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PinkMonkey Online Study Guide-Biology

8.5 Plasmids

In addition to the normal DNA chromosome, extrachromosomal genetic elements are often found in the cytoplasm of most bacterial species and in some species of eukaryotes. These elements are called plasmids. Plasmids are extrachromosomal circular DNA molecules (except the plasmid called Killer-Plasmid present in yeast cells, which is a double stranded RNA molecule). The DNA of all plasmids is double stranded supercoiled circular DNA molecule (Figure 8.8). These are capable of autonomous replication in the cytoplasm of the bacterial cell. Therefore, plasmids are also described as autonomously replicating íminichromosomesí. A bacterial cell can have one to many copies of a plasmid. Plasmids represent extra genes.

Some plasmids are called episomes. An episome is a plasmid with a dual ability to replicate, i.e. it can replicate autonomously in the cytoplasm like other plasmids or it can also integrate itself into the DNA of the bacterial chromosome and behave as part of the chromosome.


Role of plasmids

Many plasmids contain important and essential plasmid genes which are helpful under certain conditions. For example, a bacterial cell possessing R Plasmid develops resistance to certain antibiotics. Similarly, presence of some plasmids gives the bacterial cell the ability to produce toxins (toxigenicity), or the ability to use unusual chemical compounds as nutrients. Moreover, plasmids and episomes prove to be good vehicles for gene manipulations (genetic engineering).

Figure 8.8 Bacterial cell showing a plasmid

Table of Contents

8.0 Introduction
8.1 Packaging of Hereditary Material
8.2 The Structure of DNA
8.3 Replication Of DNA In Eukaryotes
8.4 Replicatin of Pokaryotic Chromosome
8.5 Plasmids
8.6 RNA: Structure and Types
8.7 The Genetic Code
8.8 The Central Theme of Protein Synthesis

Chapter 9





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