The Basic Structure of Nucleic Acids (DNA and RNA) are the complex informational macro-molecules present in all living cells and viruses. They carry genetic information about the cell.

Nucleic Acids Structure

Each nucleotide unit comprising the structure of nucleic acids consists of:

–  a 5 – Carbon Sugar

–  a Nitrogenous base attached to the sugar and

–  a Phosphate group

Carbon sugar is Deoxyribose sugar in DNA and Ribose sugar in the case of RNA.

 

Deoxyribose sugar has a Hydrogen(H) attached to the 2nd Carbon instead of Hydroxyl(-OH) in Ribose sugar.

The Nitrogenous base attached to the sugar are Purines or Pyrimidines.

A purine is a heterocyclic aromatic organic compound which has a pyrimidine ring attached to an imidazole ring. The derivatives of purines that appear in nucleic acids are Adenine(A) and Guanine(G). Other derivatives of purines are found – hypoxanthine, xanthine, theobromine, caffeine, uric acid, and isoguanine.

Pyrimidine is an aromatic heterocyclic compound similar to pyridine, which consists of six-membered heterocyclics with two nitrogen atoms at positions 1 and 3 in the ring. Pyrimidine nucleobases found in nucleic acids are Cytosine(C), Thymine(T), and Uracil(U). Pyrimidine also consists of other diazenes – Pyrazine and Pyridazine.

The Nitrogenous base covalently attached to the C1 position of sugar is referred to as Nucleoside. Nucleoside lacks a Phosphate group.

Formation of Nucleotide

A phosphodiester bond joins together the 5′ Carbon sugar, Nitrogenous base, and Phosphate group to form the nucleotide sequence (polymer called polynucleotide). The hydroxyl groups on 5′ & 3′ carbon link the phosphate group and an alternating sequence of sugar and phosphate is formed.

In the nucleotide sequence, one end is referred to as the 5′ end whereas the other as 3′ end. The end at which the third carbon of sugar is free (i.e not linked to any phosphate group) is the 3′ end while at the 5′ end the fifth carbon is free.

The nitrogenous bases (Purines and Pyrimidines have a unique property to form hydrogen-bonded pairs composed of one purine and the other pyrimidine molecule such as Guanine –  Cytosine, Adenine – Thymine.

In DNA, the Adenine of one chain is linked with the Thymine of another chain by two hydrogen bonds. Similarly, the Guanine of one chain is linked to the cytosine of another nucleotide chain by three hydrogen bonds.  The underlying reason for this property of nitrogenous bases is the right spatial interaction for stable base pairing between the purines and pyrimidines.

In RNA, the pyrimidine base Thymine is replaced by Uracil, which links Adenine with two hydrogen bonds. Adenine, Cytosine and Guanine are found in both DNA and RNA. Thymine is found only in DNA whereas Uracil is found only in RNA.

This property gives DNA its double-stranded nature. Mostly, the DNA is double-stranded except for some viruses which possess a single strand of DNA as their genome. The two strands of DNA are complementary and antiparallel to each other.

RNA is usually single-stranded but in some cases may fold back upon itself to form double helical regions.

Structure of DNA

James Watson and Francis Crick in 1953, proposed the double helix model for DNA utilizing x-ray diffraction data from the DNA crystals. It is the only universally accepted model of DNA to date.

 

      The model predicted that DNA exists as a helix of two complementary anti-parallel strands, wound around each other in a rightward direction and held together by H-bonding between the bases in adjacent strands.
       In this model, the bases are in the interior of the helix aligned at nearly 90 degrees angle relative to the axis of the helix. On the exterior, the double helix contains two deep grooves between ribose-phosphate chains.
       The grooves being of unequal sizes are termed major and minor grooves. This difference is due to the asymmetry of the deoxyribose rings and the structurally distinct nature of the upper surface of the base pair relative to the bottom of the surface.

       Watson and Crick concluded that the length of each complete twist of the helical molecule is 34nm i.e. there are 10 base pairs of 0.34 nm in each helical turn. DNA has a diameter of about 2nm.

     The structure of DNA under normal conditions is found in B form, which is the double-helical structure with 10 base pairs every 34nm. At high salt and alcoholic concentrations, the shape gets distorted to form Alpha and Z-forms.

       The base pair composition of DNA from cells of all organisms should have equality in quantity between Purines and Pyrimidines. Therefore Adenine(A) is always equal to Thymine(T) and Cytosine(C) is always equal to Guanine(G) in composition (Chargaff’s Rule)