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Meiosis and Sexual Life Cycles - 1
We have just finished looking at the process of mitosis, which ensures that eachcell of an organism has the same DNA as the original fertilized egg or zygote(absent mutations).
Transmitting chromosomes and genetic information from generation to generationis equally important. A critical role of heredity is to maintain and obtain variationamong members of a species. These variations are the result of the specificgenes we inherit from our parents. We did not always know that genes werelocated on chromosomes. We didn't even know how genetic information wastransmitted from parent to offspring. The mechanism for transmitting geneticinformation was first proposed by Gregor Mendel in the mid-1800's. It was prettymuch forgotten until the early 1900's when Mendel's papers were "discovered"about the time other researchers were drawing the same conclusions based onsimilar research. Soon after, Walter Sutton showed that Mendel's principles ofinheritance applied to chromosomes and that chromosomes are the units ofheredity. We shall discuss Mendel's principles and inheritance patterns soon, butfirst we'll look at how chromosomes are transmitted from generation togeneration by the process of meiosis and sexual reproduction.
Although asexual reproduction, which uses mitosis to make new individuals(genetically the same as the parent) is common in protists, plants, fungi and someanimals, most organisms produce offspring by a process of sexual reproduction, inwhich a gamete from one parent joins a gamete from the other parent to form azygote (or fertilized egg). This process results in offspring that have acombination of parental chromosomes.
However, each generation of a species retains the same chromosome number asthe preceding generations. Meiosis is the process that ensures that each newgeneration has the same chromosome number as the preceding generation.
Meiosis is a process that reduces chromosome number by half and occurs at justone stage in an organism's life cycle (to form gametes in animals, or to start thegamete producing stage in plants, or for some organisms to restore theappropriate chromosome number for the assimilative stage of its life history).Sexual reproduction then restores the "typical" number of chromosomes forthe next generation. In this context, we need to look not just at the process ofmeiosis, but also take a look at the sexual life cycles of organisms.
In addition to providing a mechanism to reduce chromosome number for sexualreproduction, meiosis has a second, most important function for living organisms:maintaining genetic variation. Each time meiosis occurs, followed by, at somepoint, sexual reproduction, the new individual is genetically different from eitherparent. Because meiosis is involved with genetic variation and is needed for sexualreproduction, we will discuss this important genetic function of meiosis as well asits chromosome reduction function in this section.
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So how does meiosis work?To answer the question of how meiosis works, we need to first revisit thechromosome. If we look at the chromosomes of most eukaryotic organismscarefully, it can be seen that for each individual chromosome, a secondchromosome can be found that physically matches it in length and shape. This isbest seen with the karyotype. Closer inspection of the DNA shows that thematching chromosomes have very similar, but not identical DNA. These matchingchromosomes, with their similar DNA, form the basis of the variation we see in thegenetic traits of living organisms, as well as being a way that we can reducechromosome number during meiosis and still have the appropriate geneticinformation in each cell. The matching chromosome pairs are called homologouschromosome pairs, or homologues (homologs).
Cells that contain pairs of homologous chromosomes are called diploid (2n).When a cell has chromosome pairs, we refer to the diploid number ofchromosomes, again meaning that each chromosome has a match, or homologue inthat cell. For humans, the diploid number of chromosomes is 46. Again, these 46chromosomes are comprised of 23 homologous pairs of chromosomes.
Following meiosis, the product cells will not have pairs of chromosomes; there willbe one of each pair in each cell formed, so the chromosome number will have beenreduced by half, and is said to be haploid (n).
By the way, "ploid" as a general term also means a "set", so we can also say that adiploid cell has two sets of chromosomes, or two of each kind of chromosome. Ahaploid cell has one set of chromosomes, or one of each kind. (It's possible to havemore than 2 chromosomes of each kind. Polyploids are quite common inagriculture as a result of plant breeding. Polyploids are less common in animals.)
