Refining the theory

The hypothesis was modified into the one gene-one polypeptide

hypothesiswhen it was realised that proteins could consist of more than one polypeptide chain, each determind by the action of a different gene.

For example, haemoglobin has four polypeptide chains, two identical alpha and two identical beta chains. These two different types of polypeptide are determined by two separate genes. Sickle-cell anaemia is caused by a mutation in a single gene which results in just one amino acid being changed in the beta chain of haemoglobin.

Quick check:

1. How does phenylketonuria indicate that genes exert their effects through the production of specific enzymes?

2. How does Beadle and Tatum`s experiment on neurospora support the one gene-one protein hypothesis?

3. Explain why the one gene-one protein hypothesis needed to be modified in the light of conditions such as sickle-cell anaemia.

4. Describe Beadle and Tatum’s experiment on Neurospora which led to the onegene-one enzyme hypothesis.

5. Explain how certain inherited metabolic disorders indicate that genes exert their effects through enzymes.

6. Discuss why the one gene-one enzyme hypothesis had to be modified.

7. Divide the text into an introduction, principal part and conclusion.

8. Express the main idea of each part.

9. Give a title to each part of the text and summarize the text in brief.