Fig. 10: DNA Replication by Complementary Base Pairing: Producing an RNA Primer

DNA polymerases cannot begin a new DNA chain from scratch. They can only attach new nucleotides onto 3' OH group of a nucleotide in a preexisting strand. Therefore, to start the synthesis of the leading strand and each DNA fragment of the lagging strand, an RNA polymerase complex called a primosome or primase (not shown here) is required. The primase, which is capable of joining RNA nucleotides without requiring a preexisting strand of nucleic acid, first adds several comlementary RNA nucleotides opposite the DNA nucleotides on the parent strand. This forms what is called an RNA primer. The free ribonucleoside triphosphates line up by complementary base pairing with the nucleotides on each parent strand of the unwound DNA in the replication fork and are then joined together by the primase.

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