Viruses 1.Identify the structures of viruses 2.Explain the differences between the lytic and lysogenic cycles of viral reproduction 3.Recognize the medical importance of viruses.

What is a virus? You have probably had the flu sometime during your life. Nonliving particles called viruses cause influenza Viruses are composed of nucleic acid enclosed in a protein coat – Viruses are smaller than the smallest bacterium cell

What is a virus? Most biologists consider viruses to be nonliving because they do not possess all of the criteria for life. Viruses do not carry out respiration, grow, or develop. All that viruses can do is replicate – make copies of themselves– and they can’t even do that without the help of a living cell A cell in which a virus replicates is called a host cell.

What is a virus? Some viruses are named after the disease they cause (rabies viruses and polio viruses) Other viruses are named for the organ they infect (Human Immunodeficiency Virus –HIV) A bacteriophage, or phage for short, is a virus that infects a bacterium

Viral Structure Envelopes are composed primarily of the same materials found in the cell membrane of all cells.

Viral Structure Some relatively large viruses such as human flu viruses have an additional layer that surrounds the capsid called an envelope.

Viral Structure Viral nucleic acid is either DNA or RNA and contains instructions for making the virus. Some viruses have only 4 genes while others have hundreds.

Viral Structure The protein arrangement (a) also plays a role in determining what cell can be infected and how viruses infects the cell.

Replication Before a virus can replicate, it must enter a host cell A virus recognizes and attaches to a host cell when one of its proteins interlocks with a molecular shape that is the receptor site on the host cell’s cell membrane Each virus has a specifically shaped attachment protein. Therefore, each virus can usually only attach to a few kinds of cells. In general, viruses are species specific, and some are cell type specific.

Replication Attachment is specific The species specific characteristics of viruses is significant for controlling the spread of viral diseases.

Lytic Cycle

Lytic Cycle 1 st step What is the 1 st step called? What happens during the first step? Attachment Virus attaches to host cell

What is the 2 nd step called? What happens during the 2 nd step? Entry Virus injects nucleic acid into host cell Lytic Cycle 2 nd step

What is the 3 rd step called? What happens during the 3 rd step? Replication Host cell replicates viral nucleic acid Lytic Cycle 3 rd step

What is the 4 th step called? What happens during the 4 th step? Assembly New viral particles are being put together Lytic Cycle 4 th step

What is the 5 th step called? What happens during the 5 th step? Lysis and Release Host cell breaks open & releases viruses Lytic Cycle 5 th step

Examples of Viruses that follow the Lytic Cycle Common cold Influenza (flu) Rabies What do they have in common? The symptoms of the viral infection progress fairly rapidly

Lysogenic Cycle

Lysogenic Cycle 1 st Picture What is the 1 st picture called? What happens in the 1 st picture ? Attachment and Entry Injects nucleic acid into host cell

Lysogenic Cycle 2 nd to 6 th Picture What happens from the 2 nd to the 6 th picture? Provirus Viral nucleic acid inserts into host cell chromosome; called a provirus

Lysogenic Cycle 7 th picture What is the 7 th picture called? What happens in the 7 th picture? Cell replication Viral nucleic acid is replicated with host cell

Lysogenic Cycle 8 th to 2 nd to 3 rd picture What happens from the 8 th to 2 nd to 3 rd picture? Viral nucleic acid exits host chromosome and enters lytic cycle

Lysogenic Cycle 5 th picture What happens in the 5 th picture? Host cell breaks open & releases viruses

Provirus A provirus usually does not affect the functions of a host cell and the host cell carries out its own metabolic activity. Every time the host cell reproduces, the provirus is replicated also

Provirus Every cell that originated from the infected host cell has a copy of the provirus The lysogenic phase can continue for many years before becoming a lytic virus

Examples of Lysogenic Viruses Hepatitis B Chicken pox HIV

Retroviruses Many viruses, such as the HIV virus that causes AIDS are RNA viruses (their genetic material is only RNA) Once inside the host cell, the RNA is converted into DNA The newly synthesized DNA is inserted into the host cell’s chromosome and becomes a provirus (and follows the lysogenic cycle)

Retrovirus

Review Which characteristic of life do viruses have in common with cells? A.Viruses can grow B.Viruses can respire. C.Viruses can replicate. D.Viruses can develop.

Review Which is not a component of viruses ? A.Capsid B.Nucleic acid C.Protein coat D.Phage

Review In the lytic cycle, after the host’s metabolic machinery makes viral nucleic acid and viral proteins, the next phase is-- A.Lysis and release B.Replication C.Assembly D.Attachment