Lesson Overview

1
Lesson Overview

• 20.1 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

• Viruses differ widely in terms of size and
  structure.
• Most viruses are so small they can be seen only
  with the aid of a powerful electron microscope.

4
Structure and Composition

• Most viruses have proteins on their surface
  membrane or capsid that bind to receptor proteins
  on the host cell.
• The proteins trick the cell to take the virus,
  or in some cases just its genetic material, into
  the cell.
• Once inside, the viral genes are eventually
  expressed and may destroy the cell.

5
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.

6
Viral Infections

• What happens after a virus infects a cell?
• Inside living cells, viruses use their genetic
  information to make
• multiple copies of themselves.
• Some viruses replicate immediately, while others
  initially
• persist in an inactive state within the host.

7
Types of Viral Infections

• Lytic infection
• Lysogenic Infection

8
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 is an example of a
  bacteriophage that causes such an infection.

9
Lytic Infections

• Bacteriophage T4 has a DNA core inside a protein
  capsid that binds to the surface of a host cell.

10
Lytic Infections

• The virus injects its DNA into the cell.

11
Lytic Infections

• Controlled by viral genes, the host cells
  metabolic system makes copies of viral nucleic
  acid.

12
Lytic Infections

• The host cell lyses, releasing hundreds of virus
  particles that go on to infect other cells.

13
Lytic Infections

• A lytic virus is similar to an outlaw in the
  Wild West of the American frontier in the demands
  the virus makes on its host.
• First, the outlaw eliminates the towns existing
  authority.
• In a lytic infection, the host cells DNA is
  chopped up.

14
Lytic Infections

• Next, the outlaw demands to be outfitted with
  new equipment from the local townspeople.
• In a lytic infection, the viruses use the host
  cell to make viral DNA and viral proteins.

15
Lytic Infections

• Finally, the outlaw forms a gang that leaves the
  town to attack new communities.
• In a lytic infection, the host cell bursts,
  releasing hundreds of virus particles.

16
Lysogenic Infection

• Some bacterial viruses cause a lysogenic
  infection.

17
Lysogenic Infection

• In a lysogenic infection a host cell is not
  immediately taken over.

18
Lysogenic Infection

• The viral nucleic acid is inserted into the host
  cells DNA.

19
Lysogenic Infection

• The viral DNA is then copied along with the host
  DNA without damaging the host.

20
Lysogenic Infection

• Viral DNA multiplies as the host cells multiply.

21
Lysogenic Infection

• In this way, each generation of daughter cells
  derived from the original host cell is infected.

22
Lysogenic Infection

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

23
Lysogenic Infection

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

24
Lysogenic Infection

• Influences from the environmentradiation, heat,
  etctrigger the prophage to become active.

25
Lysogenic Infection

• It then removes itself from the host cell DNA,
  directs the synthesis of new virus particles, and
  now becomes an active lytic infection.

26
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.

27
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.

28
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.

29
Common Cold Video- How the Flu Virus infects

• A virus infects a host cell and then uses it to
  make more viruses
• The viruses then infect more cells, until the
  host is weakened or killed
• Here is a video that explains how a common cold
  virus infects cells
• http//www.youtube.com/watch?vRpj0emEGShQ

30
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.

31
HIV

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

32
HIV

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

33
How HIV Infects the Cell Video

• HIV virus attacks a cell and then uses it to make
  more viruses
• http//www.youtube.com/watch?vl-2ilZA-_aw

34
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.

35
Viruses and Cells

• All viruses are parasites. Parasites depend
  entirely upon other living organisms for their
  existence, harming these organisms in the
  process.
• Viruses must infect living cells in order to
  grow and reproduce, taking advantage of the
  nutrients and cellular machinery of their hosts.

36
Viruses and Cells

• Viruses have many of the characteristics of
  living things. After infecting living cells,
  viruses can reproduce, regulate gene expression,
  and even evolve.

37
Viruses and Cells

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

38
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.