In the DNA double helix, where are the nitrogenous bases located relative to the sugar-phosphate backbone?
AOn the outer surface, facing the solvent
BPointing inward, facing the bases on the opposite strand
CAlternating between the inside and outside depending on the base type
DAttached to the phosphate group on the exterior
The sugar-phosphate backbone runs along the outside of the helix, while the bases point inward toward the center, where they pair with complementary bases on the opposite strand via hydrogen bonds. This arrangement protects the genetic information while exposing the backbone to the aqueous environment.
Question 2 True / False
Adenine (A) can form hydrogen bonds with guanine (G) in a normal DNA double helix because both are purines and have compatible shapes.
TTrue
FFalse
Answer: False
Base pairing is determined by complementary hydrogen-bond donor/acceptor geometry, not by purine/pyrimidine classification. A pairs only with T (two hydrogen bonds) and G pairs only with C (three hydrogen bonds). A and G are both purines but cannot pair with each other; a purine must pair with the pyrimidine of the appropriate complementary type.
Question 3 Short Answer
Why is the antiparallel orientation of the two DNA strands important for replication?
Think about your answer, then reveal below.
Model answer: DNA polymerase can only synthesize new DNA in the 5'-to-3' direction. Because the template strands run antiparallel, each strand can serve as a template read 3'-to-5' while new DNA is synthesized 5'-to-3'. This also means the two new strands are made differently: one continuously (leading strand) and one in fragments (lagging strand, Okazaki fragments).
The antiparallel rule is not just structural — it directly constrains how replication machinery works. Both strands of the double helix carry independent, readable sequence information, and the complementary base-pairing rules ensure each strand can template the faithful synthesis of its partner.