Nucleic acids are important cell constituents of high molecular weight. They are formed out of units known as nucleotides. Nucleic acids regulate the metabolism, growth, reproduction, and eventual destiny of the cell. They are constituents of the chromosomes which are the carriers of the units of heredity called the genes.
Each nucleotide consists of a purine or pyrimidine base linked to a pentose sugar which in turn is esterifies with phosphoric acid to form a structure: base-sugar-phosphate. The pentose sugars found in nucleotides are ribose and 2-deoxyribose. The pyrimidine bases are all derivatives of the parent compound pyrimidine. The pyrimidine derivatives commonly found in nucleotides and in nucleic acids are cytosine, uracil and thymine. Purine bases are derivatives of the parent compound purine. The most important purine bases are adenine and guanine. When a purine or a pyrimidine base condenses with a sugar a nucleoside is formed. When the sugar unit of the nucleoside is esterified with phosphoric acid a nucleotide is formed. Nuclotides containing ribose sugar are ribo-nucleotides and those containing deoxyribose are called deoxyribo-nucleotides.
Two types of nucleic acids occur in the cells (i) deoxyribonucleic acid (DNA) and (ii) ribonucleic acid (RNA). They are large polymeric molecules, the monomeric units of which are mononucleotides; nucleic acids are also referred to as polynucleotides.
In DNA, the individual nucleotide units are deoxyribonucleotides and the chief nitrogen bases are adenine (A), guanine (G), cytosine (C), and thymine (T). In RNA, the monomers are ribonucleotides and the chief bases are adenine, guanine, cytosine and uracil (U) instead of thymine.
The constituent nucleotides in the nucleic acids are linked together by ester linkages between the phosphate group on carbon-5' of the sugar of one nucleotide and 3-hydroxyl group of the sugar of the next nucleotide in the sequence. In other words, the polynucleotide chains are composed of monomer units of nucleotides linked by 3', 5'-phosphodiester bonds. The arrangement of nucleotides in nucleic acids shows a definite pattern which is very important for their function.
DNA (Deoxyribonucleic Acid)
Deoxyribonucleic acid or DNA consists of two polynucleotide chains arranged in a double helix.
The helical structure for DNA was proposed by James Watson and Francis Crick, hence it is known as the Watson-Crick model of DNA double helix. The two strands of DNA are twisted around each other in the form of a double helix, something like a double spiral staircase.
Fig.2.3.6 Watson-Crick Model of DNA Double Helix
RNA (Ribonucleic Acid)
Like DNA, RNA is a long-chain molecule made of repeating nucleotide units linked by phosphate diester bonds. There are two differences in the building units of RNA and DNA: (i) the sugar composed of RNA is not deoxyribose but ribose; (ii) although three out of four of the bases, adenine, guanine, and cytosine, are the same in RNA as in DNA, the fourth base, thymine, is replaced in RNA by uracil.
Unlike DNA, which is associated with the chromosomes in the cell nucleus, RNA is found in the nucleus as well as in the cytoplasm of the cell. There are various types of RNA found in the cell. These are:
Ribosomal RNA (rRNA): This is found mainly in the cytoplasm in the minute ribonucleoprotein particles called ribosomes. This type of RNA accounts for up to 80 per cent of the total RNA found in the cell. The rRNA is intimately associated with ribosomal structure.
Transfer RNA (tRNA) :This contains 75 to 80 nucleotides and has a molecular weight of about 25,000. Several unusual bases are found in the transfer RNA molecules. Transfer RNA molecules function as carriers of activated amino acids during protein synthesis.
Messenger RNA (mRNA): This class of RNA is of high molecular weight and accounts for only about 1 per cent of the total RNA of the cell. Its function is to convey genetic information from the cell nucleus to the protein-synthesising centres in the cell, where, in association with ribosomes and transfer RNA, it engages in the complex process of protein synthesis. All types of RNA are single-stranded.
Functions of Nucleic Acids
The nucleic acids RNA and DNA are components of the most important structures in the cell, namely the chromosomes. Chromosomes are the carriers of the hereditary units called genes. It has been proved beyond doubt, that the hereditary material or the gene is DNA which regulates all activities of the cell and the organism. DNA regulates cell metabolism by controlling the synthesis of proteins (enzymes) through the messenger RNA.
The most important feature of living organisms is their ability of self-duplication. Every organism achieves this by cell division. When a cell divides, the two daughter cells are genetically identical. The precise manner in which the DNA molecule undergoes replication ensures that the hereditary material is duplicated in such a way that the daughter cells are genetically identical. The role of various RNAs is to assist in the process of protein synthesis. On the whole the nucleic acids are probably the most important chemical constituents of cells; without them cells cannot survive.
ATP is a nucleotide containing adenine, ribose and three phosphates, and is the energy molecule of the cell. It belongs to a category of high-energy compounds that release energy when the bond between the second and the third PO4 is broken. The presence of these high energy bonds makes it possible for ATP to store and release energy for cellular chemical reactions. Hydrolytic cleavage of the third phosphate not only releases energy to do cellular work but also generates adenosine diphosphate (ADP). ADP can be converted back to ATP when the third phosphate is restored, thereby serving as an energy depot or energy currency.