Lecture 25: Gene Control In Eukaryotes

Question 1

What is differentiation?

Answer 1

Differentiation is the process whereby, a single cell, the fertilised ovum, gives rise to the fully developed organism.

Question 2

Give 2 reasons why a single cell can give rise to such a wide variety of cell types?

Answer 2

1. Segregation of genetic information: genetic info could be divided unequally amongst daughter cells such that liver cells receive genes to make liver proteins and muscle cells receive the muscle genes etc.
2. Differential gene expression: cells have the full complement of DNA from the ovum but different cell types utilise different parts of the genome and this gives them their unique features.

Question 3

What is evidence to support differential gene expression?

Answer 3

1. The DNA content of different cell types are the same (germs cells are the exception)
2. Hybridisation studies show close relatedness of DNA of different tissues
3. Fusing a chick erythrocyte with a rat hepatocyte produces a HETEROKARYON, which make chicken albumin (a liver protein). Therefore chick erythrocytes contain liver genes.

Question 4

This is a big mumma…prepare yourself

What features control gene expression in eukaryotes that ultimately govern cell differentiation?

Answer 4

1. DNA methylation is what inhibits gene transcription. Eg genes that are expressed in most cell types are UNmethylated
2. In eukaryotes: DNA is bound to histone proteins. The structure of chromatin effects gene expression. Really condensed heterochromatin prevents gene expression, relaxed heterochromatin enable gene activation.
3. Non-histone proteins: include regulatory proteins that control gene expression.
   A activator protein binds to enhancer sequence to increase the rate of transcription.
   Repressor proteins bind to elements called silencers thereby preventing transcription.
4. The activity of regulatory proteins may be affected in the following ways:
   A) through the binding of an affector molecule
   B) through protein-protein interactions and modification
5. Following transcription, mRNA quite often is processed into a ‘mature’ form through the removal of the introns and splicing together of exons
6. Translation may be effected through the action of RNA binding proteins
7. Post-transitional modification of proteins (such as translation initiation proteins or regulators) can effectively regulate protein production