Deep thought Questions

1. Use your knowledge about the structure of microtubules to explain how these cytoskeletal elements can be used to 'pull' the chromatids to the cell poles during cell division. Why might natural selection have favoured the use of microtubules (rather than microfilaments or intermediate filaments) for this function? Use the following terms in your answer: filamentous structures, assembly, disassembly, dynamic and shortening.

2. The same chromosome can look very different depending on when in the cell cycle it is observed. Why is that? How many DNA molecules does each chromosome contain at its most condensed? Use the following terms in your explanation: chromatin, histone, chromatid, centromere, chromosome.

3. What are the similarities and differences between the G2 and the M phase checkpoints? What are the phenotypic outcomes for the cell when either of the two checkpoints fails but the cell continues to divide?

4. What is meant by a repressible operon? In the case of the trp operon, how is repression exerted?  If someone asked you to genetically engineer a strain of E. coli to produce large amounts of tryptophan, what would be the easiest way to do that?

5. Describe the minimal suite of genes that must be encoded by a non-enveloped virus with an RNA genome. Explain why these need to be encoded and others do not.

6. Beadle started his career working on Drosophila, but changed his study system to bread mould (Neurospora crassa) for his studies in biosynthetic pathways. Why might it be easier and less time-consuming to use bread mould (a haploid organism) to study the effects of induced mutation on enzyme function?

7. The Central Dogma of Molecular Biology describes the flow of information in a cell. Briefly name and describe the two key processes that the cell performs in order to transmit and utilize genetic information. A full answer should include naming the template and the end product of each process, and the location of each process inside a eukaryotic cell.

8. A mutant yeast strain is found with a mutation affecting a tRNA^Tyr. The wild type normally produces a tRNA that recognizes the codon5'-UAC-3', and is charged with the amino acid Tyrosine (Tyr) - tRNA^Tyr. The mutant's tRNA is still charged with Tyr, but the anticodon is mutated and now has the sequence 5'-UUA-3'. What effect will this have on translation in these yeast cells? How will the proteins produced be different?

9. Unfertilized eggs of several animal species contain many mRNAs that should not be translated until fertilization occurs. Describe a molecular strategy employed by the egg to ensure timely activity of the proteins encoded by such mRNA.