Overview
This lecture covers the core mechanisms and stages of DNA replication in the context of the cell cycle, comparing prokaryotic and eukaryotic processes, key enzymes involved, and the significance of telomeres and telomerase.
The Cell Cycle and DNA Replication
- DNA replication occurs in the S (synthesis) phase of the cell cycle.
- Mitosis (cell division) takes place during the M phase.
- DNA must be relaxed (euchromatin) to be accessible for replication.
DNA Structure and Key Features
- DNA is an anti-parallel double helix made of nucleotides: sugar, phosphate, and nitrogenous base.
- Complementary base pairing: A-T (2 bonds), G-C (3 bonds; more stable).
- Human DNA is linear and found in nucleus and mitochondria; mitochondrial DNA is inherited maternally.
Central Dogma and Protein Synthesis
- DNA replication: copying DNA (S phase).
- Transcription: DNA → RNA.
- Translation: RNA → protein (in cytoplasm).
Telomeres and Telomerase
- DNA replication cannot fully extend to chromosome ends, causing telomere shortening with each cycle.
- Telomerase (a reverse transcriptase) extends telomeres in eukaryotes, preventing genetic loss.
- Prokaryotes do not require telomerase due to circular DNA and short lifespan.
DNA Replication Mechanism
- Replication starts at origins of replication (multiple in eukaryotes, one in prokaryotes).
- Helicase unwinds DNA; single-stranded DNA binding proteins stabilize unwound strands.
- DNA polymerases synthesize new DNA strands using parental strands as templates.
- DNA replication is semi-conservative: each new DNA has one old and one new strand.
Leading vs. Lagging Strand
- Leading strand: synthesized continuously in 5'→3' direction; needs one primer.
- Lagging strand: synthesized discontinuously in Okazaki fragments; needs multiple primers, later joined by DNA ligase.
- Primase creates RNA primers; DNA polymerase extends DNA from primers.
Topoisomerase Function
- Topoisomerase relieves torsional strain (supercoiling) during DNA unwinding.
- Inhibitors of bacterial topoisomerase (e.g., quinolones) are used as antibiotics.
Enzymes Involved
- Prokaryotes: DNA polymerase III synthesizes DNA; DNA polymerase I removes primers and fills gaps.
- Eukaryotes: DNA polymerases α, δ, ε synthesize DNA; RNase H removes RNA primers; DNA ligase seals nicks.
- DNA polymerase γ replicates mitochondrial DNA (maternal inheritance).
Key Terms & Definitions
- Euchromatin — relaxed, transcriptionally active DNA.
- Heterochromatin — tightly packed, transcriptionally inactive DNA.
- Telomere — repetitive DNA at chromosome ends, protects from shortening.
- Telomerase — enzyme that extends telomeres.
- Okazaki fragments — short DNA segments on the lagging strand.
- Topoisomerase — enzyme that relieves supercoiling during DNA replication.
- DNA ligase — enzyme that joins Okazaki fragments.
Action Items / Next Steps
- Draw and memorize a comparison table of prokaryotic vs. eukaryotic DNA replication enzymes.
- Review central dogma and DNA replication steps.
- Prepare for the next topic: DNA repair mechanisms.