VirusesViruses

At the boundary of life, between the At the boundary of life, between the macromolecules (which are not alive) and the macromolecules (which are not alive) and the prokaryotic cells (which are), lie the viruses and prokaryotic cells (which are), lie the viruses and bacteriophages (phages). bacteriophages (phages).

These twilight creatures are These twilight creatures are parasitesparasites responsible for causing many diseases in living responsible for causing many diseases in living things (herpes and HIV in humans, for example).things (herpes and HIV in humans, for example).

Viruses are found everywhere.Viruses are found everywhere.

Viruses consist of aViruses consist of a core of nucleic acid, core of nucleic acid, either DNA or RNA, and a protective either DNA or RNA, and a protective coat of protein molecules and coat of protein molecules and sometimes lipids. sometimes lipids.

Viruses -The Boundary of Life

In isolation, viruses and In isolation, viruses and bacteriophages show none of the bacteriophages show none of the expected signs of life. They do not expected signs of life. They do not respond to stimuli, they do not grow, respond to stimuli, they do not grow, they do not do any of the things we they do not do any of the things we normally associate with life. normally associate with life.

Strictly speaking, they should not be Strictly speaking, they should not be considered "living" organisms at all. considered "living" organisms at all. However, they are more complex than However, they are more complex than a lifeless collection of macromolecules a lifeless collection of macromolecules and they do show one of the most and they do show one of the most important signs of life: the ability to important signs of life: the ability to reproduce at a fantastic rate reproduce at a fantastic rate but only in a host cellbut only in a host cell..

Bacteriophages attack bacteria Bacteriophages attack bacteria (prokaryotes)(prokaryotes)

viruses attack eukaryotic cells.viruses attack eukaryotic cells.

Viruses and bacteriophages invade cells Viruses and bacteriophages invade cells and use the host cell's machinery to and use the host cell's machinery to synthesize more of their own synthesize more of their own macromolecules.macromolecules.

Once inside the host the bacteriophage or Once inside the host the bacteriophage or virus will either go into a virus will either go into a LyticLytic Cycle Cycle - - destroying the host cell during destroying the host cell during reproduction.reproduction.

or or

It will go into a It will go into a Lysogenic Cycle Lysogenic Cycle - a- aparasitic type of partnership with parasitic type of partnership with the cellthe cell

The Lytic CycleThe Lytic Cycle

The Lysogenic CycleThe Lysogenic Cycle

A A provirusprovirus is ais a DNA virus that has been inserted into a DNA virus that has been inserted into a host cell chromosome.host cell chromosome.

A A retrovirusretrovirus injects the enzyme, reverse transcriptase into the cell injects the enzyme, reverse transcriptase into the cell to copy viral RNA into DNAto copy viral RNA into DNA..

HIV is a HIV is a retrovirus injecting the enzyme, reverse retrovirus injecting the enzyme, reverse transcriptase into the cell to copy viral RNA into DNAtranscriptase into the cell to copy viral RNA into DNA..

Viruses are host specific – a protein on the surface of the virus has a shape that matches a molecule in the plasma membrane of its host, allowing the virus to lock onto the host cell.

HIVHIV doesn’t target just any cell, it goes right for the cells that doesn’t target just any cell, it goes right for the cells that want to kill it. “Helper" T cells are HIV's primary target. These want to kill it. “Helper" T cells are HIV's primary target. These cells help direct the immune system's response to various cells help direct the immune system's response to various pathogens. pathogens.

HIV undermines the body's ability to HIV undermines the body's ability to protect against disease by depleting T protect against disease by depleting T cells thus cells thus destroying the immune systemdestroying the immune system..

The virus can infect 10 billion cells a day, The virus can infect 10 billion cells a day, yet only 1.8 billion can be replaced daily.yet only 1.8 billion can be replaced daily.

After many years of a constant battle, the body After many years of a constant battle, the body has insufficient numbers of T-Cells to mount an has insufficient numbers of T-Cells to mount an immune response against infections. At the point immune response against infections. At the point when the body is unable to fight off infections, a when the body is unable to fight off infections, a person is said to have the disease person is said to have the disease AIDSAIDS..

It is not the virus or the disease that ultimately It is not the virus or the disease that ultimately kills a person; it is the inability to fight off kills a person; it is the inability to fight off something as minor as the common cold. something as minor as the common cold.