Nucleic acid

Nucleic acid was first discovered by Friedrich Miescher and called as nuclein. Nucleic acid is phosphorus containing organic acid. It is of two type; ribonucleic acid and deoxyribonucleic acid.

Structure of nucleic acid

Nucleotide is a monomeric unit of nucleic acid. Hence, nucleic acid is also known as polynucleotides. A nucleotide has three components.

1. Nitrogenous bases
2. Five-carbon sugar, and
3. Ion of phosphoric acid

Components of nucleic acids

Nitrogenous bases

Nitrogenous bases are heterocyclic, water insoluble aromatic compounds. There are two types of nitrogenous bases in the nucleic acid.

• Purines, and
• Pyrimidines

Purines

Purines are of two types.

1. Adenine (A) (6 aminopurine)
2. Guanine (G) (6 oxy 2 aminopurine)

Pyrimidines

Pyrimidines are of three types.

1. Cytosine (C)
2. Thymine (T), and
3. Uracil (U)

Note: Cytosine and thymine are present in DNA, while cytosine and uracil are present in RNA

Sugars

5-carbon monosaccharide (pentose), riboses are found in the nucleic acid..

• In ribonucleic acid (RNA), ribose sugar is found.
• In deoxyribonucleic acid, 2' deoxyribose is found.

Note: Carbon of sugar are written with prime (') to differentiate them from the carbon present in nitrogenous bases.

Structure of nucleic acid

Structure of double stranded (ds) deoxyribonucleic acid (DNA)

Watson and Crick first described the structure of DNA double helix in 1953. In deciphering the structure of dsDNA, they have used the X-ray diffraction data of Franklin and Wilkins. For this discovery Watson, Crick and Wilkins were awarded 1962 Noble Prize of Medicine. dsDNA described was B-form of DNA.
In doing so, Watson and Crick utilized the principle of nitrogenous base pairing given by Erwin Chargaff. According to Chargaff's rule:

1. Guanine pairs with cytosine with 3 H-bond (G≡C) and adenine pairs with thymine (A=T).
2. The molar ratio of adenine and thymine is one (A/T=1) and molar ratio of guanine and cytosine (G/C=1). Thus, sum of adenine and guanine is equal to the sum of cytosine and thymine (A + G = C + T).

Structure of double stranded DNA

Features of B form of DNA (dsDNA)

• Two long polynucleotide strands coiled around a central axis.
• Strands are wrapped in right-handed helix.
• Strands are antiparallel:
  • One strand is oriented in 5' → 3' direction.
  • Other is in the 3' → 5' direction.
• Strand interact by hydrogen bonds between complementary base pairs.
• G forms 3 hydrogen bond with C (G≡C).
• A forms 2 hydrogen bond with T (A=T).
• Angle of interaction between base pairs result in Major and Minor grooves.
• Helix diameter is 20 Å.
• Helix rise per base pair is 3.4 Å.
• Length of helix pitch is 36 Å.
• 10.5 base pair are per helical turn.
• Base pairs are in the inside.

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First published on 13-04-2021
Last updated on 05-05-2021