Question

Genetics Worksheet Week 3: Gene Regulation and Epigenetics 1. Duchenne muscular dystrophy is caused by a mutation in a gene that is 2.5 million nucleotides in length and encodes a protein called dystrophin. The dystrophin protein itself is 3684 amino acids in length. Calculate below the approximate size of the mRNA that encodes dystrophin. Approximately what percentage of the gene that encodes dystrophin is intron sequence? The human genome encodes a much greater variety and number of proteins than the Drosophila genome, but the human genome does not contain significantly more genes than the Drosophila genome. Propose an explanation for the difference in variety and number of proteins between the two species. 2. 3. You have identified a mutation in mRNA guanylyltransferase, the enzyme responsible for the 5'-to-5' addition of a guanine residue to mRNA in eukaryotic cells. In this mutant, you observe a widespread reduction in protein expression. What is missing from mRNA in these cells, and why does it result in reduced protein expression? 4. What are two ways in which eukaryotic promoters and enhancers are different? What is one way in which they are similar? 5. What would be the expected effect on GAL gene expression of a mutation in the GAL80 gene (such that no GAL80 is produced in the cell)? What about a mutation in GAL4? 6. You are studying the regulation of a eukaryotic gene, and have identified three DNA sequences (Sequences A, B, C) that are required for normal expression of the gene. Sequence A is found less than 200 base pairs upstream of the start site of transcription Sequence B is found approximately 3 kilobases upstream, while Sequence C is found in the second intron of the gene. Using recombinant DNA techniques that we will discuss soon, you systematically reverse the orientation (5' to 3' direction) of each of these sequences and determine the effects of this manipulation on gene expression. In the case of Sequence A, you find that flipping the sequence into the reverse orientation completely abolishes expression of your gene. However, reversing the orientations of Sequences B and C has no effect on gene expression Finally, you mutate the three sequences. Mutations in Sequence A lead to complete loss of gene expression. Mutations in sequence B cause a reduction in expression, but not total loss. Mutations in Sequence C lead to inappropriate expression of the gene in tissues where normally it is not expressed. Which of these sequences is likely to be the promoter for your gene? Why? a. The other two sequences most likely function as enhancer elements. What is the evidence that they are enhancers? b. Which enhancer element likely binds to a repressor of transcription, and why? c. 9. What would be the expected effects of a loss-of-function mutation in PRC2? 10. What would be the effect of deleting the Dnmt3 gene in honeybees? What about the effects of expressing too much Dnmt3 in a developing queen? 11. Most DNA methylation in eukaryotes occurs at CpG dinucleotides, but occasionally, individual cytosine nucleotides are also methylated to form 5-mC. Considering what you know about the mechanism by which methylation is maintained at CpG nucleotides, do you think methylation at individual C nucleotides would be maintained in the same way? Explain your reasoning.

Answer 1

1). an enhancer is a piece of DNA that enhances gene transcription. a promoter is a piece of DNA which acts to initiate or gene transcription.

an enhancer binds with transcription factors. while a promoter binds with transcription and RNA polymerase enzymes.

an enhancer can be upstream or downstream from the site where transcription is initiated while a promoter always upstream from the site where transcription is initiated.

an enhancer doesn't need to be close to the site where transcription is initiated while a promoter does need to be close to the site where transcription is initiated.