Final answer:
RNA nucleotides serve as primers providing a free 3'-OH group for DNA polymerase to start DNA replication. These RNA primers are later replaced by DNA nucleotides, ensuring continuity and accuracy of the newly synthesized DNA strand.
Step-by-step explanation:
RNA nucleotides are added first during DNA replication because DNA polymerase, the enzyme responsible for adding nucleotides to the growing DNA strand, cannot initiate the synthesis of a new strand on its own. It requires a primer with a free 3'-OH group to which it can attach nucleotides, creating a phosphodiester bond between the 3'-OH end of the primer and the 5' phosphate of the new nucleotide. Since DNA polymerase can only extend in the 5' to 3' direction and cannot create a primer, an RNA primer is synthesized by the enzyme RNA primase. This primer provides the necessary free 3'-OH group for the DNA polymerase to start adding DNA nucleotides. Once a segment of DNA has been synthesized, the RNA primer is removed and replaced with DNA nucleotides, which are also added in the 5' to 3' direction, ensuring continuity and accuracy in the newly formed DNA strand.
The importance of the RNA primer in DNA replication is evident in the process of synthesizing Okazaki fragments on the lagging strand. Each fragment begins with an RNA primer, allowing DNA polymerase to fill in the gaps with DNA nucleotides. Finally, an enzyme removes the RNA primer and replaces it with DNA nucleotides, and DNA ligase seals the gaps between fragments to complete the replication process.