Biochemistry

Chapter 17

From Gene to Protein

Rob Swatski

Associate Professor of Biology

HACC – York Campus

Overview: The Flow of Genetic Information

DNA information = specific sequences of nucleotides DNA  protein synthesis

Proteins: link genotype & phenotype Gene expression: DNA directs protein synthesis - 2 stages: transcription & translation

How was the fundamental relationship between genes & proteins discovered? - examine evidence from studies of metabolic defects

1909: British physician Archibald Garrod 1st suggested that genes dictate phenotypes with enzymes

- symptoms of inherited disease reflects inability to synthesize a certain enzyme - required understanding that cells synthesize & degrade molecules using metabolic pathways

Neurospora bread mold

Nutritional Mutants in Bread Mold

George Beadle & Edward Tatum exposed Neurospora to x-rays - created mutants that could not survive on minimal medium (cannot synthesize certain molecules) Identified 3 classes of arginine-deficient mutants - each lacked a different enzyme needed to make arginine Developed the “one gene – one enzyme hypothesis” - each gene directs the synthesis of a specific enzyme

EXPERIMENT

Growth:

Wild-type cells growing and dividing

No growth:

Mutant cells cannot grow and divide

Minimal medium

RESULTS

Classes of Neurospora crassa

Wild type Minimal medium (MM) (control) MM  ornithine

Growth

Class I mutants

No growth

Class II mutants

Class III mutants

Condition

MM  citrulline

MM  arginine (control) Summary of results

Can grow with or without any supplements

Can grow on ornithine, citrulline, or arginine

Can grow only on citrulline or arginine

Require arginine to grow

CONCLUSION

Gene (codes for enzyme)

Class I mutants (mutation in gene A) Precursor

Enzyme A

Wild type Precursor

Class II mutants (mutation in gene B) Precursor

Enzyme A

Class III mutants (mutation in gene C) Precursor

Enzyme A

Gene A...