MITOSIS AND MEOSIS

Mitosis and Meiosis

I.       These two processes function to pass chromosomes from one cellular generation to the next in a very carefully controlled manner.

II.     Mitosis and Meiosis are both correctly described as nuclear division; they are never correctly called cell division, or any kind of reproduction. It is possible (and often quite normal) for nuclei to divide when cells don't. And organisms reproduce; nuclei and cells divide.

III.   Mitosis

A.      Mitosis is the division of a nucleus to produce two genetically identical daughter nuclei.

B.       Mitosis is utilized for any function which requires the production of more cells with identical genetic information. These processes include the vase majority of cell production during the growth of an organism, the cell division needed for healing and repair, and the division of nuclei when an organism is in the process of asexual reproduction. Note: Mitosis is not asexual reproduction, nor can it be called asexual cell (or even nuclear) division.

C.       Because the vast majority of the cells in a multicellular organism were produced by mitotic cell division, those cells all have identical nuclei. They obviously don't all look or function alike. Cells mature through a process called differentiation in which select sets of genes are turned on and off, resulting in changes in the structure and function of the cell.

IV.    Meiosis

A.      Meiosis is the division of one diploid (usually) nucleus to produce four haploid (usually) nuclei, all genetically different. Though the vast majority of the time the chromosome number reduction is from diploid to haploid, in some cases it may be from, say, hexaploid (eg., wheat) to triploid.

B.       Meiosis performs a key task necessary in a sexual life cycle. Since fertilization (which is the actual sexual event in the life cycle) automatically doubles chromosome number by combining the chromosomes of an egg and a sperm, it is essential that some event occur somewhere in the life cycle which reduced the chromosome number to compensate.

C.       Though Meiosis is part of a sexual life cycle, it is not ever correctly described as, for instance, "sexual cell division," and certainly not as "sexual reproduction." It is the life cycle, and specifically the fertilization event, which constitute sexual activity, not Meiosis.

D.      In animal life cycles, the meiotic cell division in the life cycle immediately precedes the development of gametes (eggs and sperm). However, this need not at all be the case. Plants have a somewhat different sexual life cycle from animals which includes all of the same events, including Meiosis, production of gametes, and fertilization, but also includes an additional phase between Meiosis and gamete production.

V.      There are three differences between what Mitosis accomplishes and what Meiosis accomplishes.

A.      Mitosis divides one nucleus into two; Meiosis divides one nucleus into four.

B.       Mitosis conserves chromosome number; Meiosis reduces it in half (usually from diploid to haploid).

C.       Mitosis produces genetically identical daughter nuclei; Meiosis produces genetically different daughter nuclei.

VI.    Errors in nuclear division can produce chromosome anomalies.

A.      Nuclei whose chromosomes do not form normal sets (eg, which have extra or missing chromosomes in one or more of the chromosome sets) are aneuploid.

1.      Aneuploidies typically result from nondysjunction in either Meiosis I or Meiosis II.

2.      If a nucleus which should be diploid has three of a chromosome instead of the normal two, that nucleus is trisomic. For example, if the nucleus of a human cell has three of chromosome 18, that would be trisomy 18. This term is also used for an organism, all of whose cells are trisomic. Trisomy 21 can refer to a human, all of whose cells have three twenty-first chromosomes instead of the expected two.

3.      If a nucleus which should be diploid has only one of a chromosome in stead fo the normal two, that nucleus is monosomic. An individual may be described as monosomic if all (or most) of the cells in that individual are monosomic. For example, Turner's Syndrome female humans are monosomic for the X chromosome.

4.      Other things being equal, it is significantly worse to be missing chromosome material than to have extra chromosome materials. Of course, the specific genes on the extra or missing chromosome material also impact on how serious the effects of the anomaly will be.

B.       A nucleus which has extra sets of chromosomes (like the wheat mentioned above) is polyploid.

1.      Polyploidy typically results from fertilization inv0lving gametes with unreduced chromosome number. In other words, an error in chromosome reduction during Meiosis produces a gamete which still has the diploid number of chromosome sets, which then participates in fertilization with a haploid gamete.

2.      An autopolyploid has more than two sets of chromosomes from the same species. In other words, it has more than two homologous sets of chromosomes. Commercially grown bananas are autotriploids; they have three sets of banana chromosomes. Autopolyploids frequently have difficulty performing Meiosis because the pairing mechanism in Prophase I requires one-on-one pairing between two partners, and having more than two homologues in the same nucleus creates confused pairing.

