Lecture on DNA Replication

Introduction to DNA

• DNA is the ultimate director for cells and codes for traits.
• DNA replication is the process of making more DNA.

Location and Timing of DNA Replication

• Location:
  • In eukaryotic cells, DNA replication occurs in the nucleus.
  • Prokaryotic cells, which lack a nucleus, also undergo DNA replication with some differences.
• Timing:
  • Occurs before cell division (mitosis or meiosis) during interphase in eukaryotic cells.

Key Enzymes in DNA Replication

• Helicase:
  • Known as the unzipping enzyme.
  • Unwinds and separates the two strands of DNA by breaking hydrogen bonds.
• DNA Polymerase:
  • Known as the builder enzyme.
  • Replicates DNA molecules to build new DNA strands.
• Primase:
  • Known as the initializer enzyme.
  • Creates RNA primers to guide DNA Polymerase on where to start.
• Ligase:
  • Known as the gluing enzyme.
  • Joins DNA fragments together.

Steps in DNA Replication

• Initiation:
  • Starts at a specific origin identified by DNA sequences.
  • Helicase unwinds DNA, and single-stranded binding proteins (SSB) keep strands apart.
  • Topoisomerase controls supercoiling to prevent overwinding.
• Primer Synthesis by Primase:
  • Primase lays down RNA primers on both DNA strands.
• Elongation by DNA Polymerase:
  • DNA Polymerase starts adding nucleotides to the 3’ end of the primer, building in the 5’ to 3’ direction.
  • Leading Strand:
    • Continuously synthesized towards the replication fork.
  • Lagging Strand:
    • Synthesized in Okazaki fragments away from the replication fork.
    • Requires multiple primers and subsequent joining by ligase.
• Termination:
  • RNA primers replaced with DNA.
  • Ligase seals gaps between Okazaki fragments.
  • Result: Two semi-conservative DNA molecules, each with one old and one new strand.

Directionality in DNA

• DNA strands are anti-parallel, running in opposite directions (5’ to 3’ and 3’ to 5’).
• Important for understanding how enzymes like DNA Polymerase function.

Proofreading and Error Correction

• DNA Polymerase has a proofreading function, greatly reducing errors.
• Ensures accurate DNA replication, preventing incorrect gene coding.

Further Exploration

• Understanding DNA replication has medical applications, including treatments for diseases like cancer and bacterial infections.
• Encouragement to explore further reading on the topic.

"Stay curious!" - The Amoeba Sisters