|
|
This is only a preview of the paper
Click here to register and get the full text.
Existing members click here to login
|
|
|
The Webster's dictionary defines meiosis as: the cellular process that results in the number of chromosomes in gamete-producing cells being reduced to one half and that involves a reduction division in which one of each pair of homologous chromosomes passes to each daughter cell and a mitotic division. Nothing like fitting an intricate, complicated process into one sentence, I am sure glad I do not have that job! The point I am trying to make is that meiosis deserves more than a one sentence explanation. In order to understand meiosis one must first know that there are two parts to meiosis. There is meiosis I and meiosis II. In meiosis I there are four phases; Prophase I, Metapase I, Anaphase I, and Telophase I. In Prophase I, homologous chromosomes pair and form synapses, a step unique to meiosis (this means it does not happen in mitosis). The paired chromosomes are called bivalents (in our book the author calls them tetrads), and the formation of chiasmata caused by genetic recombination becomes apparent. The genetic recombination (also called crossing over) is when the non-sister chromatids of the paired chromosomes do some swapping of parts. A good example to explain this phase would be to imagine the little plastic colored easter eggs that are around during easter. Each egg has two halves that make a whole and they are all different colors. Now, take a blue egg and and orange egg, put them side by side (non-sister chromatids) then take one half of the blue egg and attatch it to the orange egg and vice versa with the orange egg and you now have a genetic recombination of easter eggs. In Metaphase I, the bivalents (or tetrads) , each composed of two chromosomes (four chromatids) align at the metaphase plate. The orientation is random, with either parental homologue on a side. This means that there is a 50-50 chance for the daughter cells to get either the mother's or father's homologue for each chromosome. Just think that the chance of getting a certain trait or hair color from either our mother or father depends upon chance at the metaphase plate. Like it says in our book: it is a throw of the dice as to which side of the plate a certain chromosome ends up on. The way I like to look at this would be to imagine two people sitting across from eachother each with opposite ends of a magnet, whatever side the north(maternal chromosome) end of the magnet ends on, the south(paternal chromosome) side will be on the other side and vice versa. In Anaphase I, the paired homologous chromosomes now start to move apart from eachother by microtubule spindles to their respective poles. Each of the daughter cels is now a haploid (containing 23 chromosomes instead of 46) but each chromosome stil has two chromatids. Now imagine the magnets that are connected at the metaphase plate like I mentioned earlier. Imagine taking some string and attatching it to each side of the magnet that is together the string representing the spindles. Once this is done the homologous chromosomes (magnets) are puled apart and up to their respective poles with the sister chromatids still staying together. Last but not least is the Telophase I phase. This is where once the sister chromatids have reached their respective poles, the cell now undergoes cytokinesis dividing into two different and seperate daughter cells. This would be like putting the half blue and orange egg at one pole and the other half blue and orange egg at the other pole and then putting the magnets (sister chromatids) into the eggs (daughter cells) to form two seperate daughter cells. The next four phases are a part of the Meiosis II phase. The phases are: Prophase II, Metaphase II, Anaphase II, and Telophase II. There is little pause between Meiosis I and Meiosis II. The difference this time is that the chromatids of each chromosome are no longer identical because of recombination. All the phases are the same except that instead of being homologous chromosomes being lined up and seperated, they are sister chromatids. When cytokeninesis occurs for the second time, there are four haploid cells instead of two. In Telophase II, the chromosomes, which had doubled prior to Meiosis I, re-condense but do not pair since there is no homologue.
Approximate Word count = 2864
Approximate Pages = 11.5
(250 words per page double spaced)
|
|
|
|
|
|