genetic engineering
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How does genetic engineering work?

The desired gene can be removed from the DNA of the donor organism using enzymes called restriction endonucleases. These are enzymes which chop up DNA strands, cutting them at specific sites, which means they can be used to remove very specific genes. Certain types of restriction endonucleases are particularly useful because they leave small regions of DNA sticking out at each end of the required gene. These are known as sticky ends, and make it much easier to attach the gene into another piece of DNA. stage 1
   
Sometimes the required gene is synthesised - an artificial gene is made. If the mRNA molecule transcribed from the required gene can be isolated, it can be used to produce a gene by effectively reversing the transcription process. Using another specialised enzyme known as reverse transcriptase, the DNA sequence of the gene can be built up from the mRNA. These artificial genes are particularly useful when we want a new organism to make very specific proteins – for example they are used to create artificial genes coding for human insulin production which are inserted into bacteria. stage 2
   

The second step is to prepare a vector molecule to carry the DNA into the host cell – often a bacterium. A bacterial plasmid ( a small circular strand of DNA often found in bacteria in addition to their main DNA) is often used as a vector. These plasmids often carry a marker gene – for example, coding for resistance to a particular antibiotic. This means that it is relatively easy to demonstrate later which organisms have been successfully engineered by growing them in a medium containing the antibiotic. Only the genetically-engineered organisms will be resistant, and so only they will grow.

The bacterial plasmid is opened up using restriction enzymes which leave sticky ends which correspond to those of the new gene.

 stage 3
   
The third step is to join the new gene into the bacterial plasmid. The sticky ends are lined up and the gene is attached – annealed – using enzymes called DNA ligases to act as ‘genetic glue’ and join the pieces of DNA together. stage 4
   
The final step is to incorporate the engineered DNA into the bacterium or other cell where it is required. This is known as transformation. Once the plasmid is inside the host bacterium (or other cell) it will be expressed and a new protein made. Microorganisms are the most commonly used organisms in genetic engineering because they are relatively easy, quick and cheap to culture and there are fewer ethical issues about their usage. However increasing numbers of other types of cells are being engineered on a regular basis as genetic modification techniques develop. stage 5

This form of recombinant DNA technology was the first form of genetic manipulation to be developed.

More and more ways of getting desirable genes into the genetic material of other organisms have been devised in recent years.

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