Cloning Vectors. To ensure the survival and propagation of foreign DNAs, they must be inserted into a vector that can replicate inside bacterial cells and be passed on to subsequent generations of the bacteria. The vectors used for cloning are derived from naturally occurring bacterial plasmids or bacteriophages. Plasmids (page 74) are small circular DNA molecules found within bacteria. Each contains an origin of replication (page 88) and thus can replicate independently of the bacterial chromosome and produce many copies of itself. Plasmids often carry genes that confer antibiotic resistance on the host bacterium. The advantage of this to the scientist is that bacteria containing the plasmid can be selected for in a population of other bacteria simply by applying the antibiotic. Those bacteria with the antibiotic resistance gene will survive, whereas those without it will die. Figure 7.2 shows mRNA 5'
chopped mRNA 5' DNA 3'
DNA with breaks 5' DNA 3'
Reverse transcriptase using dATP, dCTP, dGTP, dTTP, (T)n primer
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