Question

Watson and Crick model of DNA is a
A. B-form DNA with a spiral length of $34\mathop A\limits^ \circ$and a diameter of $20\mathop A\limits^ \circ$
B. A-form with a spiral length of $15\mathop A\limits^ \circ$ a diameter of $20\mathop A\limits^ \circ$
C. Z-form DNA with a spiral length of $34\mathop A\limits^ \circ$and a diameter of $20\mathop A\limits^ \circ$
D. B-form DNA spiral length of $28\mathop A\limits^ \circ$ and a diameter of $14\mathop A\limits^ \circ$

Answer 1

The Watson and Crick model of DNA is a double stranded DNA molecule. It was discovered in 1953. This discovery in history gave rise to modern molecular biology. DNA stands for Deoxyribonucleic Acid, it has two sugar- phosphate backbones present on the outside that are held together by hydrogen bonds between pairs of nitrogenous bases on the inside.

Complete answer:
This model given by Watson and Crick is the hereditary molecules which are responsible for carrying parental characters to offspring. The DNA is in the B- form and this is the most common conformation of DNA.

Option A: B-form DNA with a spiral length of $34\mathop A\limits^ \circ$and a diameter of $20\mathop A\limits^ \circ$ - Model given by Watson and Crick is a B-form DNA .The spiral length of which is $34\mathop A\limits^ \circ$ and its diameter of $20\mathop A\limits^ \circ$. One turn of the spiral has 10 base pairs. Thus, the correct option is this only.

Option B: A-form with a spiral length of $15\mathop A\limits^ \circ$ a diameter of $20\mathop A\limits^ \circ$ - This is not the correct answer as DNA was not A- form that was given by Watson and Crick.

Option C: Z-form DNA with a spiral length of $34\mathop A\limits^ \circ$ and a diameter of $20\mathop A\limits^ \circ$ - This is again not correct because the spiral length and diameter measurement is correct but form of DNA was not Z form. So it is not the correct answer.

Option D : B-form DNA spiral length of $28\mathop A\limits^ \circ$ and a diameter of $14\mathop A\limits^ \circ$ - This answer is also not correct because form of DNA is B but measurement of spiral length and diameter is not correct. Hence this is not the correct answer.

Hence, the correct answer is option (A).

Note: Three forms of DNA are A-form, B-form, Z-form.

A-form: It is a right handed double helix structure like that of B- form, Dehydrated DNA takes an A form which protects DNA in many extreme situations.
B-form: It is the most common form, most DNA has this form under normal physiological conditions, it is a right handed double helix structure.
Z-form: In this form there is a left -handed DNA helix found by Andres Wang.