Overview
Meiosis is the process of cell division that creates 4 haploid (one set of chromosomes) cells from 1 diploid (two sets of chromosomes) eukaryotic cell. It is essential for sexual reproduction and occurs in all eukaryotes that produce sexually.
Stages of meiosis (consists of Meiosis I and Meiosis II):
Extra notes:
Stages of meiosis (consists of Meiosis I and Meiosis II):
- Meiosis I:
- Interphase I - DNA is replicated into two identical chromatids attached at the centromere region (S-phase); organelles and membranes duplicated (Gap 2); DNA replication only happens in meiosis I
- Prophase I - most important stage of meiosis; homologous chromosomes are lined up through synapsis; crossing over (genetic exchange of chromosomes) may occur; chromosomes are condensed and attach to the nuclear envelope; later on, chromosomes thicken and the nuclear envelope dissolves; centrioles migrate from one another
- Metaphase I - chromosomes align on the metaphase plate; centromeres are turned towards opposite poles; homologous chromosomes are randomly assorted (genetic variation)
- Anaphase I - microtubules pull the chromosomes to opposite poles; sister chromatids stay together (unlike mitosis)
- Telophase I and Cytokinesis I (both usually happen at the same time) - spindle fibers continue to move to opposite ends, eventually forming two haploid daughter cells; in plant cells, the cell plate forms.
- Meiosis II:
- Interphase II - sometimes seen, but there is no DNA replication
- Prophase II - spindle fibers appear; nuclear membrane and nucleus break up; unlike meiosis I, no crossing over occurs
- Metaphase II - chromosomes are lined on the metaphase plate; kinetochore fibers point to opposite poles (similar to metaphase I)
- Anaphase II - sister chromatids seperate (unlike in Meiosis I) and begin moving to the opposite sides of the poles; spindle fibers not connected to chromatids lengthen and elongate the cell
- Telophase II and Cytokinesis II - cleavage furrow/ cell plate separates cells; 4 daughter cells are formed.
Extra notes:
- The products (daughter cells) are haploid because the sperm and egg join together, doubling the chromosome number to have the correct number needed (reduction division)
- Only specialized cells undergo meiosis to make gametes
- Daughter cells are related but not identical to parents
- To sexually reproduce, cells need to undergo meiosis and mitosis
- There is a greater variation in offspring due to recombination during sexual reproduction, as opposed to a highly identical population from asexual reproduction (only mitosis occurs)