Reverse Transcriptase PCR dilemma: when to use oligodT and when to Random hexamer

During reverse transcription we can have confusion that when to use oligodT and when to use the Random hexamers. Let us think about how the things work.

The first thing is we use oligodT in cDNA priming when we have the primer in 3 prime end. Cause oligo dT starts annealing from the Poly A site. and if the mRNA is long in size then it cant reach the 5 prime end and drop

Ranom heamers anneal randomly and anneal in any place but it can't annal in extreme 3 prime ends.

So often may labs use a mixture of oligodT and random hexamer to prime cDNA but it is not recommended when synthesize full length cDNA or cDNA library.

Another thing is

"Oligo dT and random primers anneal at low temperature and normally the reaction is recommended to be carried out at ~42°C. However, the use of longer primers such as oligo dT

(20–25) or random decamers, enables the RT reaction to occur at higher temperature (50°C)