Question

Define and indicate the significance of (a) Okazaki fragments, (b) DNA ligase, and (c) primer RNA during DNA replication.

Short answer

Answer: The three components related to DNA replication are (a) Okazaki fragments, (b) DNA ligase, and (c) primer RNA. Okazaki fragments are short sequences of nucleotides produced on the lagging strand, allowing complete replication of both DNA strands. DNA ligase is an enzyme that connects the Okazaki fragments, ensuring accurate copies of the original DNA molecule. Primer RNA facilitates the initiation of DNA replication on both the leading and lagging strands by providing a starting point for DNA synthesis.

Step-by-step solution

(a) Okazaki fragments

Okazaki fragments are short sequences of nucleotides (around 100-200 nucleotides in eukaryotes, and 1,000-2,000 nucleotides in prokaryotes) that are produced on the lagging strand of DNA during replication. This is because DNA replication happens in the 5' to 3' direction, and the lagging strand is synthesized discontinuously in small stretches, which are later joined together. The significance of Okazaki fragments is that they allow the progression of DNA replication on the lagging strand in the 3' to 5' direction, ensuring complete replication of both DNA strands.

(b) DNA ligase

DNA ligase is an enzyme that plays a crucial role in joining the Okazaki fragments on the lagging strand during DNA replication. It forms phosphodiester bonds between the individual fragments, sealing the gaps and creating a continuous DNA strand. The significance of DNA ligase is that it ensures the completion of replication by connecting the discontinuous fragments on the lagging strand, making sure that the newly synthesized DNA molecules are intact and accurate copies of the original DNA molecule.

(c) Primer RNA

Primer RNA, or RNA primer, is a short sequence of RNA nucleotides that is synthesized by an enzyme called primase. The RNA primer acts as a starting point for DNA synthesis by providing a free 3'-OH group to which the DNA polymerase can add new DNA nucleotides. Unlike DNA polymerase, primase can synthesize the RNA primer without needing a pre-existing strand with a free 3'-OH group. The significance of primer RNA is that it allows the initiation of DNA replication on both the leading and lagging strands and ensures proper synthesis of the new DNA molecules. Once the replication is complete, the RNA primers are removed and replaced with the corresponding DNA nucleotides, and DNA ligase seals the remaining gaps.