Biology · Molecular Basis of Inheritance · NEET
The pitch is 3.4 nm. Pitch means the length of one complete turn of the helix. The value 0.34 nm is smaller. It is the distance between two consecutive (next-door) base pairs, not the pitch. Remember: 3.4 nm is the big number (one full turn), 0.34 nm is the small number (one step). This exact mix-up is a very common NEET trap.
One turn (pitch) is 3.4 nm and has about 10 base pairs. Divide the turn length by the number of base pairs in it: 3.4 nm / 10 bp = 0.34 nm per base pair. So 0.34 nm is simply the pitch shared equally among the 10 base pairs of one turn.
There are roughly 10 base pairs (bp) in each turn of the DNA double helix. NCERT uses the word 'roughly', so treat it as about 10 for NEET calculations.
Pitch = length of one full 360 degree turn = 3.4 nm. Distance between base pairs = the small rise from one base pair to the next = 0.34 nm. Pitch is 10 times the distance between base pairs because one turn holds about 10 base pairs.
Because NEET uses it to calculate the length of DNA. Length of DNA = number of base pairs x 0.34 nm. NEET 2020 and NEET 2022 both asked this. If you swap 0.34 nm for 3.4 nm your final answer becomes 10 times too big or too small, so you must know which number is which.
The Watson-Crick B-DNA helix is right-handed. NCERT states the two chains are coiled in a right-handed fashion. This point is often asked together with the pitch (3.4 nm) in one statement question.
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 x 10^9 bp, then the length of the DNA is approximately
If the length of a DNA molecule is 1.1 metres, what will be the approximate number of base pairs?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The pitch is the length of one complete turn of the DNA helix. In B-DNA it is 3.4 nm (3.4 x 10^-9 m).
It is approximately 0.34 nm. This comes from dividing the pitch (3.4 nm) by the roughly 10 base pairs present in one turn.
Roughly 10 base pairs per turn, according to NCERT.
The Watson-Crick B-DNA double helix is right-handed.
Multiply the total number of base pairs by 0.34 nm. For example, 6.6 x 10^9 bp x 0.34 nm gives about 2.2 metres of DNA per mammalian cell.
Two reasons in NCERT: the hydrogen bonds between complementary bases, and the stacking of one base pair over the next in the double helix.
