Gene Expression & Regulation: A Comprehensive Worksheet

1. The Genetic Code (a) No, not all amino acids are encoded by the same number of codons. Some amino acids are encoded by multiple codons, while others are encoded by only one codon. (b) The amino acid leucine is encoded by the most codons. (c) The amino acids methionine and tryptophan are encoded by only one codon each. (d) The amino acids methionine and tryptophan are encoded by only one codon each. (e) The tRNA anti-codons that can bind to tyrosine codons are UAC and UAU. (f) Methionine is special because it serves as the start codon for protein synthesis.

2. Regulation of Gene Expression Mutations can alter the function of an operon. Predict how the following mutations would affect the Lac Operon gene transcription (i) in the presence of lactose and (ii) in the absence of lactose? (a) Mutation of the Operator so that the repressor can no longer bind to it. (i) In the presence of lactose, mutation of the Operator so that the repressor can no longer bind to it would result in increased gene transcription of the Lac Operon. (ii) In the absence of lactose, mutation of the Operator so that the repressor can no longer bind to it would have no effect on gene transcription of the Lac Operon as the repressor would not be bound to the operator region.

(b) Mutation of the Promoter so that it is no longer seen by RNA polymerase. (i) In the presence of lactose, mutation of the Promoter so that it is no longer seen by RNA polymerase would result in decreased gene transcription of the Lac Operon. (ii) In the absence of lactose, mutation of the Promoter so that it is no longer seen by RNA polymerase would have no effect on gene transcription of the Lac Operon as there would be no lactose to induce gene expression.

(c) Mutation of the Promoter for the lacI repressor so that it is no longer bound by RNA polymerase. (i) In the presence of lactose, mutation of the Promoter for the lacI repressor so that it is no longer bound by RNA polymerase would result in increased gene transcription of the Lac Operon. (ii) In the absence of lactose, mutation of the Promoter for the lacI repressor so that it is no longer bound by RNA polymerase would have no effect on gene transcription of the Lac Operon as the repressor would not be bound to the promoter region.

3. Gene Expression You want to express a human keratin protein, so you need to integrate the human gene into a bacterial plasmid. If you cut out the human keratin gene directly from the human DNA genome and insert it into the plasmid, will the bacteria be able to express this gene for you? If not, why not? What additional steps would you need to do to get the bacteria to express your gene? No, the bacteria will not be able to express this gene because bacterial genomes do not contain introns and therefore do not have the machinery to splice introns. To get the bacteria to express the gene, additional steps would be needed such as screening a cDNA library or making a cDNA from the keratin mRNA using reverse transcriptase, and then inserting the keratin cDNA into the bacterial expression plasmid.

4. Operon You are an undergrad research assistant in a UC Berkeley lab and are working in bacteria on the metabolism of the monosaccharide, those. Metabolism of those requires the enzymes X, Y, and Z. The genes encoding these enzymes are part of one operon, and the product of gene N (N protein) regulates the transcription of these three genes. In a normal cell, protein A is always produced. A diagram of this operon is shown below.

(a) Give a brief definition of an operon. (b) In a cell where there is a high level of N protein, you detect no transcription of genes X, Y, and Z. You conclude that the N protein is? (c) In further study, you discover that transcription of the N gene is controlled by protein A (protein A is the product of gene A shown in the diagram), and those binds to the N protein. You examine the transcription of genes N, X, Y, and Z in cells where gene A is normal (A+) and where gene A is not functional (A-) and in the presence (+) and absence (-) of those. The data was shown in a table. (i) Given the results above, what does protein A do? (ii) Given the results above, what does theose do?

(a) An operon is a functioning unit of genomic DNA that contains a cluster of genes under the control of a single promoter. It includes the genes encoding the proteins needed for a specific metabolic pathway or cellular process, as well as regulatory elements that control their transcription. (b) In a cell where there is a high level of N protein and no transcription of genes X, Y, and Z, it can be concluded that the N protein is acting as a repressor, inhibiting the transcription of these genes. (c) (i) Protein A activates the transcription of the N gene. (ii) Theose binds to the N protein and affects its ability to regulate the transcription of genes X, Y, and Z. The specific effect of theose on the N protein and its role in gene regulation would need further investigation.