Biology · Molecular Basis of Inheritance · NEET
Length of DNA = (total number of base pairs) x (distance between two consecutive base pairs). The distance between two next base pairs is 0.34 nm, which is 0.34 x 10^-9 metre. So Length = number of bp x 0.34 x 10^-9 m. This is the single formula NEET tests again and again.
In the Watson-Crick double helix, one full turn is 3.4 nm long and has 10 base pairs. So each base pair takes 3.4 nm / 10 = 0.34 nm of space along the length. NEET expects you to know this value by heart because it is the key number in every DNA length numerical.
Just reverse the formula. Number of base pairs = total length / 0.34 nm. Make sure both are in the same unit (metres). Example: for a 1.1 m DNA, bp = 1.1 / (0.34 x 10^-9) = about 3 x 10^9 bp. This was a real NEET 2022 question.
A human diploid cell has 6.6 x 10^9 base pairs. Multiply by 0.34 nm: 6.6 x 10^9 x 0.34 x 10^-9 m = 2.244 m, which is about 2.2 metres. This 2-metre-long DNA is packed inside a tiny nucleus of only about 10 micrometres, which is why DNA packaging with histones is needed.
3.3 x 10^9 bp is the HAPLOID content of human DNA (in a sperm or egg, one set of chromosomes). 6.6 x 10^9 bp is the DIPLOID content (in a normal body cell, two sets). NEET length numericals for a typical body cell use 6.6 x 10^9 bp, giving about 2.2 m. Watch the wording carefully.
One nanometre (nm) is one billionth of a metre, so 1 nm = 10^-9 m. Therefore 0.34 nm = 0.34 x 10^-9 m. Always convert to metres before multiplying, or your final answer will be off by a huge power of 10 and match a trap option.
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.
It is 0.34 nm, which equals 0.34 x 10^-9 metre. This comes from one helical turn being 3.4 nm and holding 10 base pairs, so 3.4 / 10 = 0.34 nm per base pair.
About 2.2 metres. A diploid human cell has 6.6 x 10^9 base pairs, and 6.6 x 10^9 x 0.34 x 10^-9 m = 2.244 m. This long thread is packed into a nucleus only about 10 micrometres wide.
NCERT gives E. coli DNA length as 1.36 mm. Number of bp = 1.36 x 10^-3 m / (0.34 x 10^-9 m) = 4 x 10^6 bp, which matches E. coli's 4.6 x 10^6 bp figure. Same divide-by-0.34-nm method.
Both. In molecular biology, DNA length is usually stated as the number of base pairs (bp). To get the physical length in metres, multiply the bp count by 0.34 nm. NEET may ask for either, so learn to switch between them.
Pitch (3.4 nm) is the length of one complete turn of the helix. 0.34 nm is the rise per single base pair. Pitch = 0.34 nm x 10 base pairs. Do not swap these two values in a numerical.
