Gene editing: CRISPR, base editing and prime editing

Prelims and MainsCurrent affairs on this: Genomics, gene editing and cell therapy

Gene editing changes DNA at a chosen place. CRISPR Cas9, adapted from a bacterial defence against viruses and awarded the 2020 Nobel Prize in Chemistry, has two parts: a guide RNA that matches the target sequence, and the Cas9 enzyme that cuts both strands of the DNA there. The cell then repairs the cut, and the repair is the edit: quick repair usually disables the gene, and with a template the cell copies a new sequence in. Base editing avoids the double strand cut: a disabled Cas9 carries an enzyme that chemically changes one letter into another, so a single point mutation can be corrected. Prime editing goes further, carrying a template and a reverse transcriptase that writes a new sequence directly. Delivery is the practical problem: the machinery must reach the right cells, by a harmless virus, a lipid particle, or by editing cells outside the body.

How a CRISPR edit happens

1 Target
A guide RNA is designed to match the DNA sequence to be changed.
2 Cut
Cas9 binds the guide, finds the match and cuts both strands.
Without the cut: Base editing changes one letter chemically; prime editing writes a new sequence with a reverse transcriptase; both avoid the double strand break.
3 Repair
The cell repairs the break: quick repair disables the gene; repair from a supplied template inserts a new sequence.
4 Deliver
The machinery reaches the cells by virus, lipid particle or editing outside the body.
5 Check
Sequencing confirms the edit and looks for off target cuts.

The enzyme only cuts; the edit is the cell's repair, which is why the newer methods skip the cut.Source: Standard molecular biology

Medicine: the first approved CRISPR therapy, for sickle cell disease and beta thalassaemia, edits a patient's own blood stem cells outside the body to switch fetal haemoglobin back on; India, with a large sickle cell burden in tribal districts, has its own therapy in trials.
Crops: a plant edited without foreign DNA (SDN 1, a disabled gene; SDN 2, a small templated change) is exempt in India from the genetically modified approval process since 2022, while SDN 3, with inserted foreign DNA, is regulated as genetically modified; the first two genome edited rice varieties were released in 2025.
The limits: off target cuts elsewhere in the genome, mosaicism when not every cell is edited, and immune reactions to Cas9; base and prime editing reduce the first.
Germline editing, changing embryos so the edit is inherited, is banned or unapproved almost everywhere, and the 2018 case of edited babies in China is the reference point for why.
India's framework: the Indian Council of Medical Research guidelines on biomedical research, the Department of Biotechnology guidelines for genome edited plants (2022) and organisms, and the Genetic Engineering Appraisal Committee (GEAC) for anything that counts as genetically modified; GenomeIndia, which sequenced 10,000 Indian genomes, is the reference genome the edits will be measured against.
A gene drive is engineered DNA that passes to far more than half of an organism's offspring and so spreads through a wild population; it is proposed for collapsing mosquito populations and opposed for resistance and for effects that cannot be reversed.

Mains: Gene editing separates two questions that genetic modification mixed together, whether foreign DNA is added and whether a gene is changed, and India's 2022 rules answered the first, which is why edited crops can reach farmers years faster than genetically modified ones.

UPSC has asked

Prelims 2019: what the Cas9 protein does
Prelims 2018: the field CRISPR Cas9 belongs to

Further reading: Human genome editing (WHO)