BOTANY Higher Secondary Second Year - Textbooks Online
BOTANY Higher Secondary Second Year - Textbooks Online
BOTANY Higher Secondary Second Year - Textbooks Online
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namely major and minor grooves. The backbone of the helix is formed of<br />
sugar and phosphate molecule. The nitrogenous bases are attached to sugar<br />
molecules. The two nucleotide strands are held together by unstable<br />
hydrogen bonds. Erwin Chargaff in 1949 showed that<br />
(i) The bases pair in specific manner. Adenine always pairs with thymine<br />
and guanine pairs with cytosine.<br />
(ii) Total amount of purine nucleotides is always equal to the total amount<br />
of pyrimidine nucleotides i.e.[A] + [G] = [T] + [C].<br />
(iii) The proportion of adenine is equal to thymine and so also of guanine<br />
is equal to cytosine. But the [A] + [T] need not necessarily be equal<br />
to [G] + [C].<br />
These empirical rules regarding the composition of bases in DNA is<br />
collectively known as Chargaff’s law or Base pair rules. There are two<br />
hydrogen bonds between adenine and thymine (A= T) and there are three<br />
hydrogen bonds between guanine and cytosine (G≡C) pairing. The two<br />
strand run antiparallely in opposite directions ie. they run in opposite<br />
direction 5’ to 3’ end and 3’ to 5’ end. The two strands are interwined in<br />
clockwise direction. The width of DNA molecule is 20 Å. The strand<br />
completes a turn every 34 Å along its length. There are ten nucleotides<br />
per turn. The internucleotide distance is 3.4 Å. Watson and Crick model<br />
of DNA is called B-form DNA. The chains in B-form DNA are in right<br />
handed orientation.<br />
Functions of DNA<br />
It controls all the biochemical activities of the cell. It carries genetic<br />
information from one generation to other. It controls protein synthesis<br />
and synthesize RNAs.<br />
Replication of DNA<br />
DNA is the genetic material of almost all the organisms. One of the<br />
active functions of DNA is to make its copies which are transmitted to the<br />
daughter cells. Replication is the process by which DNA makes exact<br />
copies of itself. Replication is the basis of life and takes place during the<br />
interphase stage. Watson and Crick suggested the semiconservative method<br />
of replication of DNA. This has been proved by Messelson and Stahl’s in<br />
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