Inserting recombinant DNA into cells

Coordination, Response and Gene Technology · Gene technology · note 2 of 5

Spec 8.19

A Level only

A Level only: only in the full A Level, not the AS Level. AS Level students can skip it. What the labels mean

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Inserting recombinant DNA into cellsSpec 8.19

In short

Recombinant DNA must enter host cells, which are then selected using marker genes. Bacteria take up plasmids after treatment with calcium chloride and a heat shock. In plants, Agrobacterium or a gene gun delivers the DNA, and in animals it is microinjected into a fertilised egg or delivered by a virus.

Ways of inserting recombinant DNA
Host	Method	How it works
Bacteria	Heat shock with calcium chloride; electroporation	Cells are treated with ice-cold calcium chloride and heated briefly to about 42 °C, which makes the cell membrane permeable to plasmids. Electroporation uses a brief electrical pulse to the same effect.
Plants	Agrobacterium tumefaciens	The gene is put into the Ti plasmid of this bacterium, which naturally transfers part of its plasmid DNA into plant cells, where it is integrated into the plant genome. Whole plants are grown from the transformed cells by tissue culture.
Plants	Gene gun	Microscopic gold or tungsten particles coated with the DNA are fired into plant cells.
Animals	Microinjection	DNA is injected through a fine needle into the nucleus of a fertilised egg, which is then implanted in a surrogate mother.
Animals / human cells	Viral vectors or liposomes	A virus modified so it cannot cause disease carries the gene into cells; liposomes are lipid vesicles that fuse with the cell surface membrane.

Only a small fraction of host cells take up the DNA. Marker genes are included in the plasmid so that cells that took it up can be identified. These may code for antibiotic resistance (cells that survive on agar containing the antibiotic carry the plasmid) or for a fluorescent protein such as GFP (cells glow green under UV light).

Flow chart of making a transgenic bacterium: mix plasmid and bacteria, calcium chloride and heat shock, plate on antibiotic agar, select colonies, grow in a fermenter, with panels for Agrobacterium and the gene gun. — From recombinant plasmid to transgenic bacteria, selected using a marker gene.

Exam tip:

Say why a marker gene is needed: not every cell takes up the plasmid, so a marker lets you pick out the ones that did.

Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026

Frequently asked questions

How is recombinant DNA made?

A restriction endonuclease cuts the gene and a plasmid at the same recognition sequence, leaving complementary sticky ends. The gene and plasmid are mixed, so the sticky ends pair by complementary base pairing. DNA ligase then forms phosphodiester bonds between the sugar–phosphate backbones, producing a recombinant plasmid that can be put into a host cell.

What do restriction enzymes and DNA ligase do?

Restriction enzymes (restriction endonucleases) cut DNA at specific base sequences, hydrolysing phosphodiester bonds and often leaving single-stranded sticky ends. DNA ligase does the opposite job, forming phosphodiester bonds to join DNA fragments together. Together they allow a gene to be cut out and joined into a vector.

How are genes put into bacteria and plant cells?

Bacteria take up plasmids after treatment with calcium chloride and a heat shock, which makes their membranes permeable. Plant cells can be transformed by Agrobacterium, which transfers DNA from its Ti plasmid, or by a gene gun that fires DNA-coated metal particles into the cells.

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