Meiosis is the process by which the sex cells (sperm and eggs) may be produced in eukaryotes. During the process 1 cell divides twice to form 4 daughter cells that are non-identical genetically.
Meiosis is broken into meiosis I and meiosis II. The first step of meiosis I is prophase I. During this phase, the nuclear envelope disappears and chromatin (DNA) condenses to form chromosomes. Homologous chromosomes (for example, chromosome 1 from your mom and...
Meiosis is the process by which the sex cells (sperm and eggs) may be produced in eukaryotes. During the process 1 cell divides twice to form 4 daughter cells that are non-identical genetically.
Meiosis is broken into meiosis I and meiosis II. The first step of meiosis I is prophase I. During this phase, the nuclear envelope disappears and chromatin (DNA) condenses to form chromosomes. Homologous chromosomes (for example, chromosome 1 from your mom and chromosome 1 from you dad) line up in the cell. A process called "crossing over" then occurs as homologous portions (coding for the same genes) are exchanged between the homologous chromosomes. This will, in the end, create new gene combinations in the daughter cells. The next step is metaphase I; the homologous chromosomes like up along the metaphase plate in the middle of the cell. The homologous chromosomes are then pulled to opposite poles of the cell during anaphase I. During telophase I, the nuclear envelope reappears and the cell membrane forms a cleavage furrow where the cell starts to pinch closed to form two daughter cells.
Meiosis II then begins and proceeds similarly to mitosis. During prophase II, the nuclear envelope again disappears and the chromatin condenses into chromosomes. There is no crossing over in prophase II. The chromosomes then line up along the center of the cell during metaphase II and the sister chromatids (if you picture the chromosome as an "X" they are the identical components that make up the right and left parts of the X) separate and move toward opposite poles of the cell. Finally, during telophase II, the nuclear envelope is reformed, the chromosomes de-condense to their chromatin form, and another cleavage furrow forms. The two daughter cells resulting from meiosis I then divide to form four daughter cells in total. Because of the crossing over in prophase I and the random nature of which chromosomes end up in which daughter cell, they are non-identical. Also, because there are two rounds of division without DNA replication in between, the daughter cells have half of the genetic information as their parent cell had. The parent cell was diploid and had two copies of each chromosome (1 from mom and 1 from dad), but the daughter cells resulting from meiosis are haploid and contain only 1 copy of each chromosome. During fertilization, these sex cells fuse with the sex cells of the opposite sex (sperm and egg) to produce a new diploid cell (zygote).
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