Question

# How long would a B DNA molecule be if it was free of supercoiling and had...

How long would a B DNA molecule be if it was free of supercoiling and had 9.4 million nucleotides? Express answer in angstroms. Also, if the chromosome was in A DNA form instead, what is one thing that would be different about the alternative form of DNA?

ANS: 1) In case of B DNA form, the rise per base pair along axis = 3.32 Angstroms

B DNA had 9.4 million nucleotides = (9.4 x 1000000) nucleotides

Therefore, length of B DNA (when free of supercoiling) = (3.32 x 9.4 x 1000000) Angstroms = 31208000 Angstroms

ANS: 2) A DNA form is different in some properties from B DNA form. In case of A DNA form, the rise per base pair along axis is 2.3 Angstroms. The diameter of DNA duplex is 20 Angstroms in A DNA form where as in B DNA form it is 23 Angstroms. Other properties are given in a table below.

 Properties A DNA B DNA Rotation/ Base Pair (BP) 33.6o 34.3o Mean BP/ turn 10.7 10.4 BP tilt 20o - 6o Pitch/ turn of helix 24.6 Angstroms 33.2 Angstroms Mean propeller twist +18o +16o Suger pucker C3' - endo C2' - endo Major groove Narrow and deep Wide and deep Minor groove Wide and shallow Narrow and deep