I have heard it was the opinion of others that it [the plague] might
be distinguished by the party's breathing upon a piece of glass, where,
the breath condensing, there might living creatures be seen by a microscope,
of strange, monstrous, and frightful shapes, such as dragons, snakes, serpents,
and devils, horrible to behold.
A Journal of the Plague Year, 1721
Campbell and Reece, Chapter 18
(Continues some material from the last lecture)
Second semester (Bio 106) starts with Diversity (the march through the kingdoms).
The difficulty (previous lecture) in defining a virus as alive makes it
virtually impossible for an introductory textbook to have a good place to
cover viruses, so here they are.
TRANSPARENCY (Fig. 18.1) Being small (Protein and DNA) they pass through
fine filters, hence an old term, "filterable"
Bacteriophage (phage-eat) protein and DNA
TRANSPARENCY Fig. 18.4 lytic cycle - bacterial cells lyse.
Viruses cause lots of disorders: measles, smallpox, chicken pox, mumps,
rabies, flu = influenza, herpes, AIDS, mononucleosis, polio, colds, rubella
(German measles), yellow fever, hepatitis
Some very fundamental terminology:
Antigen - non-self protein (virus coat)
Antibody to antigen made by B lymphocytes (white blood cells)
Vaccines - active immunity (like disease)
memory cells of immune system
Edward Jenner 1796 "encowment" cowpox, smallpox
Smallpox is so completely eliminated that one issue is whether to get rid
of lab virus.
Passive immunity - give antibodies
1918 20 million died worldwide
1957 Asian bad
1968 Hong Kong bad (70,000 died in 6 wks)
change (mutate) also exchange with birds (ducks)
TRANSPARENCY (Fig. 18.2) Structurally (and chemically) not all viruses are
as simple as bacteriophage, and sometimes RNA is the hereditary material
TRANSPARENCY (Fig, 18.3) Not always does cell burst. Here you see how the
virus takes over the cells machinery to make its own DNA and to make proteins
etc after transcription of its DNA into RNA
TRANSPARENCY (Fig. 18.7) The retrovirus, HIV (Human Immunodeficiency Virus
that leads to AIDS, acquired immunodeficiency syndrome) deserves special
attention. Its hereditary material is RNA. It is called a retrovirus because
it comes pre-packaged with functional reverse transcriptase enzyme to make
DNA out of RNA. The DNA that is made gets incorporated into the cell's own
That last phenomenon is one of the underlying themes of this chapter which
in many ways is a hodge-podge - How do cells get changed genetically?
MONERA bacteria, also blue-green algae (cyanobacteria)
(algae = aquatic plants)
TRANSPARENCY Fig. 7.4
Here is a picture
of E. coli (Escherichia coli), the most famous bacterium from
genetics and molecular studies - this picture was found at the microbe
prokaryote, (karyon as in "karyotype," refers to the nucleus)
genophore - bacterial chromosome
Reproduction by fission, "Multiply and Divide"
also DNA transferred: (1) transformation (last lecture example of DNA from
smooth transforming rough) , transduction (from phage TRANSPARENCY [Fig.
18.13]), conjugation (like mating, TRANSPARENCY [Fig. 18.15])
Plasmids - little circles of DNA - very useful in molecular biology and
easy to identify since they carry antibiotic resistance
(relates to fundamental topic of gene regulation) lac operon - genes for
enzymes for metabolism of lactose, the disaccharide in milk TRANSPARENCY
(Fig. 18.21 A & B). When lactose is present, allolactose pulls repressor
off of operator so that RNA polymerase can move from promoter to make mRNA
for genes (lacZ, lacY and lacA) that code for their respective enzymes (beta-galactosidase,
permease and transacetylase, enzymwes for lactose metabolism. The bacterium
"does not bother" making lactose metabolizing enzymes unless lactose
(the sugar in milk) is present. Note that one mRNA is for 3 proteins, never
the case in eukaryotes.
The 1965 Nobel Prize
in Physiology and Medicine was shared by FRANÇOIS JACOB and JACOUES
MONOD who established the operon model.
Two other stories:
DNA is very stable, and slow mutational changes over long time periods were
once thought to be the only agents of evolutionary change. Over the last
few decades, and related to the idea that retroviruses can orchestrate insertions
into DNA, was the idea of transposons TRANSPARENCY (Fig. 18.16). In the
lab (or in nature) genes (or hunks of DNA) can insert. For instance, in
fruit flies, genes can be inserted to make new transgenic Drosophila. If
the "jumping gene" inadvertently jumps into the middle of another
gene, it will ruin it.
The 1983 Nobel Prize
in Physiology and Medicine went to BARBARA MC CLINTOCK for her discovery
of mobile genetic elements.
Various spongiform encephalitis syndromes (where the brain degenerates)
were once thought to be caused by slow viruses, but now are thought to be
caused by protein without any hereditary material. Such disorders include
Creutzfeld-Jacob Syndrome in humans, scrapie in sheep and bovine "mad
cow disease." TRANSPARENCY (Fig. 18.10) shows how this might happen.
The 1997 Nobel Prize
in Physiology and Medicine went to STANLEY B. PRUSINER for his discovery
of Prions - a new biological principle of infection
For further study: Here is a site entitled "Communicable
Disease Surveillance and Response (CSR)" which is of relevance
to this and other chapters. And here is Disease
information from the Centers for Disease Control, also relevant to several
chapters, especially this one.
This page was last updated 7/23/02
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