Posted by Cassandra Brooks
[Entry posted at 4th February 2010 07:13 PM GMT]
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Repeat untranscribed DNA sequences are generally thought to be genetic junk at best, harmful at worst, but in ribosomes they are essential to repairing DNA damage, according to a study published this week in Science.
Ribosomal DNA (rDNA) codes for the RNA that makes up a major component of ribosomes, the site of protein synthesis. Ribosomal RNA (rRNA) comprises 80% of the RNA found in a typical cell and is highly conserved from bacteria to humans. Virtually every species has a different number of these untranscribed repeats in their rDNA from only a few in bacteria to thousands in plants and animals. Why these repeat copies were present in the cell was not known, but now researchers at the National Institute of Genetics (NIG) in Mishima, Japan, have shown that these repeat arrays are critical in two processes that facilitate DNA repair.
"For the first time, this study suggests a possible reason for this rather special organization of the rDNA," said Frank Uhlmann, a molecular biologist with Cancer Research UK who was not involved in the study. "And it will surely prompt further investigation."
Research led by Satoru Ide at NIG compared a wildtype yeast strain with 110 repeats to mutant strains with 80, 40 and 20 repeats. They exposed all strains to two mutagens, methyl methanesulfonate (MMS) and ultraviolet radiation (UV), and found that the lower copy strains were much more sensitive to DNA damage.
"For the first time, this study suggests a possible reason for this rather special organization of the rDNA," said Frank Uhlmann, a molecular biologist with Cancer Research UK who was not involved in the study. "And it will surely prompt further investigation."
Research led by Satoru Ide at NIG compared a wildtype yeast strain with 110 repeats to mutant strains with 80, 40 and 20 repeats. They exposed all strains to two mutagens, methyl methanesulfonate (MMS) and ultraviolet radiation (UV), and found that the lower copy strains were much more sensitive to DNA damage.
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Read more here/Leia mais aqui: The Scientist
Vol. 327. no. 5966, pp. 693 - 696
DOI: 10.1126/science.1179044
The ribosomal RNA (rDNA) gene repeats are essential housekeeping genes found in all organisms. A gene amplification system maintains large cluster(s) of tandemly repeated copies in the chromosome, with each species having a specific number of copies. Yeast has many untranscribed rDNA copies (extra copies), and we found that when they are lost, the cells become sensitive to DNA damage induced by mutagens. We show that this sensitivity is dependent on rDNA transcriptional activity, which interferes with cohesion between rDNA loci of sister chromatids. The extra rDNA copies facilitate condensin association and sister-chromatid cohesion, thereby facilitating recombinational repair. These results suggest that high concentrations of heavily transcribed genes are toxic to the cells, and therefore amplified genes, such as rDNA, have evolved.
1 National Institute of Genetics, Sokendai, 1111 Yata, Mishima, 411-8540 Japan.
2 Graduate University for Advanced Studies, Sokendai, 1111 Yata, Mishima, 411-8540 Japan.
3 Department of Molecular Biology, Graduate School of Biological Sciences, Nara Institute of Science and Technology, Ikoma, Nara 630-0101, Japan.
* To whom correspondence should be addressed. E-mail: takobaya@lab.nig.ac.jp
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NOTA DESTE BLOGGER:
O DNA 'lixo' nunca foi considerado 'lixo' pelos teóricos e proponentes da teoria do Design Inteligente: sempre achamos que deveria ter uma função (telos) que não sabíamos. É, parece que a pesquisa aí dos japocas corrobora mais o DI do que o acaso, necessidade, mais mutações filtradas pela seleção natural (e múltiplos outros mecanismos evolutivos) ao longo de bilhões de anos, y otras cositas mais.
