If one strand of DNA has a C attached to a sugar with a free 5’ phosphate group, the other strand will have a _____ attached to a sugar with a free _____ group.

Question

C; 5’ phosphate.

C; 3’ hydroxyl.

G; 3’ hydroxyl.

G; 3’ phosphate.

G; 5’ phosphate.

Answer 1

Hi Trinity,

The answer to this question would be G, 3' hydroxyl

The reason it is G is because of the rule: C-G, A-T. You can look up the structures to see that this rule reflects the fact that: a) one must be pyrimidine and other purine b) able form the same number of hydrogen bonds (2 for AT and 3 for CG) so that the two strands can align together properly

The reason for 3' hydroxyl is because two strands are antiparallel: If one strand has 5' at one end, then other strand has 3' at the same end. Also, phosphate is at 5' C of sugar and hydroxyl is at 3' C of sugar.

Hope that helps!