Role of DNA Polymerases in Replication | RNA Primase | DNA Ligase | Lecture 9 | Dr. Muhammad Naveed

Role of DNA Polymerases, RNA Primase & DNA Ligase in DNA Replication:
DNA polymerases play a critical role in DNA replication, the process by which a cell creates an identical copy of its DNA. Here are their primary functions:
1. DNA Synthesis: DNA polymerases are enzymes responsible for synthesizing new DNA strands by adding complementary nucleotides to the existing template strands. They use the parental DNA strands as a template and add nucleotides according to the base-pairing rules (A with T, C with G).
2. Proofreading and Error Correction: To maintain accuracy, DNA polymerases have proofreading capabilities. They possess a 3' to 5' exonuclease activity, allowing them to detect and remove incorrectly paired nucleotides. This proofreading function helps in minimizing errors in the newly synthesized DNA.
3. Initiation and Elongation: DNA polymerases are involved in both the initiation and elongation phases of DNA replication. During initiation, they recognize specific sequences and start the synthesis of the new DNA strand. Throughout elongation, they continuously add nucleotides to the growing DNA strand.
4. Processivity: DNA polymerases have a high processivity, meaning they are capable of adding multiple nucleotides per binding event without dissociating from the template DNA. This property is crucial for efficient DNA replication.
5. Interaction with other Proteins: DNA polymerases interact with various other proteins and enzymes involved in DNA replication, such as helicases, single-strand binding proteins, and DNA ligase, coordinating the complex process of DNA replication.
Different types of DNA polymerases are present in cells, each with specific functions and roles in various stages of replication and DNA repair processes. The overall contribution of DNA polymerases is fundamental in ensuring accurate and complete replication of the genetic material during cell division.
4. RNA primase
RNA primase plays a crucial role in the process of DNA replication by synthesizing short RNA primers that serve as starting points for DNA synthesis. Here's a breakdown of the role of RNA primase in replication:
1. Initiation of DNA synthesis:
DNA replication begins at specific sites on the DNA molecule called origins of replication. RNA primase is responsible for synthesizing short RNA primers complementary to the DNA template at these initiation sites.
2. Primer synthesis:
DNA polymerases, the enzymes responsible for adding new nucleotides to the growing DNA chain, require a 3' hydroxyl (-OH) group to initiate synthesis. RNA primase synthesizes a short RNA primer with a free 3' OH group, providing the starting point for DNA polymerase to add nucleotides.
3. Formation of RNA-DNA hybrid:
The RNA primer forms a short RNA-DNA hybrid with the template DNA strand. DNA polymerase then recognizes this RNA-DNA junction and starts synthesizing a complementary DNA strand.
4. DNA synthesis by DNA polymerase:
DNA polymerase extends the RNA primer by adding complementary deoxyribonucleotides to form a continuous DNA strand.
As DNA polymerase moves along the template, it displaces the RNA primer.
5. Removal of RNA primers:
Once the DNA strand is synthesized, the RNA primers are no longer needed.
Enzymes called DNA polymerase I or DNA polymerase δ, along with other proteins, remove the RNA primers and replace them with DNA nucleotides.
6. Ligase action:
Gaps are left behind after the removal of RNA primers.
DNA ligase joins the adjacent DNA fragments by catalyzing the formation of phosphodiester bonds, creating a continuous DNA strand.
In summary, RNA primase is essential for the initiation of DNA replication by synthesizing short RNA primers that serve as starting points for DNA polymerases. These primers are later replaced by DNA, and the gaps between them are sealed by DNA ligase, resulting in the formation of two complete and identical DNA molecules.
About Dr. Muhammad Naveed
(HoD, Biotechnology, University of Central Punjab, Lahore)
With distinction, Dr. Muhammad Naveed obtained a Ph.D. degree in Biotechnology (Genomics & Bioinformatics) from Quaid-e-Azam University, Islamabad. He has won Ph.D. indigenous & IRSIP scholarships from HEC. He has done Pre-Doc research at the University of Ghent, Belgium. HEC awarded him the best Ph.D. (IRSIP) Scholar of the Year in 2013 & QAU honored him as a “Distinguished Alumni” in 2017. He is doing research projects in Bioinformatics, Molecular Biotechnology and vaccine designing, and Drug designing against infectious diseases. He has supervised 90 MSc. and 78 MPhil. & 01 Ph.D. students. He has published 150 Research articles with 1120 impact factors, 5260 citations, 01 book, 06 book chapters, and filed 05 Patents. He was awarded the distinguished “Researcher of the Year” in 2016 (UoG) and 2018, 2019 & 2021 (UCP).
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