The Structure of RNA

(Cooper, 1997 p. ) 

Genetic information is stored in DNA - Information transfer carried out by RNA.
(Cell has 8-10 times more RNA at any one time than DNA)
The evolutionary choice of DNA as the information storage molecule may be reflected in the differences between DNA and RNA.

RNA Structure

 The nitrogenous base,Uracil, is used in place of thymine (Fig. 1) but it acts just like thymine in pairing with adenine .

 When RNA is formed using the DNA template (coding strand)

  • The A in the DNA is transcribed to a U in RNA
  • The T in the DNA is transcribed to an A in RNA
  • The G in the DNA is transcribed to a C in RNA
  • The C in the DNA is transcribed to a G in RNA
    • The use of Ribose in RNA instead of Deoxyribose (Fig. 2).

     In the chemical structure of ribose, at the 2' C there is a hydroxyl group instead of a hydrogen.

     The 2' hydroxyl is left free when ribonucleotides are linked to form RNA. This leaves RNA less chemically stable with a tendency for hydrolytic cleavage, especially in strong base (Fig. 3). In contrast, the key to the genetic code is stability over a long period of time, therefore DNA has the advantage.

    For same reason of instability, RNA has been more difficult to study than DNA.
     

     Differences between DNA and RNA:
    1.  Uracil, is used in place of thymine and Ribose instead of 2'-deoxyribose.
    2.  RNA is much shorter than DNA (recall that 1 DNA molecule spans the length of the entire chromosome.  RNA may span a few thousand bases of DNA.
    3.  With a few exceptions ( a few viruses) RNA occurs in single stranded form.

      4. The nitrogenous bases can form intra-strand hydrogen bonds with complementary base pairs (Fig. 4). This is important in forming the secondary structure of RNA where stem/loop or hairpin structures form because the chains fold back on themselves, creating loops and small base-paired stretches between complementary regions.

      One of the most remarkable and unexpected discoveries of recent years is that some folded RNAs have nucleolytic activity. The RNA enzymes, called ribozymes are able to cleave specific phosphodiester bonds in a manner analogous to protein enzymes.

    There are three basic classes of RNA
    Messenger RNA
    Transfer RNA
    Ribosomal RNA
    References:
    Cooper, Geoffrey M. (1997) The Cell: A Molecular Approach; ASM Press, Washington, D.C. / Sinauer Associates, Inc., Sunderland, MA.