Gel electrophoresis used in the Southern and Northern blotting techniques. (a) Gel electrophoresis is used to size-fractionate a DNA (Southern blotting) or RNA (Northern blotting) mixture. The samples are then transferred to (i.e., are blotted onto) a nitrocellulose membrane, where they are detected using short radiolabeled oligonucleotide probes that base-pair to the samples on the membrane. (b) Northern blot analysis of RNA isolated from various human tissues. For each sample, approximately 10 μg of total RNA was separated on a 1.2% agarose-formaldehyde gel, transferred to a membrane, and hybridized to a ³²P-labeled probe—an mRNA for human platelet endothelial cell adhesion molecule (PECAM-1). The same blot was also probed with a cDNA (complementary DNA, a DNA copy of an mRNA sequence; see Chapter 7) for glyceraldehyde 3-phosphate dehydrogenase (GAPDH) to control for the amount of material in each lane. (GAPDH mRNA is used as a control because it is found in all tissues, in almost equal amounts.) Note the differences in PECAM-1 RNA levels detected in the different tissues; two bands are observed for PECAM-1 in each lane because there are two distinct forms of the mRNA for this gene.