Met one question in the exam paper!

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CyLMH
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Met one question in the exam paper!

Post by CyLMH » Wed Nov 01, 2006 11:52 am

Why is DNA a better molecule than RNA for the purpose of long-term storage of genetic information?

Anybody can give a explanatory answer?

Thanks!!

druid
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Post by druid » Wed Nov 01, 2006 12:39 pm

1)
RNA uses ribose whereas DNA uses deoxyribose. Deoxyribose lacking -OH on 2-C atom cannot attack its own carbon backbone chain, thus rendering DNA more chemically stable than RNA and much greater lenghtes of DNA can be maintained without breakage.

2)
RNA's bases are C=G, A=U
DNA's bases are C=G, A=T

Suppose now, there's spontaneous deamination of Cytosine base
(deamination occurs at rate= 100 bases per cell per day, so enough rapid)
But Cytosine + H2O -> Uracil + NH3.
Thus, even mutation has occured, excision repair system (ERS) of the cell ( droven by AP glycosylases) will not detect it, because uracil is naturally occuring nucleotide.
Therefore, using Thymine (methylated uracil) secures that on deamination event the mutation will be detected by excision system.

3) The double helix is ideal for reparation

Molecular Biology of the Cell, 4rd ed, Alberts:
http://www.ncbi.nlm.nih.gov/entrez/quer ... on.827#834

The DNA Double Helix Is Readily Repaired

The double-helical structure of DNA is ideally suited for repair because it carries two separate copies of all the genetic information—one in each of its two strands. Thus, when one strand is damaged, the complementary strand retains an intact copy of the same information, and this copy is generally used to restore the correct nucleotide sequences to the damaged strand.

An indication of the importance of a double-stranded helix to the safe storage of genetic information is that all cells use it; only a few small viruses use single-stranded DNA or RNA as their genetic material. The types of repair processes described in this section cannot operate on such nucleic acids, and the chance of a permanent nucleotide change occurring in these single-stranded genomes of viruses is thus very high. It seems that only organisms with tiny genomes can afford to encode their genetic information in any molecule other than a DNA double helix.

Each cell contains multiple DNA repair systems, each with its own enzymes and preferences for the type of damage recognized. As we see in the rest of this section, most of these systems use the undamaged strand of the double helix as a template to repair the damaged strand.

CyLMH
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Post by CyLMH » Wed Nov 01, 2006 12:55 pm

^o^
Thanks a lot.
Really helpful!!

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