DNA Replication Lecture Notes

Introduction to DNA

• DNA is the ultimate director for cells.
• Codes for traits and is essential for cell division.

DNA Replication

• Definition: Making more DNA to ensure daughter cells receive a copy.

• Where:

  • Eukaryotic cells: occurs in the nucleus.
  • Prokaryotic cells: do not have a nucleus but still replicate DNA.

• When:

  • Occurs before cell division (mitosis or meiosis).
  • Takes place during interphase in eukaryotic cells.

Key Players in DNA Replication

• Enzymes: Many key players are enzymes (usually end in -ase).
  • Helicase: Unzips the DNA by breaking hydrogen bonds between bases.
  • DNA Polymerase: Replicates DNA by building new strands.
  • Primase: Creates an RNA primer to guide DNA polymerase.
  • Ligase: Glues DNA fragments together.

DNA Replication Process

1. Origins: Begins at specific DNA sequences called origin.
2. Unwinding:
   • Helicase unwinds the DNA.
   • SSB Proteins: Bind to single strands to keep them separated.
   • Topoisomerase: Prevents supercoiling of DNA.
3. Primer Formation:
   • Primase synthesizes RNA primers on both strands.
4. Strand Directionality:
   • DNA strands are anti-parallel (5’ to 3’ and 3’ to 5’).
   • DNA polymerase can only build new strands in the 5’ to 3’ direction.
   • Leading Strand: Built continuously in the 5’ to 3’ direction.
   • Lagging Strand: Built in fragments (Okazaki fragments) due to the 5’ to 3’ restriction.
5. Fragment Joining:
   • Ligase seals gaps between Okazaki fragments after RNA primers are replaced with DNA.

Final Outcomes

• Result: Two identical double helix DNA molecules from one original molecule.
• Semi-conservative replication: Each new DNA molecule contains one old strand and one new strand.

Importance of Accuracy

• DNA polymerase has proofreading ability to minimize errors in base pairing.
• Errors can lead to incorrect gene coding and potentially harmful proteins.

Medical Significance

• Understanding DNA replication can lead to treatments for harmful cells, such as bacteria and cancer.

Conclusion

• Explore beyond the basics for a deeper understanding of DNA replication and its implications.
• Encouragement to stay curious and continue learning.