Chapter 8 The Genetics of Bacteria and Their Viruses

José A. Cardé Serrano, PhDUniversidad Aventista de las AntillasBiol 223Agosto 2010

Viruses and Bacteria in GeneticsThe Genetics of VirusesThe Genetics of BacteriaMechanisms of Genetic Exchange in

Bacteria

Bacteria and viruses have made important contributions to the science of genetics.

Small sizeRapid reproductionSelective media (e.g., antibiotics)Simple structures and physiologyGenetic variabilityComplete genome sequences

Their small size, short generation time, and simple structures have made bacteria and viruses valuable model systems for genetic studies.

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.

Partícula Ácido nucleico Capa de proteínas

▪ Cápsula▪ capsómeros

Parásito celular obligado

Pequeño Infeccioso

Dimitri Ivanowsky Científico ruso Virus del Mosaico

del Tabaco 1892 agente causante de

la enfermedad no era retenido por un filtro de porcelana

Martinus Beijerinck 1898 Científico holandés Resultados similares a

Ivanowsky Aportación

Conceptualización ▪ Agente causante ES

más pequeño que una bacteria.

▪ Acuño el término Virus

Parásito intracelular obligado Infeccioso

Su genoma puede ser DNA o RNACapaz 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 El virus recién producido en la

célula hospedera. 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

Double-stranded DNA genome Genome contains, 48,502

base pairs and about 50 genes May be lytic or lysogenic

Phage

Phage -DNA integration

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.

Temperature-sensitive (ts) mutationsPlaque morphology (rapid lysis; lysis

inhibition)Host range

Viruses are obligate parasites that can reproduce only by infecting living host cells.

Bacteriophages are viruses that infect bacteria.

Bacteriophage T4 is a lytic phage that infects E. coli, reproduces, and lyses the host cell.

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.

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.

One main chromosome with a few thousand genes.

Variable number of plasmids and episomes.

Asexual reproduction by simple fission.

Parasexual processes occur.

Colony color and morphologyNutritional mutants for energy

sourcesPrototrophs and auxotrophsDrug and antibiotic resistance

Bacteria usually contain one main chromosome.

Wild-type bacteria are prototrophs; they can synthesize everything they need to grow and reproduce given an energy source and some inorganic molecules.

Auxotrophic mutant bacteria require additional metabolites for growth.

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: ?

In transduction, a bacteriophage transfers DNA from a donor cell to a recipient cell.

In generalized transduction, a random fragment of bacterial DNA is packaged in the page head in place of the phage DNA.

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 F Factor (Fertility Factor) R Plasmids (Resistance Plasmids) Col Plasmids (synthesize compounds that kill

sensitive cells)

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.

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.

Transformation involves the uptake of free DNA by bacteria.

Conjugation occurs when a donor cell makes contact with a recipient cell and then transfers DNA to the recipient cell.

Transduction occurs when a virus carries bacterial genes from a donor cell to a recipient cell.

Plasmids are self-replicating extrachromosomal genetic elements.

Episomes can replicate autonomously or as integrated components of bacterial chromosomes.