Lecture Notes: Structure and Replication of DNA

Basic Structure of DNA

• Nucleotide: The simplest component of DNA, specifically a deoxy ribonucleotide.
  • Backbone: Consists of a phosphate group attached to a five-prime (5') carbon of a sugar (deoxyribose).
  • Nitrogenous Bases: Adenine (A), Thymine (T), Guanine (G), Cytosine (C).
• Phosphodiester Linkage: Connects nucleotides.
• Directionality: DNA is read from the 5' to 3' direction.

Historical Background

• James Watson and Francis Crick (1953): Identified DNA's structure.
  • Proposed DNA strands are anti-parallel.
  • Established complementary base pairing: A with T, G with C.
  • Their discovery was based on Rosalind Franklin's work, who was not credited due to her untimely death.

DNA Replication Mechanisms

• Semi-conservative Replication: Parental DNA separates, each strand serves as a template for a new strand.
  • Each new DNA molecule consists of one old and one new strand.
• Conservative Replication: Both parental strands are copied, results in one original and one completely new DNA molecule.
• Dispersive Replication: New DNA is a mixture of old and new segments.

Meselson-Stahl Experiment (1958)

• Used isotopes Nitrogen-15 and Nitrogen-14 to support the semi-conservative model.
• Showed each parental DNA strand is copied entirely.

DNA Replication Process

• Enzymes Involved:
  • DNA Polymerase: Adds nucleotides at the 3' end.
  • Helicase: Unwinds the double helix.
  • Primase: Provides a starting point (primer) for synthesis.
  • Ligase: Joins Okazaki fragments on the lagging strand.
• Leading vs Lagging Strand:
  • Leading strand synthesized continuously.
  • Lagging strand synthesized in Okazaki fragments, which are later joined.

Challenges with Replication

• Telomeres and Telomerase: Prevent chromosome shortening by adding repeating nucleotide sequences.
• Replication Bubbles: Occur in multiple sites on eukaryotic chromosomes.

DNA Accuracy and Mutations

• DNA Polymerase III: Has proofreading capabilities to correct errors during replication.
• Mutations: Occur despite proofreading, some are beneficial and lead to evolutionary advantages.

Conclusion

• Understanding DNA replication is crucial for understanding cellular processes.
• Much of this knowledge is derived from studies on E. coli replication, which shares similarities with human DNA replication.