Lecture 2: Cell cycle 2 Flashcards
Give three men who got the Nobel Prize for research on how cells regulate cell proliferation.
Lee Hartwell
Tim Hunt
Paul Nurse
What is Saccharomyces cerevisiae?
Budding yeast, a model organism. It grows to a certain size, then grows a bud. the bud grows bigger then splits off as a new cell.
What Schizosaccharomyces pombe?
Fission yeast, another model organism. It divides by elongating and splitting into two once it reaches a certain length.
Discuss the evolutionary similarity of S. cerevisiae and S. pombe.
They are not very close to each other evolutionary. We humans have the same closeness to each yeast as they have to each other. This means that the system observed in the yeast is expected in humans too, because it has been conserved over a huge period of evolutionary history.
What did Nurse use in his experiments to study the cell cycle?
Cell division cell mutants (cdc mutants).
- Short cdc mutants are formed when the cell divides before it has elongated.
- Long cdc mutants are formed when the cell elongates but does not divide, so just gets longer and longer.
Explain complementation.
Complementation is a process by which a plasmid containing the wild type (normal) genes is introduced into a cdc mutant, ‘transforming’ it. Then the yeast that divides normally after being transformed is selected. These yeast must have been mutants because of a problem with genes which were replaced by normal genes in the plasmid. Then the plasmid is isolated from the yeast and it sequence is determined. This means that it is possible to find which gene was mutated to give that specific cdc mutant.
What were Nurse’s finding from the complementation experiment?
There are about 30 different cdc mutants. There are about 70 genes that cause elongated length. One of these genes turns out to be a key regulator of cell division cell, cdc2 (a protein kinase). Cdc2 is the gene, the protein kinase is encoded by the cdc2 gene.
Which amino acids in proteins does cdc2 protein kinase phosphorylate?
Serine and threonine (both have -OH groups)
What did Nurse notice about the relationship between the cdc2 level and cdc2 activity that was strange?
The cdc2 level stays constant, but the cdc2 activity changed during the cell cycle.
What are cyclins?
Regulators of the periodicity of the cell cycle (like cdc2 protein kinase).
Through which experiment by Hunt were cyclins discovered?
Cyclins were discovered by Hunt in an experiment involving sea urchin oocytes. These oocytes are large and readily available and are all arrested at the same stage in the cell cycle as they wait to be fertilised.
In the experiment Hunt added sperm to the oocytes, causing all the oocytes to start division at the same time, all in the same stage. This made it much easier to extract the protein whose levels were changing during the cell cycle. Instead of adding sperm, changing the pH level could make the oocytes think they’d been fertilised and start dividing at the same time.
The oocytes were preloaded with a radioactive marker.
What were the results of Hunt’s experiment?
It was found that the amount of cyclins present peaks just before mitosis., in the G2 stage. The minimum amount of cyclin is present in the G1 stage.
How do cyclins and cdc2 act together?
They are needed together to regulate the cell cycle periodicity. The cyclin physically binds to the cdc2 protein kinase to make it active. This means the cyclin is a regulatory subunit for cdc2 protein kinase and so cdc2 protein kinase is a Cyclin Dependent Kinase (cdk).
What happens when cdks are activated by specific cyclins?
When cyclin dependent kinases are activated by specific cyclins, they phosphorylate proteins, which either activates or inactivates them.
What happens after the cdk has done its work?
It is broken down by peptidases
