1. (16 pts) The results of a wide range of studies indicate that there is a very

Question

1. (16 pts) The results of a wide range of studies indicate that there is a very precise order of phosphorylation/dephosphorylation events controlling the activity of the mitotic cyclin-dependent kinase (M-cdk), and thus also the entry of the cell into mitosis. You have obtained some strains of S. pombe from a collaborator for the purpose of carrying out some biochemical studies on M-cdk. Unfortunately, during the process of shipping, the labels identifying each strain were lost and your collaborator has just left for Tanzania on a month-long vacation. The strains you requested are: WT, cdc250, wee1", and cdc25 € (a C-terminal deletion mutant of cdc25, lacking tyrosine phosphatase activity). You must identify them correctly before you can begin the biochemical experiments you had planned. See Figure 1, below. Strain 1 Strain 2 Strain 3 Strain 4 15 30C 30C 37C Figure 1. Phenotypes of S.pombe strains at 30"C and 37°C. Numbers were randomly assigned to the four strains. Cultures of all strains were started at 30°C. Once established, the culture was split into two and one aliquot was moved to 37ºC and the other half was kept at 30°C. After four hours, all eight cultures were stained with DAPI, highlighting their nuclei, and observed in the microscope. Images of the cells from each sample are presented above. a) Identify the strains in Figure 1. b) Why does the phenotype change in only two of the four strains upon a shift in temperature to 37°C? c) Does the result for Strain 1 at 37°C reflect an increase or decrease in mitotic cyclin/CDK activity? Explain. d) Why is there an increase in cell size in strain 4 at 37°C?

Explanation / Answer

Ans a) The strain in Figure 1. are the S.Pombe, It is also known as fission yeast and mostly used for the traditional brewing. It is an unicellular eukaryotic and has rod-shaped cells that measures around 3 to 4 micrometres in diameter. The genome of the micro-organism has around 14.1 million base pairs and nearly contains 4,970 protein coding genes along with 450 number of non-coding RNAs.

Ans b) The phenotypic change was observed only in two out of the four strains as there was a shift in the temperature to 37°C because those were the strains where activity took place leading to the events of the cellular division. It was observed that mitosis took place in those cells.

Ans c) It shows an increase in the mitotic cyclin/Cdk activity because it shows that it is undergoing the process of mitosis. The cyclin concentration increases as mitosis takes places and hence when the strain 1 is shifted to 37°C, it leads to increase in the activity.

Ans d) There is an increase in the size of the cell of the strain 4 at 37°C because it is the stage when the cell is increasing so that it prepares itself for the process of cell division. It starts with the process of accumulation of the proteins that is needed for the division process

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