Biology For Dummies
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In order to make sure that inserted genes don’t get broken down, it helps if scientists can send the DNA into the genetically modified organism’s cells in an acceptable form that they won’t destroy. One way that scientists do this is with the use of vectors.

A vector is a DNA molecule that can carry foreign genetic material into a cell so that the foreign genetic material can be copied or used by the cell.

Scientists choose the best vector based on the type of cell they want to modify and the size of the DNA they want to insert. If they want to change the code of a bacterium, they typically use a plasmid because bacteria normally contain plasmids and even have mechanisms for sharing them. If they want to change the code of a eukaryotic cell such as a mouse or a human, they might use an artificial chromosome or even a virus to deliver the DNA.

When a DNA molecule contains DNA from more than one source, it’s called recombinant DNA.

If a recombinant DNA molecule containing bacterial and human genes is put into bacterial cells, the bacteria read the human genes like their own and begin producing human proteins that scientists can use in medicine and scientific research. This table lists a few useful proteins that are made through genetic engineering.
Some Beneficial Genetically Engineered Proteins
Protein Benefit
Alpha-interferon Used to shrink tumors and treat hepatitis
Beta-interferon Used to treat multiple sclerosis
Human insulin Used to treat people with diabetes as a safer alternative to pig insulin
Tissue plasminogen activator (tPA) Given to patients who’ve just had a heart attack or stroke to dissolve the blockage that caused the attack
Here’s how scientists go about putting a human gene into a bacterial cell in order to make human proteins for therapeutic purposes:
  1. They choose a restriction enzyme that forms sticky ends when it cuts DNA.

    Sticky ends are pieces of single-stranded DNA that are complementary to other pieces of single-stranded DNA. Because they’re complementary, the pieces of single-stranded DNA can stick to each other by forming hydrogen bonds.

  2. They cut the human DNA and bacterial DNA with the same restriction enzyme.

    When you cut bacterial DNA and human DNA with the same restriction enzyme, all the DNA fragments have the same sticky ends.

  3. They combine human DNA and bacterial DNA.

    Because the two types of DNA have the same sticky ends, some of the pieces stick together.

  4. They use the enzyme DNA ligase to seal the sugar-phosphate backbone between the bacterial and human DNA.

    DNA ligase forms covalent bonds between the pieces of DNA, sealing together any pieces that are combined.

genetic engineering
Genetic engineering of a bacterial cell with a human gene.

About This Article

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About the book author:

René Fester Kratz, PhD, teaches biology at Everett Community College. Dr. Kratz holds a PhD in Botany from the University of Washington. She works with other scientists and K?12 teachers to develop science curricula that align with national learning standards and the latest research on human learning.

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