Nucleic acids are macromolecules present in ail living cells either in Free State, or in combination with other substances.
These are polymers consisting of units called nucleotides; they are hence called polynucleotide. Nucleic acids are of two types:
1. Deoxyribonucleic Acid (DNA)
DNAs are found in the chromosomes in the nucleus of plant and animal cells. In prokaryotes also DNA, forms the chromosomes. Some viruses, especially animal viruses have it as their genetic material. Furthermore, it is also found in mitochondria of plant and animal calls and in chloroplasts of photosynthetic organisms.
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2. Ribonucleic Acid (RNA)
RNAs mainly found in the cytoplasm of cells. There are various types of RNAs (rRNA, TRNAs, and mRNA) involved in the expression of genetic information.
Each nucleotide in nucleic acids is composed of pentose sugar, pentose nitrogenous base and phosphoric acid.
Pentose sugar
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In ribonucleic acids, the sugar is ribose; in deoxyribonucleic acids it is deoxyribose. These two sugars differ in their chemical nature on carbon 2 as shown below.
Nitrogenous base
All nitrogenous bases derive from two heterocyclic bases, purine and pyrimidine. l. Purine base
Two principal purine bases found in deoxyribonucleic acids as well as ribonucleic acids are adenine and guanine.
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2. Pyrimidine bases
Cytosine and uracil are found in ribonucleic acids; cytosine and thymine, in deoxyribonucleic acids.
Nucleosides
Nucleosides are formed from the linkage of a purine or pyrimidine base with ribose or deoxyribose sugar. This linkage joins nitrogen 9 of the purine base, or nitrogen 1 of the pyrimidine base with carbon 1 of pentose.
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The nucleosides are called ribonucleosides or deoxyribonucleosides depending upon whether ribose or deoxyribose sugars are bonded with nitrogenous bases respectively. The following table indicates the nomenclature of the main nucleosides.
Nucleotides are the phosphoric esters of nucleosides. Depending on the nature of the pentose sugar one will have ribonucleotides and deoxyribonucleotides.
A ribonucleoside has 3 positions of carbon, which can be phosphorylated (2′, 3′ and 5′) while a deoxyribonucleoside can be phosphorylated only in two positions (3′ and 5′). This results in the formation of nucleosides -mono phosphate.
A second phosphate group can be bound to the phosphate of a nucleoside- monophosphate to form a nucleoside-di-phosphate. Likewise a third phosphate group can also be attached to the second forming nucleosides-tri-phosphate.
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Primary structure of DNA
In deoxyribonucleic acids the nucleotides are joined by 3′-5′ phosphodiester bonds; in other words each phosphate group (except those present at the end of chains) esterifies to the 3′ hydroxyl group of a pentose and to the 5′ hydroxyl group of the next pentose.
Therefore the polydeoxyribonucleotide chain consists of alternating deoxyribose and phosphate residues. Secondary structure of DNA
DNA in solution state takes the form of a secondary structure. Watson and Crick in !953 proposed the secondary structure of DNA in the form of the famous double helix model for which they shared the Nobel Prize in 1962 along with Wilkins.
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They worked out the double helical model of DNA basing and the following observations made available to them.
a. It was known through base analyses that there is as much adenine as thymine and as much guanine as cytosine (A/T and G/C = 1). Therefore, the sum of purines is equal to the sum of pyrimidines (A+G = C+T). It is known as Chargaffs rule.
b. X-ray diffraction studies (Wilkins, 1952), suggested a helical configuration.
According to this model, DNA has a double stranded structure where two polydeoxyribonucleotide chains twisted around one another in a double helix. Both! The helices are held together by means of hydrogen bonds formed between the nitrogen bases.
The diameter of the DXA molecule is 2oA°(2nm). The length of the DNA in one complete turn is 34A°(3.4nm), which incorporates 10 base pairs. Therefore the distance between two adjacent base pairs is 3.4 A°
Messenger RNA (mRNA)
It is the RNA formed during the protein synthesis. Five to ten percent of cellular RXA is of this type. The molecular weight of m RNA varies from 30000-1000000. It is short lived. DNA transfers the genetic information to ribosome through this type of RNA during the protein synthesis.
Ribosomal RNA (r RNA)
The most stable form of RNA in the cell is the r RXA. About 80% of cellular RNA is of this type. The molecular weight of r RNA ranges from 40000-1000000. It may have some folds to have a complex structure; r RNA units along with protein constitute the protein synthesizing factory or the ribosome.
Transfer RNA (tRNA)
It is smallest form of RNA made of only 75 to 100 nucleotides. It is also known as the soluble RNA. It forms about 10-15% of total cellular RNA. The molecular weight of t RNA varies from 25000-30000. It transfers the amino acids from the cytoplasm to the ribosome.
In 1964 Holley gave the detailed structure of t RNA through the ‘Clover leaf model’. In that model it was proposed that t RNA has three loops and a lump. The anticodon loop has the complementary base sequence with respect to a codon of mRNA facilitating the attachment of t RNA with the later.
Other two loops are TfC loop or ribosomal binding loop and DHU loop or amino acyl synthesize binding loop. The 3′ end of t RNA ends with CCA-OH, which acts as the amino acid attachment site. The other end ends with G.