Southern and Northern Blot Analyses

Northern analysis is usually used to identify and quantitate specific mRNAs in a sample. Southern analysis is used to determine whether or not a gene of interest is present as well as its copy number. Other uses for Southern analysis include identifying restriction fragment length polymorphisms and changes in heterozygosity. 

In both Southern and Northern analyses restriction-digested DNA fragments, mRNA, and polyA mRNA are separated by size when electrophoresed on agarose gel. The separated molecules are transferred, by electroblotting or capillary blotting, on to a nylon or nitrocellulose membrane. The immobilized RNA or DNA is reacted with a radiolabeled, chemiluminescent, or fluorescent probe that is complementary to the DNA/RNA of interest, unbound probe is washed off, and the membrane exposed, in the case of radioactive probes, to radioautographic film to visualize the sample of interest. 

---

Northern and Southern blot analyses are carried out as shown in Figure 2.7 and as described below. Restriction enzyme digested DNA (Southern) or total RNA or poly(A) mRNA (Northern) is subjected to agarose gel electrophoresis, which separates DNA and RNA molecules based on their size. The separated molecules are then transferred to a nylon or nitrocellulose membrane by capillary action transfer... 

---