recap

14.1 recap

Studies of mutations in humans and bread molds led to our understanding of the one-gene, one-polypeptide relationship. In most cases, the function of a gene is to code for a specific polypeptide.

learning outcomes

You should be able to:

  • Justify the selection of various model organisms used for genetic studies.

  • Explain how experimental design has played an important role in enabling researchers to elucidate information about metabolic pathways.

  • Analyze data to link genes to their products.

Question 1

What is a model organism, and why is Neurospora a good model for studying biochemical genetics?

A model organism is easy to grow and manipulate in the laboratory and has representative characteristic(s) common to a larger group of organisms (e.g., a mouse for mammals).

Question 2

How were the experiments on mutant strains of Neurospora set up to determine the order of steps in a biochemical pathway?

Srb, Horowitz, Beadle, and Tatum treated wild-type Neurospora with X rays to cause mutations. Strains that could not grow without the addition of an amino acid (arginine) were isolated. The strains were separately incubated without arginine but with intermediates along the proposed biochemical pathway that makes arginine. Growth in one of the intermediates meant that the mutation for that strain must occur in a gene encoding an enzyme that acts before that substance in the pathway. In this way, the collection of mutant strains led to identification of the intermediates in the pathway and their order, and ultimately to the enzyme that acts to convert each intermediate into the next.

Question 3

Beadle and Tatum’s experiments showed that a biochemical pathway could be deduced from mutant strains. In bacteria, the biosynthesis of the amino acid tryptophan (T) from the precursor chorismate (C) involves four intermediate chemical compounds, which we will call D, E, F, and G. Here are the phenotypes of various mutant strains. Each strain has a mutation in a gene for a different enzyme; + means growth with the indicated compound added to the medium, and 0 means no growth. Based on these data, order the compounds (C, D, E, F, G, and T) and enzymes (1, 2, 3, 4, and 5) in a biochemical pathway.

Enzymes: 4 → 2 → 3 → 1 → 5
Compounds: C → F → E → D → G → T

Mutant strain Addition to medium
C D E F G T
1 0 0 0 0 + +
2 0 + + 0 + +
3 0 + 0 0 + +
4 0 + + + + +
5 0 0 0 0 0 +

Now that you have seen the evidence for the one-gene, one-polypeptide relationship, how does it work? That is, how is the information encoded in DNA used to produce a particular polypeptide?