Plasmid Definition image information

Structure of a plasmid

Structure / October 27, 2018

Plasmids are sometimes called "vectors", because they can take DNA from one organism to the next. Not all vectors are plasmids, however. We commonly use engineered viruses, for example bacteriophage lambda, which can carry large pieces of foreign DNA.

Why do we use the word "vector, " which we've been trying to forget ever since we took Physics 100? The word has a connotation of taking something from one place to another. A mosquito is said to be a "vector" for malarial parasites, and a velocity "vector" in physics indicates a direction in which an object is travelling. In molecular biology, a "vector" is a piece of DNA that may be introduced into a cell, usually after we've played around with it a bit in a test tube.
Orientation One important concept is that depending on the cloning strategy employed, a gene could be inserted into the plasmid in either of two orientations:

Left to right orientation

Right to left orientation

Perhaps we don't care which orientation we obtain as our final product, but we should note that there is a fundamental difference between the two. The arrow in the diagram shows the direction of transcription/translation of the "red gene" coding sequence, and the two orientations differ with respect to the outside markers Amp and ori.

How do we isolate a plasmid we want?
  • We introduce the reclosed (ligated) products into E. coli, a process called "transformation", and select for bacteria resistant to a drug (such as amipicillin, for example).
  • We screen the individual bacterial colonies to find one that contains a plasmid of the correct structure.

All you need is here