Viruses & Bacteria
Part I: Viruses.
Ed.: 328-331; 333-335 (Know Fig. 18.6).
Ed.: 334-337; 339-341 (Know Fig.
Ed.: 381-384; 387-388 (Know Fig. 19.7).
Part II: Exchange of genes to increase genetic diversity in bacteria. 6
Ed.: pp. 561-564
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edition pages on bacterial genetics”.
Viruses are infectious particles with a simple structure.
They consist only of a nucleic acid genome,
a small group of proteins, and sometimes a membrane envelope.
(The word genome means the
nucleic acid that stores genetic information for an organism; for us, the total of all our DNA).
of the proteins are arranged into a capsid that surrounds and protects the genome.
proteins that make up the capsid are called capsomeres.
Viruses can’t live independently; they must
infect a host to reproduce.
Viruses don’t have the energy sources, protein machinery, or raw
materials needed to reproduce their genomes, direct synthesis of their proteins, or actually make the
Instead, they rely completely on their host for these functions.
Viruses turn infected host
cells into factories that are dedicated to producing more virus particles, or virions.
infect animals, plants, and bacteria.
We already saw an example of a bacteriophage (phage), or a
virus that infects bacteria, in the Hershey-Chase experiment.
They used the T2 phage to show that
DNA is the genetic material.
We saw that only the DNA genome was injected into the host cell: all
the enzymes and macromolecule building blocks needed to make more virus particles, using the
viral genome as a template, were provided by the bacterial cell.
Viruses that infect animals and plants can he genomes can vary in several ways.
genomes can be either DNA or RNA, and can be single or double stranded (see examples in Table
) or 19.1 (8
), though you don’t have to know the specific viruses in the table.) Viruses
can have a variety of shapes and structures.
For instance, tobacco mosaic virus is rod shaped, with a
coiled RNA genome surrounded by a capsid.
Adenovirus, an animal virus that can cause colds and
respiratory disease in people, also has a genome surrounded by a capsid.
In this case, however, the
genome is made of DNA, and the capsid surrounding it forms an icosahedron, with 20 faces.
Glycoproteins, which will bind to the target cell, protrude from the capsid.
We’ll examine the life cycle of influenza virus in detail, as an example.