126 ❯ STEP 4. Review the Knowledge You Need to Score HighBecause gene expression in eukaryotes involves more steps, there are more places where
gene control can occur. Here are a few examples of eukaryotic gene expression control:
Transcription:controlled by the presence or absence of particular transcription factors,
which bind to the DNA and affect the rate of transcription.
Translation:controlled by factors that tend to prevent protein synthesis from starting. This
can occur if proteins bind to mRNA and prevent the ribosomes from attaching, or if the
initiation factors vital to protein synthesis are inactivated.
DNA methylation:addition of CH 3 groups to the bases of DNA. Methylation renders DNA
inactive.Barr bodies,discussed in Chapter 10, are highly methylated.
These are only a few of the examples of gene expression control that occur in eukaryotes.
Do not get lost in the specifics.The Genetics of Viruses
Avirusis a parasitic infectious agent that is unable to survive outside of a host organism.
Viruses do not contain enzymes for metabolism, and they do not contain ribosomes for
protein synthesis. They are completely dependent on their host. Once a virus infects a cell,
it takes over the cell’s machinery and uses it to produce whatever it needs to survive and
reproduce. How a virus acts after it enters a cell depends on what type of virus it is.
Classification of viruses is based on many factors:Genetic material:DNA, RNA, protein, etc.?
Capsid:type of capsid?
Viral envelope:present or absent?
Host range:what type of cells does it affect?All viruses have a genome (DNA or RNA) and a protein coat (capsid). A capsidis a pro-
tein shell that surrounds the genetic material. Some viruses are surrounded by a structure
called a viral envelope,which not only protects the virus but also helps the virus attach to
the cells that it prefers to infect. The viral envelope is produced in the endoplasmic reticu-
lum (ER) of the infected cell and contains some elements from the host cell and some from
the virus. Each virus has a host range,which is the range of cells that the virus is able to
infect. For example, the HIV virus infects the T cells of our body, and bacteriophages infect
only bacteria.
A special type of virus that merits discussion is one called a retrovirus. This is an RNA
virus that carries an enzyme called reverse transcriptase.Once in the cytoplasm of the cell,
the RNA virus uses this enzyme and “reverse transcribes” its genetic information from RNA
into DNA, which then enters the nucleus of the cell. In the nucleus, the newly transcribed
DNA incorporates into the host DNA and is transcribed into RNA when the host cell
undergoes normal transcription. The mRNA produced from this process gives rise to new
retrovirus offspring, which can then leave the cell in a lytic pathway. A well-known exam-
ple of a retrovirus is the HIV virus of AIDS.
Once inside the cell, a DNA virus can take one of two pathways—a lytic or a lysogenic
pathway. In a lytic cycle,the cell actually produces many viral offspring, which are released
from the cell—killing the host cell in the process. In a lysogenic cycle,the virus falls dor-
mant and incorporates its DNA into the host DNA as an entity called a provirus.The viralBIG IDEA 3.A.1
DNA (and some-
times RNA) is the
primary source
of heritable
information.