Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32What is a gene? The Central Dogma ReplicationDNA RNA protein•DNA stores the primary sequence information for all of the RNAs and proteins in the cell.•A ‘gene’ is that unit of DNA which encodes and is capable of expressing a particular RNA/protein.Despite the great diversity of life on Earth, all cells share the same underlying process of gene expression.1) What experimental approaches have scientists used to identify genes?2) How do they determine a gene’s function?3) Once a gene has been identified, how do we study its structure and function at a molecular level?In the coming lectures, we will look at genes from a somewhat different perspective.The first scientist to identify genes experimentally was Gregor Mendel working at St. Thomas's Abbey in Brno, now a part of the Czech Republic. In 1856-1863, Mendel performed carefully designed breeding experiments on the pea plant Pisum sativum.Mendel wanted to study the inheritance of physical traits, and to do so he made three practical decisions:•He elected to study characters that occurred in two clearly distinguishable forms, e.g. round peas vs. wrinkled peas.•He established pure-breeding lines for each of his traits. •He controlled which individuals mated by using a fine brush to transfer pollen from one flower to another.MONOHYBRID CROSSApproximately 3:1 ratioof round:wrinkledRound peas Wrinkled peasAll roundMendel's interpretationWatson, Fig. 1-1Inheritance of traits is under the control of factors (= genes) that occur in pairs.Inheritance of traits is under the control of factors (= genes) that occur in pairs.Mendel's heritable factors were paired because pea plants (like humans) are diploid, i.e. the same genetic loci are present on pairs of homologous chromosomes.Nuclear genome of a female human is composed of 23 distinct chromo-some pairs.Note that Mendel would have seen something very different if he had studied inheritance in bacteria. •Bacteria, indeed most prokaryotes, have a single chromosome, and in general carry a single copy of each gene.•In contrast, many eukaryotes (incl. virtually all animals and plants) are diploid.Mendel's interpretationWatson, Fig. 1-1Genes can exist in multiple forms (alleles).In Mendel’s experiments, when two different alleles were paired, one member of the pair was dominant.Mendel’s discovery of dominant and recessive alleles had another major implication, i.e. two individuals with the same phenotype could have different genotypes.Mendel's interpretationWatson, Fig. 1-1Individuals could be homozygous or heterozygous for a dominant allele, but would express the same dominant trait.Individuals could be homozygous or heterozygous for a dominant allele, but would express the same dominant trait.Watson, Fig. 1-1Factors are segregated during gamete formation in both genders … and recombine randomly in offspring of the next generationFactors are segregated during gamete formation in both genders … and recombine randomly in offspring of the next generationA “Punnett Square”**By inferring that genes are segregated during gamete formation, Mendel conceived in abstract terms of the cellular process that we now call meiosis.How is genomic DNA inherited from cell to cell?MITOSISMEIOSISGenetically identical cellsSister chromatidsHomologous chromosomesNOT genetically identicalDNA replicationDNA replicationNearly all animals and plants have more than one pair of homologous chromosomes. During Meiosis I, the maternal (red) and paternal (blue) copies of each chromosome undergo independent assortment. Chromosome SegregationGametesOR++Equally likely outcomes.Although he knew nothing about chromosomes, Mendel did see the independent assortment of traits.When he performed genetic crosses involving two traits controlled by two genes, the combination of traits in the F2 generation indicated that all four types of gamete (RY, Ry, rY, and ry) were produced with equal likelihood.One important feature of meiosis that Mendel failed to elucidate was crossover between homologous chromosomes, which results from homologous recombination (see Lecture 20). Crossover results is the formation of recombinant chromosomes that contain a mixture of maternal and paternal alleles.1) What experimental approaches have scientists used to identify genes?2) How do they determine a gene’s function?3) Once a gene has been identified, how do we study its structure and function at a molecular level?In the coming lectures, we will look at genes from a somewhat different perspective.Mendel and other early geneticists identified genes by looking for naturally occurring variations in phenotype. Hence, they could only study alleles that arose spontaneously in Nature.1) What experimental approaches have scientists used to identify genes?2) How do they determine a gene’s function?3) Once a gene has been identified, how do we study its structure and function at a molecular level?In the coming lectures, we will look at genes from a somewhat different perspective.Mendel described genes based on the traits they produced, which gave him some limited insight into gene function.In 1926, Hermann Muller discovered that X-rays damage DNA and produce a high rate of genetic mutation. These mutations are effectively random, i.e. they can occur anywhere on the chromosome.Soon thereafter, other scientists found that certain chemical compounds also result in high rates of mutation.These discoveries led to a new way of studying genes that came to dominate 20th century genetics. Today, this approach is called forward genetics.1) A scientist uses radiation or chemical mutagens to induce a high level of mutation in an experimental organism. This procedure is known as mutagenesis.2) Organisms carrying the induced mutations are screened, i.e. systematically examined for abnormal phenotypes that might result from the mutations.