During cloning, it is necessary to determine which of the transformed bacteria actually contain a plasmid that has your DNA of interest inserted into it. To detect this DNA sequence, a complementary DNA sequence may be used as a molecular probe [local]. You may want to find out what conditions cause the mRNA for the gene to be produced; this can be measured with a similar molecular probe. Alternately, to determine if a bacterial strain is producing a protein of interest, an antibody may be used as a molecular probe.

This site [local] contains a comprehensive treatment of molecular probes. One of the aspects about molecular probes that causes confusion is the trivial names that are used to describe many of these techniques. Three examples below are widely used in biochemistry and molecular biology.

Southern blot--detection of DNA sequence after electrophoretic fractionation. The molecular probe may be either DNA or RNA.(Animation Library)

Northern blot --detection of RNA sequence after electrophoretic fractionation. The molecular probe is a complementary DNA.

Western blot--detection of a protein after electrophoretic fractionation. The molecular probe is an antibody.