Upload
api-19824406
View
73
Download
3
Embed Size (px)
Citation preview
Lecture 10Lecture 10Introduction to bacteria & Introduction to bacteria &
virusesvirusesBiology, Campbell & Reece 7th Edn. Ch 18
By
Dr Mohamed AbumareeMolecular Reproductive Biologist & Immunologist
College of MedicineKing Saud bin Abdulaziz University for Health Science
2009
Bacteria:Bacteria:
1. Prokaryotic cells
2. Smaller and simpler
than eukaryotic cells
Viruses:Viruses:
1. Smaller and simpler
than bacteria
2. Lack the structures &
metabolic machinery
found in cells
SomeSome viruses can crystallizecrystallize
VirusesViruses are not cellsnot cells
VirusesViruses are infectious particlesinfectious particles
VirusesViruses consist of nucleic acidnucleic acid enclosed in a protein coat (capsid) with or without an protein coat (capsid) with or without an envelopeenvelope
Structure of VirusesStructure of Viruses
Viral genomes consist of:Viral genomes consist of: Double–stranded DNA Single–stranded DNA Double–stranded RNA Single–stranded RNA
DNA DNA or or RNA virusRNA virus is usually organized as a
single linear or circular molecule of nucleic single linear or circular molecule of nucleic
acidsacids
ViralViral GenomesGenomes
Capsids and EnvelopesCapsids and Envelopes CapsidCapsid is a protein
shell encloses the viral
genome
CapsidCapsid takes rod or
polyhedral or a complex
shape (like T4)
CapsidCapsid are built from a large number of protein subunits called capsomerescapsomeres
Tobacco mosaic virus: Tobacco mosaic virus: a rigid, rod–shaped capsid
Adenoviruses:Adenoviruses: infect the respiratory tracts of animals, have a polyhedral capsid
A membranous A membranous
envelopeenvelope
surrounds the
capsidscapsids to help
viruses viruses infecting infecting
the hoststhe hosts
Viral envelopeViral envelope is derived from the host cells and the
virus its self
Viral envelopeViral envelope contains phospholipids, proteins,
enzymes & glycoproteins
Bacteriophages (phages): Bacteriophages (phages): the most complex capsids that are found among
viruses infecting bacteria
Viruses:Viruses:
1.1. Reproduce onlyReproduce only within a host cell
2.2. Do not reproduceDo not reproduce as an isolated virus
3.3. Infect a limited/broadInfect a limited/broad range of host cells
4.4. Infect a singleInfect a single species (such as, measles
virus & poliovirus infect only humans)
Viral Reproductive Cycles
Viral infection of eukaryotesViral infection of eukaryotes is usually limited to particular tissues (such as, cold cold virusesviruses infect respiratory tract or AIDS virusAIDS virus binds to white blood cells)
DNA virusesDNA viruses use host DNA polymerases to synthesize new genomes
RNA virusesRNA viruses use viral polymerases, which use RNA as a template
A viral infection begins:A viral infection begins:
1.1. When When the viral genome enter enter the host cell
2.2. The viral genome commandeersThe viral genome commandeers its host to
provides the component needed for making the viral
proteins
3.3. The viral reproductive cycleThe viral reproductive cycle damages the cell that
causing some of the symptoms associated with viral
infections
4.4. The viral progenyThe viral progeny (children) exit the infected cell
to infect another cells (spreading the viral infection)
PhagesPhages are the best understood & the most complex of all viruses
Some Double–stranded DNADouble–stranded DNA viruses reproduce by 2 alternative mechanisms:
1) The lytic cycle 2) The lysogenic cycle
Reproductive Cycles of Phages
The Lytic CycleThe Lytic Cycle A lytic cycleA lytic cycle causes the death of the host cellthe host cell
During the last stage of infection:During the last stage of infection:1. The bacterium lyses
2. The newly produced phages are released
3. The newly produced phages infect a healthy cell
The lytic cycleslytic cycles can can destroydestroy an entire bacterial
population in just a few hours
The Lysogenic Cycle The lysogenic cyclelysogenic cycle does not destroy the host
Temperate phagesTemperate phages can use both reproducing cycles within a bacterium
Lambda (λ),Lambda (λ), a temperate phage resembles T4, but its tail has only one short tail fiber
What triggers the What triggers the switchover from the switchover from the
lysogenic to the lytic mode?lysogenic to the lytic mode?
An environmental signal, such as An environmental signal, such as radiation or the presence of certain radiation or the presence of certain
chemicals are the triggerschemicals are the triggers
Animal VirusesAnimal Viruses
An animal virusAn animal virus have a viral envelope
Viral glycoproteinsViral glycoproteins are projecting from the envelope that bind to the host cell
The reproductive cyclereproductive cycle does not kill the host cell
Enveloped viruses enter the host cell by:
(1) Fusion of the envelope with the cell′s plasma membrane OR
(2) Endocytosis
RNA viruses are found in: Some phages Most plant viruses Most animal viruses
Single–stranded RNA viruses directly serve as mRNA & thus can be translated into viral protein immediately after infection
Retroviruses are RNA animal viruses
Retroviruses have reverse transcriptase (enzyme), which transcribes an RNA → DNA, the opposite of the usual direction, so called retroviruses (retro means “backward”)
HIV (human immunodeficiency virus) is a retrovirus that causes AIDS (acquired immunodeficiency syndrome)
HIV & other retroviruses are HIV & other retroviruses are enveloped viruses contain enveloped viruses contain
1.1. 2 identical molecules of single–2 identical molecules of single–stranded RNA stranded RNA
2.2. 2 molecules of reverse transcriptase2 molecules of reverse transcriptase
The structure of HIVThe envelope glycoproteins enable the virus to
bind to specific receptors on certain white blood cells