OpenStax Biology 2e Instructor Answer Guide
Biology 2e
Unit 3: Genetics
Chapter 16: Gene Expression
Visual Connection Questions
1. In E. coli, the trp operon is on by default, while the lac operon is off. Why do you think that
this is the case?
Tryptophan is an amino acid essential for making proteins, so the cell always needs to have
some on hand. However, if plenty of tryptophan is present, it is wasteful to make more, and the
expression of the trp receptor is repressed. Lactose, a sugar found in milk, is not always
available. It makes no sense to make the enzymes necessary to digest an energy source that is
not available, so the lac operon is only turned on when lactose is present.
, OpenStax Biology 2e Instructor Answer Guide
2. In females, one of the two X chromosomes is inactivated during embryonic development
because of epigenetic changes to the chromatin. What impact do you think these changes
would have on nucleosome packing?
The nucleosomes would pack more tightly together.
3. An increase in phosphorylation levels of eIF-2 has been observed in patients with
neurodegenerative diseases such as Alzheimer’s, Parkinson’s, and Huntington’s. What impact
do you think this might have on protein synthesis?
Protein synthesis would be inhibited.
Review Questions
4. Control of gene expression in eukaryotic cells occurs at which level(s)?
d. epigenetic, transcriptional, posttranscriptional, translational, and posttranslational levels
5. Post-translational control refers to:
b. regulation of gene expression after translation
Biology 2e
Unit 3: Genetics
Chapter 16: Gene Expression
Visual Connection Questions
1. In E. coli, the trp operon is on by default, while the lac operon is off. Why do you think that
this is the case?
Tryptophan is an amino acid essential for making proteins, so the cell always needs to have
some on hand. However, if plenty of tryptophan is present, it is wasteful to make more, and the
expression of the trp receptor is repressed. Lactose, a sugar found in milk, is not always
available. It makes no sense to make the enzymes necessary to digest an energy source that is
not available, so the lac operon is only turned on when lactose is present.
, OpenStax Biology 2e Instructor Answer Guide
2. In females, one of the two X chromosomes is inactivated during embryonic development
because of epigenetic changes to the chromatin. What impact do you think these changes
would have on nucleosome packing?
The nucleosomes would pack more tightly together.
3. An increase in phosphorylation levels of eIF-2 has been observed in patients with
neurodegenerative diseases such as Alzheimer’s, Parkinson’s, and Huntington’s. What impact
do you think this might have on protein synthesis?
Protein synthesis would be inhibited.
Review Questions
4. Control of gene expression in eukaryotic cells occurs at which level(s)?
d. epigenetic, transcriptional, posttranscriptional, translational, and posttranslational levels
5. Post-translational control refers to:
b. regulation of gene expression after translation
