Biology · Molecular Basis of Inheritance · NEET
James Watson and Francis Crick proposed the Double Helix model in 1953. They did not do the experiments themselves — they used the X-ray diffraction data produced by Maurice Wilkins and Rosalind Franklin, plus Erwin Chargaff's base-ratio rule (A=T and G=C). For NEET, remember the year 1953 and all four names, because match-the-scientist questions are common.
NCERT lists these features: (i) DNA has two polynucleotide chains with a sugar-phosphate backbone on the outside and bases projecting inside. (ii) The two chains are antiparallel — one runs 5'→3' and the other 3'→5'. (iii) Bases pair by hydrogen bonds (A=T by two H-bonds, G≡C by three H-bonds), so a purine always faces a pyrimidine, giving a uniform strand distance. (iv) The chains coil in a right-handed fashion; pitch = 3.4 nm, with roughly 10 base pairs per turn. Learn these four points exactly — NEET picks single features to test.
Because two chains are held together by hydrogen bonds between complementary bases. Base pairing makes the structure stable and lets each strand act as a template for the other during replication. The word 'helix' means the strands are twisted, and 'double' means there are two of them.
The normal B-form DNA of the Watson-Crick model is RIGHT-handed. This is a very common NEET trap — students confuse it with left-handed. The two chains are coiled in a right-handed fashion.
A purine (Adenine or Guanine, two rings) always pairs with a pyrimidine (Thymine or Cytosine, one ring). A big purine plus a small pyrimidine keeps the width between the two backbones the same all along the helix (a uniform distance). If two purines or two pyrimidines paired, the helix width would change and the structure would not be stable.
0.34 nm (3.4 Å) is the distance between two neighbouring base pairs (the rise). Pitch = 3.4 nm is the length of one complete turn of the helix. Since 3.4 nm ÷ 0.34 nm = 10, there are about 10 base pairs in each turn. Do not mix up 0.34 nm (one step) with 3.4 nm (one full turn).
Which of the following statements is correct?
If the distance between two consecutive base pairs is 0.34 nm and the total number of base pairs of a DNA double helix in a typical mammalian cell is 6.6 × 10^9 bp, then the length of the DNA is approximately
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is the 1953 Double Helix model: two antiparallel polynucleotide chains twisted right-handed, backbone outside, complementary bases (A=T, G≡C) paired by hydrogen bonds inside, with a 3.4 nm pitch and about 10 base pairs per turn.
About 10 base pairs per turn. The pitch (one full turn) is 3.4 nm and each base pair is 0.34 nm apart, so 3.4 ÷ 0.34 = 10 bp per turn.
The X-ray diffraction data of Maurice Wilkins and Rosalind Franklin. Watson and Crick also used Erwin Chargaff's rule that A=T and G=C in double-stranded DNA.
The two strands run in opposite directions: if one strand goes 5'→3', the partner strand goes 3'→5'. This opposite orientation is a key salient feature of the double helix.
Yes. The two chains are coiled in a right-handed fashion in the standard B-DNA of the Watson-Crick model. Left-handed coiling is a common NEET distractor and is wrong for this model.
