Chapter 18.1

What is a virus? A virus is nucleic acid wrapped in a protein coat Can be DNA or RNA Viruses are considering nonliving because they can’t reproduce on their own The cell that a virus uses to reproduce is called a host cell Viruses are named after the disease they cause or the organ they affect Viruses that affect bacteria are bacteriophages

Viral Structure The inner core is made of DNA or RNA The outer coat of protein is called a capsid If a virus has another layer of protection this is called the envelope Viruses can be composed to 4 genes or 100’s of genes 4 shapes Polyhedral Spherical Helical Phage

Attachment to host cell Viruses must attach to the host cell to enter Viruses attach using receptor sites Once a virus attaches it injects it’s DNA or RNA into the host cells Viruses tend to be species specific

Viral Replication Lytic cycle Virus attaches Virus injects nucleic acid into host cell Host cell replicated viral DNA Virus particles are assembled The host cell breaks open (dies) and releases new virus particles Lysogenic cycle Virus attaches Virus injects nucleic acid into host cell Viral nucleic acid is incorporated into host cell genome, called a provirus Host cell divides, replicating genome with viral provirus, creating new cells that have the viral DNA Proviruses can be dormant for many years. When triggered, they enter the lytic cycle

Retroviruses Retroviruses use a more complex reproduction cycle Use reverse transcriptase to make DNA from an RNA template Cancer and viruses Several cancers are causes by viruses HPV Prions are proteins with no nucleic acid that are folded incorrectly Can cause mad cow disease Viroids are just a single strand of RNA with no protein coat