Question

Which of the following is the enzyme that replaces the RNA nucleotides in a primer with DNA nucleotides? a. DNA polymerase III b. DNA polymerase I c. primase d. helicase

Short answer

The enzyme that replaces the RNA nucleotides in a primer with DNA nucleotides is DNA polymerase I.

Step-by-step solution

Understanding Primer Replacement

In DNA replication, an enzyme is needed to remove the RNA nucleotides of the primer and replace them with DNA nucleotides. This process ensures that the resulting DNA strand is entirely composed of DNA nucleotides.

Identifying the Correct Enzyme

DNA polymerase III is responsible for the synthesis of the leading and lagging strands during DNA replication, but it does not replace RNA nucleotides in primers with DNA nucleotides. DNA polymerase I, on the other hand, has a specific activity that removes the RNA primer nucleotides and fills in the gaps with DNA nucleotides. Primase is the enzyme that synthesizes the RNA primer, and helicase is the enzyme that unwinds the DNA helix.

Selecting the Correct Answer

Based on the roles of the enzymes, the one which replaces RNA nucleotides of the primer with DNA nucleotides is DNA polymerase I.

Key concepts

DNA Polymerase I

DNA Polymerase I plays a pivotal role in the process of DNA replication. It is a multi-functional enzyme with several important roles in the replication and repair of DNA. Unlike DNA Polymerase III, which primarily adds nucleotides in a rapid fashion to build a new DNA strand, DNA Polymerase I has additional jobs.

After the RNA primer is laid down by Primase to start the replication process, DNA Polymerase I has a key function called 'excision repair.' It meticulously removes the RNA nucleotides of the RNA primers that were initially laid down to start DNA synthesis. The removal is facilitated by its 5' to 3' exonuclease activity, which is essential for proofreading.

Furthermore, once the RNA nucleotides from the primers are removed, DNA Polymerase I fills these gaps with DNA nucleotides, using its 5' to 3' polymerase activity. This ensures that the DNA molecule is composed exclusively of DNA nucleotides, maintaining the integrity and continuity of the genetic information. Additionally, its 3' to 5' exonuclease activity helps in proofreading, thereby ensuring that the newly synthesized DNA is free from errors.

Primer Replacement

The primer replacement process is an essential step in DNA replication. During replication, short fragments of RNA, called primers, are placed along the DNA strands by an enzyme called Primase. These primers act as starting points for DNA synthesis.

However, RNA primers cannot remain in the DNA sequence; they must be replaced with DNA nucleotides. This task is performed by the enzyme DNA Polymerase I. It removes the RNA primer with its exonuclease activity, moving in the 5' to 3' direction, and then fills in the vacant spots with corresponding DNA nucleotides.

This action of replacing RNA with DNA is crucial because DNA molecules can only function properly if they consist entirely of DNA nucleotides. Primer replacement ensures that the genetic information is accurately transmitted and the DNA strands are completely composed of DNA nucleotides before the replication process is finalized.

DNA Synthesis

DNA synthesis is the biological process by which a cell replicates its DNA. This process is fundamental to all living organisms, as it is necessary for cell division and the transmission of genetic information to offspring. The process occurs in the S-phase of the cell cycle.

DNA synthesis is initiated by RNA primers and is carried out by DNA polymerases. Several enzymes work in tandem to ensure the fidelity and completeness of the newly synthesized DNA. DNA Polymerase III synthesizes the new DNA by adding nucleotides in a 5' to 3' direction, creating the leading and lagging strands. The lagging strand is synthesized in short segments known as Okazaki fragments.

When synthesis on the lagging strand reaches the RNA primer of the previous Okazaki fragment, DNA Polymerase I replaces the RNA primer with DNA nucleotides. DNA Ligase then joins the Okazaki fragments to form a continuous strand. DNA synthesis is a prime example of biological precision, where enzymes like DNA Polymerase I ensure that every newly synthesized DNA molecule is a complete and exact copy of the original DNA molecule.