Primer extension
Encyclopedia
Primer extension is a technique whereby the 5' ends of RNA
RNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....

 or DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...

 can be mapped.

Primer extension can be used to determine the start site of RNA transcription for a known gene. This technique requires a radiolabelled primer (usually 20 - 50 nucleotides in length) which is complementary to a region near the 5' end of the gene. The primer is allowed to anneal to the RNA and reverse transcriptase is used to synthesize cDNA to RNA until it reaches the 5' end of the RNA. By running the product on a polyacrylamide gel, it is possible to determine the transcriptional start site, as the length of the sequence on the gel represents the distance from the start site to the radiolabelled primer.
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