SBI 3U Ms.Zafar October 1st, 2012 Viruses SBI 3U Ms.Zafar October 1st, 2012

What are Viruses? Virus  Latin for poison Viruses do not fit within the 5-kingdom system Often thought of as closely related to prokaryotes A virus is a non-cellular particle made up of protein-covered genetic material that can invade living cells

What Makes Viruses Unique? Lifeless chemical if outside living cell Reproduces when it is inside living cell Occupy a position between nonliving and living matter

Size & Shape Viruses could not be identified until 1934  because of their small shape 5000 influenza viruses can be fit onto the head of a pin! Shapes and sizes vary

Components of a Virus Much less complex than the simplest living organism Inner nucleic acid core Outer protective protein coat called a capsid  makes up 95% of the total virus  gives shape Genetic material can be either RNA or DNA

Viral Diversity Not every virus is disease-causing  some are benign Viruses are selective Specific viruses enter only specific host cells Most bacteriophages have a very restricted host range Bacteriophage: Infect bacteria Host Range: The limited # of host species that a virus can infect Ex: AIDS virus has a very limited host range

Viral Replication The cell and the computer analogy Virus invades a living cell and then reproduces Viruses can reproduce in 2 ways: Lytic Cycle or Lysogenic Cycle

Whole process can take between 25-45 minutes Lytic Cycle Attachment and entrance. Either whole virus, or only DNA or RNA Synthesis of protein and nucleic acid Assembly of the nucleic acids, enzymes and proteins into new virus particles Release of new virus particles Whole process can take between 25-45 minutes

Lytic Cycle Lysis: The destruction or bursting open of a cell After lysis, the new viruses infect other cells A 100 or so clones can be released Bacteriophage that causes lysis of the host cell is said to be a virulent phage

Restriction Enzymes Restriction Enzymes are those within bacteria that recognize foreign DNA and cut up the foreign DNA

Lysogenic Cycle Bacteriophage that does not cause lysis of the host is called a temperate phage This kind of virus can coexist with the host cell In a lysogenic cycle, the virus does not kill the host cell outright

Lysogenic Cycle Virus injects nucleic acid into host Nucleic acid becomes integrated into host DNA Replicated with the host DNA  virus in dormant stage  lysogeny

RNA Viruses RNA Viruses are those that contain RNA Act as messenger RNA once inside host cell or can provide instructions to manufacture its own messenger RNA All RNA viruses are lytic – some exceptions

Retroviruses Retroviruses need to be reverse transcribed into DNA before they can be copied the right way Example: human immunodeficiency virus (HIV) that causes acquired immunodeficiency syndrome (AIDS) Lysogenic cycle Virus enters a human white blood cell and directs host cell to make DNA from viral RNA