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 |
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