Question

1. (1 points) A deletion mutation in the leader sequence of the trp operon removes the two tryptophan codons that are involved in attenuation. Predict the effect of this mutation on the expression of the trp structural genes in E. coli cells grown in media that lacks tryptophan. 2. (2 points) What protein family members are the main protein components of the RISC complex? How does the RISC complex target specific mRNAs for silencing? 3. (3 points) In bacteria, the CRISPR/cas system targets specific DNA sequences found in phage (virus) genomes and degrades those sequences. How do cas enzymes recognize these DNA sequences? 4. (3 points) Researchers have modified one particular cas enzyme, cas9, to target molecular functions other than endonuclease activity (degrading DNA) to particular loci in genomes. How is this accomplished? 5. (6 points) The picture below depicts a 9kb region of a genome to which sequencing reads generated from sequencing mRNA have been mapped. Each small horizontal line represents a single sequencing read that has been mapped to the genome sequence. The numbers below indicate positions (in bases) along this region of the genome. This region of the genome is thought to contain a gene that codes for a protein that is 582 amino acids long. a) What part or structure of this gene lies between bases 3612 and 4032? What part of this gene lies between bases 4032 and 5100? b) Predict the combined length of the 5' UTR + 3' UTR sequences of this gene.

Answer 1

Leader sequence of trp operon serves in attenuation of the transcription. When tryptophan levels are high in medium, the cell does not need expression of tryptophan operon. Transcription of leader sequence and formation of stem loop structure prevents the transcription of structural genes of the operon.
In absence of tryptophan in medium, the leader sequence is not translated in absence of tryptophan in medium to prevent expression of structural genes.
The given mutation in the leader sequence that remove two tryptophan codon from it would not allow the ribosome to pause. The ribosome would translate sequence 1 and 2 of the leader sequence. Stem pairing of these two sequence would prevent the transcription of structural genes of the operon.