Biology 212 General Genetics Spring 2007Lecture 18: Gene Expression IReading: Chap. 8 pp. 282-291; pp. 302-305Lecture Outline:1. Principles of gene expression2. Transcription3. Translation: elucidating the genetic code1. Gene Expression PrinciplesGene expression involves processes of transcription and translation, which result in the production of proteinsThe primary structure of proteins is a linear sequence of amino acids held together by peptide bondsPeptide bonds link the carboxyl group of one amino acid to the amino group of the next amino acidThere are twenty naturally occurring amino acids, the building blocks of proteinsThe linear sequence of amino acids in proteins is specified by the coding information in specific genesColinearity: the linear order of amino acids is encoded in a DNA base sequenceThe base sequence in DNA specifies the base sequence in mRNA decoded in blocks of 3 nt  amino acid sequence2. TranscriptionTranscription = production of messenger RNA (mRNA) complementary to the base sequence of specific genesmRNA differs from DNA in that it is single stranded, contains ribose sugar instead of deoxyribose and has the pyrimidine uracil in place of thymineA. RNA SynthesisThe nucleotide sequence in the transcribed mRNA is complementary to the base sequence in DNARNA is copied from the template strand which is 3’-to-5’ in the 5’-to-3’ direction = antiparallelRNA synthesis does not require a primer and proceeds by the sequential addition of nucleotides to form an mRNA chainB. Process of Transcription (Prokaryotes)Promoter = nucleotide sequence 5’ to the transcription start sitebinding site of RNA polymerase initiation factor (sigma subunit, σ)Promoter recognition by RNA polymerase is a prerequisite for transcription initiationMany promoters contain a similar DNA sequence = TATAAT = “TATA” box,at -10Another consensus promoter sequence is at -35 = TTGACATranscription termination sites are often inverted repeat sequences which can form hairpin loops in RNAC. Eukaryotic Gene StructureIn many eukaryotic genes, the coding regions are interrupted by noncoding segments = “split genes”Coding regions = exonsNoncoding regions = intronsPrimary transcript contains exons and introns; introns are subsequently removed = “splicing”D. Eukaryotic TranscriptionEukaryotic transcription involves the synthesis of RNA specified by DNA template strand to form a primary transcriptProtein-coding genes are transcribed by RNA polymerase IIseparate RNA polymerases transcribe rRNA genes (RNA polymerase I) and tRNA genes (RNA polymerase III)Primary transcript is processed to form mRNA which is transported to the cytoplasmThe first processing step adds 7- methylguanosine to 5’ end = “cap”Splicing removes introns and links exons.Additional processing involves the addition of a series of 150-200 adenines at the 3’ end of the transcript = “poly A tail”The processed transcript contains a 5’ cap (7-methylguanosine), adjacent exons, and a poly A tail3. Translation: Genetic evidence for the triplet codeBiology 212 General Genetics Spring 2007Lecture 18: Gene Expression IReading: Chap. 8 pp. 282-291; pp. 302-305Lecture Outline: 1. Principles of gene expression 2. Transcription3. Translation: elucidating the genetic code1. Gene Expression Principles• Gene expression involves processes of transcription and translation, which result in the production of proteins • The primary structure of proteins is a linear sequence of amino acids held togetherby peptide bonds o Peptide bonds link the carboxyl group of one amino acid to the amino group of the next amino acido There are twenty naturally occurring amino acids, the building blocks of proteinso each amino acid has a unique side chain = R groupo The linear sequence of amino acids in proteins is specified by the coding information in specific genes• Colinearity: the linear order of amino acids is encoded in a DNA base sequence• The base sequence in DNA specifies the base sequence in mRNA decoded in blocks of 3 nt  amino acid sequence2. Transcription• Transcription = production of messenger RNA (mRNA) complementary to the base sequence of specific genes• mRNA differs from DNA in that it is single stranded, contains ribose sugar instead of deoxyribose and has the pyrimidine uracil in place of thymine A. RNA Synthesis• The nucleotide sequence in the transcribed mRNA is complementary to the base sequence in DNA• RNA is copied from the template strand which is 3’-to-5’ in the 5’-to-3’ direction = antiparallel• RNA synthesis does not require a primer and proceeds by the sequential addition of nucleotides to form an mRNA chain 1B. Process of Transcription (Prokaryotes)Three main phases:InitiationElongationTerminationInitiation:• Promoter = nucleotide sequence 5’ to the transcription start site o binding site of RNA polymerase initiation factor (sigma subunit, σ)o Promoter recognition by RNA polymerase is a prerequisite for transcription initiationo Many promoters contain a similar DNA sequence = TATAAT = “TATA” box, at -10o Another consensus promoter sequence is at -35 = TTGACAElongation:- Growth of RNA chain 5’  3’ direction by sequential addition of nucleotides catalyzed by RNA polymerase Termination:• Transcription termination sites are often inverted repeat sequences which can form hairpin loops in RNA C. Eukaryotic Gene Structure• In many eukaryotic genes, the coding regions are interrupted by noncoding segments = “split genes”• Coding regions = exons• Noncoding regions = introns• Primary transcript contains exons and introns; introns are subsequently removed = “splicing”D. Eukaryotic Transcription• Eukaryotic transcription involves the synthesis of RNA specified by DNA template strand to form a primary transcript• Protein-coding genes are transcribed by RNA polymerase II• separate RNA polymerases transcribe rRNA genes (RNA polymerase I) and tRNAgenes (RNA polymerase III)• Primary transcript is processed to form mRNA which is transported to the cytoplasmo The first processing step adds 7- methylguanosine to 5’ end = “cap”o Splicing removes introns and links exons.o Additional processing involves the addition of a series of 150-200 adenines at the 3’ end of the transcript = “poly A tail”2The processed transcript contains a 5’ cap (7-methylguanosine), adjacent exons, and a poly A tail3. Translation: Genetic evidence for