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Virus Replication. What is a Virus?. Virus : A biological particle composed of nucleic acid and protein Intracellular Parasites : organism that must “live” inside a host. Reproduce Have nucleic acid Adapt to surroundings Have organization. Not made of cells or organelles

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What is a Virus?

  • Virus: A biological particle composed of nucleic acid and protein

  • Intracellular Parasites: organism that must “live” inside a host


Have nucleic acid

Adapt to surroundings

Have organization

Not made of cells or organelles

Can’t reproduce on own

Don’t metabolize energy

Don’t perform cellular processes

Are viruses alive?





  • All Have:

    • 1) Capsid: coat of protein that surrounds nucleic acid

    • 2) Nucleic Acid: RNA or DNA

  • Some Have:

    • Tail Fibers: Used for attachment (not legs)

  • Shapes vary

Viral Replication

  • Bacteriophages (infect bacteria) are often studied

    • Replication is similar with many animal viruses

The Lytic Cycle

1st Step: Attachment

  • Virus lands on cell membrane

  • Virus attaches to a cell receptor

  • No attachment = No infection

  • Virus acts as a “key” to the receptor

The Lytic Cycle

2nd Step: Entry

  • Virus enzyme weakens cell membrane

  • Genetic material (DNA or RNA) enters host cell


The Lytic Cycle

3rd Step: Replication

  • Virus DNA/RNA uses ribosomes to make virus proteins

  • Virus proteins created by transcription/ translation

The Lytic Cycle

4th Step: Assembly

  • New virus proteins are assembled in the cytoplasm

The Lytic Cycle

5th Step: Release

  • Virus enzyme causes cell membrane to burst

  • Viruses are released to find new host

  • Cycle repeats

The Lysogenic Cycle

1st step: Attachment = Same

2nd Step: Entry = Same

Lysogenic Cycle

3rd step: Replication

  • Virus DNA combines with cell DNA, and waits

    • Provirus = cell DNA + viral DNA

  • Cell divides by mitosis

  • Each new cell will contain the provirus

The Lysogenic Cycle

4th Step: Assembly Same, except many new viruses are being assembled in many cells

5th Step: Release Same, except many cells burst releasing many more viruses


RNA viruses

Contains the enzyme Reverse Transcriptase


1) Virus RNA enters host cell

2) Cell’s ribosomes create virus DNA

3) Virus DNA combines with cell DNA (becomes dormant)

4) Cells divide as usual

5) Once active, normal steps of transcription/translation followed


How is hiv contracted

High risk activities:

1) Sex (oral, vaginal, anal)

2) Sharing needles (tattoos, piercings, drugs)

Low risk activities:

3) Mother to child in womb

4) Breast milk

5) Blood transfusions

How is HIV contracted?

What is aids

New Exposures to HIV (2006)

What is AIDS?

Gender of those living

with HIV (2003)

  • Acquired Immunodeficiency Syndrome

  • AIDS results when:

    • 1) Less then 200 T cells per 1mm³ blood

    • 2) Multiple symptoms/infections from HIV appear

  • Therefore, the HIV virus causes the disease AIDS

Female Transmission of HIV


Male Transmission of HIV


Want more stats? Go to the Centers for Disease Control’s Website

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