An investigator monitors the production of a particular mRNA in a mouse cell line. Expression of the mRNA is induced (i.e., its concentration in the cytoplasm increases) in response to the addition of a hormone. The observed increase in the mRNA concentration is blocked by actinomycin D and by cycloheximide. What does this tell you about the requirements for increased expression of this mRNA?

In female fruit fly embryos, the Sxl protein initially generated by transcription from the P_e promoter differs somewhat from that generated by later transcription from the P_m promoter. In what part of the protein does this difference arise?

As a researcher, you wish to scan the sequences of all the genes of the mouse genome to determine how many genes have multiple sites for 3′-end cleavage. What sequence would you scan for? Would a scan for a single sequence find all of the sites?

In eukaryotes, phosphorylation of the translation initiation factor eIF2α blocks translation of virtually all mRNAs. In a mammalian reticulocyte, a deficiency in iron or heme leads to eIF2α phosphorylation to block the translation of globin mRNAs. The phosphorylation of eIF2α does not create a problem for other cellular functions in reticulocytes. Suggest why.

What is the likely fate of an mRNA transcript containing the sequence (a) AAUAAA or (b) AUUUA?

As organisms become more complex, so do the numbers and structures of introns. Introns in vertebrates range up to 100,000 nucleotides in length. These long introns often include canonical splicing signals, but the signals are not recognized by the cellular spliceosome. How are these nonproductive splicing sites suppressed?

Suggest at least three cellular mechanisms that could establish a gradient of either a protein or an mRNA during maturation of an oocyte.

In C. elegans, the Pal-1 protein specifies some developmental fates in cells where it is highly expressed. Translation of the Pal-1 mRNA is suppressed by binding of the Mex-3 protein, concentrated in anterior cells of the embryo, to the 3′UTR of the Pal-1 mRNA. What would be the probable effect of mutations that eliminated the binding of Mex-3 to this 3′UTR?

A Drosophila female embryo that is bcd⁻/bcd⁻ may develop normally, but the adult fruit fly will not be able to produce viable offspring. Explain why.

In the Drosophila ovary, a germ-line stem cell repeatedly divides. After each division, one daughter cell retains stem cell identity and the other begins to differentiate into an oocyte. The germ-line stem cell is associated with additional cells called cap cells. Describe how the asymmetric divisions might occur, and the possible role of the cap cells.

What is an induced pluripotent stem cell, and how does it differ from an embryonic stem cell?

The stem cell genes that regulate tissue regeneration tend to be highly conserved. Planaria (an aquatic flatworm) has an impressive capacity to regenerate its head and other structures when they are amputated, making this a favorite subject in grade school science labs. In the wild, Planaria eats smaller worms and eukaryotic organisms in its environment. In the lab, it can be fed clumps of bacteria mixed with pieces of liver and agar. As a biologist, you know that tissue regeneration mechanisms are likely to be conserved. You are interested in determining which Planaria genes are needed to guide head regeneration. Your reading tells you that regeneration depends on certain stem cells posterior to the animal’s photoreceptors and excluded from its pharynx. Using methods described in this chapter, how would you go about discovering the key genes?