Chapter 25

The RNA World

microRNA

• Previously thought to be “junk” DNA– Now determined to “code” for other RNA

• ENCODE project• Andrew Fire and Craig Mello– Nobel Prize in 2006 for miRNA

• Epigenetic control

miRNA pathway• Pol II makes RNA transcript• Drosha and other proteins trim the long pri-

miRNA• Exportin 5 takes pre-miRNA out of nucleus• Dicer cleaves loop and separates strands?• RISC carries guide strand and releases

passenger strand (degraded)• Basepairing with mRNA causes

1. Silencing if perfect pairing2. Knockdown if imperfect (3’ UTR)

Other RNAi Mechanisms (compare)

shRNA or siRNA

• This is the same as miRNA, but man-made• Must be designed carefully– Must be less than 30 nt• Otherwise degraded by host imune system

– Cannot have strong hairpin loop– Want complementary strand to bind to RISC– Need a viral vector (manufacturer of the RNA)

Health and Disease

• Can currently silence genes in cell culture– How to use them in organisms?• Targets, specificity?

– Learning tools in animal models

Final Thought

• Is DNA in control, or is RNA in control?– RNA world hypothesis may never have gone away