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From DNA to Protein: Genotype to Phenotype

Biology Molecular Biology Difficulty 3/5

Beginning of the lesson

From DNA to Protein: Genotype to Phenotype

From DNA to Protein

How genes are expressed — from the one-gene, one-polypeptide hypothesis through transcription, translation, and mutations.

Module 01

Genes Code for Proteins

From bread mold to hemoglobin — how Beadle and Tatum proved the link between genes and enzymes.

Background

Molecular Basis of Phenotype

The molecular basis of phenotypes was known before DNA was identified as the genetic material. Studies showed that major phenotypic differences were due to specific proteins.

Model organisms — easy to grow, show the phenomenon studied — include pea plants, Drosophila, E. coli, and Neurospora crassa.

Experiment — Beadle & Tatum (1940s)

Neurospora is haploid — mutations immediately visible. Wild-type = prototrophs (grow on minimal medium). X-ray → auxotrophs (need extra nutrients).

STEP 1: Mutagenesis Irradiate Neurospora with X-rays X-ray spores ↓ Grow on complete medium ↓ Test on minimal medium Can't grow = auxotrophs Each needs one specific nutrient STEP 2: Identify pathway Test arg mutants with intermediates Mutant 1: grows on citrulline Mutant 2: needs arginine only Mutant 3: grows on ornithine ↓ Each missing ONE enzyme One gene = one enzyme REVISED HYPOTHESIS Not all proteins are enzymes. Some have multiple chains. ↓ One gene → one polypeptide Example: Hemoglobin α & β chains = separate genes
✦ Each gene specifies one polypeptide. Some genes code for RNA; others regulate expression.
2D — From Experiment to Hypothesis

Knowledge Check — Module 1

1. Auxotrophs differ from prototrophs because they:

2. The revised hypothesis states:

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