Download - Advanced Biology Chapter 27
Advanced BiologyChapter 27
Viruses
Nature of Viruses
Viral Basic Structure: DNA or RNA Protein Coat
It is not living It is not a cell and
has no cellular structures other than having nucleic acid
Viruses vary in shape and
Size
More shapes and
Sizes
Viruses invade cells
Viruses cannot replicate on their own, they need a host
Their host vary from bacteria, protists, plants and animals (including humans)
They are usually host specific (host range) and sometimes tissue specific (tissue tropism)
Viruses can remain dormant in cells for years (chicken pox and shingles caused by varicella zoster virus)
Viruses invade the host cell and use the cells protein synthesis processes to make viral proteins and take over the cell
Lytic - When a virus kills an infected host cell
by rupturing the cell Lysogenic - When a virus enters the cell and
integrate themselves into the nucleic acids of the host cells. This allows them to be replicated along with the DNA
Lytic vs. Lysogenic Cycle
http://highered.mcgraw-hill.com/sites/0072556
781/student_view0/chapter17/animation_quiz_2.htm
AIDS (Auto Immune Deficiency Syndrome) is caused by
Human Immunodeficiency Virus Thought that HIV might be related to smallpox virus HIV targets CD4+ cells such as T-helper cells which aids in
the immune response system. HIV is a retrovirus thus it has RNA instead of DNA HIV enters the cells and goes through reverse transcriptase
(DNA made from RNA). HIV then makes another copy of DNA and incorporates
itself into the DNA of the host This allows them to use hosts protein synthesis to make
more viral proteins and more HIV particles.
HIV
Fig. 27.6 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Attachment
HIVEnvelope
DNA
RNA
Ribosome
Assembly
Transcription
CD4 receptor
Nucleus
6. Complete HIV particles are assembled and HIV buds out of the cell. As the disease progresses HIV-infected T–helper cells, but not macrophages, are killed by a poorly understood mechanism.
5. Transcription of the DNA results in the production of RNA. This RN A can serve as the genome for new viruses or can be translated to produce viral proteins.
3. Reverse transcriptase catalyzes, first, the synthesis of a DNA copy of the viral RNA, and, second, the synthesis of a second DNA strand complementary to the first one.
4. The double-stranded DNA is then incorporated into the host cell's DNA by a viral enzyme.
1. The gp120 glycoprotein on the surface of HIV attaches to CD4 and one of two coreceptors on the surface of a CD4+ cell.
CD4+ cell Viral RNA
gp120glycoprotein
Rupturedcapsid
2. The viral contents enter the cell by endocytosis.
Entry into CD4+ Cells
ReversetranscriptaseViral RNA
Virus exitsby budding.
Double-stranded DNA
Host cell'sDNA
CCR5 or CXCR4coreceptor
Replication and Assembly
Nucleus
CD4 receptor
CCR5 or CXCR4coreceptor
CD4+ cell Viral RNA
Viral RNAReversetranscriptase
DNA
Host cell'sDNA
RNA
Transcription
Ribosome
Assembly
Double-stranded DNA
Rupturedcapsid
See page 532, Table 27.1
Three basic types of flu: Type A (causes most serious flu epidemics in
humans but can infect other mammals and birds)
Type B (humans only) Type C (humans only)
Subtypes are based on their protein spikes emerging from their capsid
Other Viral Diseases
Called the H1N1 killed 20-50 million people Asian Flu of 1957 (H2N2) over 100,000
Americans Hong Kong Flu of 1968 (H3N2) 70,000
Americans Flues are different strains that change/evolve
constantly and can sometimes change over from a virus in one species to another (birds to humans)
Various Flu Epidemics
Any virus that can transfer from one species to
the next is considered an emerging virus Some of these include:
Hantavirus – Deer mouse to humans (pneumonia 1993)
Hemorrhagic fever (filovirus) –causes hemorrhagic fever and death rates over 50% Ebola virus is one of the worst with 90% death rates
in Africa (unknown source) Summer of 1995 – Zaire – killed 245 out of 316 infected
Emerging Viruses
Ebola Virus
Fig. 27.8
SARS (Severe acute respiratory syndrome)
Respiratory infection Acts like pneumonia 8% fatality rate Outbreak in 2003 Unknown source
Maybe civets (weasel-like) in China Or bats
Vaccines are becoming available
SARS
Some viruses can cause cancer
Hepatitis B and liver cancer connection Human Papillomavirus (HPV) and cervical
cancer
Cancer
Prions are proteins that cause disease
(proteinaceous infectious particle) – Stanley Prusiner – 1970
Prions can cause diseases such as TSE (transmissible spongiform encephalopathies)
In this case, neurons die out and an empty space makes the brain look like a sponge.
Mad Cow Disease Chronic Wasting Disease Creutzfeldt-Jakob Disease
Prions
Viroids are small RNA molecules that are
circular and they do not have a protein coat. Viroids somehow replicate without any helper
cells They only infect plants 10 million coconut palms killed in Philippines Relatively new infectious agent that is still
being studied
Viroids