VirusesHigher Human Biology

Lesson AimsTo describe the structure of a virusTo examine the process of viral replication

Different types of virusesInfluenza virusAIDS virus

Different Shaped Viruses

SizeSmaller than bacteria20-300 nanometres (nm)

Structure of a VirusA virus contains nucleic acid (DNA or RNA)Surrounded by a protective coat (capsid)

Entering the Host CellDifferent viruses employ different methods of entering the host cellProcess depends on antigenic sites on the virus binding with certain complementary molecules in the membrane or the specific host cellFollowing binding the nucleic acid is introduced into the host cell

Binding With the Host Cell

Assembly of New VirusesOnce in the host cell the virus assumes control of the cells biochemical machineryHost cell supplies energy (ATP), nucleotides, amino acidsHost then makes many identical copies of viral nucleic acid and protein coats

ReleaseEither 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

RetrovirusA retrovirus contains RNAIt also contains reverse transcriptase an enzyme which produces viral DNA from viral RNAThe 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

AIDSThe envelope surrounding the HIV particle fuses with the membrane of the helper T-cell and the virus enters the host cellViral DNA becomes incorporated into the host cells DNA where it can remain dormant for many yearsViral mRNA is transcribed and it directs synthesis of new viral particles inside the host cellsThese escape from the infected helper T cell by budding.The original T cells membrane is left perforated which causes destruction of the cell

The Facts You Need To Know page 5from a virus can only be seen.to e.g. the viral nucleic acid