Mitosis and meiosis share some similarities but also some significant differences, chiefly that mitosis produces genetically identical diploid daughter cells where as meiosis produces genetically different haploid cells.
Mitosis and meiosis are both forms of division of the nucleus in eukaryotic cells. They share some similarities, but also exhibit distinct differences that lead to different outcomes. The purpose of mitosis is cell regeneration, growth, and asexual reproduction, while the purpose of meiosis is the production of gametes for sexual reproduction.
Mitosis is a single nuclear division that results in two nuclei that are genetically identical to the original parent nucleus. In meiosis, the two nuclear divisions result in four nuclei that are not genetically identical and contain one chromosome set only that usually divide into four new haploid daughter cells. Furthermore, the two nuclei in mitosis have the same number of sets of chromosomes, one set in the case of haploid cells and two sets in the case of diploid cells. In contrast, the nuclei resulting from meiosis is half the number of chromosome sets in the original cell, which is diploid.
The main differences between mitosis and meiosis occur during meiosis I where the homologous chromosome pairs become associated with each other and are bound together with the synaptonemal complex. As a chiasmata develops, crossing over occurs between homologous chromosomes resulting in recombinant chromosomes.
When the tetrad is broken up and the homologous chromosomes move to opposite poles, the ploidy level is reduced from two to one. For this reason, meiosis I is referred to as a reduction division. There is no such reduction in ploidy level during mitosis.
Comparing Meiosis and Mitosis: Meiosis and mitosis are both preceded by one round of DNA replication; however, meiosis includes two nuclear divisions. The four daughter cells resulting from meiosis are haploid and genetically distinct. The daughter cells resulting from mitosis are diploid and identical to the parent cell.
Meiosis II is much more similar to a mitotic division. In this case, the duplicated chromosomes (only one set, as the homologous pairs have now been separated into two different cells) line up on the metaphase plate with divided kinetochores attached to kinetochore fibers from opposite poles. During anaphase II and mitotic anaphase, the kinetochores divide and sister chromatids, now referred to as chromosomes, are pulled to opposite poles. The two daughter cells of mitosis, however, are identical, unlike the daughter cells produced by meiosis. They are different because there has been at least one crossover per chromosome. Meiosis II is not a reduction division because, although there are fewer copies of the genome in the resulting cells, there is still one set of chromosomes, as there was at the end of meiosis I. Meiosis II is, thus, referred to as equatorial division.
Practice Questions
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Key Points
• For the most part, in mitosis, diploid cells partition into two new diploid cells, while in meiosis, diploid cells partition into four new haploid cells.
• In mitosis, the daughter cells have the same number of chromosomes as the parent cell, while in meiosis, the daughter cells have half the number of chromosomes as the parent.
• The daughter cells produced by mitosis are identical, whereas the daughter cells produced by meiosis are different because crossing over has occurred.
• The events that occur in meiosis but not mitosis include homologous chromosomes pairing up, crossing over, and lining up along the metaphase plate in tetrads.
• Meiosis II and mitosis are not reduction divisions like meiosis I because the number of chromosomes remains the same; therefore, meiosis II is referred to as equatorial division.
• When the homologous chromosomes separate and move to opposite poles during meiosis I, the ploidy level is reduced from two to one, which is referred to as a reduction division.
Key Terms
chiasmata: a point at which paired chromosomes remain in contact during the first metaphase of meiosis
reduction division: the first of the two divisions of meiosis, a type of cell division
ploidy: the number of homologous sets of chromosomes in a cell
synaptonemal complex: a ladder-like series of parallel threads visible in electron microscopy adjacent to and coaxial with pairing chromosomes in meiosis
equatorial division: a process of nuclear division in which each chromosome divides equally such that the number of chromosomes remains the same from parent to daughter cells
meiosis: cell division in sexually-reproducing organisms used to produce the gametes, such as sperm or egg cells
mitosis: cell division that results in two identical daughter cells
asexual: reproduction of cells without the use of gametes producing identical copies
eukaryotes: cells with a nucleus enclosed within membranes
nuclear division: the division of the nucleus in both mitosis and meiosis
homologous chromosomes: are made up of chromosome pairs of approximately the same length. One homologous chromosome is inherited from the organism’s mother; the other is inherited from the organism’s father
haploid: half the number of chromosomes
diploid: a full set of chromosomes, 46 in humans