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Molecular Genetics Notes

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Biological Sciences
BIOSC 0160
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2/11 Chapter 15/16: Molecular Genetics One Gene­One Polypeptide • Beadle/Tatum o Deduced gene-protein relationship from mutants in mold • Extension to polypeptides o Many proteins contain several polypeptides encoded by different genes • Exceptions/Modifications o Control sequences and introns  These are non-coding sequences o tRNAs and rRNAs  Non-proteins coded for by genes  Used in protein synthesis o Overlapping reading frames  Some genes overlap with others (i.e. viruses) • Definition of gene o No single definition o Working definition: a region of DNA whose final product is either a polypeptide or an RNA molecule Overview • Central Dogma o DNA  RNA  Proteins *Describes the flow of information • Revised… Transcription Translation o DNA RNA Proteins 2/11 • Transcription o Conversion of DNA into an mRNA intermediate o The ‘m’ in mRNA means messenger; carries information to ribosomes o Still in the same “language” • Translation o “Reading” mRNA information and turning it into amino acid sequences o Involves protein synthesis o Occurs in ribosomes • Posttranslational Modification o Occurs after protein synthesis o Same proteins get modified (e.g. addition of a sugar group to make a glycolprotein) The Genetic Code • The “dictionary” linking DNA and RNA information and amino acids • Codons o Sets of three RNA nucleotides o Listed left to right in the 5’ to 3’ direction o A, C, T, U (64 possible codons) o Stand for:  20 amino acids  1 start signal (AUG)  3 stop signals (UAA, UAG, UGA) • Reading Frame o Sends the right message o Start signal establishes the frame 2/11 o No overlap in codons o A frame shift drastically alters the resulting protein THE CAT ATE THE RAT THE ATA TET HER AT • Redundancy o Only two amino acids have a single codon (tryptophan and methionine) o The other 18 have between two and six codons; this is why the code is redundant • Universality o Code is nearly universal among species o Leads us to believe that it must have arisen early in life’s history (unique common ancestor?) o Few exceptions  Certain single-celled eukaryotes (though most codons are still the same)  Mitochondrion and chloroplast codons (also mostly the same) Transcription • DNA converting to RNA • Process o Binding and Initiation  RNA polymerase unzips DNA strands  RNA polymerase binds to a promoter upstream (the 3’ end) of the sequence to be transcribed  Protein transcription factors also bind to yield transcription initiation complex (at or around the promoter) o Elongation  RNA moves along DNA, making complementary RNA 2/11 o Elongation cont.  Many RNA polymerase molecules do this at the same time on one piece of DNA, allowing the cell to transcribe multiple genes at one time o Termination  Special DNA sequence called terminator is transcribed  Resultant RNA sequence functions as a termination signal by interfering with the polymerase’s ability to hold onto it
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