The Control of Gene Expression: Gene Expression is Controlled by a Number of Features - Regulation of Transcription and Translation - RNAi

Question 1

What effect does RNAi have on gene expression?

Answer 1

• Inhibits the translation of mRNA produced from target genes in eukaryotes and some prokaryotes

Question 2

When does RNAi occur?

Answer 2

• RNAi occurs when small, double stranded RNA molecules stop mRNA being translated
• Small interfering RNA (siRNA) and microRNA (miRNA) are involved in RNAi

Question 3

What type of RNA are involved in RNAi?

Answer 3

• Small interfering RNA (siRNA)

- Micro RNA (miRNA)

Question 4

How does siRNA work?

Answer 4

• mRNA is converted into siRNA

• Once mRNA is transcribed, it leaves the nucleus and enters cytoplasm
• RNA dependent RNA polymerase makes a double stranded mRNA
• mRNA associates with dicer enzyme and is cut into smaller double stranded RNA called siRNA

• siRNA binds to target mRNA

• siRNA associates with a protein to form a siRNA-protein (RISC) complex and one siRNA strand is broken down in the process
• siRNA-protein complex binds to target mRNA using complementary base pairing

• Target mRNA cut

• The siRNA-protein complex cut the mRNA into fragments so can no longer be translated
• Fragments move into a processing body which contains ‘tools’ to degrade mRNA fragments

Question 5

What protein cuts mRNA into smaller double stranded siRNA?

Answer 5

• Dicer enzyme

Question 6

What is formed when siRNA associates with proteins?

Answer 6

• siRNA-protein complex

- AKA RISC protein

Question 7

How does miRNA in mammals work?

Answer 7

• Specificity

• In mammals, the miRNA isn’t usually fully complementary to target mRNA
• This makes it less specific than siRNA and so can target more than one mRNA molecule

• Processing miRNA

• When miRNA is first transcribed, it exists as a double strand
• It’s then processed into 2 single strands by enzymes in the cytoplasm

• Target mRNA

• One strand associates with proteins and binds to target mRNA in the cytoplasm
• miRNA physically blocks the translation of target mRNA
• mRNA moved into a processing body, where it can be stored or degraded
• When stored it can be returned and translated another time