Question

when watson and crick built and proposed the dna structure, they determined three important features. what were the three features they proposed? select all that apply.
a) two outside strands consist of alternating deoxyribose and phosphate.
b) two outside strands consist of alternating ribose and phosphate.
c) guanine and cytosine bases pair to each other by two hydrogen bonds.
d) guanine and cytosine bases pair to each other by three hydrogen bonds.
e) guanine and adenine bases pair to each other by three hydrogen bonds.
f) cytosine and thymine bases pair to each other by two hydrogen bonds.
g) adenine and thymine bases pair to each other by three hydrogen bonds.
h) adenine and thymine bases pair to each other by two hydrogen bonds.

Explanation:

1. For the sugar - phosphate backbone: DNA has a backbone of alternating deoxyribose (not ribose, ribose is in RNA) and phosphate groups, so option A is correct and B is incorrect.
2. For base pairing: Guanine (G) pairs with Cytosine (C) via three hydrogen bonds, and Adenine (A) pairs with Thymine (T) via two hydrogen bonds. So option D (G and C with three H - bonds) and option H (A and T with two H - bonds) are correct. Option C (G and C with two H - bonds) is wrong, option E (G and A pairing) is wrong, option F (C and T pairing) is wrong, option G (A and T with three H - bonds) is wrong.

Answer:

A) Two outside strands consist of alternating deoxyribose and phosphate.
D) Guanine and cytosine bases pair to each other by three hydrogen bonds.
H) Adenine and thymine bases pair to each other by two hydrogen bonds.