Noesis
6.2.13Genetic Engineering & CRISPR

Explain base editing and prime editing

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🇮🇳 Yeh note Hinglish mein padho →

WHY do we need them?

Base Editing

The two flavours

HOW it works (step by step)

  1. The guide RNA brings the Cas9-deaminase complex to the target.
  2. Cas9 unwinds the DNA, exposing a small single-stranded loop called the R-loop (the non-target strand).
  3. The deaminase acts on a base in the exposed window (~ positions 4–8 of the protospacer).
  4. Chemistry of CBE: deaminase removes an amino group from cytosine → uracil (U). The cell reads U as if it were T.
  5. Chemistry of ABE: deaminase converts adenine → inosine (I). The cell reads I as if it were G.
  6. A nickase cuts the opposite (unedited) strand. The cell, seeing a nick, uses the edited strand as the template to "repair" the other strand — locking in the edit.
Worked example Worked example: fixing a disease mutation with ABE

Suppose a pathogenic mutation created a G→A change on the coding strand, so it reads A where healthy DNA has G.

  • Why use ABE? We must turn A back into G. ABE does exactly A→G.
  • Design the guide so the mutant A sits in the editing window (positions 4–8).
  • Why the window matters? Deaminase only reaches bases in the exposed R-loop; outside the window there's no edit.
  • Result: A→G on target strand, nick on other strand, cell copies the fix. Disease letter corrected — no DSB.
Figure — Explain base editing and prime editing

Limitation of base editing

Prime Editing

The pegRNA — the clever part

HOW it works (step by step)

  1. pegRNA targets the site; Cas9 nicks one strand (single cut, not double).
  2. The nicked DNA 3′ end pairs with the PBS on the pegRNA.
  3. The reverse transcriptase uses the RT template to synthesize new DNA carrying the edit, directly onto the nicked strand.
  4. Now there's a flap of new (edited) DNA competing with the old flap. Cell resolves it, keeping the edited flap.
  5. A second nick on the other strand (guided by an extra "nicking sgRNA") encourages the cell to make the other strand match the edit.
Worked example Worked example: inserting 3 bases with prime editing

Goal: insert CTG into a gene (base editors cannot do this).

  • Why prime editing? Only prime editing can add/remove bases.
  • Design pegRNA whose RT template encodes the original sequence plus CTG.
  • Why put it in the RT template? Because whatever the RT copies becomes the new DNA — so we simply "write" the extra letters there.
  • Nick → prime → RT copies template (with insert) → flap resolution → CTG inserted. No DSB, no donor.
Worked example Worked example: a transversion C→G

Base editors can't do C→G. Prime editing can.

  • Why? Prime editing doesn't rely on deamination chemistry; it physically rewrites the strand from the RT template.
  • Encode G where C was in the RT template → done.

Comparison

Feature Cas9 (classic) Base Editing Prime Editing
Double-strand break? Yes No (nick only) No (nick only)
Needs donor DNA? Yes (for HDR) No No
Point substitutions via HDR (inefficient) only transitions any substitution
Insertions/deletions random indels No Yes (small)
Key enzyme added — deaminase reverse transcriptase

Common Mistakes (Steel-man + Fix)

Flashcards

What key feature makes base & prime editing safer than classic Cas9?
They avoid double-strand breaks (use a nick or dead Cas9), reducing indels/translocations.
What conversion does a Cytosine Base Editor (CBE) perform?
C→T (and G→A on the complementary strand), via cytidine deamination.
What conversion does an Adenine Base Editor (ABE) perform?
A→G (and T→C on the complementary strand), via adenosine deamination.
What chemical intermediate does CBE make, and how is it read?
Uracil (U), read by the cell as thymine (T).
What chemical intermediate does ABE make, and how is it read?
Inosine (I), read by the cell as guanine (G).
Which mutation types can base editors do — and NOT do?
Can do transitions (C↔T, A↔G); cannot do transversions or insertions/deletions.
What two extra enzymes/RNAs define prime editing?
A reverse transcriptase (fused to Cas9 nickase) and a pegRNA.
What two functional parts sit in a pegRNA's 3′ extension?
The PBS (primer binding site) and the RT template (carries the edit).
Why can prime editing make any substitution plus small indels?
The desired edit is written in the RT template (RNA) and reverse-transcribed into DNA — no chemistry restriction.
What is "bystander editing" in base editing?
Unwanted editing of other identical bases (e.g. extra C's) within the editing window (~positions 4–8).
Which enzyme family provides the deaminase in CBE vs ABE?
CBE uses APOBEC-type cytidine deaminase; ABE uses engineered TadA adenosine deaminase.
Does prime editing need a separate donor DNA?
No — the template is inside the pegRNA and copied by reverse transcriptase.
Recall Feynman: explain to a 12-year-old

