bases of the singled DNA chain [for example, let the nucleotide be an adenine

bases of the singled DNA chain [for example, let the nucleotide be an adenine nucleotide, containing adenine, deoxyribose and phosphoric acid. The adenine of this nucleolide will unite only with thymine base of the DNA chain, so that base pairing will be belween adenine (A) and thymine (T). Recall that it has been already stated that adenine (A) can unite only with thymine (T) and cytosine (C) can do so only with guanine (G), fig. 713.5]. Next, these newly paired nucleotide (which have been paired with the DNA chain) will be sewn up longitudinlly by appropriate enzymes, (DNA polymerase). Thus, a new chain of DNA strand is syntrhesized. This means, where there were originally two strands of DNA, there are now four slrands. Moreover, because of the base pairing these two pairs are exact replica of each other. This may be technically called duplication of the DNA; the phenomenon is also called copying of the genetic code Thus, the duplicalion of the DNA starts during the interphase of the cell division. Subsequently when the celldivides into two daughter cells, one double helix goes into each cell Now, at this stage, a short version of RNA synthesis will be made. As an example, mRNA will be discussed. The first step of mRNA synthesis, is unlocking ('unzipping') of the two strands of DNA in the double hetix. Bui the separation of the two strands is not complete, it is only partial. For example the upper half of the double helix may undergo unwinding, the lower half remaining locked up with each other. So in this example, in the upper region of the DNA molecule, where the strands have been unlocked or unzipped, the strands are separated. In one of the strands, a complementary strand, exactly as described for DNA duplicate formation, occurs. This complementary chain is the mRNA molecule, which now separales from the DNA strand. The upper halves of the DNA strands wind with each other again and the mRNA molecule leaves the nucleus (via the pores of the nuclear membrane) to enter the cytoplasm where it stays. Of the two strands of the DNA molecule, only one gives rise to the complementary strand the other remaining idle. This strand on which the complementary strand develops is called Ihe 'information strand' [It is lo be understood that, the complementary strand develops by base pairing with the information strand of the DNA molecule] The whole phenomenon of mRNA synthesis on the information strand of DNA is lechnically called "transcription'Throughout their life span, the DNA molecules transcribe the mRNA molecules mostly, although the mRNA accounts for only 1 % of the RNA in the cells. This is because, the mRNA molecules are rapidly destroyed and have to be replaced repeatedly, whereas the tRNA and rRna molecules are relatively stable. (III) Prevention of mutalion Question arises, if during Ihe duplication process, accidentally there is any alternation of the DNA strand (mutalion), the allered DNA strand will be subsequently copied by the daughler cells and the characteristics of the offsprings as well as the individual will change What prevents this ? Mutation of the DNA strand does occur in a man's indi-vidual life span, particularly after X-ray and UV exposure. Assume, a portion of a particular strand has been allered (undergone mutalion). That mutated portion of the strand is now excised and removed, and a new healthy and correct segment inserted into the gap (created by the excision and removal of the sick portion) of the strand. Enzymes have been identified which can recognize the sick segment. Other enzymes cause scission ( = excision ) of the sick segment. Polynucleotide ligase is thus an well known enzyme which is concerned with the repair of damaged DNA molecule. FUNCTIONS OF THE RNA RNAs synthesize proteins. For details, see protein synlhesis. Of the building blocks of a nucleolide, the purine or pynmidine base can be synlhesized in our body from different ingredients (for details, see melabolism of purine/ pyrimidine bases, below). The pentose sugar is oblained from glucose metabolism via penlose phosphate palhway The phosphoric acid is obtainable from phosphorous of organic compounds VIRUS Strictly speaking, the viruses cannol be called living mailers, bul they cannol be said to be totally non living too Virus contains only a single strand of nucleic acid plus little protein Oulside an animal or plant body they remain in this way without metabolism or reproduction for many many years but once introduced into the body of a suitable host (the cell of a plant or animal) they duphcate (reproduce) repeatedly. With some virus, this reproduction leads to death of the host cell, while with other virus this may not be so. Some viruses are well known for their ability to produce diseases in man, examples are, mumps, chicken pox, small pox, rabies, and so on. A recenlly discovered virus, HLTV III also called HIV, is the cause of AIDS. Another important virus is hepatitis B virus, cause of a form of hepatitis. Apart from the fact that this hepatitis itself is dangerous, studies indicate that about 80% of primary liver cancers occur only in persons who suffered from this viral hepatitis in the past. With some virus the nucleic acid strand is RNA.Strangely enough, DNA molecules can be transcribed from such RNA, the phenomenon being called reverse transcription.