DNA is a polymer of deoxyribonucleotides (or simply deoxynucleotides).
It is composed of monomeric units namely deoxyadenylate (dAMP), deoxyguanylate (dGMP), deoxycytidylate (dCMP) and deoxythymidylate (dTMP) (It may be noted here that some authors prefer to use TMP for deoxythymidylate, since it is found only in DNA).
Schematic representation of polynucleotides
- The monomeric deoxynucleotides in DNA are held together by 3c,5cphosphodiester bridges.
- DNA (or RNA) structure is often represented in a short-hand form.
- The horizontal line indicates the carbon chain of sugar with base attached to C1c.
- Near the middle of the horizontal line is C3c phosphate linkage while at the other end of the line is C5c phosphate linkage.
Chargaff’s rule of DNA composition
- Erwin Chargaff in late 1940s quantitatively analysed the DNA hydrolysates from different species.
- He observed that in all the species he studied, DNA had equal numbers of adenine and thymine residues (A = T) and equal numbers of guanine and cytosine residues (G = C).
- This is known as Chargaff’s rule of molar equivalence between the purines and pyrimidines in DNA structure.
- The significance of Chargaff’s rule was not immediately realised. The double helical structure of DNA derives its strength from Chargaff’s rule
- Single-stranded DNA, and RNAs which are usually single-stranded, do not obey Chargaff’s rule.
- However, double-stranded RNA which is the genetic material in certain viruses satisfies Chargaff’s rule
DNA DOUBLE HELIX
The double helical structure of DNA was proposed by James Watson and Francis Crick in 1953 (Nobel Prize, 1962).
- The DNA is a right handed double helix. It consists of two polydeoxyribonucleotide chains (strands) twisted around each other on a common axis.
- The two strands are antiparallel, i.e., one strand runs in the 5c to 3c direction while the other in 3c to 5c direction. This is comparable to two parallel adjacent roads carrying traffic in opposite direction.
- The width (or diameter) of a double helix is 20 A° (2 nm).
- Each turn (pitch) of the helix is 34 A° (3.4 nm) with 10 pairs of nucleotides, each pair placed at a distance of about 3.4 A°.
- Each strand of DNA has a hydrophilic deoxyribose phosphate backbone (3c5c phosphodiester bonds) on the outside (periphery) of the molecule while the hydrophobic bases are stacked inside (core).
- The two polynucleotide chains are not identical but complementary to each other due to base pairing
- The two strands are held together by hydrogen bonds formed by complementary base pairs .The A-T pair has 2 hydrogen bonds while G-C pair has 3 hydrogen bonds. The G { C is stronger by about 50% than A = T.
- The hydrogen bonds are formed between a purine and a pyrimidine only. If two purines face each other, they would not fit into the allowable space. And two pyrimidines would be too far to form hydrogen bonds. The only base arrangement possible in DNA structure, from spatial considerations is A-T, TA, G-C and C-G.
- The complementary base pairing in DNA helix proves Chargaff’s rule. The content of adenine equals to that of thymine (A = T) and guanine equals to that of cytosine (G = C).
- The genetic information resides on one of the two strands known as template strand or sense strand. The opposite strand is antisense strand. The double helix has (wide) major grooves and (narrow) minor grooves along the phosphodiester backbone. Proteins interact with DNA at these grooves, without disrupting the base pairs and double helix
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