Which molecule provides a 3' hydroxyl group for DNA polymerase to attach new nucleotides to?

Which molecule provides a 3' hydroxyl group for DNA polymerase to attach new nucleotides to?

• A. Helicase
• B. DNA ligase
• C. Primase
• D. Sliding clamp

Answer: (C)

DNA polymerase can only add new nucleotides to the 3' end of a growing strand because the incoming nucleotide’s phosphate is attached to the 3' hydroxyl of the existing strand, forming the new phosphodiester bond. A free 3' OH is required to start this linkage, so synthesis needs a primer to provide that starting point. Primase creates a short RNA primer, supplying the necessary 3' hydroxyl for DNA polymerase to extend from. Once elongation begins, the primer is removed and replaced with DNA. The other enzymes don’t provide a 3' hydroxyl: helicase unwinds the DNA, DNA ligase joins fragments after synthesis, and the sliding clamp increases polymerase processivity but doesn’t supply the starting hydroxyl.

DNA polymerase can only add new nucleotides to the 3' end of a growing strand because the incoming nucleotide’s phosphate is attached to the 3' hydroxyl of the existing strand, forming the new phosphodiester bond. A free 3' OH is required to start this linkage, so synthesis needs a primer to provide that starting point. Primase creates a short RNA primer, supplying the necessary 3' hydroxyl for DNA polymerase to extend from. Once elongation begins, the primer is removed and replaced with DNA. The other enzymes don’t provide a 3' hydroxyl: helicase unwinds the DNA, DNA ligase joins fragments after synthesis, and the sliding clamp increases polymerase processivity but doesn’t supply the starting hydroxyl.