Retrotransposons are commonly found within the brain causing changes in gene expression which ultimately leads to differences in the function of neurons and human behaviours (Singer et. al, 2010). Long interspersed repeated sequences (LINE-1 or L1 elements) are a class of retrotransposons that make up about 20% of mammalian genomic DNA, although only about 150 of these elements are thought to be capable of activity (Singer et. al, 2010). These L1 elements move about during the development of the central nervous system as well as during adult neurogenesis and take place individually in certain cells (Singer et. al, 2010). This means that some cells may contain the retrotransposons while other cells next to them are void of the elements. The effects L1 elements have on the regulation of genes depends on where they are inserted in the gene (Singer et. al, 2010). Singer et. al (2010) found that L1 elements placed in the sense strand resulted in a decreased rate of transcription by that specific gene as compared to the antisense strand where no change occurred. The activation and inactivation of L1 elements also influences regulation of neuronal genes which is demonstrated through the diversity of behaviours of organisms (Singer et. al, 2010).
L1 elements possess the needed promoters for transcription, are affected by epigenetics, and are turned on and off at different times. Each of these situations has an effect on the expression of the gene containing the retrotransposon, and in some cases, other genes surrounding this one may also be affected (Singer et. al, 2010). Although retrotransposons are generally shorter sequences, they are still able to start transcription within a gene which then influences activity further along the pathway, possibly triggering other genes (Singer et. al, 2010). Retrotransposons have also been found to be a target for DNA methylation and histone modification, where the retrotransposon is silenced in both scenarios (Singer et. al, 2010). If these elements are reactivated, other genes my actually experience changes in their expression (Singer et. al, 2010). Retrotransposons promote regulation of genes at their own timing which influences the other genes around them. When they are silenced, genes are able to transcribe normally at specific times in conjunction with the other genes around them. Once the L1 elements are activated, they interrupt this schedule causing changes in the levels of expression and in turn, differences in the functions of the genes (Singer et. al, 2010).
Without changing the genetic material of an organism, Singer et. al (2010) believes that L1 elements in neuronal cells may causes random variation in behavioural phenotypes. Not only do retrotransposons occur during the development of neurons and last into adulthood, but retrotransposons have the ability to insert themselves into many different locations within the genes (Singer et. al, 2010). This leaves lots of room for alterations to the expression and regulation of neuronal genes, resulting in multiple phenotypes from the same genome (Singer et. al, 2010). L1 elements are affected by environmental factors, either increasing or decreasing the number of retrotransposons that occur in the organism in order to better the chances of survival (Singer et. al, 2010). With this in mind retrotransposons have the ability to allow organisms to respond and change quickly to the environment.
Below is a video that provides some more information on transposons, specifically talking about LINES and SINES. It sheds some light on the relationships and ancestry between different animals and their connection through transposable elements.
(https://www.youtube.com/watch?v=_Ol492CLkdY)
Video Reference:
Wowcunning 2009, Evolution: genetic evidence - transposons, online video, viewed 1 May 2014, <https://www.youtube.com/watch?v=_Ol492CLkdY>.
Very eloquently written! Do you know what triggers a retrotransposon to copy and paste itself into the DNA? In the video, the narrator mentioned that LINEs and SINEs are useless to the genome and can sometimes cause significant damage. Any idea of when they could do this? When would retrotransposons be good for us? Nice post!
ReplyDeleteRetrotransposons seem to be triggered by RNA polymerase II or transposase enzymes. This causes reverse transcription to occur and begin the copy and pasting process. Retrotransposons are good when they produce new phenotypes that don't harm the host organism. In these cases retrotransposons create diversity between organism of the same species, potentially developing new traits that may be more beneficial than already existing ones. I tried to discuss LINEs and SINEs in this week's post, so hopefully that clears things up a bit.
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