DNA replication and gene expression in archaeans more closely resemble eukaryotic cells than the same processes in bacteria. However both Bacteria and Archaea are prokaryotic. What does this suggest?

Question

A.  Eukaryotic cells are descended from modern Archaeans.
B.  Bacterial cells are not related to Archaeans.
C.  Eukaryotic cells shared a more recent common ancestor with Archaeans than with bacterial cells.
D.  There must have been 2 origin events, one producing the bacteria and the other producing Archaeans and eukaryotic cells.

Clarify Question
· What is the key concept addressed by the question?
· What type of thinking is required?
 
Gather Content
· What do you already know about phylogenetic trees? How does it relate to the question?
 
Choose Answer
· Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?

  Reflect on Process
· Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?

Answer 1

C.  Eukaryotic cells shared a more recent common ancestor with Archaeans than with bacterial cells.

Clarify Question
· What is the key concept addressed by the question?
        o You observe that Eukaryotes are more similar to Archaea than to Bacteria. Which hypothesis best explains this observation?
· What type of thinking is required?
        o This question is an “evaluate” question. You must use your previous knowledge, along with logic and reason, to determine which hypothesis best explains the observed phenomenon.
 
Gather Content
· What do you already know about phylogenetic trees? How does it relate to the question?
        o How do phylogenetic trees help explain how organisms are related to each other? These trees use similarities and differences to map out relatedness. If two organisms are very similar, they are likely to be closely related. How would you draw a phylogenetic tree of the three domains? Are all three equally related? If so, they would all branch out of the same node. Are two domains more similar to each other? If so, first one domain branches off, then the other two.
        o Scientists used to think that Bacteria and Archaea were closely related to each other because they are both prokaryotes. These two domains were grouped together, and Eukarya was their “cousin” on the tree. Does modern evidence agree with this? What would a modern tree of the three domains look like?
 
Choose Answer
· Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?
        o You can evaluate some of these options using probability. What is the probability that life originated on Earth twice? Is it greater or less than the probability that Eukaryotes are related to Archaea?
        o Next, think about what you know of phylogenetic trees. If Eukaryotes are more similar to Archaea, does that mean that Archaea are not related to Bacteria at all? Does it mean that Eukaryotes evolved from modern-day Archaeans?

  Reflect on Process
· Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?
        o This question asked you to pick the hypothesis that best explains why Eukaryotes are more similar to Archaea than to Bacteria.
        o If you got the correct answer, great job!
        o If you got an incorrect answer, where did you get stuck?
            · Did you think that Eukaryotes descended from modern Archaeans because they are so similar? Eukaryotes did not descend from modern Archaeans, just like you did not descend from your sibling. The two domains descended from a common ancestor, and that common ancestor was more recent than the common ancestor with Bacteria. Think of Eukarya and Archaea as sisters, both descended from a mother. Bacteria is their cousin, related to them through everyone’s grandmother.
            · Phylogenetic trees are sometimes difficult to understand at first. If you are struggling with this concept, talk to your instructor or TA and try to do more practice problems. The best way to build understanding and confidence for this concept is practice!