Viruses Versus Living Organisms

Biology

Virus Consists of genetic material surrounded by

a protein coat. Viruses are much smaller than bacteria. Can only be seen through an electron

microscope.

Viruses Exhibit a variety of shapes and sizes. They cannot survive alone. They can reproduce only when they are

inside living cells. Once inside the cell, a virus uses the host

cell to produce many copies of itself. Viruses cannot reproduce on their own,

they are not considered living organisms.

Capsid The protein coat of a virus is called the

capsid. The proteins of the capsid bind to receptors

on the surface of a cell. This is the first step in the infection of a cell

by a virus.

Bacteriophage Most viruses are specific to the cells they

infect. Plant viruses only infect plant cells. A virus that infects bacteria is called a

bacteriophage. Bacteriophages are among the most

common viruses.

Bacteriophage They use bacterial cells to reproduce by attaching

their legs to a bacterium and then injecting their genetic material into it.

The virus takes control on the bacterial cell and directs it to produce more viruses.

A single infected cell may make thousands of viruses.

These new viruses go on to infect other cells. Viruses are responsible for colds, measles,

rabies, and chicken pox.

Two different processes Can occur once the genetic material of a

virus in inside its host cell. They are the lytic infection and the

lysogenic infection.

Lytic infection Injects its DNA directly into the cell. The host cell is unable to tell the difference between the viral

DNA and its own. The host cell begins to make mRNA from the genes of the

virus. The mRNA is then translated into thousands of proteins that

destroy the DNA of the cell. The cell then makes thousands of copies of the virus’ DNA

which gets assembled into new virus particles. The cell bursts and releases the virus particles.

Lysogenic viruses

Do not destroy the host cell right away. These viruses insert their DNA into the host’s DNA. The inserted DNA is called a prophage. The prophage can exist in the DNA of the host cell for many

generations without becoming active. At some point, the DNA of the prophage becomes active and

removes itself from the DNA of the host. It then directs the production of new viruses by entering the

lytic cycle.

Are viruses alive? NO. Living things must be able to live

independently of other organisms. However, viruses do share many

characteristics with living things.

Differences Between Cells and Viruses Living Cells Reproduce through cell

division Genetic code is DNA Use energy Respond to environment Change with time Develop and grow

Viruses Reproduce inside host Genetic code can be DNA

or RNA Do not use energy Do not respond to

environment Change with time Do not develop and grow

Vaccines We have vaccines for measles, mumps, and

polio. A piece of the antigen ( the part that the body that

reacts to) is injected into the body. The immune system then produces antibodies

against it. If you later come into contact with the pathogen,

your body will have a defense against it.