Bài giảng Biology - Chapter 15: Genes and How They Work

Genes and How They WorkChapter 151The Nature of GenesEarly ideas to explain how genes work came from studying human diseases.Archibald Garrod studied alkaptonuria, 1902Garrod recognized that the disease is inherited via a recessive alleleGarrod proposed that patients with the disease lacked a particular enzymeThese ideas connected genes to enzymes.2The Nature of GenesEvidence for the function of genes came from studying fungus.George Beadle and Edward Tatum, 1941studied Neurospora crassaused X-rays to damage the DNA in cells of Neurosporalooked for cells with a new (mutant) phenotype caused by the damaged DNA3The Nature of GenesBeadle and Tatum looked for fungal cells lacking specific enzymes.The enzymes were required for the biochemical pathway producing the amino acid arginine.They identified mutants deficient in each enzyme of the pathway.45The Nature of GenesBeadle and Tatum proposed that each enzyme of the arginine pathway was encoded by a separate gene.They proposed the one gene – one enzyme hypothesis.Today we know this as the one gene – one polypeptide hypothesis.6The Nature of GenesThe central dogma of molecular biology states that information flows in one direction: DNA RNA proteinTranscription is the flow of information from DNA to RNA.Translation is the flow of information from RNA to protein.78The Genetic CodeDeciphering the genetic code required determining how 4 nucleotides (A, T, G, C) could encode more than 20 amino acids.Francis Crick and Sydney Brenner determined that the DNA is read in sets of 3 nucleotides for each amino acid.910The Genetic Codecodon: set of 3 nucleotides that specifies a particular amino acidreading frame: the series of nucleotides read in sets of 3 (codon)only 1 reading frame is correct for encoding the correct sequence of amino acids11The Genetic CodeMarshall Nirenberg identified the codons that specify each amino acid.RNA molecules of only 1 nucleotide and of specific 3-base sequences were used to determine the amino acid encoded by each codon.The amino acids encoded by all 64 possible codons were determined.1213The Genetic Codestop codons: 3 codons (UUA, UGA, UAG) in the genetic code used to terminate translationstart codon: the codon (AUG) used to signify the start of translationThe remainder of the code is degenerate meaning that some amino acids are specified by more than one codon.14Gene Expression Overviewtemplate strand: strand of the DNA double helix used to make RNAcoding strand: strand of DNA that is complementary to the template strandRNA polymerase: the enzyme that synthesizes RNA from the DNA template15Gene Expression OverviewTranscription proceeds through:initiation – RNA polymerase identifies where to begin transcriptionelongation – RNA nucleotides are added to the 3’ end of the new RNAtermination – RNA polymerase stops transcription when it encounters terminators in the DNA sequence16Gene Expression OverviewTranslation proceeds throughinitiation – mRNA, tRNA, and ribosome come togetherelongation – tRNAs bring amino acids to the ribosome for incorporation into the polypeptidetermination – ribosome encounters a stop codon and releases polypeptide17Gene Expression OverviewGene expression requires the participation of multiple types of RNA:messenger RNA (mRNA) carries the information from DNA that encodes proteinsribosomal RNA (rRNA) is a structural component of the ribosometransfer RNA (tRNA) carries amino acids to the ribosome for translation18Gene Expression OverviewGene expression requires the participation of multiple types of RNA:small nuclear RNA (snRNA) are involved in processing pre-mRNAsignal recognition particle (SRP) is composed of protein and RNA and involved in directing mRNA to the RERmicro-RNA (miRNA) are very small and their role is not clear yet19Prokaryotic TranscriptionProkaryotic cells contain a single type of RNA polymerase found in 2 forms:core polymerase is capable of RNA elongation but not initiationholoenzyme is composed of the core enzyme and the sigma factor which is required for transcription initiation2021Prokaryotic TranscriptionA transcriptional unit extends from the promoter to the terminator.The promoter is composed ofa DNA sequence for the binding of RNA polymerasethe start site (+1) – the first base to be transcribed22Prokaryotic TranscriptionDuring elongation, the transcription bubble