Which enzyme seals the sugar-phosphate backbone during DNA replication?

• A. DNA polymerase
• B. Helicase
• C. Primase
• D. DNA ligase

Answer: (D)

Joining the sugar-phosphate backbone after DNA synthesis is carried out by DNA ligase. During replication, DNA polymerase can build new DNA, but on the lagging strand it does so in short pieces called Okazaki fragments. After primers are removed and gaps filled in, nicks remain between fragments. DNA ligase repairs these nicks by forming phosphodiester bonds, sealing the backbone and producing a continuous DNA strand. This reaction uses energy (ATP in eukaryotes, NAD+ in some prokaryotes). The other enzymes have different roles: helicase unwinds the double helix, primase lays down RNA primers to start synthesis, and DNA polymerase adds nucleotides but does not seal the backbone.