Editing Northern blot (section)

===Probes===
Probes for northern blotting are composed of nucleic acids with a complementary sequence to all or part of the RNA of interest. They can be DNA, RNA, or oligonucleotides with a minimum of 25 complementary bases to the target sequence.<ref name="Trayhurn1996" />  RNA probes (riboprobes) that are transcribed in vitro are able to withstand more rigorous washing steps preventing some of the background noise.<ref name="Streit2009" />  Commonly cDNA is created with labelled primers for the RNA sequence of interest to act as the probe in the northern blot.<ref name=Liang1995>Liang, P. Pardee, A. B. (1995) Recent advances in differential display. Current Opinion Immunol. 7: 274–280.</ref>  The probes must be labelled either with radioactive isotopes (<sup>32</sup>P) or with [[chemiluminescence]] in which [[alkaline phosphatase]] or [[horseradish peroxidase]] (HRP) break down chemiluminescent substrates producing a detectable emission of light.<ref name=Engler-Blum1993>{{cite journal | last1 = Engler-Blum | first1 = G. | last2 = Meier | first2 = M. | last3 = Frank | first3 = J. | last4 = Muller | first4 = G. A. | year = 1993 | title = Reduction of Background Problems in Nonradioactive Northern and Southern Blot Analysis Enables Higher Sensitivity Than 32P-Based Hybridizations | journal = Anal. Biochem. | volume = 210 | issue = 2| pages = 235–244 | doi = 10.1006/abio.1993.1189 | pmid = 7685563 }}</ref>  The chemiluminescent labelling can occur in two ways: either the probe is attached to the enzyme, or the probe is labelled with a ligand (e.g. [[biotin]]) for which the ligand (e.g., [[avidin]] or [[streptavidin]]) is attached to the enzyme (e.g. HRP).<ref name="Streit2009" />  X-ray film can detect both the radioactive and chemiluminescent signals and many researchers prefer the chemiluminescent signals because they are faster, more sensitive, and reduce the health hazards that go along with radioactive labels.<ref name="Engler-Blum1993" />  The same membrane can be probed up to five times without a significant loss of the target RNA.<ref name="Yang1993" />