moves down the DNA template at a rate of 50 nucleotides/sec.The transcription bubble consists ofRNA polymeraseDNA templategrowing RNA transcript2324Prokaryotic TranscriptionTranscription stops when the transcription bubble encounters terminator sequencesthis often includes a series of A-T base pairsIn prokaryotes, transcription and translation are often coupled – occurring at the same time252627Eukaryotic TranscriptionRNA polymerase I transcribes rRNA.RNA polymerase II transcribes mRNA and some snRNA.RNA polymerase III transcribes tRNA and some other small RNAs.Each RNA polymerase recognizes its own promoter.28Eukaryotic TranscriptionInitiation of transcription of mRNA requires a series of transcription factorstranscription factors – proteins that act to bind RNA polymerase to the promoter and initiate transcription29In eukaryotes, the primary transcript must be modified by:addition of a 5’ capaddition of a 3’ poly-A tailremoval of non-coding sequences (introns)Eukaryotic pre-mRNA Splicing30Eukaryotic pre-mRNA SplicingThe spliceosome is the organelle responsible for removing introns and splicing exons together.Small ribonucleoprotein particles (snRNPs) within the spliceosome recognize the intron-exon boundariesintrons – non-coding sequences exons – sequences that will be translated31tRNA and RibosomestRNA molecules carry amino acids to the ribosome for incorporation into a polypeptideaminoacyl-tRNA synthetases add amino acids to the acceptor arm of tRNAthe anticodon loop contains 3 nucleotides complementary to mRNA codons323334tRNA and RibosomesThe ribosome has multiple tRNA binding sites:P site – binds the tRNA attached to the growing peptide chainA site – binds the tRNA carrying the next amino acidE site – binds the tRNA that carried the last amino acid3536tRNA and RibosomesThe ribosome has two primary functions:decode the mRNAform peptide bondspeptidyl transferase is the enzymatic component of the ribosome which forms peptide bonds between amino acids37TranslationIn prokaryotes, initiation of translation requires the formation of the initiation complex includingan initiator tRNA charged with N-formylmethioninethe small ribosomal subunitmRNA strandThe ribosome binding sequence of mRNA is complementary to part of rRNA3839TranslationElongation of translation involves the addition of amino acidsa charged tRNA binds to the A site if its anticodon is complementary to the codon at the A sitepeptidyl transferase forms a peptide bondthe ribosome moves down the mRNA in a 5’ to 3’ direction404142TranslationThere are fewer tRNAs than codons.Wobble pairing allows less stringent pairing between the 3’ base of the codon and the 5’ base of the anticodon.This allows fewer tRNAs to accommodate all codons.43TranslationElongation continues until the ribosome encounters a stop codon.Stop codons are recognized by release factors which release the polypeptide from the ribosome.4445TranslationIn eukaryotes, translation may occur on ribosomes in the cytoplasm or on ribosomes of the RER.Signal sequences at the beginning of the polypeptide sequence bind to the signal recognition particle (SRP)The signal sequence and SRP are recognized by RER receptor proteins.46TranslationThe signal sequence/SRP holds the ribosome on the RER.As the polypeptide is synthesized it passes through a pore into the interior of the endoplasmic reticulum.4748Mutation: Altered GenesPoint mutations alter a single base.base substitution mutations – substitute one base for anothertransitions or transversionsalso called missense mutationsnonsense mutations – create stop codonframeshift mutations – caused by insertion or deletion of a single base4950Mutation: Altered Genestriplet repeat expansion mutations involve a sequence of 3 DNA nucleotides that are repeated many timestriplet repeats are associated with some human genetic diseasesthe abnormal allele causing the disease contains these repeats whereas the normal allele does not51Mutation: Altered GenesChromosomal mutations change the structure of a chromosome.deletions – part of chromosome is lostduplication – part of chromosome is copiedinversion – part of chromosome in reverse ordertranslocation – part of chromosome is moved to a new location5253Mutation: Altered GenesToo much genetic change (mutation) can be harmful to the individual.However, genetic variation (caused by mutation) is necessary for evolutionary change of the species.54