Lecture 10 May 24 2009 Dr Abumaree

Lecture 10 Introduction to bacteria & viruses Biology, Campbell & Reece 7th Edn. Ch 18 By

Dr Mohamed Abumaree Molecular Reproductive Biologist & Immunologist College of Medicine King Saud bin Abdulaziz University for Health Science 2009

 Bacteria: 1. Prokaryotic cells 2. Smaller and simpler than eukaryotic cells  Viruses: 1. Smaller and simpler than bacteria 2. Lack the structures & metabolic machinery found in cells

Structure of Viruses  Some viruses can crystallize  Viruses are not cells  Viruses are infectious particles  Viruses consist of nucleic acid enclosed in a protein coat (capsid) with or without an envelope

Viral Genomes  Viral genomes consist of:  Double–stranded DNA  Single–stranded DNA  Double–stranded RNA  Single–stranded RNA  DNA or RNA virus is usually organized as a single linear or circular molecule of nucleic acids

Capsids and Envelopes  Capsid is a protein shell encloses the viral genome  Capsid takes rod or polyhedral or a complex shape (like T4)

Capsid are built from a large number of protein subunits called capsomeres

 Tobacco mosaic virus: a rigid, rod–shaped capsid  Adenoviruses: infect the respiratory tracts of animals, have a polyhedral capsid

 A membranous envelope surrounds the capsids to help viruses infecting the hosts  Viral envelope is derived from the host cells and the virus its self  Viral envelope contains phospholipids, proteins, enzymes & glycoproteins

Bacteriophages (phages): the most complex capsids that are found among viruses infecting bacteria

Viral Reproductive Cycles  Viruses: 1. Reproduce only within a host cell 2. Do not reproduce as an isolated virus 3. Infect a limited/broad range of host cells 4. Infect a single species (such as, measles virus & poliovirus infect only humans)

 Viral infection of eukaryotes is usually limited to particular tissues (such as, cold viruses infect respiratory tract or AIDS virus binds to white blood cells)  DNA viruses use host DNA polymerases to synthesize new genomes  RNA viruses use viral polymerases, which use RNA as a template

A viral infection begins: 1. When the viral genome enter the host cell 2. The viral genome commandeers its host to provides the component needed for making the viral proteins 3. The viral reproductive cycle damages the cell that causing some of the symptoms associated with viral infections 4. The viral progeny (children) exit the infected cell to infect another cells (spreading the viral infection)

Reproductive Cycles of Phages  Phages are the best understood & the most complex of all viruses  Some Double–stranded DNA viruses reproduce by 2 alternative mechanisms: 1) The lytic cycle 2) The lysogenic cycle

The Lytic Cycle  A lytic cycle causes the death of the host cell  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 cycles can destroy an entire bacterial population in just a few hours

The Lysogenic Cycle  The lysogenic cycle does not destroy the host  Temperate phages can use reproducing cycles within a bacterium

both

 Lambda (λ), a temperate phage resembles T4, but its tail has only one short tail fiber

What triggers the switchover from the lysogenic to the lytic mode? An environmental signal, such as radiation or the presence of certain chemicals are the triggers

Animal Viruses

 An animal virus have a viral envelope  Viral glycoproteins are projecting from the envelope that bind to the host cell  The reproductive 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 enveloped viruses contain 1. 2 identical molecules of single– stranded RNA 2. 2 molecules of reverse transcriptase

The structure of HIV The envelope glycoproteins enable the virus to bind to specific receptors on certain white blood cells