Topic 28 Flashcards
UTR
Untranslated Region: A region of mRNA that is not translated into protein. It can be found at the 5’ (5’ UTR) or 3’ (3’ UTR) end of the mRNA.
Polyadenylation signal
A specific sequence of nucleotides in the DNA (usually AAUAAA) that signals the addition of a poly(A) tail to the mRNA.
Polyadenylation site
The site on the mRNA where the poly(A) tail is added.
RNA interference (RNAi)
A biological process in which RNA molecules inhibit gene expression by targeting mRNA molecules for degradation.
miRNA (microRNA)
A small non-coding RNA molecule that functions in RNA interference by targeting mRNA for degradation or translational repression.
shRNA (short hairpin RNA)
An artificial RNA molecule with a hairpin loop structure that can trigger RNA interference.
siRNA (small interfering RNA)
A class of double-stranded RNA molecules that can trigger RNA interference.
Guide strand
The strand of a small RNA molecule (like siRNA or miRNA) that directs the RNA-induced silencing complex (RISC) to the target mRNA.
DICER
An enzyme that cleaves double-stranded RNA into small interfering RNA (siRNA) or microRNA (miRNA).
R2D2
A protein that binds to Dicer and helps in the processing of double-stranded RNA.
Argonaute
A family of proteins that play a central role in RNA interference by binding to small RNA molecules and targeting them to complementary mRNA molecules.
RISC (RNA-induced silencing complex)
A multi-protein complex that contains Argonaute proteins and small RNA molecules. It is responsible for the silencing of target genes.
Transcriptional Termination in Eukaryotes (Major Events)
Polyadenylation Signal Recognition:
RNA polymerase II transcribes through a polyadenylation signal sequence (usually AAUAAA) in the pre-mRNA.
Cleavage of the Pre-mRNA:
A cleavage factor complex recognizes the polyadenylation signal and cleaves the pre-mRNA downstream of the signal.
Poly(A) Tail Addition:
Poly(A) polymerase adds a string of adenine nucleotides (poly(A) tail) to the 3’ end of the cleaved mRNA.
Termination of Transcription:
The poly(A) tail and other factors destabilize the RNA polymerase II complex, leading to its dissociation from the DNA template and termination of transcription.
Diagram of Sequence Elements at the 3’ End of Pre-mRNA:
5’ … UTR … Polyadenylation Signal (AAUAAA) … Cleavage Site … 3’ UTR …
Purpose of RNA Interference (RNAi):
RNA interference is a gene silencing mechanism that regulates gene expression at the post-transcriptional level. It plays a crucial role in various cellular processes, including:
Defense against viral infections: By degrading viral RNA, RNAi can protect cells from viral replication.
Regulation of gene expression: RNAi can fine-tune gene expression levels by targeting specific mRNAs for degradation or translational repression.
Development and differentiation: RNAi is involved in controlling cell fate and tissue development.
Mechanism of RNAi:
Initiation:
Double-stranded RNA (dsRNA) is processed by the enzyme Dicer into small interfering RNA (siRNA) or microRNA (miRNA) duplexes.
Loading into RISC:
One strand of the siRNA or miRNA duplex, known as the guide strand, is incorporated into the RNA-induced silencing complex (RISC).
Target Recognition:
The guide strand within RISC binds to complementary sequences in target mRNA molecules.
mRNA Degradation or Translational Repression:
RISC can either cleave the target mRNA, leading to its degradation, or inhibit its translation.
