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