کاری از محمد صادق حیرانیانDefining Cloning
“Cloning” is a loaded term that can be used to mean very different things.
Cutting a piece of DNA from one organism and inserting it into a vector where it can be replicated by a host organism. (Sometimes called subcloning, because only part of the organism’s DNA is being cloned.)
Using nuclear DNA from one organism to create a second organism with the same nuclear DNA
Restriction Enzymes
Restriction Enzymes (also called Restriction Endonucleases) are proteins that cleave DNA molecules at specific sites, producing discrete fragments of DNA.
Restriction Enzymes (RE) were first isolated by Nathans and Smith in 1970.
Why Restriction Enzymes?
Why would bacterial cells contain proteins that cleave DNA at specific sequences?
Generally restriction enzymes are thought to protect bacterial cells from phage (bacterial virus) infection. Bacterial cells that contain restriction enzymes can “cut up” invasive viral DNA without damaging their own DNA.
Joining DNA Fragments
In 1972, Paul Berg and colleagues made the first “artificial” recombinant DNA molecule.
Demonstrated that the DNA of Simian virus 40 could be linearized by EcoR1
Created a circular DIMER of Simian virus DNA by joining two linearized fragments
Also inserted pieces of Lambda phage DNA into linearized Simian 40 virus molecule.
Isolating Genes
Herbert Boyer and Stanley Cohen built on the work of Berg, Nathans and Smith to use restriction enzymes to isolate a single gene, place it into a plasmid vector.
Bacterial cells were then transformed with the recombinant plasmid.
The bacteria host cells replicated the plasmid, producing many copies of the gene, thus amplifying it.
The practical application was that expensive human protein products, like insulin, which were used to treat disease, could eventually be produced from recombinant molecules in the laboratory using bacteria or another host.
Human protein products like insulin could be used in very large quantities from the recombinant molecule. Patients no longer had to use insulin isolated from pigs or cows.
Plasmid Vectors
Plasmids are circular pieces of DNA found naturally in bacteria.
Plasmids can carry antibiotic resistance genes, genes for receptors, toxins or other proteins.
Plasmids replicate separately from the genome of the organism.
Plasmids can be engineered to be useful cloning vectors.
Plasmid Vectors (continued)
Plasmid vectors can be designed with a variety of features:
Antibiotic resistance
Colorimetric “markers”
Strong or weak promoters for driving expression of a protein
Performing the Restriction Digests
You will need to set up a restriction digest of your plasmid vector and your DNA of interest
Restriction enzymes all have specific conditions under which they work best. Some of the conditions that must be considered when performing restriction digest are: temperature, salt concentration, and the purity of the DNA
Purify your DNA Fragments
The insert of interest that you want to clone into your plasmid needs to be separated from the other DNA
You can separate your fragment using Gel Electrophoresis
You can purify the DNA from the gel by cutting the band out of the gel and then using a variety of techniques to separate the DNA from the gel matrix
Ligation
Ligation is the process of joining two pieces of DNA from different sources together through the formation of a covalent bond.
DNA ligase is the enzyme used to catalyze this reaction.
DNA ligation requires ATP.
Transforming Bacteria
After you create your new plasmid construct that contains your insert of interest , you will need to insert it into a bacterial host cell so that it can be replicated.
The process of introducing the foreign DNA into the bacterial cell is called transformation.
Competent Host Cells
Not every bacterial cell is able to take up plasmid DNA.
Bacterial cells that can take up DNA from the environment are said to be competent.
Can treat cells (electrical current/divalent cations) to increase the likelihood that DNA will be taken up
Two methods for transforming: heat shock and electroporation
Selecting for Transformants
The transformed bacteria cells are grown on selective media (containing antibiotic) to select for cells that took up plasmid.
For blue/white selection to determine if the plasmid contains an insert, the transformants are grown on plates .
What did the cells take up?
Plasmid only
Plasmid with insert cloned
Foreign DNA from the environment
Nothing
Expressing your cloned gene
Even if your plasmid contains insert, it may not be able to generate functional protein from your cloned DNA.
The gene may not be intact, or mutations could have been introduced that disrupt it.
The protein encoded by the gene may require post-translational modifications (i.e., glycosylation or cleavage) to function.
Also, some enzymes are a complex of peptides expressed from separate genes.
Expressing your cloned gene
Expression of a cloned gene can be accomplished by:
The E. coli host
Mammalian cells (if the plasmid used is designed for expression in mammalian cells)
Using an in vitro using a cell-free system.
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