Domain Archaea

Studies on 16S ribosomal RNA sequences by Carl Woese and colleagues allowed the construction of phylogenetic trees for the procaryotes, showing their evolutionary re-latedness. Figure 7.1 shows how the major procaryotic groups are thought to be related, based on 16S rRNA data. The work of Woese also revealed that one group of procaryotes differed from all the others. As described in Chapter 3, the Archaea are now regarded as being quite distinct from the Bacteria (sometimes called Eubacteria) these form the three domains of life (Figure 3.1). As can

Bacteria may acquire genes from other organisms by a variety of genetic transfer mechanisms (see Chapter 11). This is known as horizontal or lateral gene transfer, to distinguish it from vertical inheritance, in which the parental genotype is passed to the offspring.

Archaea are procary-otes differing from true bacteria in cell wall and plasma membrane chemistry as well as 16S rRNA sequences.

. Together with the Eucarya, be seen in Table 7.1, archaea

Table 7.1 The three domains of life: Archaea share some features with true bacteria and others with eucaryotes

Archaea

Bacteria

Eucarya

Main genetic

Single closed circle

Single closed circle

True nucleus with

material

of dsDNA

of dsDNA

multiple linear

chromosomes

Histones

Present

Absent

Present

Gene structure

Introns absent

Introns absent

Introns present

Plasmids

Common

Common

Rare

Polycistronic

Present

Present

Absent

mRNA

Ribosomes

70S

70S

80S

Protein synthesis

Not sensitive to

Sensitive to

Not sensitive to

streptomycin,

streptomycin,

streptomycin,

chloramphenicol

chloramphenicol

chloramphenicol

Initiator tRNA

Methionine

N-formyl

Methionine

methionine

Membrane fatty

Ether-linked,

Ester-linked,

Ester-linked,

acids

branched

straight chain

straight chain

Internal organelles

Absent

Absent

Present

Site of energy

Cytoplasmic

Cytoplasmic

Mitochondria

generation

membrane

membrane

Cell wall

Muramic acid

Muramic acid

Muramic acid

absent

present

absent

share some features in common with other bacteria and some with eucaryotes. Extending nucleic acid analysis to other genes has shown that members of the Archaea possess many genes not found in any other type of bacteria.

The domain is the highest level of taxonomic grouping.

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