Question

Although they were once classed together, Archaea and Bacteria have now been separated into two different domains. Why was this change necessary?

A: The Archaea only have cell membranes and the Bacteria only have cell walls.

B: The two groups are composed of species with different cell sizes.

C: The Archaea have nuclei and the Bacteria do not.

D: The two groups replicate their DNA in different ways.

Answer 1

Archaea by their name describes a distinct domain of single celled organisms that are once thought to be closely related to prokaryotes such as bacteria. The arch prefixes signifies a advance capabilities of these organisms to survive in extreme conditions, in heat (thermophiles), salt concentrations (halophiles), high acidity (acidophiles) etc.

The reason for this extreme tolerance of environmental stress principally stems from their membrane structure. Instead of ester linkages found in the fatty acid (C double bond O and O) bilayer pseudo-membrane for bacteria, ether linkages (O single bond C single bond O) are used to link hydrocarbons to the glycerol back bone which composes the archaea cell membrane. The ether is less prone to degradation and less reactive than the ester counterpart. Additional the presence of peptidoglycan, a sugar molecule that is used as a structural unit for the cell walls is present within bacteria but not archaea. Antibiotics often target the peptidoglycan structural components in bacteria to prevent them from reproducing (penicilin), this gives majority of archaea cellular organisms immunity from antibiotics.

However from this evidence it is tempting to focus on A as the answer. But the question is asking why are they considered different by today's classification. It is because archaea organism are actually more closely related to eukaryotes (Complex single/multicellular organisms). This is because they share very similar molecular machinery to eukaryotes in there DNA replication and protein synthesis mechanisms, they have histone, nucleosome like structures for storing DNA (much similar to human cells) while bacteria usually only have circular strand of chromosome. And multiple types of transcription enzymes to preform this act while bacteria usually only have one type. It is because of this difference in reproduction mechanisms they are classified differently from bacteria and they are considered to be more closely related to us than to bacteria.

D would be the most correct answer to the question, A is a correct statement but does not provide sufficient relevant information to answer the question.