Goal: pick 20 nt that direct a cut to your gene of interest with maximum on-target and minimum off-target activity.
Find your target region in the genomic DNA (e.g. the codon you want to knock out).
Scan for a PAM (NGG) on either strand near that region.
Take the 20 nt immediately 5' of that PAM on the DNA as the protospacer.
Write the spacer = same sequence as the protospacer strand (i.e. identical to the strand that does not pair with the guide), so that the spacer is complementary to the target (bottom) strand. Cas9 cuts ~3 bp upstream (5') of the PAM.
Score specificity: BLAST the 20-mer against the whole genome. Reject guides with close matches elsewhere (especially matches in the PAM-proximal "seed" region, the ~10–12 nt nearest the PAM, which dominate targeting).
Check on-target efficiency: avoid extreme GC content (aim ~40–60%), avoid poly-T stretches (TTTT terminates transcription from a U6/Pol III promoter), and often prefer a G at the 5' end for the U6 promoter.
What part of the CRISPR system provides target specificity?
The guide RNA (its 20-nt spacer), by base-pairing with the target DNA.
What is the spacer of an sgRNA?
The ~20-nt designed sequence complementary to the target DNA strand.
What is the scaffold and do you change it?
The fixed crRNA:tracrRNA hairpin that Cas9 binds; you do NOT change it.
What is the PAM for SpCas9?
5'-NGG-3', located immediately 3' of the 20-nt protospacer, on the DNA.
Is the PAM part of the guide RNA?
No — the PAM is on the DNA only; the guide neither contains nor pairs it.
Where does SpCas9 cut relative to the PAM?
~3 bp upstream (5') of the NGG, making a blunt double-strand break.
Which region of the guide matters most for specificity?
The seed region — the ~10–12 nt closest to the PAM.
Why do bacteria not cut their own stored spacers?
Their CRISPR array lacks an adjacent PAM, so Cas9 ignores it.
Good GC-content range for a guide?
Roughly 40–60%; avoid extremes and poly-T (which terminates Pol III transcription).
If the only NGG is on the bottom strand, how do you design the guide?
Read the 20 nt 5' of the bottom-strand NGG; spacer = that bottom-strand sequence (U for T).
Recall Feynman: explain to a 12-year-old
Imagine a robot with scissors that can cut a giant book (your DNA) — but it's blindfolded. You give it a sticky note with a sentence written on it (the guide RNA). The robot slides along until it finds a page whose words match the sticky note, then snips there. There's one catch: the robot only cuts if a special little tag "GG" is printed right next to the sentence (the PAM) — that's how it knows this isn't its own instructions. So your whole job is: write a 20-letter sentence that appears in exactly one place, right next to a GG tag. Write it well and it cuts one spot; write it sloppily and it cuts many wrong spots.
Dekho, CRISPR ek programmable kainchi hai. Kainchi (Cas9 protein) hamesha same rehti hai — usko kahan kaatna hai ye batata hai guide RNA. Guide RNA ke do hisse hote hain: ek spacer (~20 nucleotide) jo tumhe design karna hai, aur ek scaffold (fixed hairpin) jise tum chhedte nahi. Spacer target DNA ke saath base-pairing karta hai — matlab jaise ek address likha hua sticky note, jahan wo match kare wahin cut ho jaata hai.
Sabse important rule: PAM. SpCas9 tabhi kaatta hai jab target ke bilkul 3' side pe NGG ho (N = koi bhi base). Ye PAM DNA pe hota hai, guide RNA mein nahi hota — ye ek bahut common galti hai. Bacteria isliye PAM check karte hain taaki apni hi CRISPR memory na kaat de. Design karte waqt: pehle NGG dhoondo, phir uske 5' side ke 20 nt ko protospacer banao, aur spacer usi ke jaisa likho (T ki jagah U).
Cut kahan hota hai? PAM se ~3 base upstream (5' taraf) — blunt double-strand break. Aur specificity ke liye seed region (PAM ke paas wale 10–12 nt) sabse zyada matter karta hai — yahi wo jagah hai jahan mismatch bilkul allowed nahi. Isliye guide ko poore genome ke against BLAST karke off-target check karna zaroori hai, GC content 40–60% rakho, aur poly-T avoid karo warna U6 promoter se transcription ruk jaata hai.
Yaad rakhne ka simple funda: PAM Picks the spot, Spacer Sticks to DNA, Seed Sorts the off-targets. Isko samajh liya toh guide RNA design ka pura logic aa gaya.
