Theme 3: Turning Off the Signal
• 19.2 Messenger RNA to Phenotype in Eukaryote
• Once the mRNA is in the cytoplasm, there are mul7ple
opportuni7es for gene regula7on at the levels of mRNA stability,
transla7on, and protein ac7vity
◦ Example: Regulate mRNA stability via the length of the poly
A tail
• Length of 7me in which a protein func7ons can be limited by
selec7ve degrada7on
◦ Proteasomes are very large protein complexes found in cells
able to break pep7de bonds and so, degrade unneeded
proteins
◦ During protein degrada7on, target proteins are tagged with
small ubiqui7n proteins which is facilitated by ubiqui7n
ac7va7on, conjuga7on to target substrate protein and
liga7on
• Results in a polyubiqui7n chain aGached to the
substrate target protein and allows for proteasome to
degrade tagged protein
19.2.1 Small regulatory RNAs inhibit transla>on or promote
RNA degrada>on
• Small Regulatory RNAs: OIen work by binding to transcripts and
blocking transla7on; Their small size allows easy synthesis in the
lab and scien7sts can design their sequences to target transcripts
of interest
◦ MicroRNA (miRNA): A type of small regulatory RNA
• Starts out just like the RNA transcribed from protein-
coding genes, using the same RNA polymerase for
transcrip7on and going through the same processes
of capping, splicing, and polyadenyla7on
, ▪ BUT, the processed RNA folds back upon itself
to form 1/+ hairpin sturctures stabilized by base
pairing in the stem (a.)
• Enzymes recognize processed and folded
miRNAs and cleave the stems from the
hairpin and then, cleave the stem into
small double-stranded fragments
• One strand from each RNA
fragment is incorporated into a
protein complex known as RISC
(RNA-Induced Silencing Complex)
• The single-stranded RNA (i.e.
miRNA) targets the RISC
complex to specific mRNA
molecules by base pairing
with short regions on the
target mRNA (a.)
• miRNA in RISC complex
doesn't base pair exactly with
target mRNA, but has enough
sequence in common that the
two strands anneal
• Once bound to target
mRNA, RISC complex
inhibits transla7on
• Human chromosomes may encode ~1000 miRNAs,
each of which inhibit transla7on of tens or hundreds
of mRNA molecules
◦ Small Interfering RNA (siRNA): A type of small regulatory
RNA
• Transcrip7on and processing of siRNA and miRNA are
virtually iden7cal, including incorpora7on into a RISC
complex
▪ BUT, siRNA molecules have strands that match
exactly; RISC complexes containing siRNA cleave
• 19.2 Messenger RNA to Phenotype in Eukaryote
• Once the mRNA is in the cytoplasm, there are mul7ple
opportuni7es for gene regula7on at the levels of mRNA stability,
transla7on, and protein ac7vity
◦ Example: Regulate mRNA stability via the length of the poly
A tail
• Length of 7me in which a protein func7ons can be limited by
selec7ve degrada7on
◦ Proteasomes are very large protein complexes found in cells
able to break pep7de bonds and so, degrade unneeded
proteins
◦ During protein degrada7on, target proteins are tagged with
small ubiqui7n proteins which is facilitated by ubiqui7n
ac7va7on, conjuga7on to target substrate protein and
liga7on
• Results in a polyubiqui7n chain aGached to the
substrate target protein and allows for proteasome to
degrade tagged protein
19.2.1 Small regulatory RNAs inhibit transla>on or promote
RNA degrada>on
• Small Regulatory RNAs: OIen work by binding to transcripts and
blocking transla7on; Their small size allows easy synthesis in the
lab and scien7sts can design their sequences to target transcripts
of interest
◦ MicroRNA (miRNA): A type of small regulatory RNA
• Starts out just like the RNA transcribed from protein-
coding genes, using the same RNA polymerase for
transcrip7on and going through the same processes
of capping, splicing, and polyadenyla7on
, ▪ BUT, the processed RNA folds back upon itself
to form 1/+ hairpin sturctures stabilized by base
pairing in the stem (a.)
• Enzymes recognize processed and folded
miRNAs and cleave the stems from the
hairpin and then, cleave the stem into
small double-stranded fragments
• One strand from each RNA
fragment is incorporated into a
protein complex known as RISC
(RNA-Induced Silencing Complex)
• The single-stranded RNA (i.e.
miRNA) targets the RISC
complex to specific mRNA
molecules by base pairing
with short regions on the
target mRNA (a.)
• miRNA in RISC complex
doesn't base pair exactly with
target mRNA, but has enough
sequence in common that the
two strands anneal
• Once bound to target
mRNA, RISC complex
inhibits transla7on
• Human chromosomes may encode ~1000 miRNAs,
each of which inhibit transla7on of tens or hundreds
of mRNA molecules
◦ Small Interfering RNA (siRNA): A type of small regulatory
RNA
• Transcrip7on and processing of siRNA and miRNA are
virtually iden7cal, including incorpora7on into a RISC
complex
▪ BUT, siRNA molecules have strands that match
exactly; RISC complexes containing siRNA cleave
