Chapter 20 Section 1

1

• Chapter 20 Section 1
• Viruses

2
Discovery of Viruses

• A virus is a nonliving particle made of
  proteins, nucleic acids, and sometimes lipids.
• Viruses can reproduce only by infecting living
  cells.

3
Structure and Composition

• The protein coat surrounding a virus is called a
  capsid.
• Some viruses, such as the influenza virus, have
  an additional membrane that surrounds the capsid.
  
• The simplest viruses contain only a few genes,
  whereas the most complex may have more than a
  hundred genes.

4
Structure and Composition

• Most viruses infect only a very specific kind of
  cell.
• Plant viruses infect plant cells most animal
  viruses infect only certain related species of
  animals viruses that infect bacteria are called
  bacteriophages.

5
Lytic Infections

• In a lytic infection, a virus enters a bacterial
  cell, makes copies of itself, and causes the cell
  to burst, or lyse.
• Bacteriophage T4 has a DNA core inside a protein
  capsid that binds to the surface of a host cell.

6
Lytic vs. Lysogenic Infections

7
Lysogenic Infection

• Bacteriophage DNA that becomes embedded in the
  bacterial hosts DNA is called a prophage.

8
Lysogenic Infection

• The prophage may remain part of the DNA of the
  host cell for many generations.

9
A Closer Look at Two RNA Viruses

• About 70 percent of viruses contain RNA rather
  than DNA.
• In humans, RNA viruses cause a wide range of
  infections, from relatively mild colds to severe
  cases of HIV.
• Certain kinds of cancer also begin with an
  infection by viral RNA.

10
The Common Cold

• Cold viruses attack with a very simple,
  fast-acting infection.
• A capsid settles on a cell, typically in the
  hosts nose, and is brought inside, where a viral
  protein makes many new copies of the viral RNA.

11
The Common Cold

• The host cells ribosomes mistake the viral RNA
  for the hosts own mRNA and translate it into
  capsids and other viral proteins.
• The new capsids assemble around the viral RNA
  copies, and within 8 hours, the host cell
  releases hundreds of new virus particles to
  infect other cells.

12
HIV

• The deadly disease called acquired immune
  deficiency syndrome (AIDS) is caused by an RNA
  virus called human immunodeficiency virus (HIV).
• HIV belongs to a group of RNA viruses that are
  called retroviruses.
• The genetic information of a retrovirus is
  copied from RNA to DNA instead of from DNA to RNA.

13
HIV

• When a retrovirus infects a cell, it makes a DNA
  copy of its RNA.

14
HIV

• The copy inserts itself into the DNA of the host
  cell.

15
HIV

• Retroviral infections are similar to lysogenic
  infections of bacteria. Much like a prophage in a
  bacterial host, the viral DNA may remain inactive
  for many cell cycles before making new virus
  particles and damaging the cells of the hosts
  immune system.

16
Viruses and Cells

• Some of the main differences between cells and
  viruses are summarized in this chart.

17
Viruses and Cells

• Although viruses are smaller and simpler than
  the smallest cells, it is unlikely that they were
  the first living organisms.
• Because viruses are dependent upon living
  organisms, it seems more likely that viruses
  developed after living cells.
• The first viruses may have evolved from the
  genetic material of living cells. Viruses have
  continued to evolve, along with the cells they
  infect, for billions of years.