VIRUSES SB13U Unit: Diversity of Living Things “The single biggest threat to man’s continued dominance on the planet is a virus.” —Joshua Lederberg, Nobel laureate

What is a virus? Have no cellular structure, thus NOT classified in any kingdom of living things Have no cytoplasm organelles, or cell membranes Don’t carry out respiration or other common life processes Consist of DNA or RNA and a protein capsid Capsid helps from attack of host cell enzymes and is needed for attachment onto host cell receptors Are mobile genes that parasitize cells

Origin of Viruses Because viruses CANNOT replicate without host cells, this means they must have evolved after the first cells came into existence Probably originated as fragments of nucleic acid that escaped from their original cell and survived by becoming parasites Viruses are more related to their host than to other viruses

Classification Since discovery in 1935, more than 160 groups of viruses have been identified Viruses are classified according to 1. Size and Shape 2. Animal or Plant Viruses 3. Type of diseases produced

Groups differ in shape and size ◦ shape is determined by the type of arrangement of proteins in the capsid

Viral Reproduction A virus cannot multiply on its own – depends ENTIRELY on the metabolism of the prokaryotic or eukaryotic cell to replicate its DNA or RNA

Stages of Replication A. Attachment: virus attaches to specific receptor site on plasma membrane. Lock and key model, thus can only attach to specific sites. B. Entry: can enter in 2 ways 1. Lytic cycle: viruses inject its nucleic acid into the cell 2. Envelope virus: membrane of host cell surrounds the virus creating a vacuole of viruses inside cell’s cytoplasm. When virus breaks out of vacuole, it releases nucleic acid into the cell

C. Replication: hosts metabolism replicates viral DNA or RNA D. Assembly: new virus particles are assembled E. Lysis and Release: host cell breaks open and releases new virus particles Envelope Viruses: normally released, without damage to host cell

Genome of Viruses Genome consists of either DNA or RNA May be single nucleic acid molecule or several segments May be single or double stranded May be linear or circular Is limited due to size

Examples of Viruses 1. Coronavirus Is an enveloped virus The SARS coronavirus, sometimes shortened to SARS-CoV, is the virusthat causes severe acute respiratory syndrome (SARS). On April 16, 2003 WHO indicated that the caronavirus was the cause of SARS ~8000 people were infected around the world (WHO).

2. Influenza A (H1N1) Virus - most common flu virus in declared a pandemic – according to WHO, 17,000 deaths around the world by the start of the virus was made up of genetic elements from 4 different flu viruses (North American swine influenza, north American avian influenza, human influenza, and swine influenza found in asia and Europe)

Biotechnology Because viruses enter host cells and direct the activity of the host cell’s DNA they can be useful for genetic engineers

Ex: Genetic Cloning Stages: 1. Splice “wanted” gene into genome of a virus 2. Enters host cell and directs it to multiple copies of the virus 3. Each new virus produced contains “wanted” gene