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Viruses. Chapter 19 Pgs 478-484. What is a Virus?. Viruses are particles of nucleic acid, protein, and in some cases, lipids.

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Chapter 19 Pgs 478-484

What is a virus
What is a Virus?

Viruses are particles of nucleic acid, protein, and in some cases, lipids.

A typical virus is composed of a core of DNA or RNA surrounded by a protein coat.

Viruses can only reproduce by infecting living cells.

Can only be seen with the aid of a powerful electron microscope.


  • Capsid—a virus’s protein coat

    • Includes proteins that enable a virus to enter a host cell

  • Most are highly specific to the cells they infect

    • Animal viruses—infect only certain related species of animals

    • Plant viruses—infect plant cells

    • Bacteriophages—viruses that infect bacteria

Viral infection
Viral Infection

Lytic Infection—a virus enters a cell, makes copies of itself, and causes the cell to burst.

The virus injects its DNA directly into the cell and the host cell cannot tell the difference between its own DNA and the DNA of the virus

The cell begins to make mRNA from the genes of the virus

The virus then uses the materials of the host cell to make thousands of copies of its own DNA molecule

Viral infection1
Viral Infection

Lysogenic Infection—a virus integrates its DNA into the DNA of the host cell, and the viral genetic information replicates along with the host cell’s DNA.

The lysogenic virus remains inactive for a period of time.

The viral DNA is embedded into the host’s DNA—prophage.

The prophage may remain part of the DNA for many generations

Any number of factors may active the DNA of a prophage

Next step resemble that of a Lytic Infection


Contain RNA as their genetic information.

When retroviruses infect a cell, they produce a DNA copy of their RNA.

Named from the fact that their genetic information is copied backward—from RNA to DNA instead of DNA to RNA.

Responsible for some types of cancer in animals.

The virus that causes AIDS is a retrovirus

Viruses and living cells
Viruses and Living Cells

Viruses must infect a living cell in order to grow and reproduce.

They also take advantage of the host’s respiration, nutrition, and all the other functions that occur in living cells.

Can be considered parasites.

Smaller and simpler than the smallest cells.

How big is it?

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