Structure of a virus All viruses have a protein coat that surrounds nucleic acid. The nucleic acid may be DNA or RNA.

Structure of a bacteriophage A bacteriophage is a virus that invades bacterial cells (parasitic). Phages contain DNA bound by a protein coat They have no cell membrane, cytoplasm, organelles or chromosomes DIAGRAM PAGE 50

Inside the host cell the viral DNA codes for new protein coats. The DNA replicates to make copies which are packaged in the protein coats, making new viruses. Eventually the bacterial cell ruptures releasing many new viruses.

Human Immunodeficiency Virus (HIV) HIV is a retrovirus; it contains RNA instead of DNA It is bounded by a protein coat and a lipid bilayer containing glycoprotein, which allows the virus to remain largely undetected by the human immune system.

HIV invades white blood cells called lymphocytes, specifically helper T-cells It contains the enzyme reverse transcriptase which is used to synthesise viral DNA from its RNA. The DNA replicates, making new viral RNA and protein coats, which make new viruses. The new viruses destroy the T helper cells, and as more are destroyed the immune system becomes critically compromised Leading to the development of the medical condition, AIDS, where the body is unable to destroy other pathogens

glycoprotein phospholipid Bilayer protein layer reverse transcriptase (obtained from human cell) protein layer reverse transcriptase RNA