|MadSci Network: Genetics|
There are several thoughts on why cells still retain "junk" DNA. It is only been recently that groups have actually started to prove that much of this supposed useless DNA actually does indeed play important roles in gene regulation and chromosome structure.
First, much of the "junk" DNA that doesn't code for protein is actually very important for gene structure and regulation. Promoters, regulatory regions, upstream and downstream untranslated regions, and introns, while not part of the coding region for proteins, are vitally important for the proper transcription, regulation, and translation of genes and proteins.
Secondly, the idea of epigenetics is rapidly becoming accepted as useful methods to regulate genes. "junk" DNA is thought to play an important role in this regulatory process by the formation of heterochromatin. A great deal of the genome is present in the form of heterochromatin, which is normally an inactive form of DNA that doesn't allow transcription of genes within it to occur. DNA which doesn't encode proteins could still be very important for the formation of heterochromatin and the regulation of genes via epigenetic effects.
Finally, this useless DNA also has been implicated in several structural roles within the nucleus. Repetitive sequences, like Alpha-sattelite DNA in humans, are thought to be vitally important for the formation of structures such as the centromere and telomeres within chromosomes. In addition, several other processes during mitosis and meiosis could involve non-coding DNA, such as chromosome compaction, X-inactivation, and sister chromatid pairing.
If you're interested, here are some mini-reviews concerning these processes:
Lewin, Benjamin, The Mystique of Epigenetics, Cell, Vol. 93, 301-303 Wiens and Sorger, Centromeric Chromatin and Epigenetic Effects in Kinetochore Assembly, Cell, Vol 93, 313-316Actually, as i look at these, they're all apparently from one volume of Cell that was dedicated to epigenetics. So, check out Vol. 93 of Cell for more information.
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