Archaea – Exam 3 Review Flashcards

A comprehensive set of 40 question-and-answer flashcards covering molecular markers, archaeal cell structures, membrane adaptations, shared features with other domains, major archaeal groups, and notable extremophiles.

What biological process uses mobile genetic elements to spread genes between microbes and has greatly influenced microbial evolution?

Horizontal gene transfer (HGT).

Which molecular marker is most commonly used to identify and classify prokaryotic species?

The 16S rRNA gene sequence.

The 16S rRNA gene shows an average mutation rate of ___ % per million years (≈32 changes in a 1,600-bp gene).

0.02 %.

Which bioinformatics tool is routinely used to compare 16S rRNA sequences against large databases?

BLAST (Basic Local Alignment Search Tool).

Why can’t viruses be identified with 16S rRNA sequencing?

They lack 70 S ribosomes and therefore do not possess the 16S rRNA gene.

In the sample alignment provided, which two bacteria were most closely related?

Escherichia coli and Salmonella enterica (they share 9/10 identical bases).

Are Archaea considered bacteria?

No. They form a separate domain of prokaryotes distinct from Bacteria.

What kind of environments do many (but not all) Archaea inhabit?

Extreme environments such as high heat, salinity, pressure, or acidity—hence many are extremophiles.

What polysaccharide often replaces peptidoglycan in archaeal cell walls?

Pseudomurein (pseudopeptidoglycan).

Why does the Gram-stain reaction have no taxonomic value for Archaea?

Archaea lack peptidoglycan, so Gram results are not meaningful for classification.

What type of chemical bond links archaeal membrane lipids to glycerol?

Ether bonds.

Why are ether bonds advantageous for archaeal membranes?

They are stronger and more chemically stable, allowing membranes to function in extreme conditions.

Instead of fatty acids, archaeal lipid tails are usually composed of what?

Isoprenoid hydrocarbon chains (e.g., phytanyl or biphytanyl) that can contain ring structures.

What unique membrane architecture enables some Archaea to resist harsh conditions better than bilayers?

A lipid monolayer (tetra-ether) membrane.

How do psychrophilic Archaea adjust their membranes to cold temperatures?

They insert more unsaturated tails, fewer rings, and shorter chains to maintain fluidity.

How do hyperthermophilic Archaea adjust their membranes to very high temperatures?

They use more saturated tails, add cyclopentane rings, and lengthen lipids to decrease fluidity.

Give two genome-level traits Archaea share with Bacteria.

(1) Circular double-stranded, haploid chromosomes; (2) polycistronic operons, HGT, binary fission.

Give two molecular traits Archaea share with Eukaryotes.

(1) Multiple origins of replication with eukaryote-like replication proteins; (2) histones, introns, many RNA polymerases and shared ribosomal proteins.

Which amino acid initiates protein synthesis in both Archaea and Eukarya?

Methionine (not formyl-methionine).

Name the two best-studied archaeal phyla.

Crenarchaeota and Euryarchaeota.

Which archaeal phylum contains many hyperthermophiles and acidophiles?

Crenarchaeota.

Which archaeal phylum includes most methanogens and halophiles?

Euryarchaeota.

What energy-generating process is unique to Archaea and performed by methanogens?

Methanogenesis—production of CH₄ by reducing CO₂, acetate, etc.

Are human-gut methanogens extremophiles?

No. They are mesophilic but strict anaerobes.

State one ecological/industrial benefit of methanogens in wastewater treatment.

They break down sludge and generate methane-rich biogas (renewable energy).

Why is methane accumulation in landfills considered dangerous?

Methane is highly flammable/explosive and also a potent greenhouse gas.

What surface structure replaces the cell wall in Halobacteria?

A tough proteinaceous S-layer that covers the cell membrane.

Why do Halobacterium species require extremely high salt?

Their cytoplasm contains >4 M KCl; without external salt the cells lyse osmotically.

What light-driven proton pump do Halobacteria use to generate ATP?

Bacteriorhodopsin (forms the purple membrane).

What is the function of halorhodopsin in Halobacteria?

It pumps chloride ions inward to help maintain high intracellular salt concentrations.

Do Thermoplasma species possess cell walls?

No. They have extremely tough plasma membranes and external S-layers instead.

Where are Thermoplasma typically found?

Hot, acidic, sulfur-rich springs (≈55–60 °C, pH 1–2).

Name one hyperthermophilic archaeon capable of growing above 100 °C.

Thermococcus kodakarensis (also Pyrococcus furiosus is acceptable).

What specialized enzyme do many hyperthermophiles possess to stabilize DNA at high temperatures?

Reverse gyrase (introduces positive supercoils).

What protects the surface of Archaeoglobus fulgidus in the absence of a cell wall?

A crystalline hexagonal S-layer.

Which recently discovered archaeal group is thought to be closely related to Eukarya?

Lokiarchaeota.

According to the fusion theory, eukaryotic cells arose from the merger of which two ancestral cell types?

An archaeal ancestor and a bacterial ancestor.

Which two endosymbiotic events gave rise to key eukaryotic organelles?

Fusion with an alpha-proteobacterium formed mitochondria; later fusion with a cyanobacterium formed chloroplasts.

Why can Halobacteria tolerate intense UV radiation?

High carotenoid pigments + multiple (>20) genome copies enable efficient DNA repair via recombination.

What role do sensory rhodopsins play in Halobacteria?

They act as photoreceptors that control flagellar movement and phototaxis.