Old CRISPR is like ripping a whole page out of a book to fix one typo — messy. Base editing is like a magic eraser that changes one letter (only certain swaps, like turning an 'a' into a 'g'). Prime editing is even smarter: it carries a tiny sticky note with the correct spelling, presses it against the book, and a little copier machine writes the new letters in — so you can fix any typo or even add missing letters, all without tearing the page.

Connections

  • CRISPR-Cas9 mechanism — provides the targeting backbone and the nickase used here.
  • Homology-Directed Repair (HDR) — the older, DSB-dependent precise-editing route these tools replace.
  • Reverse Transcriptase — the enzyme that lets prime editing write RNA info into DNA.
  • Deamination of bases — the chemistry (C→U, A→I) behind base editors.
  • Point mutations & genetic disease — why single-letter correction matters clinically.
  • Guide RNA design — extends to pegRNA design (PBS + RT template).

Concept Map

makes

causes

motivates

motivates

solved by

solved by

uses

fused to

no

type

type

via

via

acts in

Classic CRISPR-Cas9

Double-strand break

Random indels & danger

Need precise editing

Point mutations ~half of disease

Base Editing

Prime Editing

Impaired Cas9 nickase

Deaminase enzyme

CBE: C to T

ABE: A to G

Cytosine to Uracil read as T

Adenine to Inosine read as G

R-loop window pos 4-8

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Dekho, purana CRISPR-Cas9 ek kainchi (scissors) jaisa hai — wo DNA ko dono strands se kaat deta hai (double-strand break). Lekin ek single letter ki galti theek karni ho, to itna bada cut karna khatarnak hai — repair karte waqt random indels aa jaate hain. Isliye base editing aur prime editing aaye, jo bina DSB kiye DNA ko "search and replace" karte hain.

Base editing ek deaminase enzyme ko dead/nickase Cas9 ke saath jodta hai. Ye chemically ek base ko doosre me badal deta hai: CBE karta hai C→T (beech me uracil banta hai jise cell T padhta hai), aur ABE karta hai A→G (beech me inosine banta hai jise cell G padhta hai). Bas transitions ho sakte hain — transversions ya insert/delete nahi. Editing sirf ek chhoti window (position 4-8) me hoti hai.

Prime editing aur bhi powerful hai. Yahan Cas9 nickase ke saath reverse transcriptase juda hota hai, aur ek special pegRNA use hoti hai jisme do cheezein hoti hain: PBS (jo nicked DNA end ko pakadta hai) aur RT template (jisme aapka desired edit RNA me likha hota hai). Nick hone ke baad RT us template ko DNA me copy kar deta hai — isliye koi bhi substitution, aur chhote insert/delete bhi possible hain. Yaad rakho: edit RNA me likha hota hai aur DNA me "print" hota hai, koi alag donor DNA nahi chahiye.

Ye matter isliye karta hai kyunki zyadatar genetic diseases single-letter point mutations hain, aur ye tools unhe safely, bina DNA todhe theek kar sakte hain — future me gene therapy ka bada hissa yahi hai.

Test yourself — Genetic Engineering & CRISPR

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Connections

Explain the CRISPR-Cas9 mechanismBiology · 6.2.10Describe guide RNA designBiology · 6.2.11