Meiosis
From MyMCAT
Contents |
Introduction
Unlike, mitosis, which always results in identical progeny, meiosis causes a reductional division in which the number of chromosomes per cell is cut in half. In animals, meiosis always results in the formation of gametes (sex cells), while in other organism it can give rise to alternative cell structures like spores. Meiosis is essential for maintaining genetic diversity and occurs in all eukaryotes (including single-celled organisms) that can reproduce sexually.
Meiosis can often be thought of as just two rounds of Mitosis. Prior to the process the DNA is replicated (like in mitosis), after the first round the cells are divided evenly yielding two cells with the normal amount of DNA, and after the second round, four cells are produced (from the two cells each dividing into two) but each contains only half the DNA because there was no second round of DNA replication. The distinct difference of Mitosis and Meiosis is in the resulting DNA of the progeny.
Prior to Meiosis
As with mitosis, the process of meiosis itself does not include that of the initial DNA synthesis. Thus, when a cell begins the replication process, it first proceeds through the steps of interphase: primary growth (G1), DNA synthesis (S); and secondary growth (G2). These steps are vital to maintaining the integrity of the cell and priming its genetic contents for division. Because meiosis occurs in diploid cells, there are initially two copies of every chromosome (homologous pairs). Post DNA replication (S phase), each of the chromosomes is doubled, but the replicated copies remain attached at centromeres. As such, there are now four fragments or, more descriptive, a pair of pairs. The individual fragments are considered chromatids, those chromatids attached by a centromere are considered sister chromatids.
Meiosis I
In meiosis I, the homologous pairs in a diploid cell separate, producing two haploid cells, so meiosis I is referred to as a reductional division. A regular diploid human cell contains 46 chromosomes and is considered 2N because it contains 23 pairs of homologous chromosomes. However, after meiosis I, the pairs have been separated into the two daughter cells. (Meiosis II will then further separate these such that the sister chromatids of each chromosome will split creating a total of 4 haploid cells).
Prophase I
Homologous chromosomes pair (or synapse) and crossing over (or recombination) occurs, a step unique to meiosis. Each chromosome in a diploid cell is generally paired with its second copy (with the exception of the X and Y) and because at this stage each chromosome is also made up of two sister chromatids, the chromosomal complex is often referred to as a tetrad. It is in the tetrad stage that non-sister chromatids may cross-over to form chiasmata (the site of recombination events). This crossing over allows chromosomes to exchange homologous regions and thus increase their genetic diversity.
As with mitosis, during prophase I, chromosomes (in tetrad forms) begin to condense, the nucleoli disappears, the nuclear membrane disintegrates into vesicles, and meiotic spindles from centrioles begin to form. Microtubules invade the nuclear region after the nuclear envelope disintegrates and attach to the chromosomes at kinetochores.
Metaphase I
Homologous chromosome pairs are tugged by microtubles to align and form the metaphase plate. As kinetochore microtubules from both centrioles attach to their respective kinetochores, the homologous chromosomes align along an equatorial plane that bisects the spindle. Because the orientation of chromosomal-pair alignment is random, the process is called independent assortment, and it implies that the two resulting daughter cells will have a random distribution of each chromosome (as well as random fragments from each due to the previous recombination events).
Anaphase I
As microtubules shorten, the homologous chromosomes are pulled apart. While each chromosome still contains a pair of sister chromatids the two halves of the cell have now become haploid (as the chromosomes are no longer paired). As these the microtubules attached to chromosomes at kinetochores shorten, the remaining microtubules not attached to kinetochores lengthen, pushing the centrioles farther apart. This causes the cell to elongate in preparation for division down the center.
Telophase I
Each daughter cell now has half the number of chromosomes but each chromosome consists of a pair of chromatids. The microtubules that make up the spindle network disappear, and a new nuclear membrane wraps each haploid set. The chromosomes uncoil back into chromatin. Cytokinesis, the pinching of the cell membrane in animal cells or the formation of the cell wall in plant cells, occurs, completing the creation of two daughter cells. Sister chromatids, however, remain attached during telophase I and will only separate in meiosis II.
Often, cells may enter a period of rest known as interkinesis or interphase II. No DNA replication occurs during this stage, and it is simply a refractory period before meiosis II.
Meiosis II
Meiosis II is the second part of the meiotic process. The process is entirely analogous to mitosis. The end result is production of four haploid cells (23 chromosomes, 1N in humans) from the two haploid cells (23 chromosomes, 1N * each of the chromosomes consisting of two sister chromatids) produced in meiosis I.
In prophase II, we see the disappearance of the nucleoli and the nuclear envelope again as well as the shortening and thickening of the chromatids. Centrioles move to the polar regions and arrange spindle fibers for the second meiotic division.
In metaphase II, the centromeres contain two kinetochores, that attach to spindle fibers from the centrosomes (centrioles) at each pole. Often, the metaphase plate is rotated by 90 degrees when compared to meiosis I, perpendicular to the previous plate.
In anaphase II, the centromeres are cleaved, allowing microtubules attached to the kinetochores to pull the sister chromatids apart. The sister chromatids by convention are now called sister chromosomes as they move toward opposing poles.
The process ends with telophase II, which is similar to telophase I, and is marked by uncoiling and lengthening of the chromosomes and the disappearance of the spindle. Nuclear envelopes reform and cleavage or cell wall formation eventually produces a total of four daughter cells, each with a haploid set of chromosomes.
The Importance of Sexual Reproduction
Consider two organisms, A and B, of the same species. A has a superior method to produce energy while B has a superior method to store sugars. Obviously, having both mutations would be ideal, but, if A and B are bacteria little can be done. All of A's progeny have A's traits and likewise, alls of B's progeny have B's traits, but there is almost no way for these two different cells to combine. Sexual reproduction however offers this easily. While the combinations produced by sexual reproduction are random, they are a hybrid of both partners and thus there is the potential to produce offspring containing the best of both partners.
