Genetic engineering is the process of transferring genes from one organism to another. The desired gene is cut from one organism's genome and inserted into another organism's genome. This process involves enzymes and vectors.
Cutting and joining DNA
Different enzymes are used to cut and join DNA fragments. When two different DNA fragments are joined together they are known as recombinant DNA.
Enzyme
Function
Restriction enzymes
Recognise specific sequences of DNA and cut the DNA at this point. The fragments are left with sticky ends where they have been cut.
Ligases
Join two DNA fragments together at their sticky ends.
Note: Sticky ends are short tails of unpaired bases that are complementary to each other.
Inserting DNA
Vectors are used to insert DNA into other cells. These cells may even be in another organism. There are two types of vectors: plasmids and viruses.
Vector
Function
Plasmids
These are small, circular DNA molecules that are transferred between bacteria.
Viruses
These can insert any DNA into the cells that they infect.
Procedure
1.
Use a restriction enzyme to cut out the DNA with the desirable characteristic that you want to insert.
2.
Use the same restriction enzyme to cut open the vector DNA.
3.
Mix together the DNA fragments from steps 1 and 2 with ligases.
4.
Insert the recombinant DNA into a host cell. This may be a bacterial cell.
5.
The cells will now make the desired protein from the gene that has been inserted.
General example
A
A restriction enzyme cuts out the desired gene (1.) leaving sticky ends (2.). The same restriction enzyme also cuts open the vector (3.) and leaves sticky ends (2.). In this case, the vector is a plasmid.
B
The two fragments have sticky ends that are complementary to one another. This allows the desired gene to be inserted into the vector. Ligases join the DNA fragments together.
C
The recombinant DNA is inserted into a host cell. This may be a bacterial cell.
Specific example
1.
Use a restriction enzyme to cut out the gene for human insulin.
2.
Cut the bacterial cell vector using the same restriction enzymes.
3.
Ligate (join together) the gene and the vector.
4.
Insert the recombinant DNA into a host cell. For example, this may be a bacterial cell.
5.
Now, there will be bacteria that contain the gene for human insulin and they can be grown in a fermenter to produce lots of insulin for people with diabetes.
Modification is not always successful so some of the bacterial host cells will not have the desired gene. To find out which hosts do and which don't, you must select the cells with the desired gene.
Antibiotic resistance markers
Antibiotic resistance markers can be used to find the host cells that contain the desired characteristic. A marker gene will be inserted into the vector at the same time as the gene for the desired characteristic. This marker gene will encode an antibiotic resistance gene.
The host bacteria will be grown on a medium that contains antibiotics. This means that only the bacteria that have the resistance marker gene will be able to survive and reproduce.
Genetically modified plants
Genetic engineering can be used to increase crop yields by inserting genes with desirable characteristics. The bacterium that is often used is called Agrobacterium tumefaciens.
Agrobacterium tumefaciens inserts its genes into the DNA of plant cells. Scientists can use this ability to genetically modify plant cells to contain desirable characteristics.
Example
1.
The desired herbicide resistance gene is taken from a herbicide resistant plant cell.
2.
A plasmid is taken from Agrobacterium tumefaciens.
3.
The herbicide resistance gene is inserted into the plasmid.
4.
The plasmid is inserted back into the Agrobacterium tumefaciens bacterial cell.
5.
A new plant cell is used as the target cell.
6.
The bacterium infects the target plant and inserts its genes (including the herbicide resistance gene) into the plant. This creates a new herbicide resistant plant.
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DNA structure, discovery and inheritance
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DNA: structure, genes and chromosomes
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FAQs - Frequently Asked Questions
Which bacterium is used to genetically engineer plants?
The bacterium that is often used is called Agrobacterium tumefaciens.
How can successful modifications be selected for?
Antibiotic resistance markers can be used to find the host cells that contain the desired characteristic.
What are ligases?
Ligases are enzymes used to join two DNA fragments together at their sticky ends.
What is genetic engineering?
Genetic engineering is the process of transferring genes from one organism to another. The desired gene is cut from one organism's genome and inserted into another organism's genome. This process involves enzymes and vectors.
