DNA Structure and Replication

Objective: Describe DNA polarity and complementary strands.

DNA is a polymer of nucleotides, each with a deoxyribose sugar, phosphate group, and a nitrogenous base. In double-stranded DNA, two antiparallel strands form a helix with sugar-phosphate backbones outside and bases paired inside. Adenine typically pairs with thymine and guanine with cytosine through interactions including hydrogen bonds; base stacking also stabilizes the helix. The sequence of bases carries information, while complementary pairing allows a strand to guide synthesis of another. A strand has distinct 5′ and 3′ ends. The familiar double helix is a structural model supported by several kinds of evidence; DNA may also occur in other structures and conformations.

Objective: Explain templates and products of one replication round.

Semiconservative replication produces DNA double helices each containing one strand from the starting molecule and one newly synthesized strand. The parental strands separate, and complementary base pairing helps determine the new sequences. DNA polymerases synthesize new DNA only in the 5′ to 3′ direction while reading a template in the 3′ to 5′ direction and require a preexisting primer end. Replication begins at origins; the number and organization of origins vary across organisms. A copied sequence is usually highly similar to its template, but errors can occur and repair systems help reduce them.

Objective: Distinguish leading and lagging synthesis at a moving fork.

At a replication fork, helicase separates the parental strands, proteins help stabilize exposed templates, and topoisomerases relieve twisting stress. Since DNA polymerases extend 5′ to 3′ and templates are antiparallel, one new strand can be synthesized toward the moving fork in a more continuous manner (leading strand), while the other is synthesized in shorter Okazaki fragments away from the fork (lagging strand). Primase supplies RNA primers; primer removal, gap filling, and ligase action help join fragments. Both new strands are synthesized 5′ to 3′. These roles are simplified across diverse replication systems; enzyme names and arrangements vary.