Bài giảng Molecular Biology - Chapter 1 A Brief History

Molecular BiologyFifth EditionChapter 1A Brief HistoryLecture PowerPoint to accompanyRobert F. WeaverCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.1A Brief HistoryWhat is molecular biology?The attempt to understand biological phenomena in molecular termsThe study of gene structure and function at the molecular levelMolecular biology is a melding of aspects of genetics and biochemistry21.1 Transmission GeneticsTransmission genetics deals with the transmission of traits from parental organisms to their offspringThe chemical composition of genes was not known until 1944Gene - genetic unitsPhenotype - observable characteristics3Mendel’s Laws of InheritanceA gene can exist in different forms called allelesOne allele can be dominant over the other, recessive, alleleThe first filial generation (F1) contains offspring of the original parentsIf each parent carries two copies of a gene, the parents are diploid for that gene4Mendel’s Laws of InheritanceHomoozygotes have two copies of the same alleleHeterozygotes have one copy of each alleleParents in 1st mating are homozygotes, having 2 copies of one alleleSex cells, or gametes, are haploid, containing only 1 copy of each geneHeterozygotes produce gametes having either allele Homozygotes produce gametes having only one allele5SummaryGenes can exist in several different forms or allelesOne allele can be dominant over the other, so heterozygotes having two different alleles of one gene will generally exhibit the characteristic dictated by the dominant alleleThe recessive allele is not lost; it can still exert its influence when paired with another recessive allele in a homozygote6The Chromosome Theory of InheritanceChromosomes are discrete physical entities that carry genesThomas Hunt Morgan used the fruit fly, Drosophila melanogaster, to study geneticsAutosomes occur in pairs in a given individual (not the X or the Y chromosome)Sex chromosomes are identified as X and YFemales have two X chromosomesMales have one X and one Y chromosome7Location of Genes on a ChromosomeEvery gene has its place, or locus, on a chromosomeGenotype is the combination of alleles found in an organismPhenotype is the visible expression of the genotypeWild-type phenotype is the most common or generally accepted standardMutant alleles are usually recessive8Genetic Recombination and MappingIn early experiments genes on separate chromosomes behaved independentlyGenes on the same chromosome behaved as if they were linkedThis genetic linkage is not absoluteOffspring show new combinations of alleles not seen in the parents when recombination occurs9RecombinationDuring meiosis, gamete formation, crossing over can occur resulting in the exchange of genes between the two homologous chromosomesThe result of the crossing-over event produces a new combination of allelesThis process is called recombination10Genetic MappingMorgan proposed that the farther apart two genes are on a chromosome, the more likely they are to recombineIf two loci recombine with a frequency of 1%, they are said to be separated by a map distance of one centimorgan (named for Morgan)This mapping observation applies both to prokaryotes and to eukaryotes11Physical Evidence for RecombinationMicroscopic examination of the maize chromosome provided direct physical observation of recombination using easily identifiable features of one chromosomeSimilar observations were made in DrosophilaRecombination was detected both physically and genetically in both animals and plants12SummaryThe chromosome theory of inheritance holds that genes are arranged in linear fashion on chromosomesCertain traits tend to be inherited together when the genes for those traits are on the same chromosomeRecombination between two homologous chromosomes during meiosis can scramble the parental alleles to yield nonparental combinationsThe farther apart two genes are on a chromosome the more likely it is that recombination will occur131.2 Molecular GeneticsThe Discovery of DNA: The general structure of nucleic acids was discovered by the end of the 19th centuryLong polymers or chains of nucleotidesNucleotides are linked by sugars through phosphate groupsComposition of Genes: DNA? RNA? Protein? In 1944, Avery and his colleagues demonstrated that genes are composed of DNA14The Relationship between Genes and ProteinsExperiments have shown that a defective gene gives a defective or absent enzymeThis lead to the proposal that one gene is responsible for making one enzymeProposal not quite correct for 3 reasons:One enzyme may be composed of several polypeptides, each gene codes for only one polypeptideMany genes code for non-enzyme proteinsEnd products of some genes are not polypeptides (i.e. tRNA, rRNA)15Activities of GenesGenes perform three major rolesReplicated faithfullyDirect the production of RNAs and proteinsAccumulate mutations thereby allowing for evolution16ReplicationFranklin and Wilkins produced x-ray diffraction data on DNA, Watson and Crick proposed that DNA is double helixTwo DNA strands wound around each otherStrands are complementary – if you know the sequence of one strand, you automatically know the sequence of the other strandSemiconservative replication keeps one strand of the parental double helix conserved in each of the daughter double helices17Genes Direct the Production of PolypeptidesGene expression is the process by which a gene product is madeTwo steps are required1. Transcription: DNA is transcribed into RNA2. Translation: the mRNA is read or translated to assemble a protein Codon: a sequence of 3 nucleic acid bases that code for one amino acid within the mRNA18Genes Accumulate MutationsGenes change in several waysChange one base to anotherDeletions of one base up to a large segmentInsertions of one base up to a large segmentThe more drastic the change, the more likely it is that the gene or genes involved will be totally inactivated19SummaryAll cellular genes are made of DNA arranged in a double helixThis structure explains how genes replicate, carry information and collect mutationsThe sequence of nucleotides in a gene is a genetic code that carries the information for making an RNAA change in the sequence of bases constitutes and mutation, which can change the sequence of amino acids in the genes polypeptide product201.3 The Three Domains of LifeCurrent research theories support the division of living organisms into three domains Bacteria Eukaryota Archaea Like bacteria as they are organisms without nucleiMore similar to eukaryotes in the context of their molecular biology21ArchaeaArchaea live in the most inhospitable regions of the earthThermophiles tolerate extremely high temperaturesHalophiles tolerate very high salt concentrationsMethanogens produce methane as a by-product of metabolism22