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McMaster University
Lovaye Kajiura

WEEK 9: MONDAY, OCTOBER 28TH TO FRIDAY, NOVEMBER 1ST, 2013 CHAPTER 12: GENE EXPRESSION & REGULATION What are the 3 primary types of RNA? Messenger RNA (mRNA) carries the code for a protein from the nucleus to the cytoplasm Ribosomal RNA (rRNA) and proteins form ribosomes Transfer RNA (tRNA) molecules transport amino acids to the site of ribosomes Fig. 12-1 Cells synthesize three major types of RNA that are required for protein synthesis * -the base sequence of mRNA carries the information for the amino acid sequence of a protein; group of these bases called codons, specify thee amino acids -rRNA combines with proteins to forms ribosomes; the small subunit binds mRNA; the large subunit binds tRNA and catalyzes peptide bond formation between amino acids during protein synthesis -Each tRNA carries a specific amino acid (in this example, tyrosine [tyr]) to a ribosome during protein synthesis; the anticodon of tRNA pairs with a codon of mRNA, ensuring that the correct amino acid is incorporated into the protein Table 12-1 A comparison of DNA and RNA DNA RNA Strands Two One Sugar Deoxyribose Ribose Types of bases DNA – DNA RNA – DNA RNA- RNA A – T A – T A - U T – A U – A U - A G- C C – G C- G C- G G – C G - C Function Contains genes; the sequence of baseMRNA: carries the code for a protein- in most genes determines the amino coding gene from DNA to ribosomes acid sequence of a protein rRNA: combines with proteins to form ribosomes, the structures that link amino acids to form a protein-co tRNA: carries amino acids to the ribosomes Fig. 12-2 Genetic information flows from DNA to RNA to protein * How Is the Information Encoded in the DNA Utilized in a Cell? • The majority of genes contain information for the synthesis of proteins • proteins are synthesized via the processes of transcription and translation Table 12-2 TRANSCRIPTION TRANSLATION Process Information for the Product Major Enzyme or Type of Base pairing Process Structure Involved in required the Process Transcription A segment of one One RNA molecule RNA polymerase RNA with DNA; RNA (synthesis of RNA) DNA strand (e.g mRNA, tRNA, or bases a pair with DNA rRNA) bases as an RNA molecule is synthesized Translation (synthesis MRNA One protein molecule Ribosome (also MRNA with tRNA: a of a protein) requires tRNA) codon in mRNA forms base pairs with an anticodon in tRNA Transcription synthesis of mRNA using DNA as a template Translation synthesis of polypeptide occurs under the direction of mrNA THE GENETIC CODE (Table 12-3 – Codons of mRNA) *** pg 215 CODON - is the basic unit of the genetic code - three nucleotide sequence in mRNA, which specifies the amino acid added to the growing polypeptide or it may signal the termination of the growth of the polypeptide DICTIONARY OF THE GENETIC CODE - 64 codons - 61 triplets code for amino acids - Triplet AUG has a dual function 1) It is the start signal for translation 2) It also codes for methodine - 3 codons DO NOT code for amino acids, they signal termination The three "STOP" codons are: UAA UAG UGA The GENETIC code is NOT AMBIGUOUS! WHY? -each codon codes for only one amino acid The GENETIC code is REDUNDANT ! WHY? -2 or more codons differing only in the third base can code for the same amino acid For example: UUU and UUC code of Phe How is the information encoded in a gene transcribed into RNA? 3 STAGES OF TRANSCRIPTION Initiation of transcription starts when RNA Polymerase binds to the promoter of a gene Elongation generates a growing strand of RNA Termination of transciption occurs when RNA poymerase reaches the termination signal Fig. 12-3 Transcription is the synthesis of RNA from instructions in DNA* RNA POLYMERASE - catalyzes transcription of mRNA from DNA template - separates two DNA strands and links RNA nucleotides during base-pairing along the DNA template - adds nucleotides to 3' end, mRNA grows 5' . 3' direction Fig. 12-4 RNA Transcription in action Fig. 12-5 a, b Messsenger RNA synthesis in prokaryotic cells * How is the information encoded in the mRNA translated into a protein? 3 STAGES OF TRANSLATION Initiation of translation starts when tRNA and mRNA bind to a ribosome Elongation occurs when amino acids are added one at a time to the growing protein chain Termination occurs when a stop codon signals the end of translation Fig. 12-7 Translation is the process of protein synthesis ** Fig 12-8 Be able to dif
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