For many sexually reproducing organisms, one homologous pair of chromosomesdoes not precisely match in size in one gender, but does match in the other gender.This is the pair of sex-determining chromosomes, or the sex chromosomes. Theother pairs of chromosomes do match and are called autosomes. Humans have22 pairs of autosomes and 1 pair of sex chromosomes.
Meiosis and the Life Cycles of OrganismsJust as each cell has a cell cycle, organisms have a life cycle. For most, the lifecycle includes sexual reproduction. Meiosis is something that takes place atjust one point in any sexually reproducing organism's life cycle. Meiosisalways reduces the chromosome number (typically from diploid to haploid).
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In animals, meiosis generally occurs to form gametes: sperm or eggs. Egg andsperm are the only haploid cells of animals. In many other types of organisms,meiosis occurs at some point in the life cycle other than the direct formation ofgametes, and the products of meiosis may be spores, (as in plants) or the firstcells of the next generation (for most protists and most fungi). At some pointhowever, all organisms that sexually reproduce will make haploid gametes (spermand egg, or different genetic mating types).
Similarly, fertilization occurs at one point in an organism's life history.Fertilization occurs between two different haploid cells, called gametes, to formthe zygote, or fertilized egg. The zygote obtains half its chromosomes from thesperm and half from the egg (or half from one gamete and half from the second;yeasts, for example, have a and gametes, not sperm and egg).
Fusion of gametes restores the diploid number, and in so doing, also restoreshomologous chromosomes (one of each kind being provided by the sperm and oneof each kind coming from the egg). Since each gamete has a unique combination ofchromosomes, each zygote will be unique, and genetic variation is both maintainedand obtained within the species.
Yet, as mentioned earlier, when meiosis occurs in the life cycle of organisms is notalways the same. Let's compare three typical life cycle patterns and look at thetiming of meiosis in the life cycle of each.
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Diploid Life Cycle In animals, meiosis generally produces just haploid sex cells, or gametes, which
at fertilization start the next generation. The only haploid cells of the animalare egg or sperm, and the respective maturation processes are calledoogenesis and spermatogenesis.
When gametes fuse, the zygote grows by mitosis producing the adult stage. Allcells will be diploid and all cells are produced by mitosis. The animal life cycle isa diploid life cycle.
Haploid Life Cycle In many protists, and some fungi, haploid gametes fuse to form a zygote, but
the zygote immediately does meiosis forming single haploid cells. In protists, which remain single cell organisms, the nucleus is then haploid. At some time, a single cell may just decide to become a gamete and fuse with
another to make a zygote, or haploid cells may do mitosis to make moreindividuals asexually.
Haploid life cycles can be more complex. Fungi, and some algae may makemulticellular haploid organisms from the single-celled meiotic product bymitosis. At some time, special areas of the haploid body will become gamete-making structures (often called gametangia), and haploid gametes are formedby mitosis.
Alternation of Generations Most plants have both a multicellular haploid stage and a multicellular diploid
stage in their life histories. This is called the alternation of generations. (Sincewe are humans (and animals), we just haven't had the opportunity to becomeacquainted with these different forms of a plant's life.)
In plants, the structure in which meiosis occurs is called a sporangium. Themulticellular diploid plants, or parts of plants, that produce sporangia are calledsporophytes (spore-making plant).
Meiosis does not directly produce gametes, but produces haploid cells, calledspores that in turn, grow, by mitosis, into multicellular haploid structurescalled gametophytes (gamete-making plant). Gametophytes eventuallyproduce and contain gametes.
Which stage (sporophyte or gametophyte) is predominant in the life of a plantvaries with different types of plants. Most "higher" plants have predominantsporophytes. A pollen grain of pine tree or flower, for example is the malegametophyte stage of those plants. Mosses, in contrast have predominantgametophyte stages. It is easiest to see sporangia and spores on ferns. Thefern plant we know (and love) is the diploid sporophyte generation. Thesporangia are located on the undersides of leaves. Once one knows what a ferngametophyte looks like, they can often be spotted growing in pots or on thesurfaces of pots in greenhouses.
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On to the Process of MeiosisRemember, the purposed of meiosis is to reduc