Which enzyme seals nicks in the sugar-phosphate backbone during replication?

Which enzyme seals nicks in the sugar-phosphate backbone during replication?

• A. Topoisomerase relieves supercoiling.
• B. Ligase seals nicks in the sugar-phosphate backbone.
• C. Helicase unwinds the double helix.
• D. Primase lays down RNA primers.

Answer: (B)

In DNA replication, the final sealing of the backbone between pieces is done by DNA ligase. After the lagging strand is synthesized in fragments (Okazaki fragments), there are small gaps—nickes—between fragments that must be connected to make a continuous strand. DNA ligase catalyzes the formation of phosphodiester bonds to join these adjacent nucleotides, effectively sealing the nicks and completing the sugar-phosphate backbone. This enzyme uses energy (ATP in eukaryotes; NAD+ in some bacteria) to drive the ligation reaction, which is why it’s the one that closes those gaps. The other enzymes have different roles: topoisomerase relieves supercoiling ahead of the fork; helicase unwinds the double helix; primase synthesizes short RNA primers to start DNA synthesis.

In DNA replication, the final sealing of the backbone between pieces is done by DNA ligase. After the lagging strand is synthesized in fragments (Okazaki fragments), there are small gaps—nickes—between fragments that must be connected to make a continuous strand. DNA ligase catalyzes the formation of phosphodiester bonds to join these adjacent nucleotides, effectively sealing the nicks and completing the sugar-phosphate backbone. This enzyme uses energy (ATP in eukaryotes; NAD+ in some bacteria) to drive the ligation reaction, which is why it’s the one that closes those gaps.

The other enzymes have different roles: topoisomerase relieves supercoiling ahead of the fork; helicase unwinds the double helix; primase synthesizes short RNA primers to start DNA synthesis.