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Structure amp Function of DNA Botany Tamil
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Structure Function of DNA 12TH STANDARD BIO BOTANY Higher Secondary Second Year BIOLOGY • https://www.padeepz.net/12th-standard... • TEXT NOTES AND WITH EXPLANATION IS AVAILABLE IN THE LINK GIVEN • Structure of DNA • DNA and RNA are identified in the nucleus. • They are complex macro molecules and made up of millions of smaller units called nucleotides. • Hence, DNA is a macromolecular substance with double stranded polynucleotide. • Each nucleotide is made up of pentose sugar, a phosphate group and a nitrogenous base. • Ribose is the constituent sugar in RNA and Deoxyribose in DNA. • The nitrogenous bases are of two kinds – purines and pyrimidines. • Adenine and guanine are the purines and thymine and cytosine are pyrimidines. • The nitrogenous bases found in DNA are adenine, guanine, cytosine and thymine, whereas in RNA thymine is replaced by uracil. • The sub-unit containing only sugar and nitrogenous base is known as nucleoside. • A nucleoside combines with phosphate to form a nucleotide. Thus, four kinds of nucleotides are seen in DNA molecule. • They are adenine nucleotide, guanine nucleotide, thymine nucleotide and cytosine nucleotide. • Hence, nucleotides are building blocks of DNA. • DNA – Structure of DNA • Structure of DNA • In 1953, James Watson and Francis Crick proposed double helix DNA model on the basis of x-ray diffraction studies with photographs of DNA taken by Wilkins and Franklin. • DNA is a double stranded structure in which the two strands are coiled around each other forming a double helix. • The DNA duplex is “coil of life”. • There are two grooves found in DNA molecule namely major and minor grooves. • The backbone of the helix is formed of sugar and phosphate molecule. • The nitrogenous bases are attached to sugar molecules. • The two nucleotide strands are held together by unstable hydrogen bonds. • Erwin Chargaff in 1949 showed that • (i) The bases pair in specific manner. Adenine always pairs with thymine and guanine pairs with cytosine. • (ii) Total amount of purine nucleotides is always equal to the total amount of pyrimidine nucleotides i.e.[A] + [G] = [T] + [C]. • (iii) The proportion of adenine is equal to thymine and so also of guanine is equal to cytosine. • But the [A] + [T] need not necessarily be equal to [G] + [C]. • These empirical rules regarding the composition of bases in DNA is collectively known as Chargaff’s law or Base pair rules. • There are two hydrogen bonds between adenine and thymine (A= T) and there are three hydrogen bonds between guanine and cytosine (G≡C) pairing. • The two strand run antiparallely in opposite directions ie. they run in opposite direction 5’ to 3’ end and 3’ to 5’ end. • The two strands are interwined in clockwise direction. The width of DNA molecule is 20 A. • The strand completes a turn every 34 A along its length. There are ten nucleotides per turn. • The internucleotide distance is 3.4 A. Watson and Crick model of DNA is called B-form DNA. • The chains in B-form DNA are in right handed orientation. • Functions of DNA • It controls all the biochemical activities of the cell. • They carries genetic information from one generation to other. It controls protein synthesis and synthesize RNAs.
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