The following problems are to help you sharpen and deepen your understanding of transcription, the genetic code, and translation. These questions will be discussed in the recitation section on Oct. 17. Problems are not to be handed in and will not be graded.

1. Describe in words the role or characteristics of each of the following in prokaryotic transcription:

a) promoter region
b) -10 region
c) -35 region
d) RNA polymerase holoenzyme
e) sigma (σ) factor
f) ribonucleoside triphosphates

2. Explain the principal similarities and differences between prokaryotes and eukaryotes in the following aspects of transcription:

a) Structure of the promoter region
b) the enzyme(s) that catalyze transcription
c) the accessory protein(s) involved
d) the post-transcriptional fate of the RNA

3. You synthesize the trinucleotide AAG and polymerize this trinucleotide to form a repeating polymer (AAG)_n. If you were to translate this polymer in a system with all possible aminoacyl-tRNAs available, what products would you expect to see? (Note: There are three products.)

4. In an experiment such as that in question (3), why would you see only two products with the polymer (GUA)_n? What are they?

5. (Modified from Snustad & Simmons, Principles of Genetics) The thymine analog 5-bromouracil induces single base-pair substitutions in DNA (mutations) of the types AÛG and CÛT. Explain which of the following amino acid substitutions might be caused by 5-BU treatment and why.

Met→Val          Met→Leu         Lys→Thr          Lys→Gln          Pro→Arg          Pro→Ser

First, write out the nucleotide sequence of the mRNA segment that codes for this protein, using the table of codons in your text. Note that some nucleotide positions will be uncertain. In these positions use "X" to denote any base, "R" to denote a purine (A or G) and "Y" to denote a pyrimidine (U or C).

Now consider all possible single-base-change mutations in this region, and answer the following:

a) What fraction of all such mutations cause premature termination of translation?
b) What fraction cause a loss of a negative charge?
c) What fraction cause replacement of a hydrophobic (nonpolar) amino acid sidechain with a hydrophilic (polar) amino acid sidechain?