706_F06pset3_key - 7.06 FALL 2006 Problem Set #3 1. Are SCF...

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7.06 Fall 2006 PS 3 key 1 of 4 7.06 FALL 2006 Problem Set #3 1. Are SCF and APC the same thing as the proteasome? No. SCF and APC add ubiquitin to cyclins, which in turn signals to the proteasome to destroy the cyclin proteins. 2. What is the difference between . . . a. a centromere and a kinetochore? a centromere is a DNA sequence on every chromosome that dictates chromosome attachment to the microtubules. The kinetochore is the protein structure that binds to centromeric DNA b. cohesin and condensin? cohesin holds two sister chromatids together until anaphase. Condensin holds loops of DNA on one sister together, to compact DNA c. a sister chromatid and a homolog? sisters are identical replicates of each other, created in S phase. Homologs have the same genes along their length, but differ in the alleles at those genes (b/c one is from “mom” and the other “dad”). 3. a. What happens to G2/M cyclin protein levels if you express a version of the cyclin that lacks its destruction box? A destruction box allows cyclin to be targeted for degradation. If these cyclins are not degraded, G2/M cyclin levels remain high. b. What happens to cell cycle progression in these cells? The cell cycle stalls late in mitosis. c. Would this be equivalent to what happens in a cell expressing a version of a G2/M cyclin that has all of its lysines mutated to alanines? Yes. These scenarios are similar. Since lysines are the residues to which ubiquitin is attached, without lysines there is no ubiquitination and hence no degradation of the cyclins.
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7.06 Fall 2006 PS 3 key 2 of 4 4. Before entry into S phase, a protein called Sic1p must be ubiquitinated and degraded in order to allow the cell cycle to progress. This protein is not ubiquitinated by APC but by another ubiquitin ligase complex called Cdc34-SCF. a. How would you determine what sequence in Sic1p, Cdc34-SCF binds to? You could do PCR mutagenesis on Sic1 and search for mutants that are no longer ubiquitinated and degraded. By sequencing Sic1 on these mutants, you can determine the sequence in your mutant and hence the amino acids that have been altered from wild-type that are important for Cdc34 recognition. You could also make different deletion mutants of Sic1 and determine which domains/regions are important for Cdc34 recognition of Sic1. b.
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This note was uploaded on 01/23/2012 for the course BIOLOGY lsm1301 taught by Professor Seow during the Spring '11 term at National University of Singapore.

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706_F06pset3_key - 7.06 FALL 2006 Problem Set #3 1. Are SCF...

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