The RNA molecules in turn synthesize proeins These proteins may be retained withn the cell as structural protein

my body have come from DNAs of ancient times. A single DNA molecule carries many genes. The DNA molecules produce the RNA molecules. The RNA molecules in turn synthesize proeins These proteins may be retained withn the cell as structural proteins, or special functional proteins, e.g. contractile proteins of the muscles and enzymes, or they may be exported, as in the case of protein hormones like insulin, or serum albumin (which is synthesized in the liver and enters the blood) and so on. As, ultimately speaking, these proteins determine the characteristics of the animal (eg, whether he becomes, physically, a man or a dog, or mentally a saint or a scoundrel) the DNA molecules carry the genetic code. ELEMENTARY CHEMISTRY Structure of a nucleolide A pentose sugar (Fig.7.13.1) unites with a nitrogenous base (the nitrogenous base is either a purine or a pyrimidine base) to form a compound called nucleoside. Subsequently phosphoric acid combines with a nucleoside to form what is known as nucleotide A nucleotide, therefore, _ base + penlose + phosphoric acid. Fig. 7.13.1 A nucleotide is the fundamental unit of a DNA or a RNA molecule (building blocks). Large number of such nucleotides are linked up together to form a chain like structure, which is called a strand of nucleotides. A nucleic acid molecule, thus, is a strand of nucleotides. In case of a DNA molecule, two such strands are locked up with each other (the double helix) (fig.7.13.5) whereas in case of an RNA, there is a single strand of nucleotides. The Sugar In case of RNA nucleolides, the sugar is pentose sugar ribose, whereas with DNA it is deoxyribose and hence the names, RNA and DNA. The base There are two types of bases, viz. , (i) purine and the (ii) pyrimidine Adenine (A) and guanine (G) are the two purine bases and their structural formula are given below; both of them are derivatives of purine Cytosine (C), thymine (T) and uracil(U) are the three pyrimidine bases and all are derivatives of pyrimidine When the base adenine (a purine base) combines with ribose, the nucleoside formed (ribonucleoside) is called adenosine, similarly the other ribo nucleosides, guanosine, cylidine and undine are formed by the union of guanine, cylosine and uracil respectively with ribose. When the sugar is deoxyribose, the corresponding compounds (thai is deoxyribonucleosides) become deoxyadenosme and deoxyciti-dine. Deoxyribonucleoside of thymme is called thymidine Note that there is-no ribonucleoside of thymine or no deox-yribonucleoside of uracil. In a nucleolide, the sugar (ribose or deoxyribose) molecule remains in the middle, on one side of it, Ihe phosphoric acid moiety is attached and on the other side is attached the nitrogenous base, adenine/ guanine etc. The successive nucleotides are linked up through phosphoric acids. (fig. 7.13.4). The DNA molecule he figure 134 then, represents a single strand of nucleotides. n a DNA molecule, two such strands are locked up with each other as shown in figure 7.13.5. Mark the nitrogenous bases (purine or pyrimidine) stand out almost perpendicularly from the longitudinal axis of Ihe strand of the nucleic acid. These bases unite with their fellows from the other strand and thus the structure known as the 'double helix' (the Gk word helix means a spiral, like a corkscrew, because the individual strands are greatly coiled, it is so named) results (fig. 7.13.5) n a DNA strand, only the following bases are found : adenine, guanine, cytosine and thymine and are abbreviated as A G, C and T respectively. Further, A can unite with only T and G with only C as shown in Figure 7.23.5. RNA molecule. n an RNA molecule, there is only a single strand of nucleolides. Besides, the DNA molecules are very long, whereas RNA molecules are usually short. An RNA mole cule, however, can be folded and present an appearence of double strand There types of RNA are known : (i) messenger RNA(mRNA), (n) ribosomal RNA (rRna), (iii) transfer RNA (tRNA). They have been described further in subsequent portions. FUNCTIONS OF DNA (I) RNA Synthesis All the three types of RNA (mRNA, r RNA and IRNA) are produced by the DNA, every minutest detail of every RNA molecule is predetermined in the DNA. That is, the shape and the sequence of the bases of the RNA molecule depend absolutely on the DNA molecule on whose body it is formed (see below). These RNA molecules in turn, determine the sequence of ammo acids in a polypeptide chain during protein synthesis. This sequence (of amino acids in the polypeptide chain) is ultimately responsible for the action of protein (for example, if the amino acid sequence of a polypeptide chain of an enzyme be altered at some crucial regions, the enzyme loses its characteristic action). Further ultimately speaking these synthesized proteins are responsible for the characteristics of the individual. Therefore, the chain of events are as follows, DNA D RNA moleceules molecule has been described little below. Duplication and genetic copying During the interphase of a division, the DNA molecules duplicate as follows Each of the two strands of a DNA molecules separate out from each other. Fig.7.13.4. The inset shows a single nucleotide. Large number of such nucleolides make one strand of nucleic acid as shown in the upper pannel of the diagram. The lelters A, C, T, G represent bases. Now, fresh nucleotides are synthesized within the nucleus of the cells from the raw materials (base, sugar and phosphoric acid). These nucleotides, now, 'base pair' with the correspond