Molecular Basis of Inheritance

Molecular Basis of Inheritance

• Evidence that DNA is genetic material
  • Mendel discovered heritable factors
  • Morgan identified genes on chromosomes
  • Search for the genetic material began

Griffith's Transformation Experiment

• 1928: Griffith experimented with Streptococcus pneumoniae

• Smooth cells (with capsules): pathogenic

• Rough cells (without capsules): not pathogenic

• Heat-killed smooth cells: not pathogenic

• Heat-killed smooth cells + rough cells: pathogenic and produce live smooth cells

• Conclusion: Genetic material transferred from smooth to rough cells

Transforming Principle

• Transforming Principle: What was transferred from smooth to rough cells?
  • Protein and DNA are components of chromosomes
• Protein is denatured by heat
• DNA is the probable transforming material

Bacteriophages and DNA

• Evidence that DNA is the genetic material came from studying bacteriophages (phages)
• Phage DNA programs bacterial cells
• Hershey and Chase 1952 experiment confirmed that DNA is genetic material in bacteriophages

Evidence for DNA in Eukaryotes

• DNA content doubles before mitosis
• During cell division, double DNA is divided equally between daughter cells
• Diploid cells have twice the DNA compared to haploid cells
• DNA composition varies between different species

Chargaff's Rules

• 1947: Chargaff used paper chromatography to study DNA base composition in different species
• Adenine (A) + Thymine (T) = Guanine (G) + Cytosine (C)
• Base composition varies between species, but A=T and G=C

Watson and Crick's Double Helix Model

• Watson and Crick discovered DNA's double helix structure
• Based on evidence from others like Hershey/Chase and Chargaff
• DNA is a double helix of two strands in antiparallel orientation. A-T and G-C

DNA Replication

• DNA replicates by using itself as a template
• Two strands separate, acting as templates forming two new complementary strands.
• Replication follows the semiconservative model. Each new DNA has one old and one new strand

DNA Replication Overview

• DNA replication requires enzymes and proteins

• Extremely rapid in prokaryotes and slower in eukaryotes

  • Accuracy is high in both cases

• Origin of replication (multiple origins in eukaryotes, single in prokaryotes)

Replication of DNA

• Strand separation is driven by enzymes called helicases

• Topoisomerase reduces strain caused by strand separation

• Synthesis happens in 5' to 3' direction

• Leading strand: continuous synthesis

• Lagging strand: discontinuous synthesis, replicated in short fragments (Okazaki fragments).

• Okazaki fragments are joined by the enzyme DNA ligase

• Primase creates primers to start replication

Replication Elongation

• DNA replication proceeds with enzymes and their functions, including polymerase and primase.

Explore the key experiments and discoveries that identified DNA as the genetic material. This quiz covers Griffith's transformation experiment, the role of bacteriophages, and the contributions of scientists like Mendel and Morgan. Test your understanding of the concepts pivotal to molecular genetics.