3.      An allopolyploid has chromosomes from more than one species. Wheat is an allohexaploid. It has two each of the chromosome sets from three different species of grasses, so each nucleus has six sets of chromosomes, but each chromosome has only one homologous partner in the nucleus. For this reason, allopolyploids are often perfectly fertile, as wheat obviously is. Allopolyploids are generally formed by an accidental fertilization between two different, closely related species (producing an offspring which is technically diploid, but whose chromosomes don't match--a mule is an example of this situation). In plants, it is frequently possible for the plant to reproduce asexually, so the inability to perform Meiosis may not be a problem which prevents survival and reproduction. (This is why commercial bananas can reproduct successfully, despite being unable to do Meiosis.) Our inter-species hybrid can thus generally reproduce and thrive. Eventually, an accidental chromosome doubling event can double both chromosome sets, producing a plant which is technically tetraploid (having four sets of chromosomes) but functionally diploid (as each chromosome has only one homologous partner in the nucleus). This restores the ability to reproduce sexually, and also creates a brand new species. Clearly, this happens. It's happened twice in the natural history of Triticum (wheat).

C.       Sometimes chromosomes lose segments, or acquire extra copies of segments. These are called insertions and deletions, or duplications and deficiencies. These generally arise due to uneven crossovers during Prophase I of Meiosis. They can result from crossing over in inversion heterozygotes.

D.      Occasionally, a segment of a chromosome will break free and accidentally reattach with its ends switched around. This reversed segment of the chromosome is an inversion.

1.      If the cell is an inversion homozygote, which means that both chromosomes of a homologous pair carry the same inversion, this causes no problems for the organism (though it might lead to fertility problems with offspring, which could very easily be inversion heterozygotes).

2.      If a cell has one chromosome of a pair which carries an inversion, but the partner doesn't, the cell is an inversion heterozygote. Inversion heterozygotes are, themselves, perfectly healthy, but in Meiosis the inversion can create several problems.

a.       In Prophase I of Meiosis, chromosomes synapse on a gene-by-gene basis. So in the region of the inversion, when the homologous chromosomes pair they create an inversion loop as the frontward/backward segments attempt to pair normally. This isn't, in itself, a problem, but if a crossover occurs within the inversion loop, it will lead to duplication and deficiency.

b.       If the inverted region does not include the centromere of the chromosome, and a crossover occurs within the inverted region, it will lead to one dicentric chromatid (a chromatid which has two centromeres) and one acentric chromatid (a chromatid which has no centromere). As the pull of spindle fibers on the centromeres is what moves chromosomes around during Meiosis and Mitotis, this typically leads to disaster.

E.       Sometimes, a segment of one chromosome gets accidentally broken off and attached to a different chromosome. This is a translocation.

1.      If no genes are lost or damaged by the translocation, and all genes are present in the correct quantity (generally two), the result is a balanced translocation, which causes no problems for the individual possessing the translocation, though it may cause problems in gamete production and for the offspring.

2.      If the translocation involves trading pieces of both chromosomes involved, it's called a reciprocal translocation.

3.      As with inversions, translocations may be homozygous or heterozygous. A translocation homozygote has no problems; he or she simply has an unusual arrangement of genes on his or her chromosomes, but everything works perfectly.

4.      A translocation heterozygote has problems in synapsis, just as the inversion heterozygote does, because the chromosomes try to pair gene-to-gene, and the genes are located on different chromosomes in the two homologous pairs. Again, crossing over in the wrong place can lead to duplication and deficiencies, and to dicentric and acentric chromosomes. Note that an individual can be a translocation heterozygote and still have a perfectly balanced gene complement.

5.      Even if no unfortunate crossing over occurs, the outcome of a fertilization between someone carrying a balanced translocation and someone with the more usual chromosome complement can cause problems for the offspring. The zygote will receive a normally arranged set of chromosomes from one gamete, but may receive a translocated chromosome (but not its reciprocal partner) from the other. A rare version of Down's Syndrome (which generally results from trisomy of the twenty-first chromosome) results from this sort of problem. The zygote receives two normal twenty-first chromosomes, plus one normal fourteenth chromosome and one fourteenth chromosome which is carrying a translocated copy of most of the twenty-first chromosome. The result is a genome which has the appropriate number of chromosomes, but in which one of the fourteenth chromosomes also carries a copy of the twenty-first chromosome. Thus the child functionally has three copies of chromosome twenty-one, and all of the normal characteristics of Down's Syndrome.

F.       Note that chromosome rearrangements like inversions and translocations have important impact on speciation--the division of one species into two. What distinguishes one species from another is the inability to reproduce and produce fertile offspring, and if chromosome rearrangements arise and become "fixed" within a section of a species, that subgroup can easily become unable to reproduce with the original, parent species. This is one of the things that happens when segments of a species become isolated from other segments. Comparing the chromosomes of closely related species shows us that this is a very significant way in which they differ from each other. This is why, for example, a horse can breed with a donkey (different, but closely related species) and produce a completely healthy, even robust, hybrid: a mule. Mules are strong and healthy because the chromosomes of the donkey and those of the horse carry very complementary kinds of genes, and the two sets of genetic influences interact very well, with little missing. However, the mule is sterile, because his horse chromosomes and his donkey chromosomes have significant differences in arrangement (inversions and translocations) and thus are unable to complete Meiosis.