Genetic Engineering and bacteria

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Why genetic engineering?

Two main reasons;

1.Improving a feature of the recipient organism

  • Inserting a gene into crop plants to give the plant resistance to herbicides-weed killers-allows farmers to use herbicides as the crops are growing and so increase crop yield.
  • Inserting a growth-controlling gene, such as the myostatin gene, into livestock promotes muscle growth.

2.Engineering organisms that can synthesise useful products

  • Inserting the gene for a human hormone, such as insulin or growth hormone, into bacteria and growing the bacteria produces large quantities of the hormone for human use.
  • Inserting the gene for a pharmaceutical chemical into female sheep so that the chemical is produced in their milk means the chemical can then be easily collected.
  • Inserting genes for beta-carotene production into rice so that the molecule is present i nthe edible part of the rice plant. Beta-carotene can be turned into citamin A in people who eat it.
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Bacterial cells and plasmids used in genetic engin

Once a gene had been identified to be placed into another organism, it can be cut from DNA using a restriction enzyme and then placed in a vector.

The vast majority of genetic engineering uses bacterial plasmids as the vector.

A plasmid is a small circular piece of DNA.

Plasmids are found in many types of bacteria and are separate from the main bacteria chromosomes. 

Plasmids often carry genes that code for resistance to antibiotic chemicals.

If plasmids are cut with the same restiction enzyme as that used to isolate the gene, then complementary stick ends will be formed. Mixing quantities of plasmid and gene in the presence of ligase enzyme means that some plasmids will combine witht the gene, which then becomes sealed into the plasmid to form a recombinant plasmid.

It is important to remember that many cut plasmds will, in the presence of liase enzyme, simply reseal to reform the original plasmid.

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Transformed and Transgenic bacterial cells

Large quantities of the plasmid are mixed with bacterial cells, some of which will take up the recombinant plasmid.

The addition of calcium salts and 'heat shock'-the temperature of the culture is lowed to freezing then raised quickly to 40 degress C, increase the rate at which plasmids are taken up by bacterial cells.

Even so, the process is very efficient.

Less than a quarter of 1% of bacterial cells take up a plasmid.

Thise that do are known as transformed bacteria.

This transformation results in bacteria containing new DNA.

By definition, the bacteria are thus trangenic. 

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Bacteria conjugation

This is where genetic material may be exchanged.

Copies of plasmid DNA are passed between bacteria, sometimes even of different species.

This swapping is of concern because it speeds up the spread of antibiotic resistance between bacterial populations.

Resistant strains of bacteria e.g. MRSA, cause healthcare problems as the bacterium is commonly found on human skin, where it is not a problem.

The transfer of this bacterium to a wound results in a serious infection.

Scientists are continuously looking for new antibiotics to targeth these disease-causing organisms.

The advantage for this is that it contributes to genetic variation and survival in the presense of these chemicals.

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Bacteria transformation and pneumonia in mice

Before the discovery of the role of DNA, studies were carried out with found evidence that bacteria can take up DNA fro m their surroudning and incorporate it into their genome.

They used two strains; 1.S-strain, which quickly kills mice on infection 2.R-strain, which does not kill mice on infection.

those only infected will the S-strain were killed by a toxic protein. Therefore, S-strain must have the instructions to make that protein and the R-strain does not.

Further experiements found that a mixture of living R and dead S-strain killed the mouse. In post mortem examinations, they found the mouse had living S-strain bacteria in its blood. It was transformed as the living R-strain transformed into living S-strain.

It was then followed up by injecting separate parts of the S-strain with living R-strain. It was found that only S-strain and living R-strain resulted in death and bacterial transformation. It was later confirmed that R-strain is capable of taking up DNA from their surroundings, in this case, it included DNA from the S-strain with the gene for producing the toxin. So transformation is another method by which bacteria can acquire DNA from each other.

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