Genetic Engineering

For more than 3 billion years, mutation, crossing over, random gene mixing at fertilization, and hybridizations between species have contributed to the diversity of life on Earth. Today, we can "engineer" genetic changes through recombinant DNA technology. DNA from different species can be cut, spliced together, and inserted into bacteria, which then multiply the DNA necessary for protein production. Genetic engineering has great promise for agriculture, medicine, and industry, but it has also raised ecological, social, and ethical questions.

Recombinant DNA

Humans have actually been changing the genetics of other species for thousands of years through artificial selection of plants and animals. There are also natural processes at work such as mutation and crossing over.

With the new field of Genetic Engineering, genes are isolated, modified, and inserted into an organism. This is made possible by recombinant technology in which scientists cut DNA up, recombine the pieces, and amplify modified pieces.

This animation (Audio - Important) describes recombinant DNA.

Restriction Enzymes:

Bacteria possess restriction enzymes whose usual function is to cut apart foreign DNA molecules.
Each enzyme cuts only at sites that possess a specific base sequence.

Hamilton Smith discovered restriction enzymes when he was studying how Haemophilus influenzae bacteria defend themselves from bacteriophage attack. He discovered that bacteria have an enzyme that chops up viral DNA.

Restriction enzymes cut DNA at a specific sequence. The number of cuts made in DNA will depend on the number of times the "target" sequence occurs.

This animation (Audio - Important) describes restriction enzymes.

Modification Enzymes:

Many times the "sticky ends" that result from the cut can be used to pair up with another DNA fragment cut by the same enzyme.

DNA fragments produced by restriction enzymes are treated with DNA ligase to splice the DNA fragments together to form a recombinant DNA molecule.

This animation (Audio - Important) describes how fragments with sticky ends are created and reassembled.

Cloning Vectors for Amplifying DNA:

Plasmids are circular DNA molecules in bacteria that carry only a few genes and can replicate independently of the single "main" chromosome.
Foreign DNA can be inserted into plasmids forming recombinant plasmids.
The plasmid is a cloning vector that can be used to deliver DNA into another cell.
When the plasmid is replicated, any foreign DNA that might have become incorporated into it is also replicated, producing a DNA clone.
Modified plasmids that are capable of accepting, replicating, and delivering DNA to another host cell are called cloning vectors.

Reverse Transcriptase to Make cDNA:

Even after a desired gene has been isolated and amplified, it may not be translated into functional protein by the bacteria because introns (noncoding regions) are still present. Researchers minimize this problem by using c-DNA , which is made from "mature" mRNA transcripts. The cDNA is made from mRNA by reverse transcriptase and can be inserted into a plasmid for amplification.

This animation (Audio - Important) describes cDNA.

REVIEW: Small circular molecules of DNA in bacteria are called

REVIEW: Enzymes used to cut DNA molecules in recombinant DNA research are

REVIEW: The fragments of chromosomes split by restriction enzymes

REVIEW: Restriction enzymes
a. often produce staggered cuts in DNA that are useful in splicing genes.
b. are like most enzymes in being very specific in their action.
c. are natural defense mechanisms evolved in bacteria to guard against or counteract bacteriophages.
d. are used along with ligase and plasmids to produce a DNA library.
e. all of these