The DNA structure proposed by Watson and Crick is - 
(a) Primary
(b) Secondary
(c) Tertiary
(d) Quaternary
Explain your choice with a Reason.

Dear student,​​​​​​ 
  1. Primary structure consists of a linear sequence of nucleotides that are linked together by phosphodiester bonds.
  2. Secondary structure is the set of interactions between bases. In DNA double helix, the two strands of DNA are held together by hydrogen bonds. The nucleotides on one strand base pair with the nucleotides on the other strand. The types of DNA based on secondary structure are A –DNA, B-DNA,C-DNA and Z-DNA. Out of this, James Watson and Francis Crick had discovered the B- DNA.
  3.  Tertiary structure reflects interactions which contribute to overall three dimensional shape.  Examples for tertiary type of DNA includes cruciform, triple helices and super coils.
  4.  Quaternary structure - Here nucleic acids interact with histones to form nucleosomes, and then organize into higher-order chromatin fibres.
Thus, James Watson and Francis Crick postulated the secondary structure of DNA in 1953 in their double helical model. Hence correct option is b- Secondary. 

Regards

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