Rub

Splicing removes the introns from the precursor mRNA, generating mRNA

mRNA extracted from cells

Reverse transcriptase synthesizes DNA from mRNA

DNA polymerase completes the cDNA, free from introns solated hydrogen bonds that hold the strands together are only temporary. The enzyme DNA ligase can form covalent bonds between the sugar-phosphate residues of adjacent nucleotides, joining the vector and insert. Thus, if restriction enzymes are viewed as scissors that cut DNA into fragments, then DNA ligase is the glue that pastes the fragments together. The combined actions of restriction enzymes and DNA ligase enable a researcher to take fragments of DNA from diverse sources and then join them to generate a recombinant DNA molecule. ■ ligase, p. 172

Properties of an Ideal Vector The vector, usually a modified plasmid or bacteriophage, has an origin of replication and functions as a carrier of the cloned DNA. Most vectors encode some type of selectable marker, generally resistance to an antibiotic such as ampicillin. The selectable marker makes it possible to select for cells that contain the vector sequence, either as part of a recombinant plasmid or as an intact circular molecule. This attribute is important because when DNA is added to a host, most cells do not take up that DNA. The selectable marker is used to elim-

Table 9.3 Examples of Common Restriction Enzymes

Enzyme Microbial Source

Recognition Sequence (arrows indicate cleavage sites)

AluI

Arthrobacter luteus

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