for the same amino acid, very often, there are more than one codon For example. for alanine, there are four co dons.

that is. the RNA code, is dependent on (he base sequence of the DNA strand, or the DNAcode Now. a sequence of threee consecutive bases -can recognize an ami no acid This means, that the entire RNA strand can be divided info groups of three bases, each group specific for catching a single amino acid The base sequence within the group js vanable, eg, CCG. AAA, AGG. GAG, and so on it can be shown mathematically That there can be 64 such possible groups Each such group is called a codon. As There are only 20 ammo acids in our body, what is the necessity of having 64 codons'/answer is, for the same amino acid, very often, there are more than one codon For example. for alanine, there are four co dons. CGA, CGG. CGT & CGC For methionine, however, there is only one codon. viz. AUG. In addition, some codons act as marking point (just as full stops act for a sentence} to denote the end of a gene, it may be noted, in the i960s, it was used to be believed that, out of the 64 codons, only some were necessary while many were useless Such (80 called) useless codons were named 'nonsense codons'. As explained above, there is possibly hardly any nonsense codon. In a language, there are letters like A, B, C, D, words like cat,dog. fox and sentences like, "the quick brown fox jumps over the lazy dog'. It is to be imagined that the individual letters = the individual bases (like adenine etc) Individual words= individual ammo acids As there can be only three bases in a codon. there can thus be. according to this analogy, only three lettered words However, by changing the sequence of the letters, the words can be made altogether different For example, a, r and t can form three different words, art, rat and tar Similarly, the different codons. although they may contain the same bases (but in different sequence), may catch different ammo acids code To carry this analogy further, a sentence becomes comparable to a polypeptide chain Obviously there are two types of codone (or the code words), the Dncodons and the RNA codons As slated early, the base thymine appears on the DNA. but not in the RNA in the RNA, uracil is the base which appears in place of thymine Now. in a double helix the base adenine (A) can pair oniy wilh thymine (T) But when a mRNA is transcribed from a DNA strand, the base adeinne (A) will have to pair with uraci! (UJ Therefore, if the three letter code in the DNA is CAT. the transcnbed code m the mRNA will be CUA (whereas m the double he ln it would1 have been CTA) A DNA molecule usually contains several genes Each gene carries Ihe codons all of which-can be Iranscnbed by a single m RNA A given m RNA can help to synthesize one but only oneiype of a specific polypephde chain The sequence of codons in a DNA molecule forms genetic code Mechanism of protein synthesis As stated already there are three types of RNAs, the mRNA, rRNA and the tRNA A nbosomal RNA (rRNA) has two pails. 40S and EOS subunrfs A mRNA gels attached with the 40S subuml ofthe rRNA Protein synthesis occurs on the rRNA Individual amino acids are carried by tRNA molecules The individual ammo acids must be placed in the exact sequence Afterwards, these ammo acids are longitudinally sewn up by enzymes Delays A tRNA molecule, although consisting of a single strand, loots like a twisted hairpin (fig 7,13.7) A tRNA has a 'head end'and free end At the free end, ammo acid, present in the cytoplasm, gets attached by an enzyme called ammo acid syntheEase [AAS]. i e. AAS causes The specific ammo acid to gel fixed at the free end of the specific tRN At the head end, a tripfel base, catted anlicodon is present (for example, an anticodon may consist of AAA] Now. during prolam synthesis mRNA and rRNA together form a complex After getting attached with one aminoacid, theRNA, now called, the loaded or charged tRNA, moves towards the mRNA- rRNA complex, now its anticodon recognizes its specific codon on the mRNA For example, if the anticodon is AAA, the codon of the mRNA will be UUU with which the AAA will attach it self This is how a given ammo acid rs positioned in its exact place In this way a sequence of amino acids will line up and are eventually united by specific enzyme This is facilitated by movement of the mRNA on the ribosome (the mRNA moves away from the rRNA, freeing the space) After bringing the ammo-acid To the ribosome, the [RNA moves away, creating a vacant place where a new loaded tRNA is fixed. it has been noted that a polype ptide chain consisting of about 150 amino acids may be synthesized as quickly as m one minute History The history of discoveries in connection with DNA. RNA and related affairs are studded with illustnous names. Thus, Miescher m 19th century first studied the chemistry of pus (and subsequently head of the salmon sperm) and his findings became forerunners of modern genelrcs Since the 1950s development on this subject has become very explosive, largely due lo the works of Todd, Chargraff, Walson and Crick, Nirenberg, Kornberg and Hara Govmda Khuiana (an American of Indian origin) many of whom were awarded Nobel Prize on medicine Story loving sludenls can read some related facts m the book referred below" (*) Watson, J.D The Double Hehxh. Atheneum, New York. 1968 Introduction Definition The vitamins are "accessary food factors" which are organic in nature and must be supplied from outside to maintain the heath, growth and the state of well being of a person As a rule, only Trace quantities of