Are Viruses Alive?.

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Presentation transcript:

Are Viruses Alive?

Definition “A virus is a nonliving particle composed of a nucleic acid and a protein coat.”

Characteristics of Life Growth and Develop Homeostasis Metabolism Genetic Information Evolution Reproduction Made of Cells Respond to Stimuli

enveloped (influenza) helical (rabies) polyhedral (foot-and-mouth disease) capsid nucleic acid lipid envelope surface proteins capsid surface proteins nucleic acid capsid nucleic acid lipid envelope Surface proteins

Are they living or nonliving? LIVING NONLIVING Do not reproduce Need a HOST in order to survive No nucleus Do not grow Do not make energy Contain DNA/RNA Replicate Made of lipids, proteins, nucleic acids Undergo mutations

Viruses cause two types of infections. A lytic infection causes the host cell to burst. host bacterium The bacterophage attaches and injects it DNA into a host bacterium. The host bacterium breaks apart, or lyses. Bacteriophages are able to infect new host cells. The viral DNA directs the host cell to produce new viral parts. The parts assemble into new bacteriophages. The viral DNA forms a circle. The virus may enter the lysogenic cycle, in which the host cell is not destroyed.

A Virus attaches to the host and injects its DNA into the cell. Lytic http://www.cat.cc.md.us/courses/bio141/lecguide/unit2/viruses/lytsum.html A Virus attaches to the host and injects its DNA into the cell. B The DNA replicates in the host cell. C The parts of the virus are created. D The new viruses erupt from the host cell.

A lysogenic infection does no immediate harm. The viral DNA is called a prophage when it combines with the host cell’s DNA. Although the prophage is not active, it replicates along with Many cell divisions produce a colony of bacteria infected with prophage. The prophage may leave the host’s DNA and enter the lytic cycle.

A Virus attaches to the host and injects its DNA into the cell. B The viral DNA attaches to the host DNA. C DNA replication takes place (Interphase) D The cell undergoes mitosis E Stress causes the viral DNA to create the “weird” protein thus creating an outbreak!

RNA viruses (Ex: HIV) The genome of some viruses uses RNA instead of DNA. In order to replicate, a RNA virus uses a special enzyme to create DNA (enzyme = reverse transcriptase) Once the DNA is created then it is incorporated into the host genome (like the lysogenic cycle) The difference is the genome is now a permanent resident of the host cell

Viruses cause many infectious diseases There are many examples of viral infections. common cold

Viruses cause many infectious diseases There are many examples of viral infections. common cold influenza

Viruses cause many infectious diseases There are many examples of viral infections. common cold influenza SARS Animal viruses are prions Plant viruses are viroids

Viruses cause many infectious diseases There are many examples of viral infections. HIV-infected white blood cell HIV which is a RNA virus The body has natural defenses against viruses.

Vaccines are made from weakened pathogens. A vaccine stimulates the body’s own immune response. Vaccines prepare the immune system for a future attack by creating memory cells. Pathogen = infectious agent, or germ, is a biological agent that causes disease or illness to its host Some viruses have a limited host range and others are able to jump to different species.