Bacteria and archaea are both classified as prokaryotes. In which of the following ways are these two groups similar to each other?

Introduction / Context:
Bacteria and archaea are two major domains of life that were once grouped together but are now recognised as distinct. They share some basic cellular features but also have important biochemical and genetic differences. This question asks you to identify a true similarity between these two groups while avoiding statements that actually describe differences.

Given Data / Assumptions:

• The groups compared are bacteria and archaea.
• We focus on similarities, not differences.
• Options mention prokaryotic cells, membrane linkages, cell wall composition, start codons, and multicellularity.
• We assume standard textbook definitions of prokaryotes and the two domains.

Concept / Approach:
Both bacteria and archaea are prokaryotes, meaning their cells lack a membrane bound nucleus and membrane bound organelles. Their DNA is typically in a nucleoid region. However, they differ in other features. Bacterial cell walls often contain peptidoglycan, while archaeal walls do not; instead, archaea may have pseudopeptidoglycan or other polymers. Their membrane lipids differ: bacteria normally have ester linked fatty acids, whereas archaea often have ether linked lipids. The start codon for protein synthesis can be methionine in archaea (similar to eukaryotes) and formyl methionine in bacteria, so they are not identical in this respect. Neither group is generally multicellular with specialised tissues like animals or plants. Therefore, the safe and correct similarity is that both possess prokaryotic cells.

Step-by-Step Solution:
Step 1: Recall that prokaryotic cells lack a true nucleus and membrane bound organelles. Step 2: Recognise that both bacteria and archaea share this prokaryotic cell plan. Step 3: Examine membrane linkages: bacteria have ester linked phospholipids, while archaea typically have ether linked lipids, so they are not identical. Step 4: Consider cell wall composition: many bacteria have peptidoglycan, whereas archaea lack true peptidoglycan. Step 5: Review the start signal for protein synthesis: archaea use methionine like eukaryotes, whereas bacteria use a modified form, so they are not fully similar in this detail. Step 6: Note that neither group is usually multicellular with specialised tissues; they are mainly unicellular or simple colonies. Step 7: Conclude that the most accurate similarity is that both possess prokaryotic cells.

Verification / Alternative check:
Classification charts show three domains: Bacteria, Archaea, and Eukarya. Both Bacteria and Archaea fall under the prokaryotic category, sharing the absence of a nucleus and many organelles, but they are separated due to distinct molecular and genetic characteristics. Descriptions of bacterial and archaeal membranes and cell walls highlight these differences. This confirms that the shared prokaryotic cell structure is the clearest similarity listed.

Why Other Options Are Wrong:
Both have plasma membranes with identical ester linkages: Incorrect, archaea often have ether linked lipids, which is a key difference. Both have thick peptidoglycan cell walls in all species: Only bacteria use peptidoglycan; archaea do not. Both always use methionine as the start signal exactly like eukaryotes: Start codons and initiator tRNAs differ between domains; they are not identical. Both are multicellular organisms with specialised tissues: Both groups are primarily unicellular prokaryotes, not tissue forming multicellular organisms.

Common Pitfalls:
Because bacteria and archaea used to be grouped under the kingdom Monera, some students assume they share all basic biochemical features. In reality, they share the prokaryotic cell plan but differ significantly in membrane chemistry, cell wall composition, and aspects of gene expression. When faced with such questions, look for the broad structural similarity (prokaryotic cells) rather than specific biochemical traits that may differ.

Final Answer:
Bacteria and archaea are similar in that both possess prokaryotic cells lacking a true nucleus.