miRNAs

Question 1

How common is regulation by miRNAs?

Answer 1

Regulate pretty much every pathway in all eukaryotes

Question 2

How are miRNAs transcribed?

Answer 2

50% have their own promoters. The other 50% are from introns and exons in coding genes and other non-coding regulatory elements

Question 3

What are the 2 main mechanisms of miRNAs?

Answer 3

1. Perfect complementarity of the miRNA with the target, leading to cleavage of the target mRNA. The cleaved mRNA gets destroyed
2. Imperfect complementarity of the miRNA, leading to the repression of translation and the destabilization and degradation of the target

Question 4

What is the miRNA mechanism that is commonplace in plants?

Answer 4

Perfect complementarity with cleavage

Question 5

What is the miRNA mechanism that is commonplace in animals?

Answer 5

Imperfect complementarity with translational repression

Question 6

What is the seed sequence?

Answer 6

Small region on an miRNA with perfect complementarity to the target

Question 7

Why is relying only on seed sequences beneficial?

Answer 7

It allows one miRNA to regulate multiple targets, and is used to coordinate the expression of genes in the same process

Question 8

What are 3 other, less common mechanisms of miRNAs?

Answer 8

1. Activate translation
2. Silence transcription
3. Activate transcription

Question 9

What are the steps in miRNA biogenesis?

Answer 9

1. RNAP II transcribes it into pri-miRNA and it gets processed like mRNA
2. Parts of the pri-miRNA form hairpins and are cleaved by the Drosha/Pasha microprocessor
3. The hairpins get exported to the cytoplasm
4. Dicer cleaves the hairpin and generates a short unstable duplex
5. Argonaute loads and unwinds the duplex, degrading the passenger strand and loading the guide strand onto the RISC complex

Question 10

What is the mirtron pathway?

Answer 10

For miRNAs that don’t have their own promoter. They often arise when an intron gets cleaved during splicing

Question 11

What don’t we understand very well about miRNAs?

Answer 11

Their targeting. How they only need partial complementarity, but regulate just a handful of targets instead of hundreds. How a single mRNA can be targeted by multiple miRNAs, and how the regulated mRNA can change depending on the conditions

Question 12

What are 5 factors that influence miRNA targeting?

Answer 12

1. Developmental and environmental factors
2. N6 methylation
3. ceRNAs
4. miRNA compartmentalization
5. Concentration of the miRNA