Reading Guide
Chapter 8: Bacterial Genetics
This chapter is a continuation of the genetics theme with a focus on the different
kinds of mutations that can occur, what can promote mutations, how to repair/fix
mutations, and how to select for mutants or use bacteria to study these mutations. We will
focus on sections 8.1-8.5 for now. We will finish the last sections, 8.6-8.9, after talking
about viruses.
So let’s begin with a look at some key terms and the different types of mutations
that can occur in bacterial cells. Bacterial cells are good models to use for genetic
research since they are haploid, or have one copy of every gene. One copy of every gene
makes it easy to study the phenotype, or observable characteristics. Genetic changes can
occur as a result of two main things, mutations and gene transfer. When a change in the
genotype occurs and results in a mutation, this change is transferred to other cells in
vertical gene transfer. If the change occurs as a result of gene transfer, this is called
horizontal gene transfer.
A few other key terms to review are wild-type and auxotroph. A wild-type cell
refers to an organism that was isolated in nature, where an auxotroph refers to a cell that
is a mutant. Often this mutant term is also connected to the cell lacking the ability to
grow without a particular nutrient available. For example, E. coli can normally grow fine
on a GSA plate generating all of the necessary growth factors from glucose. If this
organism (the wild-type) is mutated and the results are an organism that lacks the ability
to produce the amino acid histidine, then this is now considered to be an auxotroph
lacking the ability to produce histidine.
Mutations in bacterial cells are often the result of spontaneous events and it is
through these mutations that organisms can respond to changes in their environment and
natural selection occurs. Spontaneous mutations can occur as a result of errors with DNA
polymerase, for example if the enzyme does not properly proofread a miss-paired base.
They can also occur if cells are exposed to UV light which promotes the formation of
thymine dimers in the DNA.
If a mutation occurs which changes the sequence of DNA by just one base, this is
called a base substitution or point mutation. The results of this mutation can be a silent
mutation (no change in the amino acid code), missense mutation (a change in the code
which results in a new amino acid) and a nonsense mutation (a change in the code which
results in a stop codon). Some mutations are the result of the addition or deletion of one
or more bases, this shifts the reading frame (the order in which you read the codons). This
is known as a frameshift mutation.
There are many different things that can promote mutation in cells. In my lecture I
focus on three different types of mutagens or things that can cause mutations. They are
chemicals such as nitrous acid, uv light, and transposons also known as jumping genes.
Transposons were first discovered by Barbara McClintock as she studied the
variegated patterns in corn. She proposed that the changes in the corn were due to the
inactivation of genes for color of the actual kernels of corn. She was right! These changes
in color are due to mobile pieces of DNA that randomly insert into existing DNA. If they
happen to insert in to a coded region for a protein, the protein will be inactivated or no
longer be made. Today we recognize that there are transposons found to infect all types
of cells from plants to animals.
Chemicals can cause mutations and one that is well studied is nitrous acid. This
chemical alters the base adenine causing it to be mispaired with the base cytosine instead
of pairing with the base thymine. Some chemicals are considered to be base analogs since
the chemical structure of the compound is similar to the actual base. When the chemical
is inserted into the DNA strand instead of the regular base, they will promote the wrong
base pairing to occur. Intercalating agents are chemicals which insert into the DNA
structure and physically cause an addition of a base when the DNA is replicated.
Radiation can also cause mutations in cells. We have seen the effects of UV light
on bacterial cells in lab. The UV light promotes the formation of thymine dimers which
changes the shape of the structure of the DNA. This change in shape prevents DNA
replication as well as protein synthesis unless repaired. X rays cause serious damage to
DNA in the form of double strand breaks. These changes to the DNA strand cannot be
repaired.
Which brings me to the last topic for this reading guide…the topic of repair of
damaged DNA. Bacterial cells can repair wrong base pairing by either the proof reading
feature of DNA polymerase or by a process called mismatch repair. Mismatch repair
requires an endonuclease to cut the DNA strand around the area where the wrong base is
inserted. This leaves a gap in the strand of DNA which can be filled in by DNA
polymerase and finally the fragments can be bonded to each other with the help of the
enzyme DNA ligase. Note that these are the same key enzymes that were needed to
synthesize the lagging strand of DNA in DNA replication.
Repair of thymine dimers caused by uv damage can occur either in the presence
of light or in the dark. The light repair mechanism is much simpler as it requires one
enzyme (photolyase enzyme) that works in the light to break the thymine dimer bond.
Dark repair requires an enzyme that recognizes the thymine dimer structure and cut the
DNA strand. The resulting gap in the DNA is filled in by DNA polymerase, the pieces
joined by DNA ligase, and you are done!
SOS repair is the last mechanism to mention. In this type of repair the cell will
activate a DNA polymerase that is able to just “randomly insert” bases without reading a
template. This mechanism works as a last ditch effort to save the cell (hence the name
SOS) due to extensive uv damage when the other mechanisms mentioned above would
not work.
I’ll add more later for the last section in this chapter (8.5) on how to select for
mutants.