L15: Regulatory RNAs (Eukaryotes) Flashcards
(23 cards)
define RNA interference (RNAi)
biological processes in which RNA molecules inhibit gene expression
what are the small RNAs in eukaryotes
- microRNA (miRNA)
- small interfering RNA (siRNA)
- piwi-interacting RNA (piRNA)
eukaryotic small RNAs - what are Drosha and Dicer
- they are enzymes
- Dicer: located close to the loop of the hairpin
- Drosha: furthest away from the loop of the hairpin
miRNA
- microRNA
- they are produced from hairpin loop RNAs
- they are processed by enzymes Drosha and Dicer
- the final products are 21-23 nt in length
miRNA - what happens to the RNA
- the small DNA duplex is denatured to give rise to a single-stranded guide RNA
- therefore, only one RNA strand is used to inhibit gene expression
miRNA - explain the process of degradation
- RNA precursor – (Dicer) –> miR:miR*
- miR* is degraded
- miR is mature miRNA
siRNA
- small interfering RNA
- produced from dsDNA precursors
- its processed by the enzyme Dicer
- the final products are 21-23 nt in length
piRNA
- piwi-interacting RNA
- they are products of long ssRNA precursors
- processing does not involve Dicer
- final products are 24-34 nt in length
- they are predominantly expressed in the animal germline
what do small RNAs do?
they inhibit the expression of homologs target genes in three ways
small RNA function - three ways
- trigger destruction of mRNA
- inhibit translation of mRNA
- induce chromatin modifications of the target gene to silence transcription altogether
small RNAs - what is it mediated by
- the effect is mediated by RNA-induced silencing complex (RISC)
small RNAs - what is RISC?
- a multiprotein complex that induces an Argonaute protein
- a small RNA is incorporated into RISC to guide the RNA to its target
small RNAs - what method is typical for siRNAs?
- when complementary base pairs between the guide RNA and target is high
- target is degraded by Argonaute
small RNAs - what method is typical for miRNAs?
- when complementary base pairs between the guide RNA and target is low
- target has translation inhabited
small RNAs - how does RISC guide the RNA?
- RISC is directed to the nucleus where it recruits chromatin modifiers to the target locus
- chromatin modifiers form heterochromatin
- this then leads to transcriptional
silencing
piRNAs - where do they arise from?
from long single-stranded transcript of piRNA clusters
piRNAs - what are the clusters?
- “transposon graveyards” with inactive fragments of transposons
- they act to target transcripts made from active transposons
- therefore it protects the cells from transposon activity
chromosomal sex determination - what gene expression problem has been created?
- unequal gene dosage
- the X chromosome contains thousands of genes essential for cell activity
- but female cells have double the X chromosome number
chromosomal sex determination - dosage compensation
- how the unequal gene dosage is “solved”
- males and females produce equal amounts of X chromosome gene products
- via X chromosome inactivation
chromosomal sex determination - X chromosome inactivation
- one X chromosome is inactivated in each female somatic cell
- chromatin is converted into inaccessible heterochromatin
piRNAs: X chromosome inactivation - early development
- both X chromosomes are active
- but soon, one X chromosome in each cell will become inactivated
- the one that gets inactivated occurs independently and is left to chance in each cell
X chromosome inactivation - what is it mediated by?
- long non-coding RNA (lncRNA): regulatory RNA that is >200 nt
- ex: Xist
X chromosome inactivation - what is it Xist
- a lncRNA located on the X-chromosome that needs to be inactivated
- it coats the X-chromosome from which it is expressed
- it then recruits factors that modify and condense chromatin
