Transcript Chapter 13
Cloning and genetic engineering by Ivo Frébort Cloning Clone: a collection of molecules or cells, all identical to an original molecule or cell To "clone a gene" is to make many copies of it - for example, in a population of bacteria Gene can be an exact copy of a natural gene Gene can be an altered version of a natural gene Recombinant DNA technology makes it possible Plasmids Naturally occurring extrachromosomal DNA Plasmids are circular dsDNA Plasmids can be cleaved by restriction enzymes, leaving sticky ends Artificial plasmids can be constructed by linking new DNA fragments to the sticky ends of plasmid Cloning Vectors Plasmids that can be modified to carry new genes Plasmids useful as cloning vectors must have – a replicator (origin of replication) – a selectable marker (antibiotic resistance gene) – a cloning site (site where insertion of foreign DNA will not disrupt replication or inactivate essential markers DNA Libraries Sets of cloned DNA fragments that together represent the genes of a particular organism Any particular gene may represent a tiny, tiny fraction of the DNA in a given cell Can't isolate it directly Trick is to find the fragment or fragments in the library that contain the desired gene Colony Hybridization Disk is treated with base or heated to convert dsDNA to ssDNA and incubated with probes Colonies that bind probe hold the fragment of interest Colony Hybridization Southern Blots The Polymerase Chain Reaction What if you don't have enough DNA for colony hybridization or Southern blots? The small sample of DNA serves as template for DNA polymerase Make complementary primers Add primers in more than 1000-fold excess Heat to make ssDNA, then cool Run DNA polymerase (usually Taq) Repeat heating, cooling, polymerase cycle DNA sequencing Chemical Cleavage Method Four reactions are used G-specific cleavage with dimethyl sulfate, followed by strand scission with piperidine G/A cleavage: depurination with mild acid, followed by piperidine C/T cleavage: ring hydrolysis by hydrazine, followed by piperidine C cleavage: same method (hydrazine and piperidine), but high salt protects T residues DNA sequencing Chain Termination Method Based on DNA polymerase reaction Run four separate reactions Each reaction mixture contains dATP, dGTP, dCTP and dTTP, one of which is P-32-labelled Each reaction also contains a small amount of one dideoxynucleotide: either ddATP, ddGTP, ddCTP or ddTTP Recombinant Proteins Plant transformation Common Strategies for Introducting Foreign Genes into Plant Cells: Agrobacterium infection Particle bombardment Electroporation http://www.hort.purdue.edu/hort/courses/HORT250/animations/Leaf%20Disk%20Animation/leafdisk1.html Regeneration of transgenic plant from crown-gall callus Tobacco plants overexpressing HvCKX genes Transgenic Animals Genes can be introduced into animals by transfection - injection of plasmid DNA into recipient cells Plasmids can be injected into fertilized eggs in mice Expression is usually variable, because the gene is inserted randomly Growth hormone transfection produces mice that are very large!