An Introduction to the Luria-Delbruck Experiment

by Marco Fumasoni

Introduction

  • Salvatore Edoardo Luria from Turin, Italy & Max Ludwig Henning Delbruck from Berlin, Germany fled to the United States.
  • Delbruck was a Physicist.

Scientific background in the early 40s

  • Genetic information is heritable.
    • Due to DNA and DNA is genetic material.
  • Evolution
    • by inheritance of acquired characteristics
    • natual selection by Darwin
    • … and modern synthesis

The Problem: bacterial cultures rapidly develop resistance to viral infection

  • H1: The virus directly induces resistance mutations. “The environment induces the mutations required for survival.”

  • H2: Mutations arise spontaneously before virus exposure.

  • To resolve this disjunction, we have to use a quantitative approach.

  • Note: CC refers to Colony Count

flowchart LR Virus --infect-->Bacteria Bacteria --replicates-->Replicas subgraph Replicas R1 -.- CC12((CC-12)) R2 -.- CC5((CC-5)) R3 -.- CC0((CC-0)) Rn -.- CCn((CC-n)) end

Measuring mutation rates: Fluctuating numbers

  • Have $n$ replicates, and do the experiment: what you get is fluctuating results. The colony counts fluctuates widely.
  • Luria finds inspiration from a casino slot machine.
  • Luria´s intuition: if the mutation happens early on, i.e. if the petri dish is “lucky”, you get a higher colony count, since more bacteria offsprings survive.

Measuring mutation rates: distributions

  • Mutation events [mutation number n per culture] follow a Poissonian curve.
  • The L-R distribution derives the number of mutant cells.

The model: S. Cerevisiae

  • Genome size: 12.5 MB, #TODO

The system

  • Point mutation (uro3, CAN1)

  • The method is called Gross chromosomal rearrandement. The second leg of the experoiment will have Chromosomal loss.

  • The plan: Fluctuation vs quick and dirty

    • We use a 96 sample plate and do parallel platting events.
    • An alternative is a 6 replica experiment. It allows use to reduce the number of replicas. It drastically reduces the number of experiments we need to do as we need to analyse multiple generations.

  • Generational Fitness:

Bonus

  • Clustered mutations look the same as early mutations: this situation is called un-determination.
  • Single genes result in Mendelian diseases. Polygenic diseases depend on multiple genes.

Experiment

Controls