SOLUTIONS TO EXERCISES, LESSON 7
The structure of a virus consists of a core, a capsid, and an envelope. The core
consists mainly of a nucleic acid, either DNA or RNA, never both together. The
capsid is a protective protein coat around the core. It is constructed of individual
subunits termed capsomers. The envelope is a lipid outer covering; it may or may
not be present depending on the virus.
Viruses are classified by the following properties: morphology, structure, and
cytopathic effects in cell cultures.
Acquired immunodeficiency syndrome (AIDS) is caused by the human
immunodeficiency virus (HIV). Epidemiological data and serological data suggest
that the viral infection began in Central Africa.
The HIV belongs to a unique class of viruses, distinguished by the presence of an
enzyme that catalyzes the formation of DNA from RNA. The catalyzing enzyme is
a reverse transcriptase, and viruses that contain it are called retroviruses. HIV is a
cytopathic retrovirus that appears to be genetically related to some of the other
The virus is approximately 110-140 nanometers in size and has an outer envelope
surrounding the core. The core contains RNA (the genetic information), reverse
transcriptase, and p24, which is a protein. The p24 protein is antigenic, and
antibodies against it are detected in the ELISA test and Western blot. The
envelope contains two important glycoproteins. Glycoprotein 41 (gp 41) spans the
membrane and is also antigenic. The other protein , gp 120, is the major outer
membrane glycoprotein of HIV. The envelope gene that codes for a portion of the
virus's outer membrane varies considerably from isolate to isolate. An effective
vaccine for AIDS would therefore need to protect against many different strains of
the virus. HIV contains a gene, called the TAT (trans-activator) gene, whose
product acts as a powerful promotor of viral DNA replication. This promotion of
viral replication at the expense of cellular replication may be an important
mechanism in virally induced cell death.