miRNA

Question 1

what happens to the ratio of noncoding to total genomic DNA as organism complexity increases?

Answer 1

• higher ratio of noncoding total genomic DNA

Question 2

heterochrony

Answer 2

• a change in where, when and how much genes are expressed, leading to changes the developmental program

Question 2

miRNA

Answer 2

• specialized microRNA that prevents expressions of specific genes through complementary base pairing

Question 3

at what stage does miRNA act to regulate genes?

Answer 3

• post-transcriptional mechanisms for gene regulation, specifically affect mRNA stability and translation

Question 4

what are miRNAs transcribed by and how does this affect how they are regulated (2)

Answer 4

• RNA polymerase II (same enzyme that transcribes mRNA)

- controlled by TFs that work with the polymerase

Question 5

what are most miRNAs transcribed from (2)

Answer 5

• noncoding DNA regions that generate short dsRNA hairpins

- can be within introns, exons, or outside of protein-coding genes altogether

Question 6

how have miRNA families formed and what determines which miRNA belong in a family? (2)

Answer 6

• duplication and divergence generated clusters and families of related miRNAs
• family members often have partially redundant functions

Question 7

what are the defining features of miRNA? (3)

Answer 7

• hairpin with a stem region, unformed loop, and unformed ss flanking regions
• 4 motifs where cuts are made during miRNA processing
• Drosha and Dicer cleavage sites within the hairpin also involved in processing

Question 8

what are the two mechanisms of mRNA regulation?

Answer 8

1. mRNA degradation

2. blocking of mRNA translation

Question 9

miRNA degradation pathway of mRNA regulation (3)

Answer 9

• miRNAs bind to mRNA targets with exact complementarity
• induces the RNAi pathway
• leads to degradation of the mRNA

Question 10

miRNA translation blocking pathway of mRNA regulation (2)

Answer 10

• miRNA binds to target with imperfect complementarity

- results in blockage of mRNA translation

Question 11

miRNA processing: Step 1

Answer 11

• transcription of primary miRNA transcript (Pri-miRNA) by RNA Pol II

Question 12

miRNA processing: Step 2

Answer 12

• microprocessor complex (Drosha +) cleaves pri-miRNA using motifs to form pre-miRNA hairpin

Question 13

miRNA processing: Step 3

Answer 13

• export of the pre-miRNA hairpin to the cytosol

Question 14

miRNA processing: Step 4

Answer 14

• dicer cleaves loop off the pre-miRNA, leaving a miRNA duplex

Question 15

miRNA processing: Step 5

Answer 15

• miRNA duplex is loaded into AGO protein forming the RNA-induced silencing complex (RISC)

Question 16

how is the RISC complex formed (4)

Answer 16

1. miRNA duplex binds to proteins to form the pre-RISC complex that is inactive
2. AGO protein activates pre-RISC complex by binding to miRNA duplex
3. AGO degrades one strand, retaining a guide strand
4. now active RISC complex recognizes mRNA sequences complementary to the guide strand and binds them

Question 17

miRNA processing: Step 6

Answer 17

• RISC uses guide RNA to bind to target mRNA and silence it