. Meiosis is the mechanism used to reduce diploid cells to haploid gametes while introducing genetic diversity. The production of gametes is a crucial process for sexually reproducing organisms. Likewise, Meiosis II (during which the second round of meiotic division takes place) includes prophase II, prometaphase II, and so on.įigure 11.2 Overview of Meiosis. 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 many 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 replication followed by two rounds of nuclear division. In meiosis, the starting nucleus is always diploid and the daughter nuclei that result are haploid. Plants use mitosis to grow as sporophytes, and to grow and produce eggs and sperm as gametophytes so they use mitosis for both haploid and diploid cells (as well as for all other ploidies). In mitosis, both the parent and the daughter nuclei are at the same “ploidy level”-diploid in the case of most multicellular animals. However, as you have learned, mitosis produces daughter cells whose nuclei are genetically identical to the original parent nucleus. Meiosis is the nuclear division that forms haploid cells from diploid cells, and it employs many of the same cellular mechanisms as mitosis. Diploid organisms inherit one copy of each homologous chromosome from each genetic contributor. Homologous chromosomes are matched pairs containing the same genes in identical locations along their lengths. 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 and many unicellular organisms are diploid and therefore have two sets of chromosomes. Therefore, sexual reproduction requires a nuclear division that reduces the number of chromosome sets by half. (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 reduce its number of chromosome sets to produce haploid gametes otherwise, the number of chromosome sets will double with every future round of fertilization. When gametes unite, they form a zygote, or fertilized egg that contains two sets of chromosomes. Sexual reproduction requires the union of two specialized cells, called gametes, each of which contains one set of chromosomes.
0 Comments
Leave a Reply. |
Details
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |