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Chapter 17: Genes and How They Work

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Chapter 17: Genes and How They Work Chapter 17: Genes and How They Work Genes generally are information for making specific proteins RNA (ribonucleic acid) Overview of Gene Expression Transcription (DNA RNA) The Genetic Code Translation (RNA protein) Differences between prokaryotes and eukaryotes in transcription and translation Modern Definition of Genes Mutations Gene Regulation . Chapter 17: Genes and How They Work Genes generally are information for making specific proteins RNA (ribonucleic acid) Overview of Gene Expression Transcription (DNA RNA) The Genetic Code Translation (RNA protein) Differences between prokaryotes and eukaryotes in transcription and translation Modern Definition of Genes Mutations Gene Regulation . • What do genes do? • How do we define a gene? • Discuss the derivation of the “one gene, one polypeptide” model, tracing the history through Garrod, Beadle and Tatum, and Pauling. Genes generally are information for making specific proteins in connection with the rediscovery of Mendel’s work around the dawn of the 20th century, the idea that genes are responsible for making enzymes was advanced this view was summarized in the classic work Inborn Errors of Metabolism (Garrod 1908) Premise: certain diseases arise from metabolic disorders . Genes generally are information for making specific proteins work by Beadle and Tatum in the 1940s refined this concept found mutant genes in the fungus Neurospora that each affected a single step in a metabolic pathway developed the “one gene, one enzyme” hypothesis Follow-up work by Srb and Horowitz illustrated this even more clearly (their work is actually what is presented in your textbook and in the figure here) . Genes generally are information for making specific proteins later work by Pauling and others showed that other proteins are also generated genetically also, some proteins have multiple subunits encoded by different genes this ultimately led to the “one gene, one polypeptide” hypothesis . • What do genes do? • How do we define a gene? • Discuss the derivation of the “one gene, one polypeptide” model, tracing the history through Garrod, Beadle and Tatum, and Pauling. Chapter 17: Genes and How They Work Genes generally are information for making specific proteins RNA (ribonucleic acid) Overview of Gene Expression Transcription (DNA RNA) The Genetic Code Translation (RNA protein) Differences between prokaryotes and eukaryotes in transcription and translation Modern Definition of Genes Mutations Gene Regulation . • How does RNA differ from DNA structurally? • What are the structural and functional differences between mRNA, tRNA and rRNA? . RNA (ribonucleic acid) RNA serves mainly as an intermediary between the information in DNA and the realization of that information in proteins . RNA RNA has some structural distinctions from DNA typically single-stranded (although often with folds and complex 3D structure) sugar is ribose; thus, RNA polymers are built from ribonucleotides -OH at the #2 C on the ribose, vs. deoxyribose in DNA uracil (U) functions in place of T . RNA (ribonucleic acid) three main forms of RNA are used: mRNA, tRNA, and rRNA mRNA or messenger RNA: copies the actual instructions from the gene tRNA or transfer RNA: links with amino acids and bring them to the appropriate sites for incorporation in proteins rRNA or ribosomal RNA: main structural and catalytic components of ribosomes, where proteins are actually produced all are synthesized from DNA templates (thus, some genes code for tRNA and rRNA, not protein) . • How does RNA differ from DNA structurally? • What are the structural and functional differences between mRNA, tRNA and rRNA? . Chapter 17: Genes and How They Work Genes generally are information for making specific proteins RNA (ribonucleic acid) Overview of Gene Expression Transcription (DNA RNA) The Genetic Code Translation (RNA protein) Differences between prokaryotes and eukaryotes in transcription and translation Modern Definition of Genes Mutations Gene Regulation . • Explain the “central dogma of gene expression”. • What is the difference between transcription and translation? • How will you keep these similar- sounding terms clear in your head? . Central Dogma of Gene Expression DNA RNA protein the gene is the DNA sequence with instructions for making a product the protein (or protein subunit) is the product . Central Dogma of Gene Expression DNA RNA is transcription making RNA using directions from a DNA template transcribe = copy in the same language (language used here is base sequence) . Central Dogma of Gene Expression RNA protein is translation making a polypeptide chain using directions in mRNA translate = copy into a different language; here the translation is from base sequence to amino acid sequence . Central Dogma of Gene Expression there are exceptions to the central dogma some genes are for an RNA final product, such as tRNA and rRNA (note: mRNA is NOT considered a final product) for some viruses use RNA as their genetic material some never use DNA some use the enzyme reverse transcriptase to perform RNA DNA before then following the central dogma . Central Dogma of Gene Expression DNA RNA protein . • Explain the “central dogma of gene expression”. • What is the difference between transcription and translation? • How will you keep these similar- sounding terms clear in your head? . Chapter 17: Genes and How They Work Genes generally are information for making specific proteins RNA (ribonucleic acid) Overview of Gene Expression Transcription (DNA RNA) The Genetic Code Translation (RNA protein) Differences between prokaryotes and eukaryotes in transcription and translation Modern Definition of Genes Mutations Gene Regulation . • What three steps must most (perhaps all) biological processes have? . • Describe the events of initiation, elongation, and termination of transcription. • Be sure to use key terms like upstream, downstream, promoter, etc. Transcription (DNA RNA) RNA is synthesized as a complementary strand using DNA- dependent RNA polymerases process is somewhat similar to DNA synthesis, but no primer is needed bacterial cells each only have one type of RNA polymerase eukaryotic cells have three major types of RNA polymerase RNA polymerase I is used in making rRNA RNA polymerase II is used in making mRNA and some small RNA molecules RNA polymerase III is used in making tRNA and some small RNA molecules . Transcription (DNA RNA) only one strand is transcribed, with RNA polymerase using ribonucleotide triphosphates (NTPs) to build a strand in the 5’3’ direction thus, the DNA is transcribed (copied or read) in the 3’ 5’ direction the DNA strand that is read is called the template strand . Transcription (DNA RNA) upstream means toward the 5’ end of the RNA strand, or toward the 3’ end of the template strand (away from the direction of synthesis) downstream means toward the 3’ end of the RNA strand, or toward the 5’ end of the template strand . Transcription Nucleotide triphosphates are added to the growing strand at the 3’ end Phosphodiester bonds are made by DNA dependent RNA polymerases Two phosphates are lost from each nucleotide triphosphate Note the antiparallel, complementary strands . Complementary Coding If the template DNA is: A-T-G-C-T-T-A-A-C-C-G-G-T-T The transcribed mRNA is: U-A-C-G-A-A-U-U-G-G-C-C-A-A . Transcription (DNA RNA) transcription has three stages: initiation elongation termination . Transcription (DNA RNA) initiation requires a promoter – site where RNA polymerase initially binds to DNA promoters are important because they are needed to allow RNA synthesis to begin promoter sequence is upstream of where RNA strand production actually begins promoters vary between genes; this is the main means for controlling which genes are transcribed at a given time promoter Transcribed DNA sense strand 3’ 3’ mRNA transcript downstream . Transcription (DNA RNA) bacterial promoters about 40 nucleotides long positioned just before the point where transcription begins recognized directly by RNA polymerase . Transcription (DNA RNA) eukaryotic promoters (for genes that use RNA polymerase II) initially, transcription factors bind to the promoter; these proteins facilitate binding of RNA polymerase to the site transcription initiation complex completed assembly of transcription factors and RNA polymerase at the promoter region allows initiation of transcription (the actual production of an RNA strand complementary to the DNA template) . Transcription (DNA RNA) eukaryotic promoters (for genes that use RNA polymerase II) genes that use RNA polymerase II commonly have a “TATA box” about 25 nucleotides upstream of the point where transcription begins actual sequence is something similar to TATAAA on the non- template strand sequences are usually written in the 5’3’ direction of the strand with that sequence unless noted otherwise . Transcription (DNA RNA) regardless of promoter specifics, initiation begins when RNA polymerase is associated with the DNA RNA polymerase opens and unwinds the DNA RNA polymerase begins building an RNA strand in the 5’3’ direction, complementary to the template strand only one RNA strand is produced . Transcription (DNA RNA) elongation transcription continues in a linear fashion, with DNA unwinding and opening along the way the newly synthesized RNA strand easily separates from the DNA and the DNA molecule “zips up” behind RNA polymerase, reforming the double helix
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