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Meiosis and Development: Cutting the Ties That Bind (Article Critique Sample)


Meiosis and development: cutting the ties that bind

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Title: Chapter 3 Meiosis and Development
Subtitle: Meiosis: Cutting the Ties That Bind
Meiosis is a special type of reproductive cell division which occurs only in the generation of thegametes or germ cells that are responsible for fertilisation and hence formation of new offspring. Meiotic celldivision reduces (halves) thechromosomal content and hence the production of four haploid cells. The overallprocess of germ cell development is called "gametogenesis” which is oogenesis and spermatogenesis andincludes not only meiosis but alsothe cellular changes that occurdifferently in male and female germ cells.
The development of a new organismfrom the joining of two single cells isa complex and carefully orchestrated process. But even before sperm meets egg, anequally elaborate set ofchoreographed steps must occur toensure successful fertilization and therefore resulting in successful sexualreproduction that will lead to formation of the offspring. Those steps, known asreproductive cell division or meiosis, split the original number of chromosomes in half so that offspring will inherit half their genetic material from one parent and half from theother. During this process, each set of homologous chromosomes pair up in a kind of chromosomal square dance a process called synapses that will lead crossing over and hence variation in secies. Chromosome 1with chromosome 1, chromosome 2 with chromosome 2, and so on down the line. The two gamete cells stick together, dancing through the phases of meiosis both meiosis I and meiosis II until it is time to segregate or separate to opposite ends of the dividing cell. When the chromosomes don’t pair or part appropriately it can result in eggs and sperm with the wrong number of chromosomes, a major cause of miscarriage and birth defects. In order to avoid these mistakes, most chromosomes use a process that allows exchange of genetic information to take place, a process known as crossing over. In this case chromosomes loop their arms with their partners and even swapping pieces of genetic material to stick together until the dance (crossing over) is over. A few chromosomes, like chromosome 4 in the fruit fly Drosophila melanogaster, are too short to make these crossovers and hence the process of crossing over is not suitable for all species of living organisms. This species have somehow, figured out another way to stay connected to their partners.
Stacie E. Hughes, Ph.D., a research specialist II at the StowersInstitute for Medical Research, identified the threads of DNA that seemed to tie these other homologous chromosomes together in pairs. The major objection that still remained was: once these chromosomes are roped into pairs, how do they manage to come apart again and segregate into independent chromosomes? This lead to a research by two scientists; Hughes and R. Scott Hawley, Ph.D., who showed that an enzyme calledTopoisomerase II is required for resolvingthese threads so homologous chromosomes can part ways and become independent once again. The finding, published in the October 23, 2014 issue of Polygenetic, explains the complexity of the meiotic process."It is not surprising there are many ways to segregate chromosomes because there are also many ways to control other molecular events, like gene expression," says Hawley, a Stowers Institute investigator. "This method of segregating shorter chromosomes may be odd and no canonical as it gets, but it does not matter because cells finally survive. since Hughes' initial discovery of DNAthreads, she and Hawley have been looking for the molecular scissors which functions to cut entangled chromosomes free without leading to programmed cell death (apoptosis). This lead to the discovery of Topoisomerase II, an enzyme which cuts and untwist tangled strands of the double helix strands of chromosomes. Previous research showed thatTopoisomerase II was involved in earlier cellular processes like DNAelongation and replication, and the enzyme persisted even during later phases of meiosis. The researchers thought that Topoisomerase II might be waiting around to do yet another job, like cutting DNA threads to allow homologous chromosomes to segregate. The researchers tested their hypothesis relatively straightforward. The researchers simply needed to remove Topoisomerase II in their specimen the Drosophila melanogasterand then look to see whether meiosis was able to proceed normally without it. However, because the enzyme was involved in so many critical cellular processes, the researchers knew that an approach like that would yield nothing more than dead fruit flies. They therefore adapted a sophisticated method known as RNA interference which uses small pieces of RNA to silence genes and eliminate Topoisomerase II at a specific time point late in meiotic process. Hughes then isolated the oocytes from the fruit flies and analyzed them using fluorescent tags that illuminate the DNA threads co...
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