Topic 15: VIRUSES & BACTERIA: some are pathogens but others have been vital tools
in molecular biology research (lecture 24).
OBJECTIVES:
1. Be able to compare and contrast lytic vs. lysogenic life cycles of bacteriophages.
2. Know how animal viruses are categorized and understand what are the unique
properties of retroviruses.
3. Know the life cycle of HIV.
4. Understand the difference between bacterial nucleoids and plasmids.
5. Know the importance of bacterial genetic recombination and the three ways it may
take place in prokaryotic cells.
Virus- RNA or DNA accompanied by protein which have the capacity to harness the
machinery of cells for replication; they are not living per se since they cannot replicate
themselves
Viruses- common features (fig. 18.2)- genome (consisting of DNA or RNA); capsid
(protein coat); viral envelope (membrane cover with glycoproteins).
Generalized life cycle of a virus (fig. 18.3)1. infection
2. host DNA polymerase is co-opted to produce more viral DNA
3. viral proteins are then transcribed & translated
4. proteins and DNA are assembled into new virus particles
5. virus particles leave cell; often killing it
Bacteriophages (phages for short)- these viruses target bacteria; they have two
different kinds of life cycles:
1. lytic cycle- characteristic of virulent viruses which always kill their host; fig. 18.4; life
cycle of the T4 phage which infects E. coli. Ultimate effect is to produce 100-200 phage
particles which are released when the bacterial cell lyses and dies.
2. lysogenic cycle- present in temperate viruses which have the capacity fot two types
of
cycles- lytic as above and lysogenic.
fig. 18.5- life cycle of the  phage; also infects E. coli.
- lysogenic cycle:  phage DNA becomes incorporated in host DNA forming a
prophage; bacterial reproduction results in the production of many
daughter cells containing the prophage
genes become expressed and new virus particles are synthesized.
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Cells then lyse.
Animal Viruses
There are a number of different types that can be differentiated on the basis of the
genetic material and how it is used (table 18.1)- double-stranded dsDNA, singlestranded ssDNA, double-stranded dsRNA, single-strand ssRNA ( three types- can serve
as mRNA, can serve as template for mRNA or can serve as template for DNA
synthesis).
Retro viruses- have single-stranded RNA (ssRNA) that serves as a template for DNA
synthesis; this is accomplished by an enzyme called reverse transcriptase. Many
retroviruses are pathogens including HIV (human immunodeficiency virus); fig. 18.7
shows HIV life cycle.
Viroids- small RNA molecules that are plant pathogens
Prions- could possibly be infectious proteins; mad cow disease in England may be
caused
by a protein which has a defective conformation; it may cause proteins with the
normal conformation to assume the abnormal conformation. The resulting
proteins are extremely stable, accumulate in brain cells and cause serious
pathologies.
Bacterial Genetics
Recall that bacteria do not have nuclei; instead the chromosomes are found in a region
called the nulceoid. Bacteria also have unique structures called plasmids which are
small circular pieces of DNA consisting of a much smaller number of genes.
Key issues relating to bacterial genetics:
1. Generation time- time for cell to replicate its DNA and divide into identical daughter
cells; this can be very fast on the order of minutes to several hours.
2. Genetic variation- within a population there may be some individual differences in
genetic make-up; that is, there may be alternative forms of certain genes called
alleles (allele = alternative gene forms which may code for slightly different
products). In a population showing genetic variation, not all individuals are
genetically alike due to the presence of allelic variation. Variation is important
because it allows for the selection of individuals resulting in changes in the
characteristics of a population.
3. Sources of genetic variation:
(a)
mutation- many mutations produce defective products; other are silent in that
they do not produce any appreciable change (so-called neutral mutations)
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(b)
genetic recombination- text definition (p. 341) is “combining of genetic
material from two individuals into the genome of a single individual”;
processes very useful in modern molecular biology techniques
transformation- direct uptake and incorporation of foreign DNA into a bacterial
cell.
transduction- phages may carry some bacterial genes; after infection these
genes may become incorporated into the genome of another bacterium
conjugation- exchange of genetic material in the form of plasmids between two
bacteria
Plasmids- circular pieces of DNA containing a small number of genes that are not a
part of the bacterial chromosome; they are self-replicating. The genes present in
plasmids are generally not required for survival of the bacterium. Often they contain
genes that code for characters which allow the bacterium to survive under certain
special conditions.
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