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BIOC33H3 Lecture Notes - Cytotoxic T Cell, Cubic Metre, Mutation Rate

Biological Sciences
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Chapter 1- Understanding HIV
Why study evolution?
oThe tools and techniques of evolutionary biology offer crucial insights into matters of life and
Human Immunodeficiency Virus (HIV) causes Acquired Immune Deficiency Syndrome (AIDS)
oHIV is an emerging virus, it rapidly evolves drug resistance and it is deadly
Evolutionary biology is the science devoted to understanding two things:
oHow populations change through time following modifications in their environment
oHow new species come into being
1.1- The Natural History of the HIV/AIDS Epidemic
List of worst epidemic in human history according to the number of deaths:
oInfluenza-50 to 100 million deaths-across the globe
oBlack Death (1347-1352)-took 30%-50% of the European population-about25 million lives
oNew World small pox-released in 1520 by European conquistadores-decimated Native
American populations across two continents
AIDS is among the worst epidemics in human history
oWas first recognized in 1981
oSo far infected more than 65 million people
o25 million have already died
oBy year 2020, a total of 90 million lives would have been claimed by AIDS
oAccording to World Health Organization, AIDS is responsible for about 4.9% of all deaths
oSub-Saharan Africa is mostly affected by this epidemic
HIV establishes a new infection when a bodily fluid holding the virus, usually a blood or semen,
carries it from an infected person directly onto a mucous membrane or into the bloodstream of an
uninfected person
oCan be passed during heterosexual sex, homosexual sex, oral sex, needle sharing,
transfusion with contaminated blood products, childbirth, and breastfeeding
An HIV infection can be acquired only from someone else who already has it!
What is HIV?
Like all viruses, HIV is an intracellular parasite that cannot reproduce on its own
oIt invades specific types of cells in the human immune system
oIt uses enzymatic machinery and energy of these cells to make copies of itself, killing the
host cells in the process
Figure 1.5 (Pg. 7) contains the life cycle of HIV in detail.
oThe life cycle includes an extracellular phase and an intracellular phase
oDuring the extracellular phase, the virus moves from one host cell to another, and can be
transmitted from host to host
oExtracellular form of a virus is called a virion, or virus particle
oDuring intracellular or parasitic phase, the virus replicates
HIV initiates its replication phase by latching onto two proteins on the surface of a host
HIV then binds to two surface proteins on the target cell called CD4 and coreceptor
This binding fuses the virion’s envelope with the host’s cell membrane and spills the
contents of the virion into the cell
These contents include the virus’s diploid genome (two copies of a single-
stranded RNA molecule) and 3 proteins:
oReverse transcriptase- transcribes the virus’s RNA genome into DNA
oIntegrase- splices the DNA genome into the host cell’s genome
oProtease- which plays a role in the preparation of new viral proteins
In HIV and other retroviruses, flow of genetic information is different than in
cells and in viruses with DNA genomes
In retroviruses, genetic information does not follow the familiar route from DNA
to mRNA to proteins

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oInstead it flows from RNA to DNA, then to mRNA to proteins
Once HIV’s genome is inserted into the host cell’s chromosomes, the host cell’s RNA
polymerase transcribes the viral genome into mRNA, and the host cell’s ribosomes
synthesize viral proteins
New virions assemble in the host cell cytoplasm, then bud off the cell
membrane and enter the bloodstream
There, the new virions may find another cell to infect in the same host, or be
transported to a new host
oSignificant feature of HIV’s life cycle is that the virus uses the host cell’s own enzymatic
machinery, its polymerases, ribosomes, and tRNAs – in almost every step
How Does HIV Cause AIDS?
HIV parasitizes immune system cells, particularly helper T cells. After a long battle against the virus,
the immune system’s supply of helper T cells is badly depleted. Because helper T cells play a crucial
role in the response to invading pathogens, this leaves the host vulnerable to a variety of secondary
Through research on how SIVsm (simian immunodeficiency virus) in monkeys, it was concluded that
the host’s own immune response contributes to the development of immunodeficiency
oHuman HIV patients treated with antiretroviral drugs plus the immunosuppressant
cyclosporine maintained higher helper T cell counts than control patients treated with
antiretrovirals alone
T cells derive from stem cells in the bone marrow
These stem cells generate precursors that mature into naive T cells in the thymus
Naive T cells are activated in lymph nodes
An activated T cell undergoes a burst of proliferation, yielding effector and memory cells
oThese circulate in the blood and move through tissues
A large fraction of the body’s memory cells reside in lymphoid tissue associated with mucus
membranes lining the nose, mouth, lungs, and especially the gut
Naive T cells and memory T cells are long lived
oBut effector cells, which actively engage in the fight against invaders, are short lived
Any given T cell lineage has a finite capacity for replication-a capacity that is reduced with each cell
oSo with each burst of replication within a T cell lineage brings that lineage closer to
Sustained immune activation during HIV infection can ultimately deplete the body’s supply of helper
T cells and lead to the collapse of the host’s defences
An untreated HIV infection exhibits distinct phases, in which the loss of helper T cells happens at
different rates and appears to be driven by different mechanisms
In the acute or initial, phase, HIV virions enter the host’s body and begin to replicate
HIV gains entry into a host cell by first latching onto the cell-surface protein CD4, then binding to a
The coreceptor used by most of the HIV strains responsible for new infections is CCR5
oThese viral strains can thus infect dendritic cells, macrophages, regulatory T cells, and
especially memory and effector helper T cells
HIV replicates explosively, and the concentration of virions in the blood climbs steeply
oAt the same time, the concentrations of CD4 T cells plummet, largely because HIV kills them
while replicating
oHardest hit are the memory helper T cells in the lymphoid tissues of the gut
oSince the gut is both large and vulnerable to penetrations by pathogens, the loss of these T
cells is a severe blow to the body’s defenses
The acute phase ends when viral replication slows and the concentration of virions in the blood
oThis slow down may be because that the virus simply runs short of host cells it can easily
In addition, the immune system mobilizes against the infection and killer T cells begin to target host
cells infected with HIV
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