1、Bacterial genetics,Chapter 8,9.1 Mutations and Mutants 9.2 Genetic Recombination 9.3 Genetic Transformation 9.4 Transduction 9.5 Conjugation 9.6 Plasmids 9.7 Transposons and Insertion Sequences 9.8 Comparative Prokaryotic Genomics 9.9 Genetics in Eukaryotic Microorganisms,Microorganisms provide rela
2、tively simple systems for studying genetic phenomena and are thus useful tools in attempts to decipher the mechanisms underlying the genetics of all organisms. Microorganisms are used for the isolation and duplication of specific genes from other organisms, a technique called molecular cloning. In m
3、olecular cloning, genes are manipulated and placed in a microorganism where they can be induced to increase in number.,Section 1. Mutation and recombination,Mutation is an inherited change in the base sequence of the nucleic acid comprising the genome of an organism. Mutation usually brings about on
4、ly a very small amount of genetic change in a cell.,Genetic recombination is the process by which genetic elements contained in two separate genomes are brought together in one unit. This mechanism may enable the organism to carry out some new function and result in adaptation to changing environmen
5、ts. Genetic recombination usually involves much larger changes. Entire genes, sets of genes, or even whole chromosomes, are transferred between organisms.,Section 2. Techniques of bacterial genetics: in vivo,In vivo : manipulate the genetic material within the organism,Genetic Transformation Transdu
6、ction Conjugation,Section 3. Techniques of bacterial genetics: in vitro,In vitro: operate genetic material in the test tube,Restriction Enzymes Molecular Cloning Amplifying DNA: PCR,Auxotroph an organism that has developed a nutritional requirement through mutation Cloning vector genetic element int
7、o which genes can be recombined and replicated Conjugation transfer of genes from one prokaryotic cell to another by a mechanism involving cell-to-cell contact and a plasmid Diploid a eukaryotic cell or organism containing two sets of chromosomes Electroporation the use of an electric pulse to induc
8、e cells to take up free DNA Gene disruption use of genetic techniques to inactivate a gene by inserting within it a DNA fragment containing an easily selectable marker. The inserted fragment is called a cassette, and the process of insertion, cassette mutagenesis,WORKING GLOSSARY,Genetic map the arr
9、angement of genes on a chromosome Genome the total complement of genes of a cell or a virus Genotype the precise genetic makeup of an organism Hybridization formation of a duplex nucleic acid molecule with strands derived from different sources by complementary base pairing Molecular cloning isolati
10、on and incorporation of a fragment of DNA into a vector where it can be replicated Haploid a cell or organism that has only one set of chromosomes Mutagens agents that cause mutation Mutant an organism whose genome carries a mutation Mutation an inheritable change in the base sequence of the genome
11、of an organism,Nucleic acid probe a strand of nucleic acid that can be labeled and used to hybridize to a complementary molecule from mixture of other nucleic acids Phenotype the observable characteristics of an organism Plasmid an extrachromosomal genetic element that has no extracellular form Poin
12、t mutation a mutation that involves one or only a very few base pairs Polymerase chain reaction (PCR) a method used to amplify a specific DNA sequence in vitro by repeated cycles of synthesis using specific primers and DNA polymerase Recombination the process by which parts or all of the DNA molecul
13、es from two separate sources are exchanged or brought together into a single unit.,Restriction enzyme an enzyme that recognizes and makes double-stranded breaks at specific DNA sequences Shotgun cloning making a gene library by closing random DNA fragments Site-directed mutagenesis a technique where
14、by a gene with a specific mutation can be constructed in vitro Synthetic DNA a DNA molecule made by a chemical process in a laboratory Transduction transfer of host genes from one cell to another by a virus Transformation transfer of bacterial genes involving free DNA Transposon a type of transposab
15、le element that carries genes in addition to those involved in transposition,Genetic Recombination,Genetic recombination involves the physical exchange of genetic material between genetic elements.,Homologous recombination results in genetic exchange between homologous DNA sequences from two differe
16、nt sources. This type of recombination is extremely important to all organisms. However, it is also very complex. Even in the bacterium Escherichia coli there are at least 25 genes involved.,A simplified version of one molecular mechanism of recombination. Homologous DNA molecules pair and exchange
17、DNA segments. The mechanism involves breakage and reunion of paired segments. Two of the proteins involved, a single-stranded binding (SSB) protein and the RecA protein. Note that there are two possible outcomes, depending on which strands are cut during the resolution process. In one outcome the re
