Question

One turn of the helix in a B form DNA is approximately

• A. 20 nm
• B. 0.34 nm
• C. 3.4 nm
• D. 2 nm

Answer 1

The correct option is C 3.4 nm

Majority of DNA present in the cytoplasm of living cells exist in double helical structure as described by Watson and Crick. Watson and Crick proposed that DNA molecule is in the form of a double helix with two polynucleotide chains coiled around one another in a spiral. This is known as B-DNA or biological DNA.

It has 10 nucleotides pairs per turn of the double helix and is right handed with a diameter of 20$\mathring{A}$. One complete turn of the double helix is 34$\mathring{A}$.The distance between adjacent stacks or nucleotides is 3.4$\mathring{A}$.

Each chain has a sequence of oligonucleotides attached together by phosphodiester bonds joining the adjacent deoxyribose molecules. The two polynucleotide chains are helically coiled round one another with the help of hydrogen bonds between nitrogen bases of opposite strands. The base pairing is specific: adenine always pairs with thymine and cytosine always pairs with guanine. Other forms of DNA are A-DNA found in high salt concentration or in dehydrated form with 11 nucleotide pairs per turn and is right handed, its diameter is 23 Z-DNA is left handed with 12 nucleotide pairs per turn. The length of DNA double helix in a typical mammalian cell is calculated comes out to be approximately 2.2metres (by multiplying the total number of bp with distance between two consecutive bp, i.e., 6.6million bp X 0.34 nanometer/bp.