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Q.
Each strand of DNA appears like a helical staircase, and each step of ascent is represented by a pair of bases. At each step of ascent, the strand turns ____
NTA AbhyasNTA Abhyas 2020
Solution:
Nucleic acids display a wide range of secondary structures. One of the secondary structures exhibited by DNA is the famous Watson-Crick model. This model says that DNA exists as a double helix. A DNA molecule has two unbranched polynucleotide strands. Each polynucleotide strand/chain consists of a sequence of nucleotides linked together by phosphodiester bonds.
The double strand of DNA is coiled upon itself around a common axis like a spiral staircase with base pairs forming steps, while the backbones of the two strands form railings. The nitrogen bases are projected more or less perpendicular to the sugar-phosphate backbone but face inside. The base pairing is specific. Adenine pair with thymine and guanine is always pair with cytosine.
At each base pair, the strand turns $36^\circ $ . One complete turn of the helical strand ( $360^\circ $ ) involves $360^\circ $ of the helical strand with about ten nucleotides on each strand of DNA. The diameter of the B-DNA is about $2\text{nm}$ . In other words, the distance between the two DNA polynucleotide strands is $2nm$ .