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Dna Sequences Occurs At Many Scales Within Genomes Discussion

2417 words - 10 pages

Today it is widely believed that there are two fundamental ways in which genomes evolve; namely evolution by (1) duplication of pre-existing regions of DNA within the genome and (2) lateral gene transfer. (Brown, 2002), (Zhaxybayeva & Doolittle, 2011). The focus of this essay will be on DNA duplication, its occurrence, and it’s consequences in genomes at a molecular and organismal level. DNA duplication refers to the process by which a region of DNA already present in an organism’s genome is duplicated in that organism. The duplication of DNA has long been considered an important factor in the evolution of genomes. (Taylor & Raes, 2004) Although the duplication of DNA itself does not directly cause gene diversification and thus genome evolution, it does provide the potential for gene diversification to occur. It is true that DNA duplication occurs at many scales within genomes. DNA duplication can occur at the chromosomal level, where an extra copy of a particular chromosome becomes present in an organism. Chromosomal duplication may arise as a result of stress - some organisms’ duplicate genes in response to a particular environment. (Sheltzer et al, 2012) Chromosomal duplication offers a quick way for some organisms to adapt to a stressful environment, but can be be costly for a cell because of results in changes to normal cell metabolism as well as increased energy expenditure. (Yona et al, 2012) In yeast it’s apparent that chromosomal duplication occurs as a quick response to stress, but is slowly replaced by more efficient solutions through gene silencing or deletion. Chromosomal duplication can happen during meiosis if homologous chromosomes fail to separate during meiosis. Similarly, aneuploidy can also arise if sister chromatids do not separate in mitosis. (Griffiths et al, 2000) However, such large-scale duplication is rarely associated with evolution in in higher eukaryotes, due to the tendency of cells not to tolerate an additional chromosome. This can be due to the “gene dosage effect” whereby an extra copy of a chromosome leads to overexpression of some proteins, often disturbing the required protein levels necessary for an organisms’ development. (Clancy & Shaw, 2008) In humans, generally only individuals with trisomy 18, 16 and 21 ever develop to term, albeit with significant abnormalities. (Griffiths et al, 2000) Humans born with Trisomy 18 (Edward's syndrome) and Trisomy 13 (Pateu syndrome) are born with severe physical defects and usually do not survive past their first year. Those who are born with trisomy 21 (Down’s syndrome) suffer from some degree of mental impairment, poor muscle tone and poor growth amongst other traits. Furthermore, most males with Down’s syndrome are infertile, further limiting evolutionary potential.
Most Prokaryotes have a single copy of their chromosome. (Thanbichler and Shapiro, 2006) Despite this, some function well with chromosomal duplications. In the context of most Prokaryotes however, a...

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