Functions of RNA mRNA (messenger)- instructions protein tRNA (transfer) - carriers that match amino acids to codons during translation rRNA (ribosomal)- part of the ribosomes- ribosomes are RNA plus proteins snRNA's (small nuclear) - part of spliceosomes- that edit mRNA's siRNA’s (small interfering) control gene expression. Information storage is apparently only function of DNA. RNA has many other functions. Some even argue that the molecular antecedent of living cells – the “ancestral molecule” - may have been RNA.
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 Also- some stretches of DNA are part of more than one gene in the sense that they code for parts of more than one kind of protein
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 unit” because the transcript includes a lot of extra base pairs that will be edited out, leaving the gene.
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-tRNA-synthetases) load each amino acid on it’s correct tRNA molecule. Requires ATP
Different synthetases for each kind of tRNA & amino acid
Ribosome E,P, and A are Binding sites for three tRNA molecules A is for Amino acid +tRNA binding site E is for “Exit”site E,P, and A are Binding sites for three tRNA molecules 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