DNA - Deoxyribonucleic acid-Nucleotides

Alligator December 24, 2021
DNA:-Dioxyribonucleic acid

Nucleotides

To know about DNA 1st we need to know about Nucleotides. Nucleotides are composition of nitrogenous base, pentose suger and a phosphate.Nucleotides perform various functions bside being monomeric units in the nucleic acid structure. Nucleotide consists Nitrogenous base, suger and phosphate Nitrogenous base + suger = Nucleosides + phosphate = Nucleotide.

Nitrogenous Bases of nucelic acid

Nitrogenous bases are of two types:-

1. Purines

Further purines are of two types:-
1) Adenine (A)
2) Guanine (G)
Something like:-

2. Pyrimedines

and Pyrimedines are of two types:-
1) Cytocine (C)
2) Thymine (T) and
3) Uracil (U)
Note:- DNA contain Thymine(T) whereas RNA contain Uracil(U)
Looks like:-

Sugers of nucleic acids

The five carbon monosaccharides (pentose) are found in the nucleic acid structure. RNA contain D-ribose while DNA contain D-deoxyribose. deoxy has one oxygen less at C2 compared to ribose.

DNA DOUBLE HELIX

The double helical structure of DNA was proposed by James Watson and Francis Crick in 1953 and for this he won nobel prize in 1962. This structure of DNA double helix is comparable to a twisted ladder. The description of DNA structure is considered as milestone in the era of modern biology.

Note:- Erwin chargaff observed that DNA had equal numbers of adinine and thymine residues(A=T) and equal numbers of Guanine and Cytocine residues (G=C) DNA double Strand:- Key Points

1. The DNA is a right handed double helix. it consists of two polydeoxyribonucleotide chains (strands) twisted around each other on a common axis (fig. B)

2. The two strands are antiparallel, i.e., one strands run in the 5' to 3' direction while the other in 3' to 5' direction. This can be compared as two parallel adjacent roads carriying traffic in opposite direction.

3. The width (diameter) of a double helix is 20 A°( 2nm)

4. Each turn of the helix is 34 A°(3.4nm) with 10 pairs of Nucleotides, each pair placed at a distance of about 3.4 A°

5. The two polynucleotide chains arenot identicle but complementry to each other because of base pairing.

6. The two strands are held together by hydrogen bonds formed by complementry base pairs. The A-T pair has 2 hydrogen bonds while G-C has 3 hydrogen bonds something like:-

7. The complementry base pairing in DNA helix proves Chargaff's rule adenime equals to thymine (A=T) and guanine equals to Cytocine(G=C)

8. The genetic information resides on one of the two strands known as templare strand or sense strand.

Conformations of DNA double helix

The double of DNA exists in at least 6 different forms:

  1. A-form
  2. B-form
  3. C-form
  4. D-form
  5. E-form and
  6. Z-form
Among these, B, A and Z forms are important. The B-form of DNA double helix, described by watson and Crick is most predominant form under physiological conditions. Each turn of B-form has 10 base pairs spanning a distance of 3.4 nm. The width of double helix in 2nm.

The A-form is also right-handed helix. It contians 11 base pairs per turn and titled 20° away from central axis.

The Z-form(Z DNA) is left handed helix and contains 12 base pairs per turn. In this form DNA moves somewhat "zig-zag" fashion hence the name Z-form.

For more detail read Comparision table.


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