Additional Genetic Variability u Mutations are subject to selective pressure u Each mutant form that survives becomes an allele, an alternate form of a gene. u Such genes can be passed along by various mechanism to create genetic variability u Recombination is the process in which one or more nucleic acids are rearranged or combined to produce a new nucleotide sequence (recombinants) Slide No. 1 Slide No. 2 u Sexual reproduction leads to recombination v Vertical gene transfer = transfer of genes from parents to progeny v In eukaryotes, sexual reproduction is accompanied by genetic recombination due to Ø Crossing over between sister chromatids during meiosis Ø Fusion of gametes Slide No. 3 u Another method of recombination is Horizontal Gene Transfer (HGT) v Occurs in Bacteria and Archaea v HGT differs from vertical gene transfer because the transfer of genes is from one independent, mature organism to another v Important in evolution of many species Ø Expansion of ecological niche, increased virulence Ø Occurs in the three mechanisms evolved by bacteria to create recombinants Ø Genes can be transferred to the same or different Slide No. 4 Slide No. 5 u Yet another method of recombination involves transposable elements u Segments of DNA move about the genome in a process called transposition v Can be integrated into different sites in the v Are sometimes called jumping genes Ø Simplest transposable elements = insertion sequences Ø Transposable elements which contain extra genes are called composite transposons Slide No. 6 1
Fig. 16.12 Slide No. 7 Slide No. 8 Bacterial Conjugation u Conjugative p lasmids (F plasmid) can transfer copies of themselves to o ther bacteria d uring conjugation u F factors contain the information for formation o f sex p ilus u The sex pilus permits the a ttachment o f F+ cells to F- cells for DNA transfer d uring bacterial conjugation Slide No. 9 Slide No. 10 u F factors have u J. Lederberg a nd E. Tatum insertion sequences (IS) that assists in plasmid integration demonstrated the transfer of g enes b etween b acteria that d epends o n v Direct c ell t o c ell c ontact mediated by t he F pilus v Unidirectional DNA transfer from donor to recipient Fig. 16.18 Fig. 16.17 Slide No. 11 Slide No. 12 2
Fig. 16.21 - u F+ x F Mating v A c opy of t he F f actor is t ransferred t o t he recipient and does not integrate into t he host v Donor genes usually not t ransferred v Plasmid is replicated by rolling c ircle method Fig. 16.19 Slide No. 13 Portions Copyright The McGraw- Hill Companies, Inc. and Copyright C. R. Cooper, Jr. Slide No. 14 u Hfr Conjugation v Donor HFr cell has F factor integrated into its v Donor genes are t ransferred t o F- recipient c ell v A c omplete c opy of t he F f actor is usually not transferred v Gene t ransfer c an be c lockwise or counterclockwise Slide No. 15 Slide No. 16 Fig. 16.22 Slide No. 17 Slide No. 18 3
Bacterial Conjugation (cont.) u F Conjugation v Results when the F factor incorrectly leaves the host v Some of the F factor is left behind in the host v Some host genes have been removed along with some of the F factor v These genes can be transferred to a second host cell by conjugation Fig. 16.23 Slide No. 19 Slide No. 20 Bacterial Transformation u Uptake of naked DNA by a competent cell followed by incorporation of the DNA into the recipient cell s genome Fig. 16.24 Slide No. 21 Slide No. 22 Transduction u The transfer of bacterial genes by viruses u Viruses (bacteriophages) can carry out the lytic cycle (host cell is destroyed) or viral DNA integrates into the host genome (becoming a latent prophage) u Two types: v Generalized v Specialized Fig. 16.27 Slide No. 23 Slide No. 24 4
Transduction (cont.) Transduction (cont.) u Generalized transduction v Any part of bacterial genome c an be t ransferred v Occurs during lytic c ycle of virulent phage v During viral assembly, f ragments of host DNA mistakenly packaged into phage head u Specialized transduction v Carried out by t emperate phages t hat have established lysogeny v A s pecific portion of bacterial genome is transferred v Occurs when prophage is incorrectly excised Slide No. 26 Slide No. 28 Generalized transduction Slide No. 25 Specialized transduction Fig. 16.28 Fig. 16.29 Slide No. 27 Slide No. 29 5