Midterm exam #2 - Fall 1997

1. What are the 3 primary evolutionary branches of life? (5 points)

Bacteria, Eukarya, and Archaea

2. List the 3 main explanations for the origin of viruses. (5 points)

  1. genetic offshoots of their hosts
  2. degenerate parasites
  3. remnants of precellular forms of life

3. How does the transfer of plasmid DNA by conjugation differ from the transfer of M13 DNA in an infection cycle? (5 points)

Conjugation results in the transfer of plasmid DNA directly - the two cells must come in contact. In an M13 infection, the transfer of the DNA is via an encapsidated intermediate, the virus particle, and so the cells do not need to come into contact.

4. Briefly describe a bacterial species (or group) of a genus different than the one you described in your take-home question. (10 points)

e.g. for Thermotoga:
All are thermophilic (65-90C), anaerobic fermentative rods (protons or sulfur used as e- acceptor to make H2 or H2S). Cells enclosed in a loose sheath, or 'toga' - this is the outer membrane of a Gram-negative-type envelope. Isolated from geothermally-heated soils (solfataras) & marine sediments

5. Briefly describe a eukaryotic species (or group) of a genus different than the one you described in your take home question. Plants, animals, or fungi are not allowed. (10 points)

 Example answer:



6. Name an electron donor and acceptor that together can be used to drive electron transport for the generation of ATP. (5 points)


electron donor __e.g. NADPH____ electron acceptor __e.g. O2___

Other electron donors include: sulfide, thiosulfate, nitrite, hydrogen, etc

Other eletron acceptors include: nitrate, nitrite, sulfate, ferric ion, etc


7. What are the 4 mechanisms used by Bacteria for motility? Describe how one of these works. (10 points)

  1. Gas-vacuole bouyancy
  2. flagella
  3. gliding
  4. however-spirochaetes-move

e.g. for gliding. Gliders move by secreting partially hydrated polysaccharides from pits in their cell walls. Absorbtion of water from the environment causes the polysaccharide to expand dramatically, driving the cell forward just like a rocket

8. What is the advantage of the 2-photosystem 'Z-scheme' photosynthetic process used by cyanobacteria (and chloroplasts) over the 1-photosystem cyclic photophosphorylation carried out by other photosynthetic organisms. (5 points)

The 2-photosystem process can be used to generate reducing power (NADPH) for carbon fixation without the consumption of ATP.

9. How did symbiosis contribute to the evolution of eukaryotes? (10 points)

Both the mitochondrion and chloroplast are derived from bacterial endosymbionts. Aerobiosis in eukaryotes is provided by mitochondrial function (amitochondrial eukaryotes are anaerobic), and photosynthesis is provided by chloroplast function (achloroplastic eukaryotes are heterotrophs).

10. Why is it believed that Bacteria have thermophilic ancestry? (5 points)

Because the early and primative branches of Bacteria are all predominantly thermophilic.

11. Imagine that you are a substitute TA for MB09 (Microbial Diversity). The students begin to ask you to look at their slides & identify what they see. Use what you've learned in class about different kinds of organisms to identify any 5 of these 6 organisms as best you can, and give your rationale. (2 point each - 10 points total)


This is a diatom - it's quite large, and has the distinct glassy, striated cell wall.


Low magnification from pond water - this cell is lime green & glides.


This is Bacillus - it's producing endospores and is aerobic.


High magnification - from an colony grown under aerobic conditions.


This is a cyanobacterium - it is green, filamentous, and gliding, and is producing heterocysts for nitrogen fixation.


High magnification from pond water - the filaments look greenish and are gliding.


This is a planctomycete - it is stalked and forming a rosette, and the cells are dividing by budding.


High magnification from pond water.


This is a prosthecate alpha-purple bacterium - it has 'arms' that look like extensions of the cell, and is dividing by fission (not budding).

High magnification from pond water.


This is a diplomond. It has 2 nuclei with condenses chromatin and flagella. It's dead because it's an anaerobe!


Low magnification from a stool sample - seems to be dead.