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