Viruses & Bacteria

I.Viruses

A. General Information

  1. Virusa particle made up of nucleic acid, protein, & in some cases lipids that can replicate only by infecting living cells
  1. Viruses are organisms that bridge the gap between living and non-living

3. All viruses are composed of the following:

-genetic material: DNA or RNA (never both)

-capsid: protein coat surrounding genetic material

4. Virulent when a virus is active causes disease

  1. Temperate when viruses have infected the host, but disease is not immediate
  1. Size of a typical virus 20-14,000 nm in length

  1. Why are viruses considered NOT ALIVE?
  1. Do not have a(n)nucleus

cytoplasm

organelles

cell membrane

2. Can not reproduce (thru mitosis or meiosis)

3. Are not cells

i- must invade other cells to reproduce

ii- OBLIGATE INTRACELLUALAR PARASITES

4. Do not have control of movement outside of host cell

i- spread by wind, water, food, blood & by other means

C. History

1. 1918 Flu Virus killed approx. 21-25 million people

2. 1935 Viruses 1st isolated with use of an electron microscope

i- 1st virus to be isolated = Tobacco mosaic virus

3. TodayResearch is conducted using live tissue cultures (living cells in a culture)

D. Viral Structure

1. Polyhedral / Icosahedron capsid is composed of 20 triangular faces & 12 corners

i- example: adenovirus, papillomavirus (human wart virus) & polio

2. Helical (Rod-shaped) capsid is helical and surrounded by a protein coat

i- example: Tobacco Mosaic Virus, mumps & rabies

3. Enveloped Virus capsid is covered by a protective lipid bilayer (envelope) & surface is covered with protein spikes

i- can be helical or polyhedral in shape

ii- example: influenza virus & herpes simplex virus

4. Bacteriophage capsid has additional structures attached (head, collar & tail) & a base plate with tail fibers

i- capsid is polyhedral & tail in helical

ii- example: T-even bacteriophage

E. Classification of Viruses

1.DNA Viruscontains only DNA inside the capsid

a)once inside the host cell, a DNA virus will synthesize RNA (which will make more viral proteins)

OR

b)DNA may join with host DNA before beginning production

2.RNA Virus contains only RNA inside the capsid

a)once inside the host cell, a RNA virus is able to directly synthesize proteins (using host cell ribosomes & building block molecules

OR

b)Retrovirus

i-contain an enzyme called reverse transcriptase which synthesizes DNA from RNA

ii-DNA then makes new RNA

iii-RNA makes proteins

example: HIV/AIDS Virus

3.Viroid a strand of RNA with no protein capsid (only a protein shell)

a)do not contain enough RNA to synthesize proteins (too small)

b)disrupt cell functions

c)use cell enzymes to make now viroids

i- found only in plants

example: potato spine tuber viroid

4. Prion infectious particle made up of protein instead of DNA or RNA

a)reproduce in host

b)take a long time to become virulent

example: Mad Cow Disease (Bovine Spongiform Encephalopathy)

F. Reproduction and Evolution

1. Lytic Cycleprocess by which a virus enters a cell, makes copies of itself and causes the cell to burst

Example: bacteriophage infecting a bacteria cell

a) 5 Phases of the Lytic Cycle

  1. Attachment - virus attaches to bacterial receptor site on cell membrane
  1. Entry - virus injects nucleic acid (DNA) into host cell (other types of viruses will enter whole & then uncoat to expose nucleic acids)
  1. Replication - Viral DNA takes control of host DNA, making more viral DNA & proteins
  1. Assembly – new phages are assembled from these proteins & viral DNA is packaged
  1. Release – LYSIS: viral enzymes dissolve the bacterial cell wall & new bacteriophages leave to infect new cells

  • # of new viruses  10’s of 1000s

2. Lysogenic Cycle process by which a virus embeds its DNA into the DNA of the host cell and is replicated along with the host cell’s DNA

a)4 Steps of the Lysogenic Cycle

1. Prophage – Bacteriophage DNA attaches itself to host DNA

2.Host cell reproduces – DNA replicates including attached prophage segment

3. External stimulation triggers action in prophage

4. Prophage becomes virulent & completes lytic cycle

b) Transductionphage takes some host DNA segments w/ it, infects new cell & introduces genetic variability to new bacteria cells

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