Primer extension

Primer extension is a technique whereby the 5' ends of RNA or DNA can be mapped.

In this technique, a oligonucleotide primer from a known section of the sequence is annealed to the mRNA. Using this annealed primer as a starting point, reverse transcriptase synthesizes complementary cDNA to the mRNA until it reaches the 5' end. By running the new sequence on polyacrimide gel, it is possible for the experimenter to know the location of the transcription start point (the 5' end), as the length of the sequence on the gel represents the distance from the transcription start point to where the primer was first placed.

References

http://www.biochem.arizona.edu/classes/bioc568/primer_extension.htm