18、combinant molecules have patches, whereas in the other the two parental molecules appear to have been cut and then spliced together.,In order to detect physical exchange of DNA segments, the cells resulting from recombination must be phenotypically different from the parents.,Detection of Recombinat
19、ion,Strains that lack some selectable characteristic that the recombinants will possess. For instance, the recipient may not be able to grow on a particular medium, and genetic recombinants are selected that can.,Various kinds of selectable and nonselectable markers (such as drug resistance, nutriti
20、onal requirements, and so on) may be used.,Complementation,This can be determined by a type of experiment called a complementation test. Complementation was first used in diploid eukaryotic organisms,In prokaryotes genetic recombination is observed because fragments of homologous DNA from a donor ch
21、romosome are transferred to a recipient cell by one of three processes: transformation, which involves donor DNA free in the environment transduction, in which the donor DNA transfer is mediated by a virus conjugation, in which the transfer involves cell-to-cell contact,Genetic Transformation,(1) Tr
22、ansformation, which involves donor DNA free in the environment,(2) Transduction, in which the donor DNA transfer is mediated by a virus,(3) Conjugation, in which the transfer involves cell-to-cell contact and a conjugative plasmid in the donor cell,Three main processes of genetic recombination in pr
23、okaryotes fragments of homologous DNA from a donor chromosome are transferred to a recipient cell,Conjugation,Transformation,Transduction,Competence A cell that is able to take up a molecule of DNA and be transformed is said to be competent.,Transformation,Binding of free DNA by a membrane-bound DNA
24、 binding protein. (b) Passage of one of the two strands into the cell while nuclease activity degrades the other strand. (c) The single strand in the cell is bound by specific proteins, and recombination with homologous regions of the bacterial chromosome mediated by RecA protein occurs.,The introdu
25、ction of DNA into cells by mixing the DNA and the cell,Transformed cell,The mechanism of bacterial transformation,Transduction involves transfer of host genes from one bacterium to another by viruses. In generalized transduction, defective virus particles randomly incorporate fragments of the cells
26、chromosomal DNA; virtually any gene of the donor can be transferred, but the efficiency is low. In specialized transduction, the DNA of a temperate virus excises incorrectly and brings adjacent host genes along with it; only genes close to the integration point of the virus are transferred, but the
27、efficiency may be high.,Concept,Transduction,In transduction, DNA is transferred from cell to cell through the agency of viruses. Genetic transfer of host genes by viruses can occur in two ways.,Generalized transduction And Specialized transduction,Specialized transduction: occurs only in some tempe
28、rate viruses; DNA from a specific region of the host chromosome is integrated directly into the virus genome - usually replacing some of the virus genes.,Generalized transduction: host DNA derived from virtually any portion of the host genome becomes a part of the DNA of the mature virus particle in
29、 place of the virus genome.,Generalized transduction,In generalized transduction, virtually any genetic marker can be transferred from donor to recipient,During a lytic infection, the enzymes responsible for packaging viral DNA into the bacteriophage sometimes accidentally package host DNA. This DNA
30、 cannot replicate, it can undergo genetic recombination with the DNA of the new host.,the DNA of lambda is inserted into the host DNA at the site adjacent to the galactose genes,On induction, Under rare conditions, the phage genome is excised incorrectly,A portion of host DNA is exchanged for phage
31、DNA, called lambda dgal ( dgal means “defective galactose“ ),Phage synthesis is completed,Cell lyses and releases defective phage capable of transducing galactose genes,Specialized Transduction,Genetic recombination involves the physical exchange of genetic material between genetic elements. It resu
32、lts in genetic exchange between homologous DNA sequences from two different sources.,Homologous recombination is extremely important to all organisms. However, it is also very complex .,Genetic Recombination,Direct contact between two conjugating bacteria is first made via a pilus. The cells are the
33、n drawn together for the actual transfer of DNA. Note the F-specific bacteriophages on the pilus,Bacterial conjugation (mating) is a process of genetic transfer that involves cell-to-cell contact.,conjugation,Conjugation involves a donor cell, which contains a particular type of conjugative plasmid,
34、 and a recipient cell, which does not.,The genes that control conjugation are contained in the tra region of the plasmid (see Section 9.8 in your text ). Many genes in the tra region have to do with the synthesis of a surface structure, the sex pilus . Only donor cells have these pili,The pili make
