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Functions of RNA
• mRNA (messenger)- instructions protein
• tRNA (transfer) - carriers that match amino
acids to codons during translation
• rRNA (ribosomal)- part of the ribosomesribosomes are RNA plus proteins
• snRNA's (small nuclear) - part of
spliceosomes- that edit mRNA's
• siRNA’s (small interfering) control gene
expression.
The “central dogma” of molecular biology
DNA
replication
transcription
mRNA
translation
protein
What is a “gene”?
• “The genetic information that codes for a
particular kind of protein”. However…
• Some “genes” code for RNA molecules
that have other functions (are not mRNA)
• Some “genetic elements” are never
transcribed into RNA or translated but still
do important things, like control gene
transcription
Transcription
• Synthesis of RNA using DNA as a pattern
• Promoter sequence at beginning and
terminator sequence at end of
transcription unit (why not call it a gene?)
• RNA polymerases- (3 different ones in
eukaryotes)
• Only template strand of DNA is transcribed
Transcription (see fig 12.5 in Brooker)
Promotor
RNA polymerase
Terminator
Transcription unit
“raw” RNA transcript
mRNA processing in eukaryotes
• Pre-mRNA (the raw transcript) is modified
before leaving nucleus
• GTP cap and poly-A tail (a long string of
adenosines) are added to 5’ and 3’ ends
mRNA processing, continued
• Non-coding introns are removed and the
remaining exons are spliced together.
Correspondence between exons and protein domains
Translation (protein synthesis)
• mRNA has the pattern (the codons)
• tRNA’s (transfer RNA) deliver the amino
acids and match anticodon to codon
• Ribosomes are where this happens.
Catalyze formation of the peptide bonds.
• Aminoacyl tRNA synthetases reload the
tRNA’s with matching amino acids
Three different representations of tRNA molecules
Different tRNA for each kind of amino acid
Anticodon
“Activating enzymes” (aminoacyl-tRNAsynthetases) load each amino acid on it’s
correct tRNA molecule. Requires ATP
Different synthetases for each kind of tRNA &
amino acid
Ribosome
E is for “Exit”site
E,P, and A are
Binding sites for three
tRNA molecules
A is for Amino acid
+tRNA binding site
P is for Peptide+tRNA
binding site
The initiation of translation
The elongation cycle of translation
The termination of translation
Polyribosomes
Coupled transcription and translation in bacteria
A summary of transcription and translation in a eukaryotic cell