3. a) Draw all the elements you would expect to see in an operon encoding 2 genes.
The operon is defined at the level of DNA. Remember it is only found in prokaryotes.
Note* the operator (O) can be on either side of the promoter (P)
b) Draw the resulting mRNA with all the needed elements. Assume this is in E. coli.
c) If a frameshift mutation occurs near the 5’ end of the coding region (ORF) of gene 1, will it
affect gene 2/ORF 2? Explain why/why not?
No it will not affect gene 2 because both genes are independently translated from the
mRNA. They are not translated as a single combined protein.
d) Why don’t eukaryotes make polycistronic mRNA? Explain in molecular terms.
Eukaryotes cannot read more than one ORF from a single mRNA because of the
dependence for ribosome binding at the 5’ cap. The small subunit binds at the cap and
then scans the mRNA until it finds the first AUG codon and translation begins and ends
at the first in-frame stop codon. Then the ribosome falls off. If there is a second ORF, the
ribosome will never get to it.
4. List the possible genotypes of single-mutant E. coli cells demonstrating the following
phenotypes for b-galactosidase production:
a) constitutive I-P+O+Z+Y+A+ or I+P+OcZ+Y+A+ (or I+P+O-Z+Y+A+)
b) repressive (non-inducible) I+P-O+Z+Y+A+ or I+P+O+Z-Y+A+ or I+P+O+Z+Y-A+ (or
IsP+O+Z+Y+A+ , although we did not cover IS in lecture, only tutorial so it won’t be
asked for on the exam)
c) inducible must be wildtype for all genes/elements