Question

Before the understanding of bacteria and archaea as distinct domains, what characteristics were used to define 'prokaryotes"? Select the cell components that were believed to be lacking before genetic and genomic analyses proved otherwise.

membrane-bound nucleus

an endoplasmic reticulum

ribosomes

a plasma membrane

Golgi apparatus

cytoskeleton

Answer 1

Solution: Archaea and bacteria were collectively classified as prokaryotes due to the similarities as both are single celled organisms and also lack nucleus or organelles.

1. Cell structure- archaea and bacteria having similar cell structure but different composition and organisation of structures found in archaea.

Both contain flagella to swim but evolution in archaeal flagella is bacterial type IV pili where base attached with subunits while in bacterial flagella is of type III secretion system where subunits are freely moving along the central pore upto tip of flagella.

2. Cell wall- archaea lacks peptidoglycan rather contain pseudopeptidoglycan having glycerol ether linked lipids. In bacterial cell wall contain peptidoglycan and lipopolysaccharide where glycerol ester linked lipids are present.

3. Membrane bound nucleus and organelles-archaea and bacteria both lack nucleus, endoplasmic reticulum, golgi apparatus, plasma membrane etc.

4. Ribosomes- Archaea having three ribosomal RNA polymerase similar to eukaryotes while bacteria only have one ribosomal RNA polymerase.

Archaea preferred as more prominent group of prokaryotes and having molecular distinction that separate archaea from bacteria.