Biotechnology: Genetic Engineering Techniques (College Board AP® Biology): Exam Questions

Figure 1. Comet assay to detect double-stranded breaks in DNA

A comet assay is a technique used to determine the amount of double-strand breaks in DNA (DNA damage) in cells. The nucleus of an individual cell is placed on a microscope slide coated with an agarose gel. An electric current is applied to the gel that causes DNA to move (electrophoresis), and the DNA is stained with a fluorescent dye. When viewed using a microscope, undamaged DNA from the nucleus appears as a round shape (the head), and the fragments of damaged DNA extend out from the head (the tail). The length of the tail corresponds to the amount of the damage in the DNA (see Figure 1).

To explain the movement of DNA fragments in the comet assay, identify one property of DNA and provide reasoning to support how the property contributes to the movement during the comet assay technique.

In a different experiment, cells are treated with a chemical mutagen that causes only nucleotide substitutions in DNA. Predict the likely results of a comet assay for this treatment.

Scientists used genetic engineering to insert genes from cold-adapted Arctic fish into tomato plants using a natural soil bacterium called Agrobacterium tumefaciens. The aim was to help tomatoes grow in colder climates by expressing a fish antifreeze protein that prevents ice crystal formation in plant cells.

To do this, scientists used a genetic material containing:

• A fish antifreeze gene

• A DNA sequence upstream of the transcription start site that is active in plant cells

The genetic material was introduced into tomato plant cells using a plasmid vector. Researchers then used gel electrophoresis to identify successfully engineered plants.

(i) Identify which genetic engineering technique was used to successfully introduce the vector into the plant cells.

(ii) Bacteria are extremely useful for DNA manipulation. Explain why.

(i) Identify the DNA sequence required for transcription in tomato cells.

(ii) Explain the role of this DNA sequence in gene expression.

(iii) Predict what would happen if the inserted gene lacked this DNA sequence.

(i) Explain how gel electrophoresis confirms the presence of the antifreeze gene.

(ii) Identify the property of DNA that allows it to be separated by gel electrophoresis.

Explain how natural selection could act on this genetically modified population of plants.

Researchers are studying a gene in a population of wild rabbits that appears to affect fur color. They use polymerase chain reaction (PCR) on a specific region of DNA from each rabbit. Then, they perform DNA sequencing to identify the nucleotide sequence of the gene in dark-furred and light-furred individuals.

Their aim is to determine whether a difference in DNA sequence correlates with the fur color variation.

Describe the purpose of PCR in the experiment.

PCR uses DNA polymerase during the procedure.

Justify the requirement for this enzyme in the process.

Explain how DNA sequencing data can be used to compare rabbits with different fur colors.

Researchers find a single base-pair difference in the DNA sequence between dark-furred and light-furred rabbits.

Justify how a single base-pair change could affect the protein and the rabbit’s fur color.