Unit – V A

Structure of RNA

Differences between RNA molecules and DNA molecule:

  1. RNA contains ribose sugar units while DNA contains deoxyribose sugar unit.
  2. RNA contains the base uracil while DNA contains thymine.
  3. RNA is single stranded, except in some viruses while DNA is double stranded structure.
  4. RNA molecules are much smaller than DNA molecules.
  5. RNA is mainly formed in the cytoplasm while DNA is mainly found in the nucleus.
  6. In RNA the ratio of nitrogen bases Purine to Pyrimidine may not be 1:1 but in DNA it is always 1:1.

RNA and its Classification:

  • RNA is a nucleic acid found in nucleus ad well as in the cytoplasm. They are further classified into a) genetic RNA and b) Non-genetic RNA. The genetic RNA acts as genetic material in case of some viruses. Non-genetic RNA is responsible for protein synthesis.
  • RNA was the first genetic material. There is now enough evidence to suggest that essential life processes such as metabolism, translation, splicing, etc.), evolved around RNA. RNA used to act as a genetic material as well as a catalyst. RNA being a catalyst was reactive and hence unstable. Therefore, DNA has evolved from RNA with chemical modifications that make it more stable. DNA being double stranded and having complementary strand further resists changes by evolving a process of repair

Structure of RNA


Generally, RNA is single-stranded structure. It may be simple and straight or folded or coiled upon itself

Chemically it is polynucleotide molecule.

Its components are ribose sugar, phosphate as phosphoric acid and nitrogen bases Purines and Pyrimidines.

Purines are Adenine (A) and Guanine (G), while pyrimidines are Uracil (U) and Cytosine (C).

In RNA molecule nucleotides are joined by a phosphodiester linkage.

RNA strand has 3′ and 5′ ends. The base pairing occurs in complementary bases only (A = U) and (G ≡ C)

In RNA molecule the base pairing is not fixed and definite, hence the ratio of nitrogen bases purine to pyridine may not be 1:1

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