DNA and Its Components

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DNA and Its Components

[CENTER]Deoxyribonucleic acid (DNA)stores genetic information .Its components,two types of nucleotide base (purines and pyrimidines),deoxyribose,and a phosphate group,are arranged in a specific chemical relationship.They determine the three-dimensional structure of DNA,from which it derives its functional consequences

A. Nucleotide bases
The nucleotide bases in DNA are heterocyclic molecules derived from either purine or pyrimidine.Five bases occur in the two types of nucleic acids,DNA and RNA.The purine bases are adenine (A)and guanine (G).The py rimidine bases are thymine (T)and,in DNA,cytosine ©or,in RNA,uracil (U).The nucleotide bases are part of a subunit of DNA,the nucleotide. This consists of one of the four nucleotide bases,a sugar (deoxyribose),and a phosphate group.The nitrogen atom in position 9 of a purine or in position 1 of a pyrimidine is bound to the carbon in position 1 of the sugar
(N -glycosidic bond).
Ribonucleic acid (RNA)differs from DNA in two respects:it contains ribose instead of deoxyribose (unlike the latter,ribose has a hydroxyl group on the position 2 carbon atom)and uracil instead of thymine.Uracil does not have a methyl group at position C5.

B. Nucleotide chain
DNA is a linear polymer of deoxyribonucleotides units.The nucleotide chain is formed by joining a hydroxyl group on the sugar of one nucleotide to the phosphate group attached to the sugar of the next nucleotide. The sugars linked together by the phosphate groups form the invariant part of the DNA.The variable part is in the sequence of the nucleotide bases A,T,C,and G.
A DNA nucleotide chain is polar.The polarity results from the way the sugars are attached to each other.The phosphate group at position C5 (the 5 _ carbon)of one sugar joins to the hydroxyl group at position C3 (the 3 _ carbon)of the next sugar by means of a phosphate diester bridge.Thus,one end of the chain has a 5 _
phosphate group free and the other end has a 3 _ hydroxy group free (5 _ end and 3 _ end,respectively).By convention,the sequence of nucleotide bases is written in the 5 _ to 3 _ direction.

C. Hydrogen bonds between bases
The chemical structure of the nucleotide bases determines a defined spatial relationship. A purine (adenine or guanine)always lies opposite to a pyrimidine (thymine or cytosine).
Three hydrogen-bond bridges form between cytosine ©and guanine (G).Two hydrogen bonds form between adenine (A)and thymine (T). Therefore,either guanine and cytosine or adenine and thymine are posed opposite each other,forming complementary base pairs G –C and A –T.Other spatial relationships are not possible. The distance between two bases is 2.90 or 3.00 Å.

D. DNA double strand
DNA consists of two opposing double strands in a double helix. As a result of the spatial relationships of the nucleotide bases,a cytosine will always lie opposite to a guanine
and a thymine opposite to an adenine.The sequence of the nucleotide bases on one strand of DNA (in the 5 _ to 3 _ direction)is complementary to the nucleotide base sequence (or simply the base sequence)of the other strand in the 3 _ to 5 _ direction.The specificity of base pairing is the most important structural characteristic of DNA. Most DNA is a right-handed helix with a diameter of 20 Å (2 _10 –6 mm).Bases on the same strand are 0.36 nm (3.6 Å)apart.A helical turn repeats itself at intervals of 3.6 nm,about 10.5 base pairs per turn.This type is the B form of DNA. Genetic information is determined by the sequence of the base pairs (bp).