Molecular Genetics ❮ 127DNA is quietly reproduced by the cell every time the cell reproduces itself, and this allows
the virus to stay alive from generation to generation without killing the host cell. Viruses
in the lysogenic cycle can sometimes separate out from the host DNA and enter the lytic
cycle (like a bear awaking from hibernation).
Viruses come in many shapes and sizes. Although many viruses are large, viroidsare plant
viruses that are only a few hundred nucleotides in length, showing that size is not the only
factor in viral success. Another type of infectious agent you should be familiar with is a prion—
an incorrectly folded form of a brain cell protein that works its magic by converting other
normal host proteins into misshapen proteins. An example of a prion disease that has been get-
ting plenty of press coverage is “mad cow” disease. Prion diseases are degenerative diseases that
tend to cause brain dysfunction—dementia, muscular control problems, and loss of balance.The Genetics of Bacteria
Bacteria are prokaryotic cells that consist of one double-stranded circular DNA molecule.
Present in the cells of many bacteria are extra circles of DNA called plasmids,which con-
tain just a few genes and have been useful in genetic engineering. Plasmids replicate inde-
pendently of the main chromosome. Bacterial cells reproduce in an asexual fashion,
undergoingbinary fission.Quite simply, the cell replicates its DNA and then physically
pinches in half, producing a daughter cell that is identical to the parent cell. From this
description of binary fission, it seems unlikely that there could be variation among bacter-
ial cells. This is not the case, thanks to mutation and genetic recombination. As in humans,
DNA mutation in bacteria occurs very rarely, but some bacteria replicate so quickly that
these mutations can have a pronounced effect on their variability.Transformation
An experiment performed by Griffith in 1928 provides a fantastic example of transformation—
the uptake of foreign DNA from the surrounding environment. Transformation occurs
through the use of proteins on the surface of cells that snag pieces of DNA from around the
cell that are from closely related species. This particular experiment involved a bacteria
known as Streptococcus pneumoniae,which existed as either a rough strain (R), which is non-
virulent, or as a smooth strain (S), which is virulent. A virulent strain is one that can lead
to contraction of an illness. The experimenters exposed mice to different forms of the bac-
teria. Mice given live S bacteria died. Mice given live R bacteria survived. Mice given heat-
killed S bacteria survived. Mice given heat-killed S bacteria combined with live R bacteria
died. This was the kicker... all the other results to this point were expected. Those exposed
to heat-killed S combined with live R bacteria contracted the disease because the live R bac-
teria underwent transformation. Some of the R bacteria picked up the portion of the heat-
killed S bacteria’s DNA, which contained the instructions on how to make the vital
component necessary for successful disease transmission. These R bacteria became virulent.Transduction
To understand transduction, you first need to be introduced to something called a phage
(Figure 11.8)—a virus that infects bacteria. The mechanism by which a phage (otherwise
known as bacteriophage) infects a cell reminds me of a syringe. A phage contains within its
capsid the DNA that it is attempting to deliver. A phage latches onto the surface of a cell
and like a syringe, fires its DNA through the membrane and into the cell. Transductionis
the movement of genes from one cell to another by phages. The two main forms of trans-
duction you should be familiar with are generalized and specialized transduction.BIG IDEA 3.C.1
Bacteria have mul-
tiple ways to
increase genetic
variation.
BIG IDEA 3.C.3
Viral infection can
introduce genetic
variation into the
host.
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