Central Dogma From DNA to Protein How Cells Read the Genome The Central Dogma Gene A and gene B can be transcribed at different rates producing different amounts of RNA within the same cell Differences between RNA and DNA nucleotide sugar is ribose uses uracil instead of thymine typically single stranded RNA molecules can fold into specific 3D shapes have catalytic regulatory and structural functions Transcription produces an RNA molecule that is complementary to the template strand of DNA The sequence of the coding strand is equivalent to the RNA products RNA polymerases mediate transcription The growing RNA chain is extended in the 5 to 3 direction RNA polymerase is responsible for both unwinding the DNA helix and catalyzing the formation of the phosphodiester bonds between nucleotides Essential Cell Biology Fifth Edition Copyright 2019 W W Norton Company Cells Produce Various Types of RNA Essential Cell Biology Fifth Edition Copyright 2019 W W Norton Company Genes are transcribed by many RNA polymerases at the same time A new RNA molecule can begin to be synthesized from a gene before the previous RNA molecule s synthesis is completed Signals in the DNA Tell RNA Polymerase Where to Start and Stop Transcription Bacterial transcription the sigma factor subunit of RNA polymerase recognizes the promoter and transcription begins the DNA is unwound Promoter Portion of DNA to which RNA polymerase binds at the start of transcription Essential Cell Biology Fifth Edition Copyright 2019 W W Norton Company Specific sequences tell bacterial RNA polymerase where to start promoter and stop terminator transcription Essential Cell Biology Fifth Edition Copyright 2019 W W Norton Company Every promoter has a certain polarity The orientation of its promoter determines which strand is the template strand Essential Cell Biology Fifth Edition Copyright 2019 W W Norton Company Transcription in bacteria The sigma subunit of bacterial RNA polymerase recognizes promoter sites in the DNA RNA polymerase along with its sigma subunit can initiate transcription on its own Bacterial cells contain a single RNA polymerase Bacterial cells lack nucleosomes Eukaryotic transcription differs from prokaryotic transcription Three types of RNA polymerases transcribe different types of genes Transcription factors are needed to initiate transcription Initiation requires dealing with nucleosomes Eukaryotic transcription General transcription factors are required for RNA pol II initiation TFIID TBP 11 TAF subunits The TBP subunit of TFIID distorts the DNA double helix TFIIB recognizes the distorted DNA Eukaryotic transcription TFIIF stabilizes interaction between RNA pol II TFIID B TFIIE recruits TFIIH TFIIH unwinds DNA at start site phosphorylates RNA pol II CTD After phosphorylation RNA pol II disassociates from the TFs and enters elongation phase Essential Cell Biology Fifth Edition Copyright 2019 W W Norton Company Eukaryotic RNAs are processed prior to export from the nucleus Essential Cell Biology Fifth Edition Copyright 2019 W W Norton Company mRNAs are capped and polyadenylated These two modifications increase the stability of a eukaryotic mRNA molecule facilitate its export from the nucleus to the cytosol Essential Cell Biology Fifth Edition Copyright 2019 W W Norton Company The coding regions exons of eukaryotic genes are interrupted by introns Essential Cell Biology Fifth Edition Copyright 2019 W W Norton Company Splicing removes the introns and is mediated splicesomes Watch Movie Splicesomes consist of multiple small nuclear ribonucleoproteins snRNPs which contain small nuclear RNAs snRNAs and proteins recognize specific RNA sequences Proteins initially bound to RNA pol II mediate RNA processing Phosphorylation of the tail of RNA polymerase II allows RNA processing proteins To assemble there The primary transcript of a gene can sometimes be spliced differently so that different exons can be stitched together to produce distinct proteins Only properly processed mRNAs are exported from the nucleus to the cytoplasm proteins that recognize the mRNA modifications bind to the RNA and interact with the nuclear pore complex to allow export FROM RNA TO PROTEIN tRNA Molecules Match Amino Acids to Codons in mRNA An mRNA Sequence Is Decoded in Sets of Three Nucleotides Specific Enzymes Couple tRNAs to the Correct Amino Acid The mRNA Message Is Decoded on Ribosomes The Ribosome Is a Ribozyme Specific Codons in an mRNA Signal the Ribosome Where to Start and to Stop Protein Synthesis Proteins Are Produced on Polyribosomes Inhibitors of Prokaryotic Protein Synthesis Are Used as Antibiotics How is RNA read groups of 3 nucleotides codons encode individual amino acids the genetic code is redundant Several different codons can specify a single amino acid three possible reading frames Transfer RNAs tRNAs link codons to amino acids 61 amino acid encoding codons tRNAs 20 amino acids some amino acids are attached to multiple tRNAs some transfer RNAs can bind multiple codons wobble in the third base pair The number of tRNAs encoded by a genome varies from organism to organism Wobble base pairing They require accurate base pairing only at the first two positions of the codon and can tolerate a mismatch or wobble at the third position Allows flexible base pairing In tRNA charging aminoacyl tRNA synthetases connect amino acids to the correct tRNA Essential Cell Biology Fifth Edition Copyright 2019 W W Norton Company Ribosomes are made of proteins and rRNAs Small subunit matches tRNA to codons Large subunit catalyzes peptide bond formation Ribozyme Ribosomes have binding sites for mRNA and tRNA A aminoacyl tRNA P peptidyl tRNA E exit The tRNA sites are designated the A P and E sites short for aminoacyl tRNA peptidyl tRNA and exit respectively B Highly schematized representation of a ribosome in the same orientation as A which is used in subsequent figures Note that both the large and small subunits are involved in forming the A P and E sites while only the small subunit contains the binding site for an mRNA Essential Cell Biology Fifth Edition Copyright 2019 W W Norton Company rRNAs catalyze the formation of the peptide bond ex of a ribozyme form the mRNA and tRNA binding sites General process of translation Peptide bonds form between an aminoacy tRNA and a peptidyl tRNA positioned in the A and P sites of the ribosome Initiation of translation Initiation of protein synthesis in eukaryotes requires translation initiation factors and a