DNA Base Editing

DNA base editing changes one nucleotide into another through a chemical reaction performed directly on DNA. It combines a programmable CRISPR targeting system with a deaminase enzyme, allowing conversion of a base without cutting both DNA strands or supplying a donor DNA template.

Targeting and chemical conversion

A guide RNA directs a catalytically impaired Cas protein to a complementary DNA sequence beside an appropriate PAM. Cas binding locally separates the strands, exposing a short single-stranded editing window in which the attached deaminase chemically modifies a base.

• The deaminase changes the base's chemical identity rather than removing and replacing the entire nucleotide.
• The Cas component is usually a Cas9 nickase or catalytically inactive Cas9, so it does not simultaneously cleave both DNA strands.

Major base-editing reactions

• A cytosine base editor deaminates cytosine to uracil, which is read like thymine; repair or replication can therefore convert a C:G pair into a T:A pair.
• An adenine base editor converts adenine into inosine, which is read like guanine; this can convert an A:T pair into a G:C pair.
• These editors mainly produce transition substitutions and cannot directly make every possible nucleotide substitution.

How the change becomes permanent

The modified base initially creates a mismatch with the opposite DNA strand. Cellular replication and repair then establish the complementary base, making the substitution permanent.

• Many base editors nick only the unedited strand, which biases cellular repair toward retaining the chemically modified base.
• Because no intended double-strand break is produced, base editing generally avoids the break-repair pathway used by conventional CRISPR cutting, although unintended edits and occasional DNA damage remain possible.

How UPSC asks this