Transcript size ?

St George's Med. School lawrence at sghms.ac.uk
Thu Feb 9 09:27:11 EST 1995


This may be a stupid question but,I frequently read in the literature that people isolate full length 
sequences from cDNA libraries (i.e. derived from mRNA) and then use these clones (eg 3kb) as 
hybridization probes to screen Northerns.
What they find are signals as large as 10, 6 and 4 kb.
HOW CAN THIS BE ?
Surely the cDNA clone/probe is derived from the same message as blotted...
OK add on a bit of  5' and 3' untranslated sequence and some polyA tail,but still, how can you get from 3 
to 10 kb ?
Knowing that the largest splice variant adds on only 500 bases more to the 3kb core sequence,the only 
thing I can think of is that the message on the Northern hasn't been processed properly and there are still 
introns present in unspliced heterologous nuclear RNA.Or alternately,contaminating DNA.
Can anyone help clarify this apparent anomaly ?


 |~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~|
 |Kevin LAWRENCE - e-mail: k.lawrence at sghms.ac.uk |
 |DEPARTMENT OF PHARMACOLOGY                            |
 |St George's Hospital Medical School - London             |
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