Structure of DNA

Chemical Nature of DNA

DNA (Deoxyribonucleic acid) is a polynucleotide, a macromolecule made of units called nucleotides. Each nucleotide consists of three sub units:

  1. a pentose (5 carbon) sugar called deoxyribose
  2. 4 nitrogenous bases: Adenine (A), and Guanine (G) are purine bases and Thymine (T) and Cytosine (C) are pyrimidine bases
  3. a phosphate group (PO4) positioned on the sugar

A base and a sugar combine to form a nucleoside, while it becomes a nucleotide when a phosphate group gets attached to the nucleoside.

Base + sugar = nucleoside

Base + sugar + Phosphate = nucleotide

So there are four nucleotides in DNA formed of sugar and nitrogenous base and phosphate.

Chargaff’s rule

The four nucleotides are not present in equal amounts in a DNA molecule. But the amount of purines (A + G) and that of pyrimidines (T + C) is always equal.

A = T and G = C

Physical Structure of DNA: DNA Double Helix

A DNA molecule is three dimensional and made of two strands helically coiled around each other. Franklin and Wilkins first showed through X-ray diffraction studies of DNA that it is a double helix.

In 1953, James Watson and Francis Crick were awarded the Nobel Prize for working out the structure of DNA. According to the Watson and Crick model:

  • DNA molecule is a double helix consisting of two strands of DNA
  • The arrangement of the two strands is antiparallel, which means that the sequence of nucleotides goes up in 5′to 3′ direction in one strand and other strand comes down in 3′ to 5′ direction.
  • The backbone of the helix is made of sugar and phosphate. Nitrogenous bases are linked to the sugar.
  •  The bases of the two strands are linked by hydrogen bonds.
  • Base pairing is very specific as per Chargaff’s rule. Adenine, a purine base always pairs with thymine, a pyrimdine base. The purine base Guanine pairs with the pyrimidine, Cytosine. These pairs of bases are called complementary bases.

There are two hydrogen bonds between A and T and three hydrogen bonds between G and C. A and T are complementary bases and so are G and C.

In the DNA helix, a complete helical turn occurs after 3.4 nm (or 34Å). This complete turn encloses 10 base pairs. Each base pair lies 0.34 nm (3.4 Å) apart. The diameter of the double helical DNA molecule is 2.0 nm.