CRISPR Lab: Interpreting Colony Color, Gene Editing, and Experimental Results

Q1. Explain why the colonies on both plates are blue.

Background

Topic: Reporter genes and blue/white screening in bacterial genetics.

This question tests your understanding of how the lacZ gene and its product (β-galactosidase) interact with X-gal to produce a blue color, and what this indicates about gene function in the context of a CRISPR experiment.

Key Terms and Concepts:

• lacZ gene: Encodes β-galactosidase, which cleaves X-gal to produce a blue product.

• X-gal: A substrate that turns blue when cleaved by β-galactosidase.

• IPTG: An inducer that activates the lac operon, leading to expression of lacZ.

Step-by-Step Guidance

1. Recall that blue colonies indicate active β-galactosidase, which means the lacZ gene is functional and being expressed.

2. Both plates contain IPTG and X-gal, so if the lacZ gene is intact and expressed, the colonies will turn blue.

3. Consider the role of Cas9: If Cas9 has not disrupted the lacZ gene, the gene remains functional.

4. Think about whether any other additives (like arabinose) would affect lacZ expression or only impact CRISPR editing efficiency.

Try solving on your own before revealing the answer!

Q2. What color would the colonies be if the colonies did not have a functional version of the lacZ gene?

Background

Topic: Blue/white screening and gene disruption.

This question tests your understanding of how loss of lacZ function affects colony color on X-gal plates.

Key Terms and Concepts:

• Non-functional lacZ: No β-galactosidase produced, so X-gal is not cleaved.

• Colony color: Indicates whether lacZ is functional (blue) or disrupted (white or colorless).

Step-by-Step Guidance

1. Recall that functional lacZ leads to blue colonies due to X-gal cleavage.

2. If lacZ is non-functional, β-galactosidase is not produced, so X-gal remains uncleaved.

3. Think about what color the colonies would be if X-gal is not cleaved.

Try solving on your own before revealing the answer!

Q3. How will the presence of arabinose in the IX/ARA plate impact the ability to edit the genomes of the cells in the experiment?

Background

Topic: Inducible gene expression systems and CRISPR genome editing.

This question tests your understanding of how arabinose can induce expression of genes (such as Cas9 or sgRNA) under the control of an arabinose-inducible promoter, affecting genome editing efficiency.

Key Terms and Concepts:

• Arabinose: A sugar that can induce expression of genes under the araBAD promoter.

• CRISPR editing: Requires expression of Cas9 and sgRNA to cut target DNA.

Step-by-Step Guidance

1. Identify which component(s) of the CRISPR system are under the control of an arabinose-inducible promoter.

2. Consider how the presence of arabinose would affect the expression of these components.

3. Think about how increased expression of Cas9 or sgRNA would impact genome editing efficiency.

4. Relate this to the expected outcomes on the IX/ARA plate compared to the IX plate.

Try solving on your own before revealing the answer!

Q4. For each sample (A–D), indicate with a + or – if the transformed cells contain the sgRNA, the donor template DNA, or both.

Background

Topic: Plasmid transformation and CRISPR components.

This question tests your ability to interpret experimental design and identify which genetic elements are present in each sample based on the plasmids used.

Key Terms and Concepts:

• sgRNA: Single guide RNA, directs Cas9 to the target DNA sequence.

• Donor template DNA: Provides a template for homology-directed repair after Cas9-induced cutting.

• pLZDonor: Plasmid containing donor template DNA.

• pLZDonorGuide: Plasmid containing both donor template DNA and sgRNA.

Step-by-Step Guidance

1. Review the table to see which plasmid was used for each sample (A–D).

2. Determine which plasmids provide sgRNA, donor template DNA, or both.

3. Assign a '+' if the component is present in the transformed cells, or a '–' if it is absent.

4. Fill in the table for each sample accordingly.

Try solving on your own before revealing the answer!

Q5. For each of the four samples, indicate if growth of colonies will be expected on the plates as well as the color of the colonies if growth occurs.

Background

Topic: Selection and screening in bacterial transformation experiments.

This question tests your understanding of antibiotic selection (spectinomycin), blue/white screening, and the effects of CRISPR editing on colony color and growth.

Key Terms and Concepts:

• Spectinomycin: Antibiotic used for selection; only transformed cells with resistance will grow.

• Colony color: Indicates lacZ gene status (blue = functional, white = disrupted).

Step-by-Step Guidance

1. For each sample, check if the cells should be resistant to spectinomycin based on the plasmid(s) they received.

2. Determine if growth is expected (Yes/No) for each sample.

3. If growth is expected, use your knowledge of lacZ status to predict the color of the colonies (blue or white).

4. Fill in the table for each sample with your predictions.

Try solving on your own before revealing the answer!

Q6. Which samples show evidence that the lacZ gene has been cut?

Background

Topic: CRISPR-mediated gene disruption and phenotypic screening.

This question tests your ability to interpret experimental results (colony color) as evidence of gene editing (cutting of lacZ).

Key Terms and Concepts:

• Gene cutting: Cas9/sgRNA complex introduces double-strand breaks at the target site.

• Colony color: White colonies suggest lacZ disruption (cutting); blue colonies suggest intact lacZ.

Step-by-Step Guidance

1. Review your predictions for colony color from the previous question.

2. Identify which samples produce white colonies, indicating lacZ has been cut/disrupted.

3. List these samples as showing evidence of lacZ gene cutting.

Try solving on your own before revealing the answer!

Q7. Of the samples that show evidence that the lacZ gene was successfully cut, which show evidence that the cut has been repaired?

Background

Topic: Homology-directed repair (HDR) and functional restoration of genes.

This question tests your understanding of how donor template DNA can repair a CRISPR-induced cut, restoring gene function (and blue color).

Key Terms and Concepts:

• Homology-directed repair (HDR): Uses donor DNA to repair double-strand breaks, potentially restoring gene function.

• Colony color: Blue colonies after editing suggest successful repair of lacZ.

Step-by-Step Guidance

1. From the samples with evidence of lacZ cutting, check which also have donor template DNA present.

2. Determine which of these samples produce blue colonies, indicating successful repair.

3. List these samples as showing evidence of repair.

Try solving on your own before revealing the answer!

Q8. Write a brief paragraph in grammatically correct English that explains the results for each sample.

Background

Topic: Experimental analysis and scientific communication.

This question tests your ability to synthesize experimental results and explain the genetic outcomes for each sample, including the roles of CRISPR components and the observed phenotypes.

Key Terms and Concepts:

• Experimental controls: Samples without sgRNA or donor DNA serve as controls.

• Phenotypic outcomes: Colony color and growth indicate gene status and editing success.

Step-by-Step Guidance

1. For each sample (A–D), summarize which components were present (sgRNA, donor DNA, Cas9, arabinose).

2. Describe the expected outcome (growth, colony color) based on the presence or absence of these components.

3. Explain how the results support or refute successful gene editing and repair.

4. Write a concise paragraph integrating these points for all four samples.

Try writing your paragraph before revealing the sample explanation!