Cell Division, Genetics, Molecular Biology

20.2 Gene ExpressionTranscription/TranslationProtein SynthesisCell Division, Genetics, Molecular Biology1Amino AcidsFrederick Sanger 1952: - proteins consist of sequence of molecules called amino acids- specific sequence of amino acids determines chemical properties of each protein- proteins produced by cell determine structure, function & development of cellProteins responsible for inherited traits Sequence of base pairs in DNA makes up genetic code of organismGenetic code determines how amino acids strung together (how proteins are made)

2Genetic CodeIn a gene: each set of 3 bases is a CODONGenetic code always interpreted in terms of mRNA codon rather than original DNA sequenceEach mRNA codon codes for a specific amino acidOnly 20 amino acids found in proteins, depend on combination of bases in codonStart and stop codons initiate or terminate protein synthesis

3

Ex) A U G C U U A A A G C C U G A4Gene ExpressionWay information in a gene is converted into a specific trait through production of a polypeptide (protein)- products of all genes = polypeptidesRNA is utilized to convert genes into proteins- messenger RNA (mRNA)- transfer RNA (tRNA)- ribosomal RNA (rRNA)2 stages of gene expression: transcription and translationTranscription: genetic information converted from DNA sequence into mRNA, carries information from nucleus to cytoplasmTranslation: genetic information from mRNA used to synthesize polypeptide chains of specific amino acids5Gene ExpressionCentral Dogma: 2 step process of transferring genetic information from DNA to RNA, then from RNA to protein

6Transcription OverviewDNA sequence copied (transcribed) into the sequence of a single stranded mRNA moleculeTHREE PROCESSES:1. Initiation: RNA polymerase binds to DNA at specific site near beginning of gene2. Elongation: RNA polymerase uses DNA as template to build mRNA molecule3. Termination: RNA polymerase passes the end of gene and stopsmRNA then released from template strandCarried through nuclear pores, into cytoplasm of cell7Transcription Initiation Only one strand of the double-stranded DNA is transcribed for each gene- template strand: one that is transcribed- either strand can serve as the template strand for different genesRNA polymerase binds to segment of DNA)- opens double helix- binds in front of gene to be transcribed

8Transcription - ElongationDNA strand to be transcribed = templateRNA polymerase moves along template strand of DNA and begins building mRNA in 5 to 3 directionPromoter not transcribedmRNA strand complementary to DNA template strand EXCEPT thymine is replaced with URACIL

9Transcription - TerminationRNA polymerase reaches end of geneTermination sequence: RNA polymerase recognizes it as endTranscription stops, mRNA disconnects from DNA template strandRNA polymerase free to bind to another promoter region- transcribe another genemRNA free and diffuses out of nucleus into cytoplasm

10

ReviewTranscription11Translation OverviewmRNA now in the cytoplasm needs to be translated - puts amino acids together according to genetic information and creates proteinsRibosomes: site of translation- composed of 2 subunits: clamp together around mRNA, moves along mRNATHREE PROCESSES:1. Initiation2. Elongation3. Termination

12Translation - InitiationOccurs when ribosome recognizes specific sequence on mRNA binds to itRibosome moves along mRNA in 5 to 3 direction- adds amino acids to polypeptide chain once it reads a codonMust start reading in correct spot on mRNA- START codon (AUG)- ensures ribosome translates code using reading frame of mRNA molecule- results in correct sequence of amino acidsTransfer RNA (tRNA) carries amino acids back to ribosomestructure of tRNA contains an ANTICODON- complementary to codon of mRNA- tells tRNA which amino acid to bind to- amino acid binds to opposite end- tRNA delivers amino acid to ribosome13

tRNA Structure14Translation - ElongationStart codon recognized by ribosome- codes for methionine2 sites for tRNA to attach- A (aminoacyl)- P (peptidyl)tRNA with anticodon complementary to start codon enters P siteNext tRNA carrying required amino acid enters the A site- peptide bond formed between both amino acidsRibosome shifts over one codon so that SECOND tRNA is now in P siteReleased first tRNA from P site and allowed THIRD tRNA to enter empty A site- like a tickertape!Released tRNAs recycled back into cytoplasm15

16Translation - TerminationRibosome reaches one of THREE STOP CODONS- UGA, UAG, UAADo not code for an amino acid, no corresponding tRNAsProtein release factor recognizes that ribosome has stalled and helps release polypeptide chain from ribosomeTERMINATION.TranslationTranscribe & Translate!

17