DNA: The Genetic Material

Historical Discovery of DNA as Genetic Material

The identification of DNA as the molecule of inheritance was a major milestone in biology. Early experiments with bacteria and viruses provided key evidence that DNA, not protein, is the genetic material.

• Frederick Griffith's Experiment: Demonstrated transformation, where harmless bacteria became pathogenic by assimilating foreign DNA.

• Hershey-Chase Experiment: Showed that DNA, not protein, enters bacterial cells during viral infection, confirming DNA as the genetic material.

Chargaff's Rules and DNA Diversity

Erwin Chargaff discovered that DNA composition varies between species and that the amount of adenine (A) equals thymine (T), and guanine (G) equals cytosine (C). These findings supported DNA's role as the genetic material.

• Chargaff's Rules:

  • Base composition varies between species.

  • In any species, A = T and G = C.

DNA Structure: The Double Helix

Building the Structural Model

Watson and Crick, aided by Rosalind Franklin's X-ray crystallography, deduced the double-helical structure of DNA. The molecule consists of two antiparallel strands forming a uniform helix.

• Key Features:

  • Two sugar-phosphate backbones on the outside.

  • Nitrogenous bases paired in the interior.

  • Antiparallel orientation of strands.

Base Pairing and Helix Uniformity

Base pairing is specific: adenine pairs with thymine, and guanine pairs with cytosine. Pairing a purine with a pyrimidine ensures the helix's uniform width.

• Hydrogen Bonds: Hold the base pairs together.

• Purine-Pyrimidine Pairing: Maintains consistent helix diameter.

DNA Replication: Mechanisms and Enzymes

Semiconservative Replication

DNA replication is semiconservative: each new molecule contains one old strand and one new strand. Experiments by Meselson and Stahl confirmed this model.

• Replication Models:

  • Conservative: Both old strands stay together.

  • Semiconservative: Each daughter molecule has one old and one new strand.

  • Dispersive: Strands are mixtures of old and new DNA.

Initiation and Enzymatic Machinery

Replication begins at origins of replication, forming replication bubbles. Multiple enzymes coordinate the unwinding and synthesis of new DNA strands.

• Key Enzymes:

  • Helicase: Unwinds the DNA helix.

  • Single-strand binding proteins: Stabilize unwound DNA.

  • Topoisomerase: Relieves strain from unwinding.

  • Primase: Synthesizes RNA primers.

  • DNA Polymerase: Adds nucleotides to the growing strand.

Leading and Lagging Strands

DNA polymerase synthesizes the leading strand continuously toward the replication fork, while the lagging strand is synthesized in fragments (Okazaki fragments) away from the fork and joined by DNA ligase.

• Directionality: DNA synthesis occurs only in the 5' → 3' direction.

• Okazaki Fragments: Short segments of lagging strand DNA.

Proofreading and DNA Repair

DNA polymerases proofread newly synthesized DNA, correcting errors. Additional repair mechanisms, such as mismatch repair and nucleotide excision repair, maintain DNA integrity.

• Nucleotide Excision Repair: Damaged DNA is removed and replaced by new nucleotides.

Telomeres and Chromosome Ends

Telomere Structure and Function

Telomeres are repetitive nucleotide sequences at the ends of eukaryotic chromosomes. They protect genes from erosion during replication but shorten with each cell division.

• Telomerase: Enzyme that extends telomeres in germ cells.

• Significance: Telomere shortening is linked to aging and cancer.

Chromatin Structure and DNA Packing

Chromatin Organization

Eukaryotic DNA is packed with proteins into chromatin, which undergoes multiple levels of folding to fit into the nucleus. The basic unit is the nucleosome, consisting of DNA wrapped around histone proteins.

• Levels of Packing:

  • DNA double helix (2 nm)

  • Nucleosome (10 nm)

  • 30-nm fiber

  • Looped domains (300 nm)

  • Metaphase chromosome (1,400 nm)

• Histone Tails: Involved in gene regulation.

Euchromatin vs. Heterochromatin

Chromatin can be loosely packed (euchromatin) or highly condensed (heterochromatin). Euchromatin is accessible for gene expression, while heterochromatin is transcriptionally inactive.

• Euchromatin: Loosely packed, active in gene expression.

• Heterochromatin: Densely packed, generally inactive.

Key Terms and Concepts

• DNA (Deoxyribonucleic Acid): The molecule that carries genetic information.

• Chromatin: DNA-protein complex in eukaryotic cells.

• Nucleosome: DNA wrapped around histone proteins.

• Telomere: Protective DNA sequence at chromosome ends.

• Mutation: Permanent change in DNA sequence.

Important Equations

• Chargaff's Rule:

• Semiconservative Replication:

Summary Table: DNA Replication Enzymes

Example: Hershey-Chase Experiment

In the Hershey-Chase experiment, radioactive labeling showed that only DNA, not protein, entered bacterial cells during viral infection, confirming DNA as the genetic material.

Example: Meselson-Stahl Experiment

Meselson and Stahl used isotopic labeling to demonstrate that DNA replication is semiconservative, with each daughter molecule containing one old and one new strand.

Additional info:

• DNA replication is essential for cell division and inheritance.

• Mutations in DNA can lead to genetic diversity and evolution.

• Chromatin structure affects gene expression and cellular function.