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David Sadava H. Craig Heller Gordon H. Orians William K. Purves David M. Hillis Biologia.blu B – Le basi molecolari della vita e dell’evoluzione From DNA to Protein: Genotype to Phenotype From DNA to Protein: Genotype to Phenotype • What is the evidence that genes code for proteins? • How does information flow from genes to proteins? • How is the information content in DNA transcribed to produce RNA? • How is RNA translated into proteins? • What are mutations? From DNA to Protein: Genotype to Phenotype - What is the evidence that genes code for proteins? The molecular basis of phenotypes was known before it was known that DNA is the genetic material. Studies of many different organisms showed that major phenotypic differences were due to specific proteins. From DNA to Protein: Genotype to Phenotype - What is the evidence that genes code for proteins? The gene-enzyme relationship has been revised to the one-gene, one-polypeptide relationship. Example: in hemoglobin, each polypeptide chain is specified by a separate gene. Other genes code for RNA that is not translated to polypeptides; some genes are involved in controlling other genes. From DNA to Protein: Genotype to Phenotype - How does information flow from genes to proteins? Expression of a gene to form a polypeptide: • transcription—copies information from gene to a sequence of RNA; • translation—converts RNA sequence to amino acid sequence. From DNA to Protein: Genotype to Phenotype - How does information flow from genes to proteins? RNA, ribonucleic acid differs from DNA: • it usually has one strand; • the sugar is ribose; • it contains uracil (U) instead of thymine (T). From DNA to Protein: Genotype to Phenotype - How does information flow from genes to proteins? RNA can pair with a single strand of DNA, except that adenine pairs with uracil instead of thymine. Single-strand RNA can fold into complex shapes by internal base pairing. From DNA to Protein: Genotype to Phenotype - How does information flow from genes to proteins? Messenger hypothesis—messenger RNA (mRNA) forms as a complementary copy of DNA and carries information to the cytoplasm. This process is called transcription. From DNA to Protein: Genotype to Phenotype - How does information flow from genes to proteins? The central dogma of molecular biology: information flows in one direction when genes are expressed (by Francis Crick). From DNA to Protein: Genotype to Phenotype - How does information flow from genes to proteins? The central dogma raised two questions: • how does genetic information get from the nucleus to the cytoplasm? • what is the relationship between a DNA sequence and an amino acid sequence? From DNA to Protein: Genotype to Phenotype - How does information flow from genes to proteins? From gene to protein From DNA to Protein: Genotype to Phenotype - How does information flow from genes to proteins? There is an exception to the central dogma. Viruses: acellular particles that reproduce inside cells; many have RNA instead of DNA. From DNA to Protein: Genotype to Phenotype - How does information flow from genes to proteins? Transcription occurs in three phases: 1.initiation; 2.elongation; 3.termination. From DNA to Protein: Genotype to Phenotype - How does information flow from genes to proteins? DNA is transcribed to form RNA (phase 1) From DNA to Protein: Genotype to Phenotype - How does information flow from genes to proteins? DNA is transcribed to form RNA (phases 2 and 3) From DNA to Protein: Genotype to Phenotype - How is the information content in DNA transcribed to produce RNA? The genetic code specifies which amino acids will be used to build a protein. Codon: a sequence of three bases. Each codon specifies a particular amino acid. Start codon: AUG—initiation signal for translation. Stop codons stops translation and polypeptide is released. From DNA to Protein: Genotype to Phenotype - How is the information content in DNA transcribed to produce RNA? The genetic code From DNA to Protein: Genotype to Phenotype - How is the information content in DNA transcribed to produce RNA? The genetic code is nearly universal: the codons that specify amino acids are the same in all organisms. Exceptions: within mitochondria and chloroplasts, and in one group of protists. This genetic code is a common language for evolution. The code is ancient and has remained intact throughout evolution. The common code also facilitates genetic engineering. From DNA to Protein: Genotype to Phenotype - How is RNA translated into proteins? tRNA, the adaptor molecule: for each amino acid, there is a specific type or “species” of transfer RNA. Functions of tRNA: • carries an amino acid; • associates with mRNA molecules; • interacts with ribosomes. From DNA to Protein: Genotype to Phenotype - How is RNA translated into proteins? Transfer RNA From DNA to Protein: Genotype to Phenotype - How is RNA translated into proteins? Ribosomes have two subunits, large and small. In eukaryotes, the large subunit has three molecules of ribosomal RNA (rRNA) and 45 different proteins in a precise pattern. The small subunit has one rRNA and 33 proteins. From DNA to Protein: Genotype to Phenotype - How is RNA translated into proteins? Ribosome structure • • • E- site: the ribosomal site harbouring deacylated tRNA on transit out from the ribosome P-site: the ribosomal site most frequently occupied by peptidyl-tRNA A-site: the ribosomal site most frequently occupied by aminoacyl-tRNA From DNA to Protein: Genotype to Phenotype - How is RNA translated into proteins? The initiation of translation (part 1) From DNA to Protein: Genotype to Phenotype - How is RNA translated into proteins? The initiation of translation (part 2) From DNA to Protein: Genotype to Phenotype - How is RNA translated into proteins? The elongation of translation (part 1) From DNA to Protein: Genotype to Phenotype - How is RNA translated into proteins? The elongation of translation (part 2) From DNA to Protein: Genotype to Phenotype - How is RNA translated into proteins? The termination of translation (part 1) From DNA to Protein: Genotype to Phenotype - How is RNA translated into proteins? The termination of translation (part 2) From DNA to Protein: Genotype to Phenotype - How is RNA translated into proteins? The termination of translation (part 3) From DNA to Protein: Genotype to Phenotype - How is RNA translated into proteins? Destinations for newly translated polypeptides in a eukaryotic cell From DNA to Protein: Genotype to Phenotype – What are mutations? From DNA to Protein: Genotype to Phenotype – What are mutations? Missense mutations: base substitution results in amino acid substitution. From DNA to Protein: Genotype to Phenotype – What are mutations? Nonsense mutations: base substitution results in a stop codon. From DNA to Protein: Genotype to Phenotype – What are mutations? Frame-shift mutations: single bases inserted or deleted—usually leads to nonfunctional proteins. From DNA to Protein: Genotype to Phenotype – What are mutations? Chromosomal mutations (part 1) From DNA to Protein: Genotype to Phenotype – What are mutations? Chromosomal mutations (part 2) From DNA to Protein: Genotype to Phenotype – What are mutations? Spontaneous and induced mutations (part 1) From DNA to Protein: Genotype to Phenotype – What are mutations? Spontaneous and induced mutations (part 2) Fom DNA to Protein: Genotype to Phenotype – What are mutations? Mutation provides the raw material for evolution in the form of genetic diversity. Mutations can harm the organism, or be neutral. Occasionally, a mutation can improve an organism’s adaptation to its environment, or become favorable as conditions change.