viral & prokaryotic genetics “simple” model systems
Post on 18-Dec-2015
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Experimental Model Systems for Genetics characteristics of good model
systemssmall genome size
E. coli: ~4 million base pairs (bp)
bacteriophage: ~45,000 bplarge population size
E. coli: ~one billion (109) per liter
bacteriophage: ~100 billion (1011) per liter
Experimental Model Systems for Genetics characteristics of good model
systemsshort generation time
E. coli:18-20 minutesO/N: 45 generations [1 => 1.76 x 1013]
bacteriophage: ~20 minutes
haploid genomegenotype => phenotype
Viruses small resistant to inactivation by
alcoholdehydration
infectivity may decrease; can’t increase reproduction: obligate intracellular parasitesuses host nucleotides, amino acids, enzymes
hostsanimals, plants, fungi, protists, prokaryotes
Viruses virus structure
virion = virus particlecentral core = genome: DNA or RNA
capsid = protein coat; determines shape
lipid/protein membrane on some animal viruses
Viruses virus classification
host kingdomgenome type (DNA or RNA)strandedness (single or double)
virion shapecapsid symmetrycapsid size+/- membrane
Viruses bacteriophage (“bacteria eater”)reproduction
lytic cycle: virulent phagesinfection, growth, lysis
lysogenic cycle: temperate phagesinfection, incorporation, maintenance
Viruses•expression of bacteriophage genes during lytic infection–early genes - immediate–middle genes•depends on early genes•replicates viral DNA
–late genes•packages DNA•prepares for lysis
Prokaryotes
•bacteria reproduce by binary fission–reproduction produces clones of identical cells
–research requires growth of pure cultures
•auxotrophic bacteria with different requirements can undergo recombination
bacteria exhibit genetic recombinationFigure 13.7
minimal
minimal
minimal
complete
minimal + Met, Biotin, Thr, Leu
minimal + Met, Biotin
minimal + Thr, Leu
Prokaryotes•recombination exchanges new DNA with existing DNA–three mechanisms can provide new DNA•transformation - takes up DNA from the environment•transduction - viral transfer from one cell to another•conjugation - genetically programmed transfer from donor cell to recipient cell
conjugation: programmed genetic exchange
programmed by the chromosome or by an F (fertility) plasmidFigure 13.11
Prokaryotes•Plasmids provide additional genes–small circular DNAs with their own ORIs
–most carry a few genes that aid their hosts•metabolic factors carry genes for unusual biochemical functions •F factors carry genes for conjugation•Resistance (R) factors carry genes that inactivate antibiotics and genes for their own transfer
Transposable Elements•mobile genetic elements–move from one location to another on a DNA molecule
–may move into a gene - inactivating it
–may move chromosome => plasmid => new cell => chromosome
–may transfer an antibiotic resistance gene from one cell to another
Regulation of Gene Expression
•transcriptional regulation of gene expression–saves energy•constitutive genes are always expressed•regulated genes are expressed only when they are needed
Regulation of Gene Expression
•transcriptional regulation of gene expression–the E. coli lac operon is inducible
Regulation of Gene Expression
•regulation of lac operon expression–the lac operon encodes catabolic enzymes•the substrate (lactose) comes and goes•the cell does not need a catabolic pathway if there is no substrate
–the lac operon is inducible•expressed only when lactose is present•allolactose is the inducer
a repressor protein blocks transcription
lac repressor blocks transcription
Figures 13.15, 13.17
promoter gene
Regulation of Gene Expression
•regulation of lac operon expression–lac repressor (lac I gene product) blocks transcription
–lac inducer inactivates lac repressor
Regulation of Gene Expression
•regulation of trp operon expression–the trp operon encodes anabolic enzymes•the product is normally needed•the cell needs an anabolic pathway except when the amount of product is adequate
–the trp operon is repressible•trp repressor is normally inactive•trp co-repressor activates trp repressor when the amount of tryptophan is adequate
positive and negative regulation
•both lac and trp operons are negatively regulated–each is regulated by a repressor
•lac operon is also positively regulated–after lac repressor is inactivated by the inducer, transcription must be stimulated by a positive regulator
induced lac operon alsorequires
activation before genesare transcribed
induced lac operon alsorequires
activation before genesare transcribed
Figure 13.19
positive and negative regulation
in bacteriophage•the “decision” between lysis & lysogeny depends on a competition between two repressors
in a healthy, well-nourishedculture
in a slow-growingnutrient-poorculture
lysis vs. lysogeny
Figure 13.20
new tools for discovery
•genome sequencing reveals previously unknown details about prokaryotic metabolism
•functional genomics identifies the genes without a known function
•comparative genomics reveals new information by finding similarities and differences among sequenced genomes