Meiosis is the nuclear division of diploid cells into haploid cells, which is a necessary step in sexual reproduction; it is a significant chance to introduce genetic diversity into gametes.
Sexual reproduction is the production of haploid cells (gametes) and the fusion (fertilization) of two gametes to form a single, unique diploid cell called a zygote. All animals and most plants produce these gametes, or eggs, and sperm. In most plants and animals, through tens of rounds of mitotic cell division, this diploid cell will develop into an adult organism.
A type of cell division called meiosis produces haploid cells that are part of the sexual reproductive cycle. It employs many of the same mechanisms as mitosis. However, the starting nucleus is always diploid, and the nuclei that result at the end of a meiotic cell division are haploid, so the resulting cells have half the chromosomes as the original.
To achieve the reduction in chromosomes, meiosis consists of one round of chromosome duplication and two rounds of nuclear division. Because the events that occur during each of the division stages are analogous to the events of mitosis, the same stage names are assigned. However, since there are two rounds of division, the major process and the stages are designated with a “I” or a “II.” Meiosis I is the first round of meiotic division and consists of prophase I, prometaphase I, and so on. Meiosis II, the second round of meiotic division, includes prophase II, prometaphase II, and so on. Through both of these stages, there is a significant opportunity for genetic diversity to be introduced, resulting in gametes that produce offspring that are not identical genetic copies of their parents.
MCAT Official Prep (AAMC)
• Meiosis is essential for nuclear reduction and introducing genetic diversity into gametes
• Sexual reproduction is the production of haploid cells and the fusion of two of those cells to form a diploid cell.
• Before sexual reproduction can occur, the number of chromosomes in a diploid cell must decrease by half.
• Meiosis produces cells with half the number of chromosomes as the original cell.
• Gametes, haploid cells used in sexual reproduction, are formed during meiosis, which consists of one round of chromosome replication and two rounds of nuclear division.
• Meiosis I is the first round of meiotic division, while meiosis II is the second round.
haploid: of a cell, having a single set of unpaired chromosomes
gamete: a reproductive cell, male (sperm) or female (egg), that has only half the usual number of chromosomes
diploid: of a cell, having a pair of each type of chromosome, one of the pair is derived from the ovum and the other from the spermatozoon
meiosis I: the first phase of meiosis
meiosis II: the second phase of meiosis
meiosis: cell division in sexually-reproducing organisms used to produce the gametes, such as sperm or egg cells
zygote: a fertilized ovum begins as a single cell but divides rapidly in the days following fertilization