Lecture Notes on DNA Replication

Introduction to DNA

• DNA is the coding molecule for traits in cells.
• Essential for cell division, ensuring daughter cells receive DNA.

Overview of DNA Replication

• Definition: Process of making more DNA.
• Where:
  • Eukaryotic Cells: Occurs in the nucleus.
  • Prokaryotic Cells: No nucleus, but they also replicate DNA (differences not covered).
• When: Occurs before cell division (mitosis or meiosis) during interphase.

Key Enzymes in DNA Replication

• General Note: Many key players in DNA replication are enzymes (often ending in -ase).

Major Enzymes

1. Helicase:

   • Function: Unzips DNA by breaking hydrogen bonds between bases.

2. DNA Polymerase:

   • Function: Builds new DNA strands.
   • Can only synthesize DNA in the 5’ to 3’ direction.

3. Primase:

   • Function: Initializes DNA replication by creating RNA primers.
   • Primer aids DNA polymerase in knowing where to start.

4. Ligase:

   • Function: Glues DNA fragments together.

5. SSB Proteins:

   • Bind to separated DNA strands to prevent them from re-annealing.

6. Topoisomerase:

   • Prevents supercoiling of DNA during replication.

Process of DNA Replication

• Origin of Replication: Specific DNA sequences where replication begins.
• Steps in DNA Replication:
  1. Helicase unwinds the DNA.
  2. SSB proteins keep strands separated.
  3. Topoisomerase prevents supercoiling.
  4. Primase synthesizes RNA primers on both strands.
  5. DNA polymerase adds nucleotides to the primer in 5’ to 3’ direction.
  6. Leading strand synthesized continuously.
  7. Lagging strand synthesized in fragments (Okazaki fragments).
  8. Ligase seals gaps between Okazaki fragments.

Directionality of DNA Strands

• Strands are anti-parallel:
  • One strand runs 5’ to 3’, the other 3’ to 5’.
• Importance of strand direction in replication:
  • DNA polymerase can only build in the 5’ to 3’ direction.

Result of DNA Replication

• Two identical double helix DNA molecules.
• Called semi-conservative: Each new molecule has one old and one new strand.

Proofreading in DNA Replication

• DNA polymerase has proofreading ability to reduce errors.
• Importance of accuracy: Incorrect matches can lead to defective proteins.

Conclusion

• Understanding DNA replication is vital for medical advancements (e.g., cancer treatments).
• Encouragement to explore further reading and stay curious.