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Question 3
A double stranded DNA solution with an absorbance of 1.0 at 260 nm has a concentration of 50 μg/ml while a single stranded DNA solution with the same absorbance would only have a concentration of 35 μg/mL. Account for the difference in the absorption properties.

The purine and pyrimidine bases in DNA show absorbance at 260 nm. In double stranded DNA, purine and pyrimidine bases are well stacked and they absrobs less UV due to better stacking interaction. But, this base stacking interaction is absent in case of single stranded DNA and they absobs more strongly than double stranded DNA.
Thus, an absorbance of 1.0 at 260 nm can be obtained by a concentration of 50 μg/ml of double stranded DNA, whereas a less amount of 35 μg/mL single stranded DNA can absorb the same amount of UV.

Question 4
Explain how the twist in DNA arises.

Sugar and phosphate of the DNA are hydrophilic whereas, nitrogen bases are hydrophobic. Thus N- bases tends to remain in the core region of the double stranded DNA axis. Moreover, N-bases of two strands attracts each other while another force, the base stacking force attracts the bases of the same DNA strand above or below of that particular base. This happens in case of each and every base of the double stranded DNA molecule and to make balance between these two attractive forces along with hydrophobic interaction, double stranded DNA get twisted....
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