Download Chapter 8 The Genetics of Bacteria and Their Viruses

José A. Cardé Serrano, PhD
Universidad Aventista de las Antillas
Biol 223
Agosto 2010
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Viruses and Bacteria in Genetics
The Genetics of Viruses
The Genetics of Bacteria
Mechanisms of Genetic Exchange in Bacteria
Bacteria and viruses have made important contributions to the science of
genetics.
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Small size
Rapid reproduction
Selective media (e.g., antibiotics)
Simple structures and physiology
Genetic variability
Complete genome sequences
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Their small size, short generation time, and
simple structures have made bacteria and
viruses valuable model systems for genetic
studies.
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Many basic concepts of genetics were first
deduced from studies of bacteria and viruses.
Viruses can only reproduce by infecting living host cells.
Bacteriophages are viruses that infect bacteria.
Several important genetic concepts have been discovered
through studies of bacteriophages.
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Partícula
 Ácido nucleico
 Capa de proteínas
▪ Cápsula
▪ capsómeros
 Parásito celular obligado
 Pequeño
 Infeccioso
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Dimitri Ivanowsky
 Científico ruso
 Virus del Mosaico del
Tabaco
 1892
 agente causante de la
enfermedad no era
retenido por un filtro de
porcelana
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Martinus Beijerinck
 1898
 Científico holandés
 Resultados similares a
Ivanowsky
 Aportación
Conceptualización
▪ Agente causante ES más
pequeño que una
bacteria.
▪ Acuño el término Virus
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Parásito intracelular obligado
 Infeccioso
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Su genoma puede ser DNA o RNA
Capaz de replicarse y dirigir la síntesis de
proteínas utilizando la maquinaria del
hospedero cuando se encuentra en el
hospedero correcto.
La progenie de los virus se
ensambla “de novo” a partir de
componentes sintetizados en la
célula hospedera.
 Medio de transmisión
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 El virus recién producido en la célula
hospedera.
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Composición estructural simple
 Cristalografía
 Virus del Mosaico del Tabaco -1935
Double-stranded DNA
genome
 Protein head
 Genome contains
168,800 base pairs and
150 characterized genes
 Lytic phage
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Double-stranded DNA genome
Genome contains, 48,502 base
pairs and about 50 genes
 May be lytic or lysogenic
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 Phage
 Phage -DNA
integration
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Genes may be mapped based on
recombination frequencies.
Host bacteria are infected with two types of
phage; progeny phage are screened for
recombination.
Map distances are calculated as the average
number of crossovers between genetic
markers.
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Temperature-sensitive (ts) mutations
Plaque morphology (rapid lysis; lysis inhibition)
Host range
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Viruses are obligate parasites that can reproduce
only by infecting living host cells.
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Bacteriophages are viruses that infect bacteria.
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Bacteriophage T4 is a lytic phage that infects E. coli,
reproduces, and lyses the host cell.
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Bacteriophage lambda () can enter a lytic pathway,
like T4, or it can enter a lysogenic pathway, during
which its chromosome is inserted into the
chromosome of the bacterium.
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In its integrated state, the  chromosome is called a
prophage, and it’s lytic genes are kept turned off.
Bacteria contain genes that mutate to produce altered
phenotypes. Gene transfer in bacteria is unidirectional—
from donor cells to recipient cells.
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One main chromosome with a few thousand
genes.
Variable number of plasmids and episomes.
Asexual reproduction by simple fission.
Parasexual processes occur.
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Colony color and morphology
Nutritional mutants for energy sources
Prototrophs and auxotrophs
Drug and antibiotic resistance
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Bacteria usually contain one main chromosome.
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Wild-type bacteria are prototrophs; they can
synthesize everything they need to grow and
reproduce given an energy source and some
inorganic molecules.
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Auxotrophic mutant bacteria require additional
metabolites for growth.
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Gene transfer in bacteria is unidirectional; genes
from a donor cell are transferred to a recipient cell,
with no transfer from recipient to donor.
Bacteria exchange genetic material through three different parasexual
processes.
Pregunta: Ocurrirá intercambio
de material genético entre
bacterias?
Hipótesis: Si ocurre intercambio.
Diseño Experimental: El Tubo U
Racional: Si incubo bacterias
con fenotipos distintos y el
intercambio ocurre, debo
obtener bacterias recombinantes
Resultados: ?
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In transduction, a bacteriophage transfers DNA
from a donor cell to a recipient cell.
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In generalized transduction, a random fragment
of bacterial DNA is packaged in the page head in
place of the phage DNA.
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In specialized transduction, recombination
between the phage chromosome and the host
chromosome produces a phage chromosome
containing a piece of bacterial DNA.
A plasmid is a genetic element that can replicate
independently of the main chromosome in an
extrachromosomal state.
 Most plasmids are not required for the survival of
the host cell.
 Plasmids in E. coli
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 F Factor (Fertility Factor)
 R Plasmids (Resistance Plasmids)
 Col Plasmids (synthesize compounds that kill sensitive
cells)
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An episome is a genetic element that is not
essential to the host and that can either
replicate autonomously or be integrated into
the bacterial chromosome.
Integration depends on the presence of IS
elements.
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Three parasexual processes—transformation,
conjugation, and transduction—occur in bacteria.
These processes can be distinguished by two
criteria: whether the gene transfer is inhibited by
deoxyribonuclease and whether it requires cell
contact.
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Transformation involves the uptake of free DNA by
bacteria.
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Conjugation occurs when a donor cell makes
contact with a recipient cell and then transfers DNA
to the recipient cell.
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Transduction occurs when a virus carries bacterial
genes from a donor cell to a recipient cell.
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Plasmids are self-replicating extrachromosomal genetic
elements.
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Episomes can replicate autonomously or as integrated
components of bacterial chromosomes.