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 X-ray diffraction data from Rosalind Franklin.

Example: The Hershey-Chase experiment used radioactive labeling to track DNA and protein in viruses infecting bacteria.

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: Adenine (A), Thymine (T), Cytosine (C), Guanine (G)

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

• Antiparallel Orientation: One strand runs 5' to 3', the other 3' to 5'

Equation:

$\text{DNA strand:} \quad 5'\text{-}ATCG...\text{-}3'$

Example: The structure allows for accurate replication and storage of genetic information.

DNA Replication and Repair

DNA Replication

DNA replication is the process by which a cell copies its DNA before cell division. It is semi-conservative, meaning each new DNA molecule contains one old and one new strand.

• Origin of Replication: Specific sequence where replication begins

• Replication Fork: 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

Equation:

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

Example: In E. coli, replication begins at a single origin; in eukaryotes, there are multiple origins.

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

• DNA Ligase: Seals repaired DNA fragments

Example: UV light can cause thymine dimers, which are repaired by nucleotide excision repair.

Mutations

A mutation is any permanent change in the nucleotide sequence of DNA. Mutations can be inherited or acquired and may have various effects on gene function.

• Types of Mutations:

  • Point Mutation: Change in a single nucleotide

  • Insertion/Deletion: Addition or loss of nucleotides

  • Silent, Missense, Nonsense: Effects on protein coding

• Consequences: Can lead to genetic diseases, altered protein function, or be neutral

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
Topoisomerase	Relieves strain ahead of replication fork

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.