Mitosis takes a diploid cell and creates a nearly exact copy. Mitosishas two main functions: (1) it leads to the creation of all of thesomatic (body) cells in humans and other living organisms; (2) inorganisms that undergo
asexual reproduction, diploid parentcells undergo mitosis to create identical daughter copies ofthemselves. Mitosis creates a daughter cell with chromosomes that areidentical to the chromosomes in its parent cell.
But humans and most other complex plantsand animals each have a unique set of chromosomes. This diversity ofchromosomes is the result of
sexual reproduction, whichinvolves the contribution of the genetic material from not one, but twoparents. During sexual reproduction the father’s haploid sperm cell andthe mother’s haploid ovum (egg) cell fuse to form a single-celleddiploid
zygote that then divides billions of times to form a whole individual.
In order for sexual reproduction to take place, however, the parents first need to
have haploid sperm or ova, also called
sex cells,
germ cells, or
gametes. Meiosis is the name for the special type of cell division that produces gametes.
Process of Meiosis
Unlike the single-cell division of mitosis,meiosis involves two cellular divisions: meiosis I and meiosis II. Eachstage of meiosis runs through the same five stages as discussed inmitosis. During the first round of division, two intermediate daughtercells are produced. By the end of the second round of meiotic division(meiosis II), the original diploid (2
n) cell has become four haploid (
n) daughter cells.
Meiosis I
Meiosis I is quite similar to mitosis.However, there are a number of crucial differences between meiosis Iand mitosis, all of which will be outlined in the discussion of eachindividual stage below.
Interphase I
Just as in mitosis, the cell undergoes DNAreplication during this intermediate phase. After replication, the cellhas a total of 46 chromosomes, each made up of two sister chromatidsjoined by a centromere.
Prophase I
The major distinction between mitosis andmeiosis occurs during this phase. In mitotic prophase, thedouble-stranded chromosomes line up individually along the spindle. Butin meiotic prophase I, chromosomes line up along the spindle inhomologous pairs. Then, in a process called
synapsis, the homologous pairs actually join together and intertwine, forming a
tetrad(two chromosomes of two chromatids each, or four total chromatids).Often this intertwining leads the chromatids of homologous chromosomesto actually exchange corresponding pieces of DNA, a process called
crossing-over orgenetic reassortment. Throughout prophase I, sister chromatids behaveas a unit and are identical except for the region where crossoveroccurred.
Metaphase I
After prophase I, the meiotic cell entersmetaphase I. During this phase, the nuclear membrane breaks down,allowing microtubules access to the chromosomes. Still joined at theircrossover regions in tetrads, the homologous pairs of chromosomes, withone maternal and one paternal chromosome in each pair, align at thecenter of the cell via microtubules, as in mitotic metaphase. The pairsalign in random order.
Anaphase I
Anaphase I differs slightly from its mitoticcounterpart. In mitotic anaphase, sister chromatids split at theircentromeres and are pulled apart toward opposite poles. In contrast,during anaphase I, the centromeres do not split: the entire maternalchromosome of a homologous pair is pulled to one end, and the paternalchromosome is pulled to the other end.
Telophase I
During telophase I, the chromosomes arrive atseparate poles and decondense. Nuclear membranes re-form around them.The cell physically divides, as in mitotic cytokinesis.
The Product of Meiosis I
Meiosis I results in two independent cells.One cell contains the maternal homologous pair, with a small segment ofthe paternal chromosome from crossover. The other cell contains thepaternal homologous pair, likewise with a small segment of the maternalchromosome. Despite the small region of crossover in the chromosomes ofeach cell, the maternal sister
chromatids are still quite similar, as are the paternal sister chromatids. Both cells formed by meiosis I contain a haploid amount of DNA.
The cells produced in meiosis I aredifferent from those produced in mitosis because both haploid membersof the meiotic pair derive from random assortments of either thematernal or paternal chromosomes from each homologous pair (with theexception of the small crossover sections). In mitosis, the cellulardivision separates sister chromatids and results in diploid cellscontaining one maternal and one paternal copy in each diploid pair.
Meiosis II
The cells produced by meiosis I quickly enter meiosis II. These cells
do notundergo DNA replication before entering meiosis II. The two cells thatmove from meiosis I into meiosis II are haploid—each have 23 replicatedchromosomes, rather than the 46 that exist in cells entering bothmitosis and meiosis I.
Meiotic division II occurs through thefamiliar phases from meiosis I and mitosis. To distinguish the phases,they are called prophase II, metaphase II, anaphase II, and telophaseII. One important difference between the events of meiosis I and II isthat no further genetic reassortment takes place during prophase II. Asa result, prophase II is much shorter than prophase I. In fact, all ofthe phases of meiosis II proceed rapidly.
During meiosis II, chromosomes align at thecenter of the cell in metaphase II exactly the way they do in mitoticmetaphase. In anaphase II, the sister chromatids separate, once againin the same fashion as occurs in mitotic anaphase. The only differenceis that since there was no second round of DNA replication; only oneset of chromosomes exists. When the two cells split at the end ofmeioisis II, the result is four haploid cells.
Of the four haploid cells, one cell iscomposed completely of a maternal homologue, another of a maternalhomologue with a small segment of paternal DNA from crossover inmeiosis I, another complete paternal homologue, and a final paternalhomologue with a small segment of maternal DNA from crossover inmeiosis I. These four haploid cells are the gametes, the sperm or eggcells, that fuse together in sexual reproduction to create newindividuals.
