Final answer:
The typical plasmid is a small, extra-chromosomal DNA molecule in bacteria ranging from 1 kilobase to 200 kilobases in size, essential for antibiotic resistance and other functions. Supercoiling helps fit large DNA molecules like the 4.6 million base pair genome of E.coli into small bacterial cells.
Step-by-step explanation:
The typical plasmid size in bacteria ranges from 1 kilobase (kb) to 200 kb. These extra-chromosomal, double-stranded DNA molecules are essential for certain functions such as antibiotic resistance and production. Plasmids are generally present in bacterial cells, and there may be one or several plasmids per cell, possibly in the range of 10 to 50 copies per cell.
When considering a large mammalian genome of more than 2 billion base pairs, plasmids can carry only very short DNA sequences, usually not exceeding 1000 base pairs. This limitation is why vectors like bacteriophage are used for larger DNA insertions.
As an example, the genome of well-studied prokaryotes like Escherichia coli (E.coli) is about 4.6 million base pairs, equivalent to 1.1 mm if linearized. To fit such a large DNA molecule inside the small confines of a bacterial cell, mechanisms like supercoiling are utilized. Supercoiling involves the DNA being over-wound or under-wound from its normal state, aided by certain proteins and enzymes like DNA gyrase.