Download Viruses

21.9 Viruses
Viruses are small
particles of RNA and
DNA containing 3 to
200 genes.
The Epstein–Barr virus
(EBV), herpesvirus 4,
causes cancer in
humans.
Learning Goal Describe the methods by which a virus infects
a cell.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Viruses
Viruses
• are small particles of
DNA or RNA that
require a host cell to
replicate and cause
infection.
• can replicate only in
cells, by taking over
the machinery and
materials necessary
for protein synthesis
and growth.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Viral Infections
A virus causes infection when
• an enzyme in the protein coat of the virus makes a
hole in the outside of the host cell;
• the virus enters the cell and the viral nucleic acid
mixes with the materials in the host cell;
• a protease processes proteins to produce a protein
coat encasing the new viral RNA or DNA; and
• the new virus particles are released from the cell,
ready to infect more cells.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Diseases Caused by Viruses
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Reverse Transcription
In reverse transcription,
• a retrovirus, which contains viral RNA but no viral
DNA, enters a cell.
• the viral RNA uses reverse transcriptase to
produce a viral DNA strand.
• the viral DNA strand forms a complementary DNA
strand.
• the new DNA uses the nucleotides and enzymes in
the host cell to synthesize new virus particles.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
HIV Virus and AIDS
The HIV-1 virus
• is a retrovirus that infects and
destroys T4 lymphocyte cells.
• leaves the immune system
unable to destroy harmful
organisms.
• is associated with an
increased chance of
developing pneumonia and
skin cancer associated
with AIDS.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Retroviruses
After a retrovirus injects its viral RNA into a cell, it
forms a DNA strand by reverse transcription.
The single-stranded DNA forms a double-stranded
DNA called a provirus, which is incorporated into the
host cell DNA.
When the cell replicates, the provirus produces the
viral RNA needed to produce more virus particles.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Retroviruses
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
AIDS Treatment
Treatment for AIDS is based on
• attacking the HIV at different points in its life cycle.
• developing nucleoside analogs that mimic the structures of
the nucleosides used for DNA synthesis.
Drugs based on nucleoside analogs used to treat HIV/AIDS
include
• AZT (3ʹ-azido-2ʹ-deoxythymidine), which is similar to
thymidine.
• ddI (2ʹ, 3ʹ–dideoxyinosine), which is similar to guanosine.
• ddC (2ʹ,3ʹ-dideoxycytidine).
• d4T (2ʹ,3ʹ-didehydro-2ʹ,3ʹ-dideoxythymidine).
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
AIDS Treatment, Nucleoside Analogs
When a nucleoside analog
such as AZT, ddI, ddC, or
d4T is incorporated into
viral DNA,
• the lack of a hydroxyl
group on the 3ʹ-carbon in
the sugar prevents the
formation of the sugar–
phosphate bonds.
• the production of viral
DNA is stopped.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
AIDS Treatment
Current treatment for HIV and
AIDS involves a combination
of drugs that include
• entry inhibitors.
• reverse transcriptase
inhibitors.
• protease inhibitors.
Lexiva metabolizes slowly to
provide amprenavir, an HIV
protease inhibitor.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Chemistry Link to Health: Cancer
When cells begin to grow and
multiply without control, they invade
nearby cells and appear as a tumor.
Tumors
• that are limited are called benign.
• that invade other tissues,
interfering with normal body
functions, are cancerous.
Cancer can be caused by
• chemical and environmental
substances.
• ultraviolet or medical radiation.
• oncogenic viruses.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
Epstein–Barr virus (EBV),
herpesvirus 4, causes
cancer in humans.
© 2016 Pearson Education, Inc.
Chemistry Link to Health: Cancer
Most cancers are caused by environmental and chemical
substances (carcinogens), including aniline dyes, cigarette
smoke, and asbestos.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Chemistry Link to Health: Cancer
Oncogenic viruses cause cancer when cells are infected.
Some cancers such as retinoblastoma and breast cancer appear to occur
within families. There is some indication that a missing or defective gene
may be responsible.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Study Check
Match the following terms to their definitions:
1) virus
2) retrovirus
3) protease inhibitor
4) reverse transcription
A. a virus containing RNA
B. small particles requiring host cells to replicate
C. a substance that prevents the synthesis of viral
proteins
D. using viral RNA to synthesize viral DNA
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Solution
Match the following terms to their definitions:
1) virus
2) retrovirus
3) protease inhibitor
4) reverse transcription
A. a virus containing RNA
B. small particles requiring host cells to replicate
C. a substance that prevents the synthesis of viral
proteins
D. using viral RNA to synthesize viral DNA
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
2) retrovirus
1) virus
3) protease
inhibitor
4) reverse
transcription
© 2016 Pearson Education, Inc.
Concept Map
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.