Basic Concept of Biotechnology 23

  Nucleic Acids Every generation of each and every species resembles its ancestors in many ways. How are these characteristics transmitted from one generation to the next? It has been observed that nucleus of a living cell is responsible for this transmission of inherent characters, also called heredity. The particles in nucleus of the cell, responsible for heredity, are called chromosomes which are made up of proteins and another type of bimolecular called nucleic acids. These are mainly of two types, thedeoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Since nucleic acids are long chain polymers of nucleotides, so they are also called polynucleotides. Why is a dog a dog and not a cat? Why do some people have blue or brown eyes and not black? From a chemical standpoint, how does the body know what particular type of protein is to be synthesized? How is this information transmitted from one generation to the next? The study of the chemistry of heredity is one of the most fasc...

  Structure of Nucleic Acids        Like all natural molecules, nucleic acids are linear polymeric molecules. They are chain like polymers of thousands of nucleotide units, hence they are also called polynucleotides. A nucleotide consists of three subunits: nitrogen containing heterocyclic aromatic compound (calledbase), a pentose sugar and a molecule of phosphoric acid. So a nucleic acid chain is represented as shown below.       In DNA molecules, the sugar moiety is 2 -deoxyribose, where in RNA molecules it is ribose. In DNA, four bases have been found. They are adenine (A), guanine (G), cytosine (C) and thymine (T). The first three of these bases are found in RNA also but the fourth is uracil (U). The sequence of different nucleotides in DNA is termed as its primary structure. Like proteins, they also have secondary structure. DNA is a double stranded helix. Two nucleic acid chains are wound about each other and held together by ...

  Watson and Crick’s double helix structure of DNA  Unlike DNA, RNA is a single stranded molecule, which may fold back on itself to form double helix structure by base pairing in a region where base sequences are complimentary. There are three types of RNA molecules which perform different functions. These are named as messenger RNA (m-RNA), ribosomal-RNA(r-RNA) and transfer RNA (t-RNA) Biological Functions of Nucleic Acids A DNA molecule is capable of self duplication during cell divisions. The process starts with the unwinding of the two chains in the parent DNA. As the two strands separate, each can serve as a master copy for the construction of a new partner. This is done by bringing the appropriate nucleotides in place and linking them together. Because the bases must be paired in a specific manner (adenine to thymine and guanine to cytosine), each newly built strand is not identical but complimentary to the old one. Thus when replication is completed, we have two DNA mol...

  Tautomerism The nitrogenous bases in nucleosides and nucleotides, with the exception of adenine and adenosine, undergo enol-keto tautomerism. Studies have shown that the predominant species in solution is the keto form. Examples are uracil and guanine. Biological Functions of Nucleic Acids DNA is the chemical basis of heredity and may be regarded as the reserve of genetic information. DNA is exclusively responsible for maintaining the identity of different species of organisms over millions of years. A DNA molecule is capable of self duplication during cell division and identical DNA strands are transferred to daughter cells. Another important function of nucleic acids is the protein synthesis in the cell. Actually, the proteins are synthesised by various RNA molecules in the cell but the message for the synthesis of a particular protein is present in DNA.

  Nucleosides and nucleotides Nucleosides are molecules formed by attaching a nucleobase to a ribose or deoxyribose ring. Examples of these include cytidine (C), uridine (U), adenosine (A), guanosine (G), thymidine (T) and inosine (I). Nucleosides can be phosphorylated by specific kinases in the cell, producing nucleotides. Both DNA and RNA are polymers, consisting of long, linear molecules assembled by polymerase enzymes from repeating structural units, or monomers, of mononucleotides. DNA uses the deoxynucleotides C, G, A, and T, while RNA uses the ribonucleotides (which have an extra hydroxyl (OH) group on the pentose ring) C, G, A, and U. Modified bases are fairly common (such as with methyl groups on the base ring), as found in ribosomal RNA or transfer RNAs or for discriminating the new from old strands of DNA after replication. Each nucleotide is made of an acyclic nitrogenous base, a pentose and one to three phosphate groups. They contain carbon, nitrogen, oxygen, hydrogen ...

  Proteins   Proteins are the most abundant macromolecules in living cells. The name protein is derived from the Greek word ‘proteios’ meaning ‘of prime importance’. These are high molecular mass complex amino acids. You will study about amino acids in the next section. Proteins are most essential class of biomolecules because they play the most important role in all biological processes. A living system contains thousands of different proteins for its various functions. In our every day food pulses, eggs, meat and milk are rich sources of proteins and are must for a balanced diet. Proteins are molecular tools that perform an astonishing variety of functions. In addition to serving as structural materials in all living organisms (e.g., actin and myosin in animal muscle cells), proteins are involved in such diverse functions as catalysis, metabolic regulation, transport, and defense. Proteins are composed of one or more polypeptides, unbranched polymers of 20 different amino ac...

  Amino Acids The hydrolysis of each polypeptide yields a set of amino acids, referred to as the molecule’s amino acid composition. The structures of the 20 amino acids that are commonly found in naturally occurring polypeptides. Amino acids are the most versatile small biomolecules. They fulfil a number of extremely important roles in biology. These include: building blocks of proteins which are polymers of amino acids, precursors of hormones, and precursors of molecules with specialized physiological functions, e.g., the neurotransmitter dopamine and the hormone thyroxine are both derivatives of the amino acid tyrosine. As the name implies, amino acids contain amino and carboxyl groups. They can be divided into groups based on acidic, basic, and neutral properties when dissolved in water. They are also classified according to solubility, e.g., hydrophilic and hydrophobic. There are 20 so-called amino acids in proteins; however, one of these, proline, is in fact an imino acid. Nin...