Download what is the sturcture of a virus?

VIRUSES
WHAT IS A VIRUS?
Viruses are particles that are not alive. They cause diseases and infections. Viruses are made
up of nucleic acids, either DNA or RNA, surrounded by a protein coat. They are smaller than the
tiniest bacterium. Most biologists agree that viruses are not alive because they don’t grow,
develop, or carry out respiration.
All viruses replicate, or make copies of themselves. Viruses need the help of living cells to copy
themselves. In order to copy itself, a virus must enter a living cell. The cell in which a virus
replicates is called a host cell.
WHAT IS THE STURCTURE OF A VIRUS?
Viruses differ in size and shape. Some viruses contain as few as four genes. Other viruses have
hundreds of genes. Every virus has an inner core of nucleic acid. This core contains the virus’s
genetic material. Some viruses contain RNA, some contain DNA. The nucleic acid contains the
instructions for making copies of the virus.
The outer protein coat surrounding the virus is called a capsid. The arrangement of proteins in
the capsid gives the virus its shape. The proteins arrangement also determines what cell can be
infected and how the virus infects the cell. Some large viruses contain another layer called an
envelope.
HOW DOES A VIRUS ATTACH TO A HOST CELL?
Before a virus can replicate, it has to enter a host cell. In order to do this, it must first attach
itself to the host cell. Every virus has a specially shape device called an attachment protein.
Because of the specific shape, each virus can only attach to a few kinds of cells. The
attachment process is like two pieces of a jigsaw puzzle fitting together.
A virus that infects a bacterium is called a bacteriophage, or phage for short. A protein located
in the tail fibers of the bacteriophage recognizes and attaches to its bacterial host cell.
DO VIRUSES INFECT MORE THAN ONE SPECIES?
Many viruses are species specific. They infect only certain species. For example, smallpox
infects only humans. Other viruses are not species specific. The virus that causes the flu
infects both human and animals. Viruses that are not species specific are more difficult for
scientist to eliminate.
VIRAL REPLICATION CYCLES
Once a virus attaches itself to the host cell, the virus enters the cell and takes over its
metabolism. Only then can the virus replicate itself. There are two ways that viruses get into
cells. First, the virus can inject its nucleic acid in to the host cell. When this takes place, the
capsid of the virus stays attached to the outside of the host cell. An enveloped virus enters a
host cell in another way. After attachment, the plasma membrane of the host cell surrounds
the virus. This produces a virus filled vacuole inside the host cell’s cytoplasm. Recall that a
vacuole is a membrane bound compartment used for temporary storage of materials within a
cell. Once the vacuole is in the cytoplasm, the virus burst out of the vacuole and releases it
nucleic acid into the cell.
WHAT IS A LYTIC CYCLE?
Once the virus is inside the host cell, the virus’s genes are expressed. The substances that
produced take over the host cell’s genetic material. When viruses take over the cell, the cell
stops producing the materials it needs to live. It uses its own enzymes and energy to make new
viruses. The new viruses then burst from the host cell and the host cell dies. The new viruses
can infect and kill other host cell. This process is call a lytic cycle.
WHAT HAPPENS DURING A LYSOGENIC CYCLE?
Not all viruses kill the cells they infect. Some viruses go through a lysogenic cycle. A lysogenic
cycle is a replication cycle in which the virus’s nucleic acid is integrated into the chromosome of
the host cell.
The lysogenic cycle begins the same way as a lytic cycle. The virus attaches to the host cell and
its nucleic acid enters the cell. Instead of taking over the genetic material of the host, the viral
DNA is included in the host’s cell chromosome. Once that happens, the viral DNA is called a
provirus. The cell continues to carry out it own metabolic activity, but every time the host cell
reproduces, the provirus is replicated as well. This means that every cell that comes from an
infected host cell has a copy of provirus.
The lysogenic cycle can last for many years. At any time, though, the provirus can be activated
and enter a lytic cycle. When that happens, the virus will replicate and kill the host cell.
Next meeting we will discuss some diseases caused by proviruses. We will discuss Retrovirus,
what is HIV, Cancer and Viruses, what is a plant virus and the origin of viruses.