Question

Which component of the CRISPR/Cas9 gene editing system is NOT of natural origin?

• A. CAS9 protein
• B. CRISPR repeats
• C. PAM sequence
• D. sgRNA

Hint:

Natural CRISPR systems use two separate RNAs (crRNA and tracrRNA); the single fused guide RNA used in the lab is an engineered construct.

Answer 1

The Correct Option is D

Step 1: Recall the natural bacterial CRISPR-Cas system.
CRISPR-Cas9 evolved in bacteria and archaea as an adaptive immune system against invading phages and plasmids. The natural locus has repeat sequences (CRISPR repeats) separated by spacers taken from past invaders, which are transcribed and processed into short crRNAs. A separate small RNA called tracrRNA base-pairs with the crRNA, and this crRNA:tracrRNA duplex guides the Cas9 protein to cut matching foreign DNA.

Step 2: Check which parts exist naturally.
The Cas9 protein is a naturally occurring bacterial nuclease. The CRISPR repeats are naturally occurring DNA sequences in the bacterial genome. The PAM (protospacer adjacent motif) sequence is a short natural sequence next to the target site that Cas9 must recognize before it can cut; it exists in the genome of the organism being targeted, not something engineered in.

Step 3: Identify the engineered part.
In the lab, researchers fuse the crRNA and tracrRNA into one continuous RNA molecule to simplify the system. This fusion product is called the single guide RNA, or sgRNA. It does not exist as a single molecule in nature; natural systems always keep the crRNA and tracrRNA as two separate, base-paired RNAs.

Step 4: Rule out the other options.
CAS9 protein, CRISPR repeats, and the PAM sequence are all pre-existing, naturally occurring parts of the bacterial defense system; genome engineers did not invent any of them. Only the single continuous guide RNA is a laboratory construct built by fusing two natural RNAs together.

Final Answer:
The sgRNA is not of natural origin, it is an engineered fusion of the natural crRNA and tracrRNA. \[ \boxed{\text{sgRNA}} \]