Week 15 Quiz

A muscle cell and a neuron from the same person carry the same genome. What best explains why the two cells look and act so differently?

Select one.

• The neuron has lost the genes for muscle proteins during development
• The two cells switch on different subsets of the same genes
• The muscle cell carries extra copies of DNA that the neuron lacks
• Each cell type gained its own mutations that built its job

In the lac operon model, when no lactose is present, the repressor sits on the DNA and the lactose-digesting genes are switched off.

True or false.

• True
• False

Gene 1's mRNA is 5′-AUG GCU UAU UGA-3′, which reads Met–Ala–Tyr, then stop. In the standard code, GCU and GCC both code for alanine (Ala). What kind of mutation changes GCU to GCC?

Select one.

• A missense mutation, because one base in the codon changed
• A nonsense mutation, because the codon was rewritten
• A silent mutation, because the amino acid stays Ala
• A frameshift mutation, because a base was replaced

On Gene 1's mRNA, 5′-AUG GCU UAU UGA-3′, the codon UAU (Tyr) becomes UAA, which is a stop codon. Which protein does the mutated gene make?

Select one.

• Met–Ala, a chain cut short by the new stop
• Met–Ala–Tyr, the same chain as before
• Met–Asp–Tyr, a chain with one swapped amino acid
• Met–Ala–Tyr–Ala, a chain that runs past the stop

The first base after AUG is deleted from Gene 1's mRNA, 5′-AUG GCU UAU UGA-3′, leaving AUG CUU AUU plus 2 leftover bases. Which statement describes this mutation?

Select one.

• It is a silent change, because the code has more than one codon per amino acid
• It changes only the second amino acid, because only one base was lost
• It is a nonsense change, because the protein is shortened to Met–Ala
• It is a frameshift, so every codon after AUG changes and the stop is lost

On the illustrative virtual gel, the size ladder has bands at 500, 1,000, 2,000, and 4,000 bp. DNA runs from the wells toward the positive end. Which ladder band travels farthest from the wells?

Select one.

• The 500 bp band, because smaller fragments slip through the gel faster
• The 4,000 bp band, because bigger fragments carry more charge
• The 2,000 bp band, because middle sizes move most freely
• All four bands travel the same distance, because all DNA is negative

On the illustrative virtual gel, the crime-scene lane shows bands at 500, 2,000, and 4,000 bp. Suspect 1 shows 1,000, 2,000, and 4,000 bp; Suspect 2 shows 500, 2,000, and 4,000 bp. Which reading is accurate?

Select one.

• Suspect 1 matches, because two of the three bands line up
• Neither suspect can be compared, because the lanes share bands
• Suspect 2 matches, and Suspect 1 is excluded by one band
• Both suspects match, because each shares the 4,000 bp band

Which statements about PCR and gel electrophoresis are accurate? Select every accurate statement.

Select every option that applies.

• PCR copies a chosen stretch of DNA in rounds of denature, anneal, extend
• Larger DNA fragments travel farther through a gel than smaller ones
• A gel sorts DNA fragments by size as they run toward the positive end
• A trace sample is copied by PCR before a gel sorts the copies
• PCR sorts DNA fragments by size so they can be read as bands

In CRISPR-Cas9 gene editing, what does the guide RNA do?

Select one.

• It cuts the DNA at the target site once it arrives
• It carries a new gene into a bacterium as a small ring
• It copies the target sequence over and over for study
• It leads the Cas9 protein to the matching DNA sequence

A described edit changes the DNA of an early human embryo. Where does it fall on the line between somatic and germline editing?

Select one.

• Somatic, because an embryo is made of body cells
• Germline, because the change can pass to future generations
• Somatic, because only one future person is affected
• Germline, because any edit made with CRISPR is germline