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Berkeley MCELLBI 140 - Lecture Notes

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1Add to reading:from p444: “Deletions remove material…”to p449 “Duplications add material…” but not pp445-447Benzer’s question:How is complementation between mutantsrelated to recombination between mutants? Benzer’s question:How are functional allelesrelated to segregational alleles? He needed AND HAD a selective genetic systemwith extremely high resolving power for small map distancesBenzer’s question:How does the genetic map between the cis-acting heritable functional unitscompare to genetic maps within the cis-acting heritable functional unitsSimple, cheap, fast, and POWERFULthe large-scale map generated byrecombination betweenmutants in different genes (as complementation groups)21,612 INDEPENDENT mutants mapped for Fig. 7.21 (and ultimately >3000)B -- Athe fine-structure map of rIIA & rIIB generated byrecombination betweenmutants in the same genes (as complementation groups)?(1) A linear series of discrete points, hence topologically no different than intergenic maps(2) Points not necessarily closer within a gene than between(3) Based on the physical size (DNA base pairs) of T4, and the total map distance around the T4 map adjacent points estimated to be ~2 bp apart -- hence recombination appears possible between adjacent bp it must be an incredibly precise process (to reconstruct wt) Remember:(1) this was at a time before the nature of the genetic code had been determined(2) A genetic map is an abstraction -- a representation of meiotic event frequency -- but the correspondence between the genetic map and DNA had to be meaningfulBenzer’s intragenic map:Benzer had..(1) defined true “point” mutations (segregational alleles)(2) developed deletion mapping(3) discovered the lack of any qualitative (topological) difference between the maps of genes vs. chromosomes(4) clarified the relationship between functional allelism (complementation, the cis/trans test) and segregational allelism (recombination)(5) discovered mutational hot spots(6) established that although genes were divisible (by recombination) they were not infinitely divisible, and the lower limit appeared to be the DNA base pair (7) Set up a genetic system (a map) that could be exploited to discover a huge amount about the genetic code and informational suppressors without any biochemistry…and all without having a clue what rIIA & rIIB did for T4!!!B -- AThe fine-structure map of rIIA & rIIBis a valuable tool for further research into the nature of genetic information.rIIB frameshifting point mutationsConsequences of deletingthe boundary (on the genetic map!)between rIIA and rIIB.informationalsuppressors(Fig. 8.32)(Fig. 8.5)3nonrevertables (deletions)revertables (point mutants)noyes yes yes yes yes yesASK: any wildtype recombinants generated from hybrid between 1011 revertable and particular nonrevertables?This is onlyone kind ofdeletion mapping(based on segregational allelism)1011Drosophila genetic mapDrosophila physical map (giant polytene chromosomes)Drosophila genetic mapDrosophila physical map (giant polytene chromosomes)(a portion of chromosome 3R)Deletion mapping based on complementationDeletion mapping based on complementationFig. 14.8w- rst- fa-Df(1)256-45)= w- (mutant) phenotype (failure to complement) hence white is within region deleted.phenotypescale: deletions noteasily visible unless >2-3 genes(can be up to ~250 genes)Bulge in the synapsedpolytene chromosomesshows what is deleted.The complementation test as an operational definitionof the gene is not quite as straightforward as it may sound.genes can have very complex complementation patternsbecause of all the various kinds of information they containthat work in cisp277 Heading: “rII regon has two genes”Is this statement compatible with the statement thatcomplementation groups are what we want to call genes?p291:A gene is not simply the DNA that is transcribed into the mRNAcodons specifying the amino acids of a particular polypeptide. Rather,a gene is all the DNA sequences needed (IN CIS) for expression of the geneinto a polypeptide product. A gene therefore includes the promotersequences that govern where transcription begins and, at the oppositeend, signals for the termination of transcription. A gene also includessequences dictating where translation starts and stops. In addition toall these features, eukaryotic genes contain introns that are spliced outof the primary transcript to make the mature mRNA. Because ofintrons, most eukaryotic genes are much larger than prokaryotic genes.So promoters are part of genes. Are they part of complementation groups?4Fig. 8.12A bacterial promoter(cis-acting information fortranscription start)51 bp transcript--->1,612 INDEPENDENT mutants mapped for Fig. 7.21 (and ultimately >3000)B -- Athe fine-structure map of rIIA & rIIB generated byrecombination betweenmutants in the same genes (as complementation groups)?There are 12 bp betweenrIIA & rIIB “genes”(T4 has 168,903 total bp)Figure 17.5: The Lactose Operon in E. colian example ofpolycistronic mRNArII-A rII-BrII makes a polycistronic mRNArII-A356671rII-B2658799723rIIA&B (promotor mutants?)rII “complementation map” of point mutants(a very different kind of “map”)three complementation groups?No, one complex complementation groupone “gene”Alternative pre-mRNA splicingis what allows DrosophilaDSCAM gene to make30,000 different proteinsFig. 8.18intronsexons6 vs. 7-8 alternativeexonsEukaryotic genesare even messier:…and the regulatory regions(non-protein-coding)can extend enormousdistances on both sides5(NATURE 184:1927-29, 1959)These arethe mutantsthat argue forone gene witha complexcomplementation pattern(14/31)“What are the genes? What is the nature ofthe elements of heredity that Mendelpostulated as purely theoretical units? …Frankly, these are questions with whichthe working geneticist has not muchconcern himself…T.H. MorganThe Relation of Geneticsto Physiology and MedicineNobel Lecture, June 4, 1934(true, but a bit of sour grapes)“What are the genes? What is the nature ofthe elements of heredity that Mendelpostulated as purely theoretical units? …Frankly, these are questions with whichthe working geneticist has not muchconcern himself…H.J.Muller, in 1932, publisheda classic review of how mutationschange gene function, withouteven knowing what genes wereat the chemical levelNobel Prize, 1934-- and he did this in


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Berkeley MCELLBI 140 - Lecture Notes

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