DNA Replication Lecture Notes

Introduction to DNA and Its Replication

• DNA is the director of cell functions and codes for traits.
• During cell division, DNA replication is necessary to ensure new daughter cells receive a copy of DNA.

Location and Timing of DNA Replication

• Where:
  • Eukaryotic cells: Occurs in the nucleus.
  • Prokaryotic cells: Occurs despite the absence of a nucleus.
• When:
  • Occurs before cell division during interphase (before mitosis or meiosis in eukaryotic cells).

Key Players in DNA Replication

• Helicase: Unzips the two DNA strands by breaking hydrogen bonds.
• DNA Polymerase: Builds the new DNA strand.
• Primase: Creates RNA primers to help DNA polymerase start building.
• Ligase: Glues DNA fragments together.

Steps in DNA Replication

1. Origin of Replication:
   • Identified by specific DNA sequences where replication begins.
2. Unwinding:
   • Helicase unwinds the DNA.
   • Single-Stranded Binding proteins (SSB) keep the strands apart.
   • Topoisomerase prevents supercoiling.
3. Primer Placement:
   • Primase places RNA primers for DNA polymerase to begin synthesis.
4. Strand Elongation:
   • DNA polymerase begins adding nucleotides in the 5’ to 3’ direction.
   • Leading Strand: Continuous synthesis in the direction of unwinding.
   • Lagging Strand: Synthesized in Okazaki fragments against the direction of unwinding due to the antiparallel nature of the strands.
5. Fragment Processing:
   • RNA primers are replaced by DNA.
   • Ligase seals the gaps between Okazaki fragments.
6. Result: Two identical DNA molecules, each with one old and one new strand (semi-conservative replication).

Additional Notes

• Proofreading:
  • DNA polymerase has a proofreading function to minimize errors, ensuring accurate DNA replication.
• DNA replication understanding aids in medical applications, including treatments for bacterial infections and cancer.

Conclusion

• DNA replication is a complex, critical process for cell division.
• Encouraged to explore further details and applications.

Stay curious and explore further readings for a deeper understanding of DNA replication.