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AIDS, acquired immunodeficiency syndrome - An incurable (at the time of this writing)
disease that results from infection by the human immunodeficiency (HIV) virus. AIDS
symptoms build up as the concentration of HIV viruses in the body increases, usually
over a period of years. Eventually, AIDS victims loose their abilities to fight infections.
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ATP (adenosine triphosphate) – A relatively stable, high energy molecule used to fuel
chemical reactions within cells. ATP is the ‘universal energy carrier’ molecule in nature,
performing energy exchange and energy transport functions in all living things. ATP is an
adenine-containing nucleoside triphosphate that releases free energy when one of its
phosphate bonds is hydrolyzed – a reaction that yields ADP, Adenosine diphosphate, as a
product. The ADP is recycled to ATP through the addition of an activated phosphate
group coupled to energy-producing reactions.
NH2
N
N
O–
–
O
P
O–
O
P
O–
O
P
O
N
CH2
N
O
O
O
O
OH
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bacteriophage - A virus that infects only bacteria. The term is usually shortened to
‘phage’.
A TEM of several bacteriophage viruses.
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capsid - The protein shell that encloses the viral genome. These are built from subunits
called capsomeres. The capsid conforms to the shape of the virus, and may be helical,
polyhedral or more complex in shape. The capsid is also called a ‘protein jacket’.
Capsid
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capsomere - The building block protein molecule found in the capsid covering of a virus.
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CD4 molecule - A molecule present on the surface of helper T-lymphocytes that is normally used for cell-cell interactions. The HIV virus uses this protein as a receptor, thus
gaining access to the T-lymphocyte through receptor mediated endocytosis.
HIV Virus
CD4 molecule
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DNA polymerase – The enzyme that catalyzes the addition of nucleotide bases to the 3’
end of replicating DNA strands. DNA polymerase follows the replication fork as it opens,
promoting the addition of new phospho-diester bonds in the DNA backbone chain (of the
leading strand), as the new bases line up. These reactions involve the expenditure of
energy. DNA polymerase also acts to check for replication errors, and repairs them as the
strands are replicating.
DNA polymerase also catalyses the replication of the lagging strand, but in this case,
the enzyme functions a short way down the strand from the replication fork, in order to
build in the 5’ to 3’ direction. This creates Okazaki fragments along the lagging strand.
DNA polymerase
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DNA virus - A virus with a genome made up of deoxyribonucleic acid, DNA. This genome
can be directly inserted into the host-cell genome where it directs the cell to replicate
copies of the virus. Viruses with a genome made from RNA are called RNA viruses.
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glycoprotein - A protein with a carbohydrate covalently bonded to it. Most secreted proteins and most proteins on the outer surface of the cell are glycoproteins.
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helper T-lymphocyte - One of the types of blood cells in the human immune system. Tlymphocytes function in cell-mediated immunity. Helper T-cells secrete protein factors
called cytokines which help to regulate B cells and T cells in the immune system.
Lymphocytes differentiate from stem cells in the bone marrow and mature in the thymus.
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HIV, human immunodeficiency virus - The virus that causes AIDS. HIV is thought to
have evolved relatively recently in Africa from a related virus. It’s considered the most
deadly virus known. HIV is spread only by direct contact with infected bodily fluids such
as blood or semen.
HIV particle
HIV entering host cell
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host range - The limited number of different types of host cells that a type of virus can
infect or parasitize.
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influenza - The condition caused by the common ‘flu’ virus with symptoms such as fever,
aches, stomach upset, and coughing. New genetic strains of influenza regularly arise in
human populations, spreading rapidly over large portions of the world.
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lysogenic cycle - A reproductive cycle used in some DNA bacteriophages that reproduces the viral genome without killing the host cell. After infecting a host bacterium, the
viral DNA becomes integrated with a host’s DNA. The bacterium continues to divide, producing many new bacteria cells, each with a copy of the viral DNA. Each of these is capable of initiating a second virus reproductive cycle inside its host cell (the lytic cycle)
where multiple copies of the entire virus are produced by the host. In the lytic cycle, the
host cell breaks apart, releasing the completed viruses into the environment – killing the
host cell.
