lecture 11

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Details of the Virus

Classified According to a. DNA or RNA

b. Enveloped or Non-Enveloped c. Single-stranded or double-stranded

Viruses contain only a few genes Reverse transcriptase

proteins to inhibit host synthesis to make coat proteins

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Obligate parasites

•Smaller than bacteria

•Can not replicate without host - autonomous

replication in the host

•Composition

-nucleic acid

-protein coat

-envelope?

•Where did they come from?

(4)

Origin of Viruses

• Do not encode ribosomes or enzymes for

energy production

• Suggests they evolved after cells

• Evolved from small segments host cell DNA

or RNA

• had regions of homology with host cell DNA -

then could recombine and increased in

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Fig 16.1 Electron micrographs showing the morphologies of a plant virus, an animal virus, and a bacterial virus.

Credit: Courtesy of Robley Williams, University of California Berkeley and Harold Fisher,

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Fig 16.2a Electron micrograph showing the structure of bacteriophage T4.

Credit: Courtesy of John Finch, Cambridge

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Viruses

• Simplest of all organisms • proteins and nucleic acids

• T phages (T1-T7) - E. coli phage • Components (Figure 16.2)

– head: contains several proteins (20 sides) - DNA in head

– tail: has 2 hollow tubes (inner needle and outer sheath) – tail fibers: uses to find a bacterial host

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Fig 16.2 Diagram showing the structure of bacteriophage T4.

20 Sided polyhedron

Double-stranded

Locate bacterium

Anchor to bacterium

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Fig 16.3 The life cycle of bacteriophage T4.

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Select Details of Phage Lifecycle

A. Bacteriophage DNA enters bacterium

B. Viral proteins bind to host polymerase/inhibit host synthesis

these proteins also aid in host polymerase recognizing viral genes C. Host DNA is degraded by viral nucleases

D. Viral genes encode coat proteins

lysozyme (to break host wall for lysis)

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Bacterial Defense

• Restriction endonucleases (restriction

enzymes) - help protect bacteria from

invasion by viruses

• can degrade C- and HMC-containing DNAs

• Viruses smart - add glucose to HMC

(13)

Mapping Phage Genome

• Normally, crossing organisms with different

alleles of a gene

• Don’t have this in viruses

• Only can been seen with electron

microscope

(14)

Phage Plaques

• Clear area on a lawn of bacteria- results

from lysis or killing of contiguous cells

• Host range - infect some bacterial strains

but not others

• Phage morphology factors:

(15)

X174

• Genes within genes - 5389 nt; should be

1795 amino acids, but there are 2300 amino

acids

• learned that they contain overlapping genes

(translated using different reading frames)

• See

Figures 16.17 and 16.18

- e.g., E gene

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HIV

Human Immunodeficiency Virus

• Virus that causes AIDS (acquired immune

deficiency syndrome)

• 30 to 40 million infected

• Prolonged infection

• Primary effect of HIV infection: reduction in

T

_H

cells

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HIV

• Results of T_H cell depletion:

– opportunistic infections (pneumonia) – tumors develop

• HIV mutates rapidly

• HIV is a retrovirus –genome RNA

• Member of lentivirus – “slow virus”

• Enveloped

• gp120 on virus binds to CD4 – T helper lymphocytes – macrophages

• gp41

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Life Cycle of HIV

FIGURE 16.20 (next slide)

1. virus attaches and penetrates host cells

2. Viral RNA converts to viral DNA (PROVIRUS)

3. Viral DNA integrates into human chromosome

4. Infected host cells produce new virus particles

5. New virus particles bud from host cell one-by-one, taking host cell's membrane along as envelope

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Fig 16.20 Overview of the life cycle of HIV.

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• Rnase H- step 5- degrades RNA in

DNA/RNA duplex

• integrase - enzyme that allows integration of

the virus into the host genome

• Long terminal repeats

– required for integration

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Fig 16.23 Map of the integrated HIV genome showing the location of regulatory genes and genes encoding important viral proteins.

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Development of HIV Disease

Time Periods:

1. weeks 1-3: virus enters body, circulates, and makes

infected person contagious

2. weeks 1-8: acute viral syndrome

– short term

– mild or severe flu-like symptoms

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Development of HIV Disease

3. 6 weeks - 6 months +: positive HIV antibody test

– seronegative: negative test

– seropositive: positive test for HIV

4. 2 yrs: onset of longer-lasting symptoms

5. 6 months-15 yrs: development of AIDS

yeast infections fungal pneumonia

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Diagnosis of AIDS

• ELISA test

• Western Blot

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HIV Therapy Strategies

• Boosting immune response insufficient

– once virus in T cells - humoral response ineffective

– cytotoxic T cells must kill infected cells, but viral antigens not displayed

• Interfere with:

– HIV attachment to Cells

– HIV integration into the host cell genome

– Virus replication

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Possible HIV Therapies

• Preventing assembly of HIV virus – HIV protease inhibitor (Crixivan)

• Inhibit reverse transcription – AZT (azidothymidine) – Epivir, Retrovir

• Gene Therapy

– Antisense RNA

– Introduce into stem cells

• Preventing entry of HIV into uninfected cells gp120/viral envelope (bind with CD4)

– CD4 on the cell surface (bind with mAb)

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