RNA (or ribonucleic acid)
RNA (or ribonucleic acid) is a single stranded structure which consists of uracil as a nitrogenous base instead of thymine and ribose sugar.
RNA is also the genetic material of some of the viruses, such as: Tobacco Mosaic viruses, QB bacteriophage, etc. Also, in all organisms RNA performs the dynamic functions of messenger and adapter. Most of the essential life processes (such as: metabolism, splicing, translation, etc.) are performed by RNA, but still DNA is considered as the genetic material instead of RNA because of the following reasons.
For being genetic material, a molecule must have to fulfill the following criteria:
(i) It should be able to generate (or make) its replica.
(ii) It should always be chemically and structurally stable.
(iii) It should provide scope for slow changes (or mutation) essential for evolution.
(iv) It should be capable to express itself in the form of ‘Mendelian Characters’.
On examining all these criteria one by one, it is found that:
> On the basis of the rule of base pairing and complementarity both DNA and RNA have the capability to make their replica (or perform duplications). While, proteins fail to fulfill this criteria (or first criteria).
> The two strands of DNA being complementary to each other, if denatured (or separated by heating) come together (or renature) when favourable conditions are returned back.
> Moreover, due to the presence of 2′-OH group at every nucleotide of RNA is a reactive group and this makes RNA labile and easily degradable. Meanwhile, RNA is also known to be catalytic in nature, therefore it is reactive.
> DNA is chemically less reactive and structurally more stable than that of RNA. Therefore, DNA is chemically and structurally more stable. Thus, DNA fulfils the second criteria. Also, the presence of thymine at the position of uracil in DNA provides additional stability to DNA.
> Both DNA and RNA are able to perform mutation. Since, RNA is unstable, so it mutates at a faster rate. As a result, viruses having RNA genome and have shorter life span. They mutate and evolve faster.
> RNA directly codes for the synthesis of proteins, hence it can very easily express characters. Instead, DNA is dependent upon RNA for synthesis of proteins. Thus, protein synthesising machinery has evolved around RNA.
Hence, except the stability criteria both DNA and RNA can function as genetic material, but DNA being more stable is preferred most for storing genetic information. But, for the transmission of genetic information, RNA is most preferable.
Therefore, with the exception of certain viruses, DNA rather than RNA carries the hereditary genetic code in every forms of biological life on Earth.
DNA is also more resilient and more easily get repaired than RNA. As a result, DNA serves as a more stable carrier of the genetic information which is very essential for survival and reproduction in every living forms.
Thus, this is the only reason why DNA is considered as a genetic material instead of RNA and protein.