DNA: The Molecular Basis of Inheritance, Replication, and Repair

DNA as the Genetic Material

Discovery and Evidence

The identification of DNA as the genetic material was a pivotal moment in biology. Early experiments by Griffith, Avery & Chase, Wilkins & Franklin, and Watson & Crick established DNA's role in heredity.

• Griffith's Experiment: Demonstrated transformation in bacteria, suggesting a 'transforming principle.'

• Avery, MacLeod, McCarty: Identified DNA as the transforming substance.

• Hershey-Chase Experiment: Used bacteriophages to show DNA, not protein, is the genetic material.

• Watson & Crick: Proposed the double helix structure of DNA based on Franklin's X-ray diffraction data.

Example: The Hershey-Chase experiment used radioactive labeling to track DNA and protein in viral replication.

Structure of DNA

Double Helix Model

DNA is composed of two antiparallel strands forming a double helix. Each strand consists of nucleotides with a deoxyribose sugar, phosphate group, and nitrogenous base.

• Nucleotides: Building blocks of DNA; each contains adenine (A), thymine (T), cytosine (C), or guanine (G).

• Base Pairing: A pairs with T, C pairs with G via hydrogen bonds.

• Antiparallel Orientation: Strands run in opposite directions (5' to 3' and 3' to 5').

Formula:

$\text{DNA strand directionality: 5' \rightarrow 3'}$

Example: The structure allows for complementary base pairing, essential for replication and repair.

DNA Replication and Repair

DNA Replication

DNA replication is a semi-conservative process, meaning each new DNA molecule consists of one old and one new strand. Replication begins at specific sites called origins of replication.

• Replication Fork: The Y-shaped region where DNA is unwound and new strands are synthesized.

• Enzymes Involved:

  • Helicase: Unwinds the DNA double helix.

  • Primase: Synthesizes RNA primers.

  • DNA Polymerase: Adds nucleotides to the growing DNA strand.

  • Ligase: Joins Okazaki fragments on the lagging strand.

• Leading vs. Lagging Strand:

  • Leading Strand: Synthesized continuously in the 5' to 3' direction.

  • Lagging Strand: Synthesized discontinuously as Okazaki fragments.

Formula:

$\text{DNA Polymerase Reaction:} \\ \text{(DNA)_n + dNTP} \rightarrow \text{(DNA)_{n+1}} + PP_i$

Example: In E. coli, replication proceeds bidirectionally from a single origin.

Proofreading and Repair

DNA polymerases have proofreading ability to correct errors during replication. Additional repair mechanisms fix damage caused by environmental factors.

• Mismatch Repair: Corrects errors missed by DNA polymerase.

• Excision Repair: Removes damaged sections and replaces them using the undamaged strand as a template.

• Enzymes: DNA ligase, DNA polymerase, and nucleases are involved in repair processes.

Example: UV-induced thymine dimers are repaired by nucleotide excision repair.

Mutations

A mutation is any permanent change in the nucleotide sequence of DNA. Mutations can be caused by errors in replication or by environmental factors such as radiation or chemicals.

• Heritable Mutations: Passed on to offspring if they occur in germ cells.

• Types of Mutations:

  • Point Mutation: Change in a single nucleotide.

  • Insertion/Deletion: Addition or loss of nucleotides, potentially causing frameshifts.

• Consequences: Can alter gene function, lead to genetic disorders, or drive evolution.

Example: Sickle cell anemia is caused by a point mutation in the hemoglobin gene.

Summary Table: Key Enzymes in DNA Replication

Enzyme	Function
Helicase	Unwinds the DNA double helix
Primase	Synthesizes RNA primers
DNA Polymerase	Adds nucleotides to the new DNA strand; proofreads
Ligase	Joins Okazaki fragments
Nuclease	Removes damaged or mismatched DNA

Additional info: These notes expand on the original outline by providing definitions, examples, and a summary table for key enzymes involved in DNA replication and repair, suitable for General Biology students.