Ans. a (Northern blot) (Ref. Harper biochemistry 27th ed., Fig. 39-5).# Northern blot = Similar to Southern blot, except that an RNA sample is electrophoresed. Useful for studying mRNA levels, which are reflective of gene expression.# *Southern = DNA# **Northern = RNA# ***Western = Protein.SOUTHERN BLOT# Southern blotting is used to analyze whether genes have been deleted or rearranged. It is also used to detect restriction fragment length polymorphisms (RFLPs).# Genomic DNA is digested with restriction endonucleases and separated by gel electrophoresis. Individual fragments can then be transferred to a membrane and detected after hybridization with specific radioactive DNA probes.NORTHERN BLOT# Northern blots are used to analyze patterns and levels of gene expression in different tissues.# In a Northern blot, mRNA is separated on a gel and transferred to a membrane, and specific transcripts are detected using radiolabeled DNA as a probe. This technique has been largely supplanted by more sensitive and comprehensive methods such as reverse transcriptase (RT)-PCR and gene expression arrays on DNA chips.# The Northern and Western blot transfer techniques are used to size and quantitate specific RNA and protein molecules, respectively.# A fourth hybridization technique, the Southwestern blot, examines protein-DNA interactions. Proteins are separated by electrophoresis, renatured, and analyzed for an interaction by hybridization with a specific labeled DNA probe.Western blot# Sample protein is separated via gel electrophoresis and transferred to a filter. Labeled antibody is used to bind to relevant protein. Confirmatory test for HIV after + ELISA.Southwestern blot# Identifies DNA-binding proteins (e.g., transcription factors) using labeled oligonucleotide probes.