6. Nucleic acids

Nucleic acids

Nucleic acids were first discovered by a Swiss biochemist, Friedrich Miescher (1869) who called them nuclein due to their acidic nature.Chemical analysis of chromosomes shows presence of two nucleic acids-DNA (Deoxyribo nucleic acid) and RNA (Ribo nucleic acid). Nucleic acid is a macromolecule & consists of many (polymer) monomeric units, called nucleotides. Each nucleotide is composed of a nucleoside and a phosphate group. Thus nucleotide is a phosphoric ester of nucleoside.

Each nucleoside consists of sugar molecule and a nitrogenous base. The relationship can be shown as given below.

Nucleic acid =many nucleotides

 Nucleotide =nucleosides + phosphate

Nucleoside =sugar + nitrogenous base

Thus nucleotide =phosphate + sugar+ nitrogenous base

Nucleic acids bear different components that are briefly discussed below.

1. Phosphoric acid : The acidic nature of nucleic acids is due to the presence of phosphoric acid. Sugar of nucleoside combines with phosphoric acid by a phosphodiester bond formed at 5th or 3rd carbon of the sugar.

2. Sugar : It is a five carbon (pentose) sugar. There are two types of pentose sugars-ribose and deoxyribose. Deoxyribose sugar has one oxygen atom less at second carbon. Ribose sugar is present in RNA while deoxyribose sugar occurs in DNA.

3. Nitrogenous bases : they have two catagories

(a) Pyrimidine:- It includes cytosine, thymine and uracil. Pyrimidine bases are made of only one ring of carbon.

(b) purine:- It includes  Adenine and guanine. Purine bases are made of two ring of carbon and nitrogen  bases of DNA contains adenine, guanine, cytosine and thymine. In RNA, uracil is present in place of thymine.

Many nucleotide monomer units join one another to give rise to polynucleotide chain.

The two adjacent nucleotides are joined by formation of phosphodiester bond (a bond that involves two ester bonds). A polynucleotide chain is often written as 5’p 3’OH. This indicates that it is a dinucleotide with phosphate group (p) attached to the 5th carbon of terminal nucleotide and hydroxyl group (OH) is present at 3rd carbon of basal nucleotide.

Structure of DNA (Deoxyribonucleic acid) :

J.D. Watson and F.H.C. Crick (1953)

Proposed double helical structure of DNA based on the results of M.H.F.Wilkins and co-workers. All these three persons were awarded Nobel Prize in 1962 for this work.

The following are some of the characteristic features of double helical structure of DNA.

(i) Each nucleotide consists of sugar, phosphate and a nitrogenous base. Many such nucleotides are linked by phosphodiester bonds to form a polynucleotide chain or strand.

(ii) Phospho diester bonds are formed between 5’carbon of sugar of one nucleotide and 3' carbon of sugar of the next nucleotide.

(iii) Nitrogenous base is attached to1’ carbon of sugar. At this place purine base is attached by its 5' position and pyrimidine by its 3' position.

(iv) Polynucleotide strand is made of backbone of sugar and phosphate forming its long axis and bases at right angles to it.

Chargaff (1950)

Made observations on the base and other contents of DNA. These observations or generalizations are called Chargaffs rule.

(1) Purine and pyrimidine base pairs are in equal amount, that is, adenine + guanine =thymine + cytosine.

(2) Molar amount of purine-adenine is always equal to the molar amount of pyrimidine-thymine. Similarly, guanine is equalled by cytosine.

(3) Sugar deoxyribose and phosphate occur in equimolar proportions.

(4) The ratio of A + T/G + C is constant for a species.

(5) Chargaffs rule states that in natural DNAs the base ratio A/T is always close to unity and the G/C ratio also to always close to unity. It indicate that A always pairs with T and G pairs with C. A and T, G and C, therefore,are complementary base pairs.

(6) Thus, if one DNA strand has A, the other would have T and if it has G, the other, would have C. Therefore, if the base sequence of one strand is CAT TAG GAC, the base sequence of other strand would be GTA ATC CTG. Hence, the two poly nucleotide strands are called complementary to one another.

(7) Two such complementary strands are joined with each another by hydrogen bonds between their complementary nitrogenous bases. There are three hydrogen bonds between cytosine and guanine and two hydrogen bonds between adenine and thymine.

(8) The two polynucleotide chains are helically coiled around the same axis in such a way that these can separate from one another only by uncoiling. Helical coiling is supposed to be right handed. Such a form of DNA is now called B-DNA

(9) The two chains or strands are antiparallel, i.e., they run in opposite directions in relation to their sugar molecules. Their 5’p 3' OH phosphodiester linkages are in opposite directions

(10) Double stranded DNA molecule has a diameter of 20Aº.

(11) The helix makes one complete turn every 34 Aº along its length.

(12) There are 10 nucleotides per turn of helix.Thus the distance between two neighbouring base pairs is 3.4 A

RNA :

RNA is ribonucleic acid formed in the nucleus and is found in the cytoplasm of the cell.

Types of RNA and their functions :

(1) r-RNA (Ribosomal – RNA) :

It is found in the ribosomes and it is usually associated with protein to form the ribosomes. It is synthesised in the nucleus by DNA. It is single stranded, comprising about 80% of the total RNA. It is metabolically stable.

Functions : (i) This forms the site for protein synthesis.

(ii) It is also supposed to help the binding of m-RNA to the ribosomes during protein  synthesis.

(2) m-RNA (Messenger RNA) :

It is carries the genetic message code from the D.N.A. to ribosomes. It is produced by the DNA ; m-RNA is also single stranded and constitutes about 15% of total RNA.

Functions : It carries the genetic information from DNA to the ribosomes where protein is synthesised.

(3) t-R.N.A (Transfer–RNA) :

It is synthesised in the nucleus by the DNA. It is also called soluble RNA. It is single stranded. There are 20 different kinds of t-RNA and each type has a specificity for a particular amino acid. It constitutes about 5% of total RNA. It has very short life.

Functions : It carries amino acids from different parts of cytoplasm to the ribosomes during protein synthesis.