How Does Genetic Engineering Work?

Genetic engineering works by transferring a chosen gene into an organism so its cells can express that gene. The resulting organism is called transgenic if the gene came from another species.

How is the gene transferred?

1. Obtain the DNA fragment. Scientists can cut the gene from DNA using restriction endonucleases, or use reverse transcriptase to make a DNA copy of its mRNA. The fragment can be amplified using PCR to make many copies.
2. Insert it into a vector. A vector carries the DNA into a host cell; a bacterial plasmid is one example. Restriction endonucleases cut the plasmid, and DNA ligase joins the inserted fragment to it, producing recombinant DNA.
3. Transfer and identify it. The vector is introduced into host cells in a process called transformation. Not every cell takes it up, so a marker gene helps identify the cells that have been transformed.
4. Express the gene. Suitable promoter and terminator regions help the host cell transcribe the inserted gene. The cell can then translate its mRNA into a polypeptide. Because the genetic code is universal, a gene from one species can be expressed in another.

For example, a gene coding for a useful protein can be inserted into a bacterial plasmid. After transformed bacteria are identified and grown, they may produce that protein.

A DNA fragment can also be made using a gene machine. PCR copies a fragment outside living cells (in vitro), whereas growing transformed host cells copies DNA inside living cells (in vivo). These are ways to increase the amount of DNA; neither guarantees that the gene will be expressed. Expression also depends on the host cell's transcription and translation machinery.

Exam mistake to avoid: inserting a gene does not mean every host cell takes it up or produces the protein. Explain how transformed cells are identified, and remember that the inserted gene needs appropriate control sequences to be expressed.