Molecular biology - Chapter 1: A brief history

Molecular BiologyFourth 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 offspringChemical composition of genes not known until 1944GenePhenotype3Mendel’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 Gene TransmissionHeterozygotes 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 allele5The Chromosome Theory of InheritanceChromosomes are discrete physical entities that carry the genesThomas Hunt Morgan used the fruit fly, Drosophila melanogaster, to study geneticsAutosomes occur in pairs in a given individualSex chromosomes are identified as X and YFemale has two X chromosomesMale has one X and one Y chromosome6Hypothetical ChromosomesEvery 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 recessive7Genetic 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 occurs8RecombinationDuring 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 recombination9Genetic 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 bacteria and to eukaryotes10Physical Evidence for RecombinationMicroscopic examination of 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 plants111.2 Molecular GeneticsThe Discovery of DNA: The general structure of nucleic acids were found by the end of the 19th centuryLong polymers or chains of nucleotidesNucleotides are linked by sugars through phosphate groupsComposition of Genes: In 1944, Avery and his colleagues demonstrated that genes are composed of nucleic acids12The Relationship between Genes and ProteinsExperiments have shown that a defective gene gives a defective or absent enzymeThese lead to the proposal that one gene is responsible for making one enzymeProposal not quite correctEnzyme may have several polypeptides, each gene codes for only one polypeptideMany genes code for non-enzyme proteinsEnd products of some genes are not polypeptides13Activities of GenesGenes perform three major rolesReplicated faithfullyDirect the production of RNAs and proteinsAccumulate mutations thereby allowing evolution14ReplicationFranklin 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 – know the sequence of one, automatically know the sequence of the otherSemiconservative replication keeps one strand of the parental double helix conserved in each of the daughter double helices15Genes Direct the Production of PolypeptidesGene expression is the process by which a gene product is madeTwo steps are requiredTranscription: copy of DNA is transcribed into RNATranslation: the RNA copy is read or translated to assemble a protein Codon: a sequence of 3 nucleic acid bases that stand for one amino acid16Genes 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 segmentAs the change is more drastic, it is more likely that the gene or genes involved will be totally inactivated171.3 The Three Domains of LifeCurrent research theories support the division of living organisms into three domains Bacteria Eukaryota Archaea living in the most inhospitable regions of the earthThermophiles tolerate extremely high temperaturesHalophiles tolerate very high salt concentrationsMethanogens produce methane as a by-product of metabolism18