02.6 Nucleic Acids

Nucleic acids are Polymers that exist as polynucleotides made up of monomers called nucleotides. They contain C, H, O, N and P. Nucleic acids are macromolecules and biopolymers. There are two types of Nucleic acids: DNA (Deoxyribonucleic acids) and RNA (Ribonucleic acids).

Structure of nucleotides

Nucleotides have 3 components; namely pentose sugar, a nitrogenous base and a phosphate group

Fig 2.15: Structure of nucleotides [©: ProtonsTalk ]

A nucleotide without a phosphate group is called a nucleoside. e.g. Adenosine, Guanosine

Pentose sugar

Pentose sugars are two types; namely Deoxy ribose and ribose (in deoxyribose one oxygen atom is less than in ribose)

Nitrogenous bases

There are two major groups of nitrogenous bases:

1.    Purines- larger in size with two rings

2.    Pyrimidines- smaller in size with a single ring

In purines, there are two types; namely Adenine, Guanine. In pyrimidines, there are three types, Thyamine, Uracil and Cytosine. Bases are commonly represented by letters A, G, T, U and C respectively.

Phosphate group

It gives the nucleic acids an acidic nature.

Formation of nucleic acids

Millions of nucleotides are joined by a phospho-di-ester bond to form polynucleotide chains by condensation between the –OH group of the phosphate of one nucleotide with the –OH attached to the 3rd carbon of pentose sugar of the other. These bonds result in a backbone with a repeating pattern of sugar-phosphate units. Nucleic acids are linear polymers of nucleotides. There are two kinds of nucleic acids depending on the type of sugar molecules involved. If the sugar molecule in the nucleotide is deoxyribose, the nucleic acid is (DNA). If the pentose sugar is ribose, then the nucleic acid is RNA. DNA contains Adenine, Thymine, Guanine and Cytosine and RNA contains Adenine, Guanine, Cytosine and Uracil.

Structure of DNA molecule (Watson and Crick model)

DNA molecules have two anti-parallel polynucleotide chains that spiral around an imaginary axis, forming a double helix. The two sugar-phosphate backbones run in opposite directions from each other, and the arrangement is referred to as anti-parallel. The sugar-phosphate backbones are on the outside of the helix, and the nitrogenous bases are paired in the interior of the helix. The two strands are held together by hydrogen bonds between the paired nitrogen bases.

Base pair-rule

Always a purine base, pairs with a specific pyrimidine base

A=T (2 hydrogen bonds)

G≡C (3 hydrogen bonds)

Hence two chains (strands) are said to be complementary to each other. These pairs are known as complementary base pairs. In this original double helical structure, one complete turn consists of ten base pairs as shown in the diagram.

 Fig2.16: The structure of the DNA and RNA molecules [©: BYJU'S ]

Functions of DNA

•      Store and transmit genetic information from one generation to the next generation

•      Store the genetic information for protein synthesis

Structure of RNA

This is normally a single-stranded nucleic acid composed of ribonucleotides containing bases, Uracil (U), Cytosine (C ), Guanine (G), and Adenine (A). Complementary base pairing between two RNA molecules or within the same molecule may occur in some. Complementary base pairing facilitates three-dimensional shapes essential for their functioning. Adenine binds with Uracil with two hydrogen bonds and Guanine binds with Cytosine with three hydrogen bonds. There are three types of RNA present in cells,

1.    Messenger RNA (mRNA)

2.    Transfer RNA (tRNA)

3.    Ribosomal RNA (rRNA)

1.   Messenger RNA

Messenger RNA is a linear molecule and is the least abundant type of RNA in cells comparatively. There are two functions;

•      Copies the genetic information stored in DNA molecules as a sequence of nitrogenous bases

•      Transports genetic information from nucleoplasm to the site of protein synthesis (ribosome) through nucleopores

2.   Transfer RNA (tRNA)

Smallest RNA molecule. Linear, but forms three-looped structures as shown in the diagram.

Fig 2.17: Structure of the tRNA molecule [©: CARLBIO PVT LTD ]

Function - transportation of amino acids to the site of protein synthesis.

3.   Ribosomal RNA

It is the most abundant type of RNA. rRNA has a complex irregular structure. It provides the site where polypeptide chains are assembled.

Differences between DNA and RNA

1. DNA is a double-stranded molecule while RNA is a single-stranded molecule. 2. DNA consists of A, T, G and C and the absence of U, while RNA consists    of A, U, G and C and the absence of T

3. Sugar molecule in RNA is ribose, while in DNA it is deoxyribose.

Nucleotides other than those found in nucleic acids

ATP, NAD+, NADP+, FAD and their functions

Functions of ATP

•       Universal energy carrier

Functions of NAD+

•       Act as a coenzyme

•       Act as an electron carrier

•       Function as an oxidizing agent during respiration

Functions of NADP+

•       Act as coenzymes

•       Act as an electron carrier

•       NADP+ act as a reducing agent in photosynthesis

Functions of FAD

•       Act as a coenzyme

•       Act as an electron carrier

References:

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