Question

You have isolated bacteria containing mutations in various parts of the CRISP-Cas system. Consider each of these mutations, and select the answer that describes where in the CRISPR-Cas system each mutant would be stuck?

Answer 1

In the CRISPR-Cas system, mutations that prevent the production of Cas proteins, alter tracrRNA, or affect endonuclease activity of Cas proteins will disrupt the bacterial immune response pathway at different stages, leading to an inability to target or cleave phage DNA.

Step-by-step explanation:

In the CRISPR-Cas system, mutations at various stages could lead to different blocks in the immune response pathway against phage DNA. If a mutation occurs in the Cas genes responsible for the CRISPR array, the bacterial cell will fail to produce the Cas proteins that are essential for the interference stage of the immune response. With ineffective Cas proteins, the CRISPR RNA (crRNA) and trans-activating CRISPR RNA (tracrRNA) cannot form a complex to target and cleave the phage DNA.

Additionally, a mutation in the tracrRNA or processing enzymes involved in crRNA maturation would prevent proper formation of the crRNA-tracrRNA-Cas protein complex, halting the system before the targeting stage. If cells cannot form the CRISPR-spacer RNA complex, for example, they won't be able to identify and destroy the matching viral DNA. Lastly, should there be a mutation in the endonuclease activity of the Cas protein, even if the crRNA-tracrRNA-Cas protein complex forms correctly, it won't be able to cleave the phage DNA, thus preventing the immunity the CRISPR system conveys.