Viruses

Higher Human Biology

Lesson Aims

• To describe the structure of a virus

• To examine the process of viral replication

Different types of viruses

• Influenza virus • AIDS virus

Different Shaped Viruses

Size

• Smaller than bacteria• 20-300 nanometres

(nm)

Structure of a Virus

• A virus contains nucleic acid (DNA or RNA)

• Surrounded by a protective coat (capsid)

Entering the Host Cell

• Different viruses employ different methods of entering the host cell

• Process depends on antigenic sites on the virus binding with certain complementary molecules in the membrane or the specific host cell

• Following binding the nucleic acid is introduced into the host cell

Binding With the Host Cell

Assembly of New Viruses

• Once in the host cell the virus assumes control of the cell’s biochemical machinery

• Host cell supplies energy (ATP), nucleotides, amino acids

• Host then makes many identical copies of viral nucleic acid and protein coats

Release

• Either by LYSIS (bursting of the host cell membrane)

• Or BUDDING (the cell membrane pinches off containing the new viral particle)

Before and After Lysis

Budding

Smallpox

Retrovirus

• A retrovirus contains RNA

• It also contains reverse transcriptase an enzyme which produces viral DNA from viral RNA

• The virus can thus replicate itself

Acquired Immune Deficiency Syndrome (AIDS)

• AIDS is a retrovirus • It attacks helper T-

lymphocytes • The AIDS virus

attaches itself by glycoprotein on its surface to receptors on the helper T cell surface

AIDS

• The envelope surrounding the HIV particle fuses with the membrane of the helper T-cell and the virus enters the host cell

• Viral DNA becomes incorporated into the host cell’s DNA where it can remain dormant for many years

• Viral mRNA is transcribed and it directs synthesis of new viral particles inside the host cells

• These escape from the infected helper T cell by budding.

• The original T cell’s membrane is left perforated which causes destruction of the cell

The Facts You Need To Know • page 5

• from “a virus can only be seen….”

• to “e.g. the viral nucleic acid…”