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Genetic Engineering. First, the nucleus of human cells are burst. Human cell. Nucleus. Genetic Engineering. The chromosomes are cut up into small fragments and the required gene identified. Fragment containing required gene. Chromosome fragments. Genetic Engineering.

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genetic engineering
Genetic Engineering

First, the nucleus of human cells are burst

Human cell


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Genetic Engineering

The chromosomes are cut up into small fragments and the required gene identified.

Fragment containing required gene

Chromosome fragments

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Genetic Engineering

Next the fragments are spread out and the required one isolated.

Segment with required gene

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Genetic Engineering



Bacterial cell wall

Bacterial chromosome

Structure of a typical bacterium

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Genetic Engineering


Plasmids are loops of DNA separate from the main chromosome. They carry genes for things like antibiotic resistance. This makes them very useful to theGenetic engineer.

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Genetic Engineering



In the above plasmid, the YELLOW gene is one that gives the bacterium resistance to one antibiotic (eg Penicillin).

The GREEN gene gives resistance to a different antibiotic (eg Tetracycline)

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Genetic Engineering


Cut here


By using special enzymes, we can make a cut in the midst of ONE of theseantibiotic resistance genes.In this example, we will cut open the ‘T’ gene

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Genetic Engineering

Prepared human gene

Next, we introduce the prepared HUMAN gene to the mixture. If all goes according to plan, the human gene will fit into the cut in the plasmidso that the green ‘T’ gene will no longer work correctly.

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Intact P gene and ‘defective’ T gene

No P or T gene

P and T

Genes intact

Genetic Engineering

As plasmids are extremely small, we cannot tell by looking which ones have gotthe human gene in the right place. We need to use a ‘shotgun’ approach andincubate thousands of plasmids with hundreds of bacterial cells

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Genetic Engineering

Required cell

Cell with P and T intact

Cell with neither P or T

Some cells will take up the recombinant plasmid, some will take up original plasmids, others will take up no plasmds at all or ones without antibioticresistance genes.

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Colonies growing from single cells that are resistant to penicillin

Genetic Engineering

Agar containingpenicillin

An agar plate containing Penicillin is used to allow only those cells which havetaken up a suitable plasmid to survive and divide. These cells must have resistanceto Penicillin

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Genetic Engineering

Next, these colonies are sub-cultured onto agar containing tetracycline. Only cells resistant to BOTH antibiotics will be able to grow. We are interested in those cells which WON’T grow in the presence of Tetracycline

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These cells must have intact T genes

These cells must have intact P genes and defective T genes

Genetic Engineering

Next, these colonies are sub-cultured onto agar containing tetracycline.

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Genetic Engineering

This colony will probably have the correct plasmid to produce the product from thehuman gene. Cells from this colony will be grown on a large scale and the mediumanalysed for the presence of the product from the human gene, eg growth hormone