Question

The length of one full turn of DNA is
A. 3.4 Angstrom
B. 34.0 Angstrom
C. 20.0 Angstrom
D. 3.04 Angstrom

Answer 1

DNA stands for deoxyribonucleic acid. It is a prevalent hereditary material in various microorganisms and higher organisms. DNA is present on chromosomes present in the nucleus, carrying hundreds or more genes. The popular double-helical structure of DNA was discovered by James Watson and Francis Crick.

Complete answer:
Nucleic acids are made up of nucleotides which in turn are made up of three components namely sugar, phosphoric acid, and a nitrogenous base. Nitrogen bases are classified into two categories namely purines and pyrimidines. Purines consist of nitrogen bases: Adenine (A) and Guanine (G). Pyrimidine consists of nitrogen bases: Thymine (T), Cytosine (C), and Uracil (U). RNA contains nitrogen bases in combination: Adenine, Guanine, Cytosine, and Uracil.
DNA contains nitrogen bases in combination: Adenine, Guanine, Cytosine, and Thymine.
Structure of DNA:
The DNA molecule is double-stranded. It is in the form of a spiral staircase i.e. the two strands are coiled around each other along an imaginary central axis. The two strands are also anti-parallel i.e., they run in the opposite direction with respect to one another. The nitrogenous bases along with hydrogen bonds make up the rungs or steps of the ladder or staircase. Distance between two steps or rungs is 3.4 Angstrom. There are ten steps in each complete turn of DNA hence the distance of one complete turn is 34 Angstrom. The diameter of the DNA molecules is 20 Angstrom.
Hence, the correct answer for the given question is option (B), i.e., 34 Angstrom.

Note: The DNA structure proposed by James Watson and Francis Crick was based on the studies of Maurice Wilkins and Rosalind Franklin. The purine guanine always pairs with pyrimidine cytosine with three hydrogen bonds and the purine adenine can pair with pyrimidine thymine or uracil (depending on RNA or DNA) with two hydrogen bonds.