Isolation of Plasmid Dna by Alkali-Lysis Method

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1. INTRODUCTION 1.1 PLASMID Plasmids are extra chromosomal genetic material. They are circular molecule of DNA that led an independent existence in bacterial cell. Plasmid has capability of self –replication or may be integrating with bacterial chromosome. Plasmid has at least one DNA sequence that can act as an origin of replication. Plasmid almost always carry one or more gene which is responsible for a useful characteristic displayed by bacteria .e.g. Antibiotic resistance gene, therefore it is used as selectable marker. All plasmid possess atleast one DNA sequence that can act as an “origin of replication”, so they are able to multiply with the cell quite independently of the main bacterial chromosome. The smaller plasmids make use of the host cell’s own DNA replicative enzymes in order to make copies of themselves, whereas some of the larger ones carry genes that code for special enzymes that are specific for plasmid replication. Size and copy number of plasmid is an important feature of plasmids for cloning purpose e.g. plasmid ranging from 6-10 Kb is very suitable for cloning and having copy number as many as 50. Plasmid may be of following type: • F-Plasmid: having ability to promote conjugal transfer of plasmid e.g. F-Plasmid of E.coli • R-Plasmid: responsible for providing resistance to host against foreign bodies such as anti bacterial resistance e.g. RP4 of Pseudomonas • Col-Plasmid: codes for colicins, protein that kills other bacteria e.g. ColE1 of E.coli and Pvocine of Pseudomonas • Degradative Plasmid: it allows the bacteria to metabolize unusual molecule such as toluene and salicylic acid e.g. TOL of Pseudomonas putida • Virulence Plasmid: it confers pathogenicity on the host bacterium e.g. Plasmid of Agrobacterium. Essential features of an ideal plasmid cloning vector • Small size: Low molecular weight, are easier

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