Ribonucleotide polymerisation

RNA consists of long strings of ribonucleotides, polymerised in a similar way to DNA, but the chains are considerably shorter than those of DNA. RNA contains ribose rather than deoxyribose and also contains uracil instead of thymidine. This has important connotations in the secondary structure of RNA which does not form the long helices found in DNA. RNA is usually much more abundant than DNA in the cell and its concentration varies according to cell activity and growth. This is because RNA has several roles in protein synthesis. There are three major classes messenger RNA (mRNA) ribosomal RNA (rRNA) and transfer RNA (tRNA). 

The somewhat unexpected efficiency of a small RNA enzyme, only five nucleotides in length, suggests that many different reactions could be accelerated by small RNAs and even be operative after the most primitive ribonucleotide polymerisation had occurred [111]. The evolution of small entities into very specific and efficient RNA enzymes is considered in terms of effects on the transition states of reactions. There are many RNAs that bind amino acids and this may be the origin of catalysers of peptide bond formation. 

Figure 20.5 Nucleotides that are required for RNA or DNA synthesis. Note that the ribonucleotide diphosphates are the precursors for the formation of deoxyribonucleotides. It is the triphosphates that are required for polymerisation to form either RNA or DNA (see text).

A further problem is the lack of a mechanism for initiating polymerisation. DNA duplication must always start from an RNA primer, i.e. a short sequence of polymerised ribonucleotides. The sequence of the primer is achieved through base pairing with the short sequence... 

As much of the terminology used in molecular biology may be unfamiliar to some readers, it is appropriate to define some of the vocabulary and this is given in an appendix to this chapter. There are two types of nucleic acids, the ribonucleic acids (RNA) and the deoxyribonucleic acids (DNA). Genetic information is carried in the linear sequence of nucleotides in DNA. Each molecule of DNA contains two complementary strands of deoxyribonucleotides which contain the purine bases, adenine and guanine and the pyrimidines, cytosine and thymine. RNA is single-stranded, being composed of a linear sequence of ribonucleotides the bases are the same as in DNA with the exception that thymine is replaced by the closely related base uracil. DNA replication occurs by the polymerisation of a new complementary strand on to each of the old strands. 

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