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GENERAL SCIENCE | Structure of DNA

GENERAL SCIENCE | Structure of DNA

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  A – Adenine G – Guanine C – Cytosine T – Thymine adenine nucleotide, guanine nucleotide, thymine nucleotide and cytosine nucleotide. Hence, nucleotides are building blocks 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 Å. The strand completes a turn every 34 Å along its length. 
There are ten nucleotides per turn. 
The internucleotide distance is 3.4 Å. 
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. 
It carries genetic information from one generation to other. 
It controls protein synthesis and synthesize RNAs

Replication of DNA 
DNA is the genetic material of almost all the organisms. 
One of the active functions of DNA is to make its copies which are transmitted to the daughter cells. 
Replication is the process by which DNA makes exact copies of itself. 
Replication is the basis of life and takes place during the interphase stage. 
Watson and Crick suggested the semiconservative method of replication of DNA. 
This has been proved by Messelson and Stahl's in their experiments on Escherichia coli using radioactive isotopes. 
The replication of chromosome in E. coli is completed in 40 minutes

During replication of DNA, the two complementary strand of DNA uncoil and separate from one end in a zipper like fashion. 
The enzyme helicase unwinds the two strands and as a result replication fork is formed. 
As the DNA unwinds, the part of the DNA that is found above the replication fork becomes supercoils. 
These are called positive supercoils. 
An enzyme called topoisomerase releases these supercoils. 
Based on separated DNA strands, new strands grow by the addition of nucleotides. 
DNA polymerase I, II and III are involved in this elongation. 
However, these enzymes are not capable of initiating DNA synthesis. 
For the synthesis of new DNA, two things are required. One is RNA primer and the enzyme primase. 
The DNA polymerase moves along the newly formed RNA primer nucleotides, which leads to the elongation of DNA. 
In the other strand, DNA is synthesized in small fragments called Okazaki fragments. 
These fragments are linked by the enzyme called ligase. 
In the resulting DNA, one of the strand is parental and the other is the newer strand which is formed discontinuously. 
Hence, it is called semidiscontinuous replication


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