Chapter 12

Suppose you discovered a temperature-sensitive mutant whose nucleus failed to accumulate certain nuclear proteins at an elevated (restrictive) temperature but continued to accumulate other nuclear proteins. What conclusions might you draw about nuclear localization and the nature of this mutation?

Humans born with three \(X\) chromosomes but no \(Y\) chromosomes often develop into females of normal appearance. How many Barr bodies would you expect the cells of these women to have? Why?

What advantage might be gained by having transcripts synthesized and processed in certain regions of the nucleus rather than randomly throughout the nucleoplasm?

If you were to find a mutant of \(E\) coli that produced continuous polypeptide chains containing both \(\beta\) -galactosidase and galactoside permease (encoded by the \(y\) gene), how might you explain how this happened?

Suppose you had conducted a series of experiments in which you had transplanted nuclei from several different adult tissues into an activated, enucleated mouse egg and found that the egg did not develop past the blastocyst stage. Could you conclude that the transplanted nucleus had lost genes that were required for postblastula development? Why or why not? What does this type of experiment tell you more generally about interpreting negative results?

How do you explain why enhancers can be moved around within the DNA without affecting their activity, whereas the TATA box can only operate at one specific site?

Suppose that you are working with a cell that exhibits a very low level of protein synthesis, and you suspect the cells are subject to a global translational-control inhibitor. What experiment might you perform to determine whether this is the case?

The signal sequences that direct the translocation of proteins into the endoplasmic reticulum are cleaved by a signal peptidase, whereas the NLSs and NESs required for movement of a protein into or out of the nucleus remain as part of that protein. Consider a protein such as hnRNPA1, which is involved in the export of mRNA to the cytoplasm. Why is it important that the transport signal sequences for this protein remain as part of the protein, whereas the signal sequence for ER proteins can be cleaved?

When methylated DNA is introduced into cultured mammalian cells, it is generally transcribed for a period before it becomes repressed. Why would you expect this type of delay before inhibition of transcription would occur?

A study in a British medical journal found there was a correlation in telomere length between fathers and their daughters and between mothers and both their sons and daughters but not between fathers and their sons. How can you explain this finding?