Gene Expression: Big Picture

two levels of gene function

molecular and trait

how are the two levels of gene function connected?

the molecular function impacts the structure and function of the cells to determine the organism’s trait

input of a gene

DNA

output of a gene

protein

gene replication

copies the organism’s ENTIRE GENOME in preparation for cell division

gene expression

from a short region of DNA, a new RNA molecule is made that can be turned into a protein

Archbold Garrod

• first proposed a relationship between genes and production of enzymes

• studied patients with metabolic defects

Archbold Garrod hypothesis

“disease is due to a missing enzyme”

Beadle and Tatum (1940s)

• worked on Neurospora crassa, common bread mold that is haploid

• reasoned that a mutation might cause a defect in an enzyme required for synthesis of an essential molecule

wild type of neurospora

“normal” N. Crassa can grow on minimal medium

mutant strains neurospora

unable to grow unless supplemented with specific substances, vitamins, or amino acids

How did Beadle and Tatum discover the fundamental relationship between genes and proteins?

• disabled genes in bread mold, looked for phenotypic changes

• investigated mutations that altered the ability of the fungus to grow on minimal medium

why did Beadle and Tatum study haploid bread mold?

because it would be easier to detect recessive mutations

complete medium for neurospora

provides all nutrients needed for growth (mutants can grow too)

minimal medium for neurospora

minimal nutrients for growth for wild type cells (inorganic salts, glucose, biotin); mutants cannot grow

hypothesis for Beadle and Tatum

in a nutritional mutant, the gene for the enzyme that synthesizes a particular nutrient has been rendered non-functional by a mutation

method for Beadle and Tatum

expose Neurospora cells to X-rays to induce mutations, then check mutants with new nutritional requirements

result of Beadle and Tatum’s experiment

mutant cells that can grow on complete medium, and not minimal medium, but that can then grow on minimal medium in the presence of a specific supplement — have mutation in pathway

“one-gene, one-enzyme” hypothesis

function of a gene is to produce a specific enzyme

in what direction are the codons read in during translation?

5’ to 3’

what is the flow of information from nucleic acids to proteins based on?

the triplet code, a series of non-overlapping three-nucleotide words

genetic information flow

1. RNA bridges the gap between DNA and protein

2. transcription produces an RNA from the information in DNA

3. mRNA codes for a protein

key purpose of genetic material

to encode the production of proteins at the proper time and in suitable amounts in the correct cell