Explain the difference between the leading and lagging strands during DNA replication and the role of Okazaki fragments.

Explain the difference between the leading and lagging strands during DNA replication and the role of Okazaki fragments.

• A. The leading strand is synthesized discontinuously opposite fork progression; lagging strand is continuous toward fork; No lagging fragments.
• B. The leading strand is synthesized continuously in the direction of fork progression; the lagging strand is synthesized discontinuously as Okazaki fragments in the opposite direction, later joined by ligase.
• C. Both strands are synthesized equally fast and both yield Okazaki fragments on the same direction.
• D. The leading strand requires Okazaki fragments; the lagging strand does not.

Answer: (B)

DNA replication relies on two ways of building new DNA on antiparallel templates, and the key is that polymerases can only extend DNA in the 5' to 3' direction. The template strand running 3' to 5' toward the fork is copied continuously in the same direction the fork is moving, producing the leading strand. The other template runs 5' to 3' toward the fork, so synthesis on that strand must proceed away from the fork in short bursts called Okazaki fragments. Each fragment starts with an RNA primer laid down by primase, and DNA polymerase extends from that primer. As the fork advances, the primers are removed and replaced with DNA, and DNA ligase seals the gaps between fragments to create a continuous lagging strand. This arrangement explains why the leading strand is synthesized continuously toward the fork, while the lagging strand is synthesized discontinuously as Okazaki fragments in the opposite direction, later joined by ligase.

DNA replication relies on two ways of building new DNA on antiparallel templates, and the key is that polymerases can only extend DNA in the 5' to 3' direction. The template strand running 3' to 5' toward the fork is copied continuously in the same direction the fork is moving, producing the leading strand. The other template runs 5' to 3' toward the fork, so synthesis on that strand must proceed away from the fork in short bursts called Okazaki fragments. Each fragment starts with an RNA primer laid down by primase, and DNA polymerase extends from that primer. As the fork advances, the primers are removed and replaced with DNA, and DNA ligase seals the gaps between fragments to create a continuous lagging strand. This arrangement explains why the leading strand is synthesized continuously toward the fork, while the lagging strand is synthesized discontinuously as Okazaki fragments in the opposite direction, later joined by ligase.