Microbio 310 1st Edition Lecture 9 Outline of Last Lecture I. 6.1 Macromolecules and GenesII. 6.2 The Double HelixIII. 6.3 SupercoilingIV. 6.4 Chromosomes and Other Genetic ElementsV. 6.5 The Escherichia coli ChromosomeVI. 6.6 Plasmids: General PrinciplesVII. 6.7 The Biology of PlasmidsVIII. 6.8 Templates and EnzymesIX. 6.9 The Replication ForkX. 6.10 Bidirectional Replication and the ReplisomeXI. 6.11 The Polymerase Chain ReactionXII. 6.12 Overview of TranscriptionXIII. 6.13 Sigma Factors and Consensus SequencesOutline of Current Lecture I. 6.15 The Unit of TranscriptionII. 6.16 Polypeptides, Amino Acids, and the Peptide BondIII. 6.17 Translation and the Genetic CodeIV. 7.2 Transcription and RNA Processing in ArchaeaV. 7.4 Shared Features of Bacteria and ArchaeaThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.VI. 7.5 Genes and Chromosomes in EukaryaVII. 7.8 RNA ProcessingVIII. 7.9 Transcription and Translation in EukaryaIX. 7.10 RNA Interference (RNAi)X. 7.11 Regulation by MicroRNAXI. 8.3 Negative Control of Transcription: Repression and InductionXII. 8.4 Positive Control of TranscriptionXIII. 8.5 Global Control and the lac OperonXIV. 8.7 Two-Component Regulatory SystemsXV. 8.8 Regulation of ChemotaxisXVI. 8.9 Quorum SensingXVII. 8.14 RNA Regulation and Antisense RNAXVIII. 8.15 RiboswitchesXIX. 8.16 AttenuationCurrent Lecture6.15 The Unit of Transcription• Unit of transcription: unit of chromosome bounded by sites where transcription of DNA to RNA is initiated and terminated• Most genes encode proteins, but some RNAs are not translated (i.e., rRNA, tRNA)– Three types of rRNA: 16S, 23S, and 5S– rRNA and tRNA are very stable– tRNA cotranscribed with rRNA or other tRNA• mRNAs have short half-lives (a few minutes)• Prokaryotes often have genes clustered together (tightly clustered genes=more efficient transcription)– These genes are transcribed all at once as a single mRNA• An mRNA encoding a group of cotranscribed genes is called a polycistronic mRNA• Operon: a group of related genes cotranscribed on a polycistronic mRNA– Allows for expression of multiple genes to be coordinated6.16 Polypeptides, Amino Acids, and the Peptide Bond• Proteins play a major role in cell function – Catalytic proteins (enzymes)– Structural proteins• Proteins are polymers of amino acids• Amino acids are linked by peptide bonds (bond between carboxyl carbon and amino nitrogen) to form a polypeptide6.17 Translation and the Genetic Code • Translation: the synthesis of proteins from RNA• Genetic code: a triplet of nucleic acid bases (codon) encodes a single amino acid– Specific codons for starting and stopping translation– Degenerate code: multiple codons encode a single amino acid– Anticodon on tRNA recognizes and pairs with codon on mRNA– Wobble: irregular (more flexible) base pairing allowed at third position of tRNA• Stop codons: terminate/stop translation (UAA, UAG, and UGA)• Start codon: translation begins with AUG• Reading frame: triplet code requires translation to begin at the correct nucleotide; read 3 nucleotides at a time in a specific order• Shine–Dalgarno sequence: ribosome binding site; ensures proper reading frame• Open reading frame (ORF): AUG followed by a number of codons and a stop codon in the same reading frame• Codon bias: multiple codons for the same amino acid are not used equally– Varies with organism– Correlated with tRNA availability– Cloned genes from one organism may not be translated by recipient organism because of codon bias• Some organelles and a few cells have slight variations of the genetic code (e.g., mitochondria of animals, Mycoplasma, and Paramecium)7.2 Transcription and RNA Processing in Archaea• Eukaryotic genes have coding and noncoding regions– Exons are the coding sequences– Introns are the intervening sequences• Are rare in Archaea• Are found in tRNA and rRNA genes of Archaea• Archaeal introns excised by special endonuclease7.4 Shared Features of Bacteria and Archaea• Bacteria and Archaea share several fundamental properties that are absent from eukaryotes– Typically single-celled, most divide by binary fission– Neither possesses a nucleus or membrane-bound organelles– Archaea and Bacteria have coupled transcription and translation (simultaneously do both)– Both possess a single, circular chromosome– Both use Shine–Dalgarno sequences to indicate translation start7.5 Genes and Chromosomes in Eukarya• Protein-encoding genes in Eukarya are often split into multiple exons (coding regions) by introns (noncoding regions)• Both introns and exons are transcribed into the primary transcript• Functional mRNA is formed by the excision of introns and splicing of exons• Eukaryotes contain much more DNA than is needed to encode all proteins required for cell functioning– 3% of DNA in human genome encodes protein– >90% of prokaryotic DNA encodes protein– Eukaryotic microorganisms have fewer introns than higher eukaryotes• Often have multiple copies of the same gene• Have multiple linear chromosomes in the nucleus7.8 RNA Processing• Ribozymes: RNA molecules with enzymatic activity– Ex: Self-splicing intron: an intron that has enzymatic activity and splices itself out of RNA– Most found in mitochondria and chloroplasts– Also found in lower eukaryotes (e.g., Tetrahymena)– Catalyze reaction only once– Vestiges of simpler form of life (Indicate that life started with RNA)7.9 Transcription and Translation in Eukarya• Protein synthesis is more complex in eukaryotes than in Bacteria– Eukaryotic ribosomes are larger than bacterial ribosomes– More initiation factors in Eukarya– Eukaryotic mRNA is monocistronic– Eukaryotic mRNA has no ribosome-binding site o Eukaryotic mRNA is recognized by its cap7.10 RNA Interference (RNAi)• Healthy cells do not contain dsRNA (ds=double stranded)– Presence of dsRNA indicative of RNA virus in cell– When a cell sees dsRNA it sends a signal to surrounding cells to self-destruct– RNA interference (RNAi): defense against dsRNA viruses• Cleaves dsRNA• Destroys ssRNA (ss=single stranded) corresponding to targeted dsRNA sequence• Found only in eukaryotes• RNAi triggered by dsRNA longer than 20 base pairs (way to regulate gene expression)– Long dsRNAs cleaved into 21–23 base pairs by Dicer