MBIO 1010 - Lecture 12

• Eukaryotes (true nucleus)

  • genetic material is housed in a nucleus
  • generally larger than prokaryotes
  • complex internal structure
  • membrane bound organelles
  • intra-cytoplasmic membranes used for transport - in the organelles
  • cytoskeleton
  • divide by mitosis and meiosis
  • Some eukaryotes (lip like) have flagella but is not the same as bacterial flagella (propeller)
• the nucleus

  holds the genetic information, multiple linear double stranded DNA chromosomes

  only in eukaryotes

  where the histones are - tightly packed with DNA

  Membranes around the nucleus
• Chloroplasts

  are in eukaryotes but we have a theory that they might have evolved from prokaryotes

  70S ribosomes which is the same that we would see in a prokaryote

  size of unicellular cyanobacteria

  Site of photosynthesis

  Chlorophyll

  surrounded by 2 membranes

  DNA and ribosomes (70S)
• Mitochondria

  site of respiration and oxidative phosphorylation

  surrounded by 2 membranes

  DNA and ribosomes (70S)

  few protists that have mitochondria

  in eukaryotes

  nearly universal in eukaryotic cells but there is a few protists that don't have mitochondria and so not a defining feature of what a eukaryote is
• The Endosymbiotic Hypothesis

  mitochondria and chloroplasts evolved from bacteria - descendent of phototropic bacteria and became apart of eukaryotes

  • Evidence - mitochondria and chloroplasts are both
  • Semi-autonomous
  • circular chromosomes
  • lack histones
  • 70s ribosomes
  • two membranes
  • outer membrane has porins
• the endosymbiotic hypothesis is the thought that these bacteria became part of eukaryotes
• mitochondria are most closely related to Rickettsia, rickettsia is the genus and proteobacteria is the phylum, obligate intracellular pathogens. ex) rocky-mountain spotted fever
• Chloroplasts are most closely related to cyanobacteria (blue-green algae)

  Note* Cyanobacteria used to be called blue-green algae but it is NOT algae its a bacteria
• Viruses are not part of the tree of life because they are not living
• Viruses

  • Acellular infectious particles - not considered living, no cytoplasmic membrane
  • obligate intracellular pathogens - cannot survive on their own (don't have enough of their own things to survive on their own) - need a host
  • reproduce inly inside of living cells
  • lack independent metabolism
  • composed of at least 2 parts - minimum requirements to be called a virus
  • nucleic acid genome (DNA or RNA)
  • protein coat (capsid)
  • together is the nucleocapsid
  • some viruses have an envelope - layer of lipid surrounding the nucleocapsid
• Viral genomes

  • DNA or RNA - Never both at the same time
  • single stranded or double stranded
  • circular or linear
  • can be in several pieces - segmented
  • at times one type of virus can be DNA or RNA but not at the same time
  • only one type of genomic nucleic acid is found in the virion of any particular type of virus
  • genome size
  • smallest ~3.6 kb for some ssRNA viruses (3 genes)
  • largest >150 kbp for some dsDNA viruses (>100 genes)
• capsid - protein coat that surrounds the genomes

  • allows transfer of viral genome between host cell
  • provides protection to the genome
  • can't just have DNA or RNA floating around, will become degraded by enzymes that will cut it up
  • made of identical polypeptides -protomer
  • helical capsids
  • protomers from a spiral cylinder
  • nucleic acid genome coiled inside
  • ex. tobacco mosaic virus capsid is made of ~2100 identical protomers
  • protomers assemble together to made capsomeres
  • protomers aggregate to form capsomeres
  • multiple types of capsids
  • helical capsid
  • icosahedral capsid
• Protomer - basic unit - made of identical polypeptides, Will come together to make capsomeres

  Capsomeres are the building block of capsid but made from protomers
• Icosahedral capsids

  • regular geometric shape with 20 triangular faces
  • exhibit symmetry
  • protomers aggregate to form capsomeres
  • ex. human papillomaviruses have form their capsids from pentamers (clusters of 5)
  • repeating shapes around forming a ball
  • ex. HPV
• Binal capsids

  • geometric head with an attached helical tail
  • ex. T4 bacteriophage of E.coli
  • genome is carried in a polyhedral head, helical tail is used to inject DNA into a host cell
  • Nucleocytoplasmic large DNA viruses
  • viruses with complex multi-layered structure
  • ex. mimivirus (infects amoebae)
  • 0.75 micrometer in diameter, 1200 kbp DNA
  • larger than some bacteria
• bacteriophage

  • viruses that infect bacteria
• Envelope is a lipid bilayer surrounding the nucleocapsid that was acquired from the host membrane

  • consists of host lipids and viral proteins -spikes
  • ex. influenza virus
  • flexible helical capsid, surrounded by an envelope
  • two major spikes: hemaglutanin (H) and neuraminidase (N)
• Viral protein is called a spike
• Genetic material on the inner most protected by the capsid or protein coat and then the envelope
• Host range

  • viruses infect all domains of life
  • animal viruses infect and multiply only inside of animal cells
  • human papillomavirus - infects human epithelial cells
  • causes benign tumors (warts)
  • most viruses are pretty specific and will only affect a single host
• Most viruses are specific to a single host species
• virus must attach to specific receptors on the host cell surface

  • ex. HIV binds to CD4
  • Chemoreceptor on surface of some human immune system cells
  • HIV infects only humans
• some viruses infect more than one species

  • ex. influenza attaches to a glycoprotein found on surface of several animal cells
  • infects humans, pigs, chickens, seals etc
• Viral replication cycle

  1. adsorption - attachment to the host cell

  • involves specific receptors on the host cell surface
  • ex. LPS, outer membrane proteins or glycoproteins
• viral replication cycle

  2. penetration and uncoating - entry into the host cell

  • bacteriophage - usually inject their nucleic acid into the cell
  • leave the capsid outside the cell as a ghost
• entry by animal viruses

  • fusion with the plasma membrane
  • endocytosis
  • binding to specific receptors triggers normal endocrytic activity
  • in either case, once inside:
  • the capsid is removed
  • viral genome is released into the cell
• viral replication cycle

  3. synthesis of viral nucleic acids and protein

  • viral genes are expressed and viral proteins are synthesized (by the host's own ribosomes)
  • viral genome is replicated (by the host's replication machinery)
• viral replication cycle

  4. assembly of new virions

  • viral proteins are assembled into capsids, and then genomes are packaged into nucleocapsids
  • viruses do not reproduce by division
• viral replication cycle

  5. release of new virion

  • two basic strategies:
  • naked viruses usually accumulate, eventually lysing the host cell to release progeny - lytic infection
  • enveloped viruses are usually released by budding
  • virion push through the cytoplasmic membrane without killing the host cell - persistent infection
• Fusion only happens for enveloped viruses

  endocytosis can occur for enveloped or naked viruses
• naked viruses cant do fusion