35、specific contact with a receptor on the recipient and then retract, pulling the two cells together. The contacts between the donor and recipient cells then become stabilized, probably from fusion of the outer membranes, and the DNA is then transferred from one cell to another.,Mechanism of DNA Trans
36、fer During Conjugation,A mechanism of DNA synthesis in certain bacteriophages, called rolling circle replication, was presented here to explains DNA transfer during conjugation .,if the DNA of the donor is labeled, some labeled DNA is transferred to the recipient but only a single labeled strand is
37、transferred. Therefore, at the end of the process, both donor and recipient possess completely formed plasmids.,DNA Transfer in Bacteria,transformation,transduction,conjugation,(1) Transformation, which involves donor DNA free in the environment,(2) Transduction, in which the donor DNA transfer is m
38、ediated by a virus,(3) Conjugation, in which the transfer involves cell-to-cell contact and a conjugative plasmid in the donor cell,Three main processes of genetic recombination in prokaryotes fragments of homologous DNA from a donor chromosome are transferred to a recipient cell,Conjugation,Transfo
39、rmation,Transduction,Transformation,A number of prokaryotes have been found to be naturally transformable, including certain species of both gram-negative and gram-positive Bacteria and some species of Archaea. However, even within transformable genera, only certain strains or species are transforma
40、ble,Competence A cell that is able to take up a molecule of DNA and be transformed is said to be competent. Competence in most naturally transformable bacteria is regulated, and special proteins play a role in the uptake and processing of DNA. These competence-specific proteins may include a membran
41、e-associated DNA binding protein, a cell wall autolysin, and various nucleases.,Competent cells bind much more DNA than do noncompetent cells as much as 1000 times more,Binding of free DNA by a membrane-bound DNA binding protein. (b) Passage of one of the two strands into the cell while nuclease act
42、ivity degrades the other strand. (c) The single strand in the cell is bound by specific proteins, and recombination with homologous regions of the bacterial chromosome mediated by RecA protein occurs.,The introduction of DNA into cells by mixing the DNA and the cell,Transformed cell,The mechanism of
43、 bacterial transformation,Artificially Induced Competence High efficiency natural transformation is found only in a few bacteria; Azotobacter, Bacillus, Streptococcus, for example, are easily transformed. Many prokaryotes are transformed only poorly or not at all under natural conditions. Determinat
44、ion of how to induce competence in such bacteria may involve considerable empirical study, with variation in culture medium, temperature, and other factors,when E. coli is treated with high concentrations of calcium ions and then stored in the cold, the transformation by plasmid DNA is relatively ef
45、ficient.,DNA Transfer by Electroporation for artificial induction of competence are being supplanted by a new method termed electroporation. Small pores are produced in the membranes of cells exposed to pulsed electric fields. When DNA molecules are present outside the cells during the electric puls
46、e, they can then enter the cells through these pores. This process is called electroporation.,Transfection Bacteria can be transformed with DNA extracted from a bacterial virus rather than from another bacterium, a process known as transfection.,In transduction, DNA is transferred from cell to cell
47、through the agency of viruses. Genetic transfer of host genes by viruses can occur in two ways.,Generalized transduction And Specialized transduction,Transduction,Transduction involves transfer of host genes from one bacterium to another by viruses. In generalized transduction, defective virus parti
48、cles randomly incorporate fragments of the cells chromosomal DNA; virtually any gene of the donor can be transferred, but the efficiency is low. In specialized transduction, the DNA of a temperate virus excises incorrectly and brings adjacent host genes along with it; only genes close to the integra
49、tion point of the virus are transferred, but the efficiency may be high.,Concept:,Specialized transduction: occurs only in some temperate viruses; DNA from a specific region of the host chromosome is integrated directly into the virus genome - usually replacing some of the virus genes.,Generalized t
50、ransduction: host DNA derived from virtually any portion of the host genome becomes a part of the DNA of the mature virus particle in place of the virus genome.,Transduction has been found to occur in a variety of prokaryotes, including certain species of the Bacteria: Desulfovibrio, Escherichia, Pseudomonas, Rhodococcus, Rhodobacter, Salmonella, Staphylococcus, and Xanthobacter, as well as the archaean Methanobacterium thermoautotrophicum.,
