Ribonucleic acid or RNA

RNA (ribonucleic acid) is the hereditary material in some viruses (retroviruses). It is a polymer of ribonucleotides and is made up of ribose sugar, phosphoric acid and nitrogenous bases (A, U, C, G).

Chemical Composition of RNA

The chemical composition of RNA differs from DNA in two respects:

  1. The pentose sugar in RNA is ribose sugar while it is deoxyribose in DNA. 
  2. Thymine in DNA is replaced by Uracil in RNA. All other nitrogenous bases are the same in both.

Types of RNA

On the basis of molecular size and function, three main forms of non-genetic RNA are:

  1. mRNA
  2. tRNA
  3. rRNA

mRNA constitutes about 3.5% of cellular RNA, tRNA is about 15% and rRNA is about 80%.

  1. mRNA (Messenger ribonucleic acid) or informational RNA or template RNA is a molecule of RNA that is transcribed from a gene and then translated by ribosomes in order to manufacture protein.
    • When a particular gene codes for an mRNA strand, it is said to be monocistronic or monogenic, e.g., eukaryotes.
    • When several genes (cistrons) are transcribed into a single mRNA molecule, it is described as polycistronic or polygenic, e.g., prokaryotes.
  2. tRNA (transfer RNA) is also known as soluble(s) RNA, adaptor RNA. Holly proposed a two-dimensional structure of tRNA in 1965, which consists of an AA-binding site (lies opposite to the anticodon site.
    • tRNA molecules have unpaired CCA-OH sequence at the 3’ end), T ψ C  loop (contains pseudouridine, the site foe attaching to ribosomes), DHU loop (contains dihydrouridine, the binding site for aminoacyl synthase enzyme), extra arm (variable site or loop which lies between T ψ C  loop and anticodon) and anticodon loop (made up of three nitrogen bases for recognizing and attaching to the codon of mRNA).
  3. rRNA (ribosomal RNA) is a component of the ribosomes, the protein synthetic factories in the cell. It is formed in the nucleolus.
    • Depending upon sedimentation coefficient eukaryotic ribosomes contain different rRNA molecules 28s, 5.8s, and 5s in eukaryotes and 23s and 5s in prokaryotes occur in a larger subunit of ribosomes while 18s in eukaryotes and 16s in prokaryotes are found in the smaller subunit of ribosomes.