DNA Replication Mechanism
Shared 12/22/2025•54 views
DNA Replication: Semi-Conservative Mechanism Each new DNA molecule consists of one original strand and one newly synthesized strand. Relies on complementary base pairing: A-T, G-C. 1. Origin of Replication (Ori) Replication starts at specific sites called "origins of replication" (Ori). Bacterial/viral DNA: Often single, circular replicon. Eukaryotic DNA: Multiple origins along linear chromosomes. Replication proceeds bidirectionally from the Ori, forming replication forks. 2. Activation of Deoxyribonucleotides Deoxyribonucleoside monophosphates (dAMP, dGMP, dCMP, dTMP) are activated. They are converted to triphosphates (dATP, dGTP, dCTP, dTTP) using ATP and phosphorylase. These triphosphates serve as both building blocks and energy source for DNA synthesis. 3. Unwinding of DNA Helix Helicase: Binds to Ori and unwinds DNA by breaking hydrogen bonds, creating a replication fork. Gyrase (Topoisomerase): Relieves supercoiling tension ahead of the replication fork. Separated strands serve as templates. 4. Synthesis of RNA Primer Primase (DNA-directed RNA polymerase): Synthesizes a short RNA primer (5-10 nucleotides) on the DNA template. DNA polymerase needs a free 3'-OH group to start synthesis, which the RNA primer provides. 5. Elongation of New DNA Strands DNA Polymerase III: Extends the new DNA strand in the 5' to 3' direction. Leading Strand: Synthesized continuously towards the replication fork as the DNA unwinds. Only one primer needed. Lagging Strand: Synthesized discontinuously in short segments (Okazaki fragments) away from the replication fork. Each Okazaki fragment requires a new RNA primer. DNA Polymerase I: Removes RNA primers and replaces them with DNA nucleotides. DNA Ligase: Joins the Okazaki fragments (and other nicks) on the lagging strand, forming a continuous strand. 6. Proofreading and DNA Repair DNA Polymerase: Has 3' to 5' exonuclease activity for proofreading, correcting errors during replication. Mistakes (wrongly incorporated bases) are removed and replaced. Additional repair enzymes identify and correct errors missed by proofreading or caused by mutagens, ensuring genetic fidelity.
