Defining Gene and Protein Relationships (Experiments) Flashcards
Gene
A segment of DNA that carries the information to produce a specific RNA molecule
Proteins derived from RNAs create the….
physical manifestation of the traits encoded by genes (phenotype)
Proteins are the link between…
genotype and phenotype
Gene Expression
The process by which DNA directs the synthesis of proteins (or just RNAs in some cases)
–> Getting the info into a useable form: “expressing” the info
Garrod (1902)
First to suggest that genes dictate phenotypes through enzymes and their controlled production
Garrod’s Study + Idea
Studied Alkaptonuria –> Hypothesized that the symptoms of the disease reflected an inability to make a particular enzyme
What causes black urine in Alkaptonuria?
The lack of an enzyme prevents the break down of homogenistic acid (HA) causing HA to accumulate and be secreted in the urine which creates a black color
“Inborn Errors of Metabolism”
Idea that recessive mutations in humans could cause defects in proteins, specifically those controlling metabolism
Garrod was the first to…
link genes with enzymes that carry out metabolic reactions
–> Got people thinking about DNA encoding enzymes!
Beadle and Tatum (1940)
Built off of Garrod’s work to find EVIDENCE that genes are connected to metabolic enzymes
What model did Beadle and Tatum use?
Neurospora crassa (Bread mold)
Why was neurospora a good model? (3)
1) Cheap/easy to grow
2) A HAPLOID species –> Made it easy to study genes as only 1 copy needed to be mutated to see a change in phenotype
3) Modest food requirements –> Can be grown in minimal medium (important for their experimentation)
Beadle and Tatum Hypothesis
If there is a 1:1 relationship between genes and specific enzymes, it would be possible to create mutants that are unable to carry out specific enzymatic reactions
–> (If enzymes and DNA are connected then certain mutations would lead to the loss of enzymatic function/production)
What mutants did Beadle and Tatum study?
Nutritional Mutants
Nutritional Mutants
Mutations that cause defects in enzymatic pathways needed for growth under normal growing conditions (in minimal medium for neurospora)
Beadle and Tatum:
Finding the Nutritional Mutants Methodology
1) Collected spores of the neurospora
2) Bombarded the spores with x-rays = some spores with random mutations
3) Offspring of x-rayed spores put in test tubes with COMPLETE medium
4) Took some growth colonies from each tube and put them in tubes with MINIMAL medium
–> The samples that had NO GROWTH in the minimal medium were the nutritional mutants (used in part 2 of the experiment)
Beadle and Tatum:
Pinpointing the “Broken” Pathway Methodology
Took the nutritional mutants and put them into two different conditions:
1) Minimal medium + full set of VITAMINS
2) Minimal medium + full set of AMINO ACIDS
(Found growth in the amino acid tubes)
3) Put the mutants into tubes containing minimal medium and just ONE amino acid to pin point which A.A. was the problem
What happened to the nutritional mutants in minimal medium?
They died
–> Couldn’t produce an essential molecule needed for growth (that the complete medium had previously covered with its contents)
How did Beadle and Tatum determine the issue with the mutants was with amino acids?
When placed in minimal medium with just amino acids added in the mutants was “rescued” –> The mutant GREW when it hadn’t in just regular minimal medium
–> Therefore, the mutation must block the synthesis of one or more amino acids
How did Beadle and Tatum determine which AA was the problematic one?
Put the mutants into tubes with minimal medium + just ONE A.A
–> Found that the mutant was “rescued” by the addition of ARGININE
= Therefore, the mutation must have impacted Arginine biosynthesis
Beadle and Tatum conducted further experimentation:
What did it show?
Showed that the mutants lacked an enzyme needed for arginine synthesis
Beadle and Tatum CONCLUSION
Proposed the “one gene—one enzyme” hypothesis
–> The function of a gene is to dictate the production of a specific enzyme
Flaws in the “one gene—one enzyme” hypothesis (3)
1) Not all proteins are enzymes –> Many genes encode non-enzyme proteins (ex: keratin)
2) Some proteins are encoded by multiple genes –> Proteins that have multiple subunits are usually encoded by different genes
–> One polypeptide is encoded by one gene
3) Many genes encode RNA molecules –> Many RNAs encoded by genes have other functions and NEVER get translated to proteins
Revised version of the “one gene—one enzyme” hypothesis:
“one gene—one RNA”
–> Gene = a segment of DNA that encodes a functional RNA
RNA is the bridge between…
DNA and protein synthesis
The different “languages” of nucleic acids and proteins:
1) Nucleic acids = nucleotides/nitrogenous bases
2) Proteins = amino acids