Describe the steps of DNA synthesis occurring on lagging strand of DNA. (IFS 2022, 10 Marks)

Introduction

DNA synthesis on the lagging strand of DNA occurs during DNA replication, a crucial process that ensures the accurate transmission of genetic information from one generation to the next. This process involves several steps that are carried out by a complex machinery of enzymes and proteins. 

Steps of DNA Synthesis on the Lagging Strand

• Helicase Unwinds the DNA: The enzyme helicase unwinds the double-stranded DNA at the replication fork to create single-stranded DNA templates for replication.
• Primase Synthesizes RNA Primer: The enzyme primase synthesizes a short RNA primer (typically 10–12 nucleotides long) on the lagging strand. This primer provides a 3' hydroxyl group to initiate DNA synthesis, as DNA polymerase can only add nucleotides to an existing strand.
• DNA Polymerase III Adds Nucleotides: DNA polymerase III adds complementary DNA nucleotides to the 3' end of the RNA primer, synthesizing DNA in the 5' to 3' direction. This forms an Okazaki fragment, a short segment of newly synthesized DNA.
• Okazaki Fragment Formation: Each Okazaki fragment is initiated with an RNA primer, followed by DNA synthesis by DNA polymerase III. The lagging strand is synthesized discontinuously in small segments because the DNA polymerase cannot move continuously along the template.
• RNA Primer Removal: The RNA primers are removed by the enzyme DNA polymerase I, which has exonuclease activity. This enzyme replaces the RNA with DNA nucleotides.
• DNA Ligase Joins Okazaki Fragments: After the RNA primers are replaced, DNA ligase seals the gaps between the Okazaki fragments by forming phosphodiester bonds, ensuring that the newly synthesized strand is continuous.
• Completion of the Lagging Strand: The lagging strand is now fully synthesized. The replication process is complete, with the lagging strand made up of a series of linked Okazaki fragments that are joined together by DNA ligase.

Conclusion

DNA synthesis on the lagging strand is a complex process that involves the coordinated action of multiple enzymes and proteins. Understanding the steps involved in DNA synthesis on the lagging strand is crucial for unraveling the mysteries of DNA replication and ensuring the faithful transmission of genetic information.