the protein product of the target gene. Oligonucleotides are designed so that they are complementary to the target sequence. The oligonucleotides are chemically synthesized, radiolabeled, and used to screen a DNA library, or other collections of cloned DNA fragments. Sequences that hybridize with the hybridization probe are further analyzed, for example, to obtain the full length sequence of the targeted gene. Normal chromosomal or gene rearrangement can be studied using this technique. Can be used to find similar sequences in other species or in the genome by decreasing the specificity of hybridization. In a mixture having different sizes of digested DNA, it is used to identify the restriction fragment of a specific size. It is useful in identifying changes that occur in genes including insertions, rearrangements, deletions, and point mutations that affect the restriction sites. Moreover it is used to identify a specific region that uses many different restriction enzymes in a restriction mapping. Also it is used to determine which recognition site has been altered due to a single nucleotide polymorphism that changes a specific restriction enzyme. Southern blotting can also be used to identify methylated sites in particular genes. Particularly useful are the restriction nucleases MspI and HpaII, both of which recognize and cleave within the same sequence. However, HpaII requires that a C within that site be methylated, whereas MspI cleaves only DNA unmethylated at that site. Therefore, any methylated sites within a sequence analyzed with a particular probe will be cleaved by the former, but not the latter, enzyme. Can be used in personal identification through fingerprinting, and in disease diagnosis. Southern blot limitations Compared to other tests, southern blot is a complex technique that has multiple steps and these steps require equipment and reagents that are expensive. High quality and large amounts of DNA are needed. Southern blot is a time consuming method and can only estimate the size of the DNA since it is a semi-quantitative method. It cannot be used to detect mutations at base-pair level. See also Gel electrophoresis of nucleic acids Restriction fragment Genetic fingerprint Northern blot Western blot Eastern blot Southwestern blot Northwestern blot References External links OpenWetWare Molecular biology techniques