No 3. Some bacteria possess a fascinating system for combatting viral infections
ID: 218794 • Letter: N
Question
No 3. Some bacteria possess a fascinating system for combatting viral infections called CRISPR (Clustered Regularly Interspersed Short Palindromic Repeats). The genetic elements of the CRISPR locus (region) of the bacterial chromosome are arranged like this: Intervening spacer DNA CAS genes Palindromic Repeats The spacer DNAs are actually sequences from viral genomes, that were copied and placed between the palindromic repeats during a viral infection. Through the action of the Cas enzymes, the viral sequences can be used to target viral RNA or DNA for destruction, as shown in the figure to the right. DNA sequences from different viruses are retained and can be used as a defense against the viruses in the future RNA Processing by Cas Enzymes Targeting of Cas-enzyme complex to virus RNA or DNA ous RNA or DNA Cleavage of vinus ONA RNA by Cas enzymes A. Which steps in the CRISPR mechanism are most similar to the Dicer and RISC steps of the iRNA process of eukaryotes? B. Describe three similarities between the functions or mechanisms of the RNA and the CRISPR. (e.g., "In both CRISPR and iRNA...") C. Other than the specific enzymes involved, identify three differences between the RNAi and the CRISPR mechanisms. (e.g., "In CRISPR..., whereas in RNA...) D. Explain the function of the palindromic repeats in the CISPR mechanism?Explanation / Answer
CRISPR is a mechanism for Adaptive immunity in the bacterial cell which provides specific immunity against any viral infection by adding the virus-encoded Spacer sequences in the bacterial genome which is cut by Cas proteins; the mechanism is somewhat similar to the eukaryotic gene regulating and RNA silencing pathway by RNA interference.
A. Step 2 in the CRISPR mechanism is similar to that of DICER in the RNAi mechanism where the spacer RNA is being cut and isolated from the bacterial genome.
Step 4 in the CRISPR mechanism is similar to that to RISC in the RNAi mechanism where the target RNA or DNA is being fragmented or digested by cas enzyme complexes.Cas enzyme complexes are functionally similar to RISC complex.
B. 1.Both the CRISPR and RNAi mechanisms are involved in Post-transcriptional silencing of the DNA, blocking the process of translation.
2. In both the mechanisms, a specific RNA sequence is modified into a enzyme complex which eventually fragments the targeted RNA/DNA strands.
3. Both the processes can be used as tools of modifying eukaryotic genome by insertion of specific required DNA sequences due to its heritability to next generation.
c. 1. Cas enzyme is used at usually every step of the CRISPR mechanism, whereas different types of proteins are used for different steps in the RNAi silencing mechanism. CRISPR RNA is required to be encoded by the bacterial genome with spacer sequences only, but for RNAi, small ds RNA can be introduced in the cell which will eventually lead the mechanism.
2. The precursor RNA for CRISPR comes from a single stranded RNA, but in case of RNAi, the precursor for RISC comes from double-stranded RNA. RNAi system targets self, specific mRNA, but CRISPR can target both DNA or RNA depending upon the genome of infecting viruses or invading plasmids.
3. CRISPR has a role in Immunity in the prokaryotic cells, it also provides memory of past viral infections by adding new spacer sequences alongwith the old one, and it has no specific cellular function. RNAi is primarily involved in the gene regulation at post-transcription level.
D. Palindromic sequences in the CRISPR genome makes a specific stable secondary structure (Hair-pin foldind) in the CRISPR rna after transcription. The nature of repeat sequences and its ability to form stable secondary structures describes the various types of CRISPER system in bacterial world. THese repeats not only provides a media for the viral genome, spacer, to be part of bacterial genome but also provide the specific recognition sites for the fragmentation of spacer only by cas genes.
Related Questions
drjack9650@gmail.com
Navigate
Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.