DNA Replication Lecture Notes

Introduction to DNA

• DNA is the ultimate director for cells, coding for traits.
• DNA replication is essential for cell division, ensuring new daughter cells receive DNA.

Where and When DNA Replication Occurs

• Where:
  • In eukaryotic cells, DNA replication occurs in the nucleus.
  • Prokaryotic cells also perform DNA replication but lack a nucleus.
  • Differences exist between DNA replication in eukaryotic and prokaryotic cells.
• When:
  • Occurs before cell division, specifically before mitosis or meiosis during interphase in eukaryotic cells.

Key Players in DNA Replication

• Helicase: The unzipping enzyme that separates the two strands of DNA by breaking hydrogen bonds.
• DNA Polymerase: The builder enzyme that replicates DNA molecules to build new DNA strands.
• Primase: The initializer that creates RNA primers to guide DNA polymerase in starting replication.
• Ligase: The gluing enzyme that connects DNA fragments.

DNA Replication Process

• Origin of Replication:
  • Specific DNA sequences identify where replication starts.
• Role of Enzymes:
  • Helicase unwinds DNA at the origin.
  • SSB proteins prevent DNA strands from re-annealing.
  • Topoisomerase prevents supercoiling during replication.
• Replication Directionality:
  • DNA strands are anti-parallel and have directionality (5' to 3' and 3' to 5').
  • DNA polymerase can only add nucleotides in the 5' to 3' direction.

Leading vs. Lagging Strand

• Leading Strand:
  • Continuously synthesized in the 5' to 3' direction.
• Lagging Strand:
  • Synthesized in fragments (Okazaki fragments) because of directionality constraints.
  • Primase frequently adds RNA primers for DNA polymerase.
  • Ligase fills gaps between fragments after RNA primers are replaced.

Result of Replication

• Produces two identical double helix DNA molecules from one original double helix.
• Process is semi-conservative: each new DNA molecule contains one original strand and one new strand.

Proofreading and Error Correction

• DNA polymerase has proofreading ability to minimize errors in base pairing.
• Incorrect DNA could lead to incorrect proteins or no protein production.

Further Exploration

• Understanding DNA replication has medical applications, including treatments targeting harmful cells.
• Encouragement to explore detailed readings on DNA replication.

End Note: Stay curious and continue to learn more about the fascinating process of DNA replication and its biological significance.