Lecture 11 Outline
What is an antibody? Why do I get a fever? (basic immunology); What is a virus? (basic virology)
Micrograph of Bacteriophage T4
Micrograph of Bacteriophage T7
Micrograph of Tobacco Mosaic Virus
Bacteriophages
bacteriophages are viruses that specifically attack bacteria
the capsid (or head) sits atop a sheath (or stalk)
the sheath ends in a baseplate from which protrude tail fibers
when a bacteriophage attacks a bacterial cell the following process ensues:
first the bacteriophage attaches to the bacterial cell membrane using its tail fibers
then viral enzymes bore a hole in the cell membrane
then the sheath contracts and the phage nucleic acids are injected into the bacterial cell
the viral nucleic acids take over cellular functions
viral proteins, like those in the capsid, are produced on bacterial ribosomes
the viral nucleic acid is also reproduced
the viral components come together to form mature virus particles
phages are self-assembling complexes
finally, the mature phages are released from the cell
this causes the cell to burst, thus this is the lytic cycle
Lytic cycle vs. lysogenic cycle
in the lytic cycle the reproduction of new virus particles continues without pauses until the infected cell bursts or lyses
in the lysogenic cycle, after the phage nucleic acids have been injected into the cell the infection processes pauses
during this pause, the viral nucleic acids are integrated into the bacterial cell chromosome (genomic DNA)
the viral DNA sequences can then be replicated along with the infected cell's DNA
at some point in time, the viral DNA genes are turned on or induced
generally environmental factors cause induction
at this point the virus takes over cellular functions, makes new virus particles and lyses the cell
The Immune Response
when an animal is invaded by a virus, bacteria or protozoan a series of defense events occur
macrophages reconnoiter the animal's blood, looking for invaders
proteins that protrude from cell surfaces carry information about the type of cell encountered
if the cell has the proper MHC complex (major histocompatability complex) then it is considered to be "self", belonging to that particular individual animal
cells that don't have the "self" signals on their surfaces are considered invaders
the macrophages eat the invader in a process called phagocytosis (eat in one bite)
after phagocytosis, the invader is generally killed (digested) by the macrophage
the macrophage does two things at this point: release pieces of the invader into the bloodstream or displays pieces of the invader on its surface
Release and the cause of fevers
bacterial cell walls contain (LPS) lipopolysaccharides (lipids [fats] attached to carbohydrates [sugar polymers])
the LPS is released from the macrophage during the digestion that follows phagocytosis
the released LPS signals other cells in the animal that the body is under attack
this causes some white blood cells to release a chemical signal called interleukin
the interleukin alerts other white blood cells and other glands and cells in the animal
a particular gland called the hypothalmus is sensitive to interleukin
the hypothalmus is the body's thermostat, controlling body temperature
when interleukin levels are high the hypothalmus releases hormones called prostaglandins
these chemicals cause the hypothalmus to raise the body's temperature - this is a fever
fever is maintained by restricting blood flow to the skin (vasoconstriction) where is it usually cooled
global metabolism rates in the body are enhanced
the body will shiver to raise its temperature
the fever process helps to kill bacteria or viruses, enhances the bodies rate of healing, and helps killer cells of the immune system
