DNA primase creates what starting point for DNA synthesis?

DNA primase creates what starting point for DNA synthesis?

• A. DNA primer
• B. RNA primer
• C. Protein primer
• D. Lipid primer

Answer: (B)

DNA synthesis cannot start from nothing; a starting point with a free 3' OH is required for DNA polymerases to add nucleotides. Primase is an RNA polymerase that lays down a short RNA primer complementary to the DNA template, creating that essential starting block. Because RNA primers provide the necessary 3' hydroxyl, DNA polymerases can extend the chain from there. After elongation begins, the RNA primer is removed and replaced with DNA, and the fragments are joined. The primer must be RNA here because RNA polymerases can start de novo without a preexisting strand, whereas a DNA primer would not be formed by primase, and protein or lipid primers aren’t nucleic acids and can’t serve as priming templates.

DNA synthesis cannot start from nothing; a starting point with a free 3' OH is required for DNA polymerases to add nucleotides. Primase is an RNA polymerase that lays down a short RNA primer complementary to the DNA template, creating that essential starting block. Because RNA primers provide the necessary 3' hydroxyl, DNA polymerases can extend the chain from there. After elongation begins, the RNA primer is removed and replaced with DNA, and the fragments are joined. The primer must be RNA here because RNA polymerases can start de novo without a preexisting strand, whereas a DNA primer would not be formed by primase, and protein or lipid primers aren’t nucleic acids and can’t serve as priming templates.