Why rna primer is synthesized in dna replication instead of dna primer
The Correct Answer and Explanation is:
Correct Answer:
RNA primers are synthesized instead of DNA primers during DNA replication because the enzyme that initiates the synthesis of a new strand, primase, can only synthesize RNA primers. DNA polymerases cannot begin synthesis on a bare template strand; they require a free 3′-OH group to add nucleotides, which the RNA primer provides.
Textbook-Style Explanation
During DNA replication, the accurate copying of the genetic material is essential for the transmission of information from one generation to the next. The process is initiated at specific sites called origins of replication, where the two DNA strands are separated to form a replication fork. DNA polymerase, the main enzyme responsible for synthesizing new DNA, cannot begin this process alone. It can only add nucleotides to an existing strand of nucleic acid that provides a free 3’-OH group. Therefore, a primer is required to provide this starting point.
This primer is made of RNA, not DNA. The enzyme responsible for synthesizing the primer is called primase, a type of RNA polymerase. Primase can begin synthesis without a pre-existing strand. It assembles a short segment of RNA—usually 5 to 10 nucleotides long—complementary to the DNA template strand. This RNA primer provides the necessary 3’-OH group that DNA polymerase needs to start elongating the new DNA strand.
One reason RNA primers are used rather than DNA primers is due to the structural and enzymatic differences between RNA polymerases and DNA polymerases. RNA polymerases, like primase, do not require a primer to begin synthesis, while DNA polymerases do. Additionally, using RNA primers helps the cell distinguish between the newly synthesized strand and the original DNA, aiding in error-checking and repair mechanisms.
After DNA synthesis is well underway, the RNA primers are removed by other enzymes, such as RNase H or DNA polymerase I, and replaced with DNA. The final gap is sealed by the enzyme DNA ligase. This step ensures that the entire DNA molecule is composed only of DNA and is continuous.
