Module 7 Lesson 5 - 2

Recombinant DNA

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Genetic engineering is used in the laboratory to manipulate genetic material. Genes from plants, animals, and bacteria can be inserted into a different organism's genetic material. When the foreign gene is inserted into the host cell, the foreign DNA replicates whenever the host cell divides. This type of gene cloning allows scientists to replicate large quantities of the desired DNA or to produce large quantities of proteins coded by the foreign DNA.

In the last lesson, an example of a goat producing silk in its milk was described. That is an example of genetic engineering. The gene for silk production from a spider was isolated and then inserted into the genetic material of the goat. This new combination of spider and goat DNA is called recombinant DNA. 

Researchers use restriction endonucleases, a specific group of restriction enzymes able to "cut" the interior of DNA molecules within specific short sequences of nucleotides called target sequences. The actual site where the DNA is cut is called the restriction site. The target sequences and restriction sites are specific for various endonucleases.


The figure above shows the target sequence of GAATTC for a restriction endonuclease in a bacterial plasmid. Bacterial plasmids are circular DNA common in bacteria. Many bacteria absorb plasmids from their surroundings.

Notice the restriction site within the target sequence. The restriction endonuclease recognizes this specific sequence. The small fragments of DNA created from this cleavage are called restriction fragments. The same endonucleases are used to cut the foreign DNA from another organism. The ends of restriction fragments are sticky and can base-pair with the restriction fragments of the foreign DNA. An enzyme called DNA ligase can be used to splice or glue these fragments together.

The bacterial plasmid now contains the new foreign DNA, and it is referred to as the recombinant DNA. When the plasmid containing the recombinant DNA enters a host bacterial cell, it can replicate with the host bacterial DNA.

Review the processes involved in genetic engineering by reading pages 647 to 648 in your textbook.

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1. The following animation describes restriction endonucleases, target sequences, sticky ends, and DNA ligase. It is an excellent visual illustration of the material you read above.
   
   
   
   

   
   


2.   BiologiX 36 video titled Genetic Engineering:  The Science of Manipulating DNA goes through genetic engineering in more detail.  As you watch the excerpt below from this video, give particular attention to how the students model the action of the restriction endonuclease using paper.