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lysogenic - Certain viruses, in the form of a prophage, can become integrated into a host
cell genome. At some point after this, the viral genes can become active causing the host
cell to produce copies of the virus that will cause the host cell to break open or lyse. This
type of infection is termed lysogenic.
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lytic cycle - A reproductive
cycle used in double
stranded DNA
bacteriophages where multiple copies of the entire virus
are produced by the host. In
the lytic cycle, the host cell
breaks apart, releasing the
completed viruses into the
environment – killing the host
cell.
viral entry into cell
virus
cell
DNA
coat protein
viral genome enters
cell
DNA
transcription
replication
RNA
translation
DNA
capsid protein
assembly
coat protein
lysing of cell membrane and
release of viral replicates
The lytic cycle of a virus.
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obligate intracellular parasite - Those parasites that can only reproduce from within the
cytoplasm of an infected host cell.
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prophage - The section of viral genome that has been inserted into a host bacterium’s
genome. In DNA viruses, this is the same DNA as inserted by the phage, while in RNA
viruses, this DNA is first produced by reverse transcription.
bacteriophage
Injected DNA
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protein jacket -The capsid, or outer protein coat, of a virus.
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provirus - The section of DNA in a host cell’s genome that arises from DNA injected by a
virus into the nucleus of a host.
Provirus
Host DNA and RNA polymerase
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retrovirus - An RNA viruses that replicates inside a host by first making a DNA template
of itself using the enzyme, reverse transcriptase. From the root, retro , meaning backward.
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reverse transcriptase - The enzyme that promotes the reverse action of transcription,
creating a DNA template from a molecule of RNA.
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RNA polymerase - The enzyme that catalyzes the transcription of messenger RNA from
a DNA template.
RNA polymerase
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RNA virus - A virus with a genome made up of ribonucleic acid, RNA.
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T-4 virus - A highly-studied bacteriophage that infects the common intestinal bacterium,
Escherechia coli . Like the T-2 and T-6 phages (which also infect E. coli ), the T-4 virus
has an icosohedral (see diagram below) ‘head’ region enclosing the viral DNA, and a
collapsing tail structure with attached fibers that allows the virus to insert its DNA into the
host cell.
T-4 bacteriophage
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temperate virus - A virus that is capable of two different reproductive modes within the
host cell: the lysogenic mode or cycle, and the lytic mode or cycle. In the lysogenic
mode, the virus reproduces without killing the host cell. After infecting a host bacterium,
the viral DNA becomes integrated with a host’s DNA. The bacterium continues to divide,
producing many new bacteria cells, each with a copy of the viral DNA. Each of these is
capable of initiating a different reproductive cycle inside its host cell (the lytic cycle)
where multiple copies of the entire virus are produced by the host. In the lytic cycle, the
host cell breaks apart, releasing the completed viruses into the environment – killing the
host cell.
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transcription - The process whereby the genetic code on DNA molecules is translated
into a corresponding code on RNA molecules. The process involves the enzyme, RNA
polymerase, which reads the DNA code and matches nucleotides in a growing strand of
RNA based on DNA-RNA base pairing rules. For each transcription unit on the DNA, a
complementary molecule of mRNA is formed as a result of transcription.
During transcription, RNA nucleotides are
matched with corresponding DNA nucleotides.
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RNA polymerase opens the two DNA strands and
reads the template strand as RNA nucleotides
are added.
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vaccine - A harmless or relatively harmless variant or derivative of a microscopic pathogen that stimulates the body’s immune system to produce antibodies against the pathogen.
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viral envelope - A membrane that covers the capsid of some viruses. These membranes
include membrane created by the host cell along with proteins and glycoproteins programmed by the viral genome.
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virus - An infective, parasitic genome of DNA or RNA covered by a protective protein
coat. Viruses are on the boundary between living organisms and simple molecules.
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viurlent viruses - Viruses that depend entirely on the lytic mode or cycle of reproduction
to propagate. In the lytic cycle, multiple copies of the virus are made by the host cell. The
host cell then breaks apart (lyses), releasing the new viruses.
Other viruses, called temperate viruses, have a dual mode of reproduction with the
possibility of either the lytic cycle, or a lysogenic cycle where the viral genome is reproduced as the host cell divides, potentially through many generations.
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