Question

Thermococcus, Methanococcus, and Methanobacterium are groups of
A. Bacteria containing a cytoskeleton and all membrane-bound organelles
B. Archaebacteria with peptidoglycan in their cell wall
C. Archaebacteria that consists of a protein homologous to eukaryotic core histones
D. Most advanced type of bacteria.

Answer 1

Thermococcus, Methanococcus, and Methanobacterium belongs to the domain Archaea.
Archaea constitute a domain of single-celled organisms.
Archaebacteria are a group of microorganisms considered to be an ancient form of life that evolved separately from the bacteria and blue-green algae, and they are sometimes classified as a kingdom.

Complete answer: Archaebacteria lack cell nuclei and are therefore prokaryotes.
1. In taxonomy, Thermococcus is a genus of thermophilic archaea in the family the Thermococcaceae.
It is an obligate heterotroph, strictly anaerobic archaeon which grows on organic substrates, primarily in the presence of elemental sulfur (S°), which is reduced to hydrogen sulfide.
2. Methanococcus is a genus of coccoid methanogens of the family Methanococcaceae that belong to archaebacteria. In taxonomy, Methanobacterium is a genus of the Methanobacteriaceae. This genus belongs not to the bacterial domain but the archaeal domain because they lack peptidoglycan in their cell walls.
3. The histones found in archaea are widespread throughout the domain but are absent in most Crenarchaeota. Archae has the same histone fold as eukaryotic histones, but N-terminal histone tails have not been identified. Small basic proteins present in most archaea share a common ancestor with the eukaryotic core histones.
4. DNA wraps around an extended polymer, formed by archaeal histone homodimers, in a quasi-continuous superhelix, with the same geometry as DNA in the eukaryotic nucleosome.

So, the correct answer is “Option C ”.

Note: Linker histones, homologous to eukaryotic H1, have not been found in Archae.
Histone is a protein that provides structural support to a chromosome.
In order for very long DNA molecules to fit into the cell nucleus, they wrap around complexes of histone proteins, giving the chromosome a more compact shape.
Some variants of histones are associated with the regulation of gene expression.
Histones are composed of mostly positively charged amino acid residues such as lysine and arginine.
The positive charges allow them to be closely associated with the negatively charged DNA through electrostatic interactions.