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Chapter 17 Central Dogma of Molecular Biology From Genes to Protein One gene – one polypeptide hypothesis One gene dictates the production of a single polypeptide Prokaryotic Cell Central Dogma of Molecular Biology transcription mRNA ribosome translation polypeptide Transcription: synthesis of mRNA under the direction of DNA - one step in prokaryotic cells Eukaryotic Cell - Two steps in eukaryotic cells 1. creating a 1o transcript transcription mRNA 1o transcript RNA Processing 2. RNA processing (editing) to create 2o transcript mRNA 2o transcript translation polypeptide ribosome Translation: synthesis of a polypeptide under the direction of mRNA Directional Triplet-code Instructions for a polypeptide 3’ A C C A A A C C G A G T 5’ DNA 3’ mRNA Transcription 3’ to 5’ direction on DNA 5’ U G G U U U G G C U C A Translation 5’ to 3’ direction on RNA trp phe gly ser polypeptide Central Dogma 3 RNA Players Involved tRNA rRNA= =type RNA of that, RNAcombined that carries with theprotein, amino acids makes to up thethe mRNA = DNA transcript ribosome structural component of a ribosome Transcription Initiation 3 steps 1) Initiation 2) Elongation 3) Termination Transcription begins at a “Promotor” region of DNA, recognized by a TATA box sequence A transcription factor binds to help RNA polymerase (RNA pol.) to bind to the DNA RNA pol. binds and begins to unwind DNA Elongation RNA pol unzips 10-20 DNA bases at a time in a 3’ 5’ direction and base pairs with the DNA using RNA nucleotides Base-pair occurs at a rate of 60 nucleotides/second A-U; G-C base-pair rules New 1o transcript (initial copy) of mRNA peels away from DNA template Termination 1) RNA sequence, not DNA sequence, is used to end transcription. Once RNA pol. creates the AAUAAA termination sequence, the primary mRNA transcript peels away from the DNA template. Transcription video mRNA 2) RNA processing occurs: (creating of 2o transcript) - 5’ cap of guanine nucleotides are added to protect mRNA from hydrolytic enzymes and to provide a starting site for ribosomes in translation - 3’ poly A tail is added to protect mRNA from hydrolytic enzymes and to help with mRNA export from the nucleus Termination RNA Processing Continues Primary transcript of mRNA is long and includes introns (non-coding regions) and exons (coding regions) mRNA Evolutionary Significance? Non-coding RNA regions were the result of non-coding DNA regions. Longer DNA increased chances of Xover during meiosis. During RNA processing, introns must be cut out (spliced) before a functional polypeptide can be made Termination RNA Processing Continues 3) RNA splicing A “spliceosome” complex recognizes intron sequences and deletes them Spliceosomes are made up of special RNA called snRNA + various proteins 2o mRNA transcript is now ready t-RNA Activation for Translation tRNA = brings in the corresponding AA Aminoacyl-tRNA coded by synthase the mRNA (enzyme) catalyzes the binding to the ribosome for of a specific amino acid to a free polypeptide tRNA. (protein) synthesis Activated t-RNA is now ready for translation. Structure of a Ribosome P= Creates peptide bonds P = Peptidyl-tRNA between AA and A holds the growing polypeptide binding chain site A= = AminoacyltRNA brings tRNA site in newbinding AA E= Free t-RNA detaches E = Exit Site from ribosome Large subunit E P A tRNA Anticodon Small subunit mRNA codon Ribosome= rRNA + protein Initiation Translation 1) Initiation 2) Elongation 3) Termination - Small ribosomal subunit attaches near the 5” end of mRNA at AUG start codon - tRNA carrying AA Methionine attaches to AUG start codon - Large ribosomal subunit attaches to mRNA w/ tRNA occupying the “P” site. Elongation Free tRNA exits and can return to bind with other AA for repeat deliveries Ribosome slides down the 3’ end of the mRNA New tRNA brings in another amino acid, based on the codon on the mRNA. Basepairing occurs. A peptide bond forms between the two adjacent AA Termination Ribosome encounters a “stop” codon: UAA, UAG, or UGA Release factor facilitates the release of both ribosomal subunits Translation Video Watch these realtime videos on DNA transcription and translation…Totally Cool Dude! http://www.dnai.org/a/in dex.html Redundancy in AA - Notice…several codons code for the same AA Evolutionary advantage? Other ways of increasing Redundancy redundancy: helps to Although A pairs with U minimize errors and G pairs with C, U protein pair can in sometimes synthesis due to with G wobble effect mutations Endo-membrane System Recall that glyco-proteins (made by attached ribosomes) are processed by the ER and golgi before being incorporated within vesicles (ie. lysosomes) , exported from the cell, or incorportated into the plasma membrane.