# 1.3: Classification: The Three Domain System (2023)

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learning goals

1. define phylogeny.
2. Name the 3 domains of the 3-domain classification system and recognize a description for each.
3. Name the four realms of the Eukarya Domain and recognize a description of each.
4. Define horizontal gene transfer.

The Earth is 4.6 billion years old, and microbial life is believed to have evolved between 3.8 and 3.9 billion years ago; In fact, 80% of Earth's history has been entirely microbial life. Microbial life remains the dominant life form on Earth. The total number of microbial cells on Earth is estimated to be on the order of 2.5 x 1030cells, making up the largest fraction of the planet's biomass.

Phylogeny refers to the evolutionary relationships between organisms. The Woese et al. The proposed three-domain system is an evolutionary model of phylogeny based on differences in the nucleotide sequences in the ribosomal RNAs (rRNA) of cells, as well as in the lipid structure of the cell membrane and its sensitivity to antibiotics. Comparison of rRNA structure is particularly useful. As rRNA molecules have the same function throughout nature, their structure changes very little over time. Therefore, similarities and differences in rRNA nucleotide sequences are a good indication of how related or unrelated different cells and organisms are.

(Video) The Three Domains of Life -Bacteria-Archaea-Eukarya

There are several hypotheses about the origin of prokaryotic and eukaryotic cells. Since all cells are similar in nature, it is generally assumed that all cells are descended from a common ancestral cell.last universal common ancestor (LUCA)🇧🇷 These LUCAs eventually evolved into three distinct cell types, each representing a domain. The three domains areyou arch, to diebacteria, e aseukaryotic.

More recently, various fusion hypotheses have begun to dominate the literature. It is proposed that the diploid or 2N nature of the eukaryotic genome arose after the fusion of two haploid or 1N prokaryotic cells. Others suggest these domainsyou archjeukaryoticarose from a common archaeal-eukaryotic ancestor, which in turn arose from a member of the domainbacteria🇧🇷 Some of the evidence behind this hypothesis is based on a "superphylum" of bacteria called PVC, whose members share some properties with archaea and eukaryotes. There is growing evidence that eukaryotes may have descended from a subgroup of Archaea. In any case, it is now accepted that there are three distinct domains of organisms in nature:Bacteria, Archaea, jeukaryotic🇧🇷 Below is a description of the three domains.

Domains?

There is a "superphylum" of bacteria called PVC, referring to the three members of that superphylum: Planctomycetes, Verrucomicrobia, and Chlamydiae. PVC members as long as they belong to the domainbacteria, shows some domain propertiesyou archjeukaryotic.

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Some of these bacteria exhibit cellular compartmentalization, with membranes surrounding parts of the cell's interior, such as B. Clusters of ribosomes or DNA, similar to eukaryotic cells. Some divide by budding or contain sterols in their membranes, again similar to eukaryotes. Some lack peptidoglycan, similar to eukaryotes and archaea. It has been hypothesized that these bacteria may be an intermediate step between a bacterial-derived ancestor (domainbacteria) and an archael-eukaryotic ancestor before its domain splityou archjeukaryotic.

## Las Archaea (Archaebakterien)

That oneyou archhave the following properties:

1. you archThey are prokaryotic cells.
2. not howbacteriait's ateukaryotic, to dieyou archthey have membranes of branched hydrocarbon chains (many of which also contain rings within the hydrocarbon chains) linked to the glycerol via ether linkages (Figure $$\PageIndex{3}$$).
3. The cell walls ofyou archThey do not contain peptidoglycan.
4. you archare not sensitive to some antibiotics that affect thebacteria, but are sensitive to some antibiotics that affect theeukaryotic.
5. you archcontain rRNA that is unique to theyou archas indicated by the presence of distinctly different molecular regions of the rRNAbacteriajeukaryotic.

you archThey usually live in extreme environments and include methanogens, extreme halophiles and hyperthermophiles. One reason for this is the ether-containing bonds in theyou archis more stable than ester-containing bonds in membranesbacteriajeukaryoticand are better able to withstand higher temperatures and stronger acid concentrations.

## That onebacteria(eubactéria)

Bacteria (also known as eubacteria or "true bacteria") are prokaryotic cells that are common in human daily life and are much more common than archaebacteria. Eubacteria are found almost everywhere, killing thousands upon thousands of people every year, but they also serve as antibiotic producers and digestive organs in our stomachs. That onebacteriahave the following properties:

1. bacteriaThey are prokaryotic cells.
2. While theeukaryotic, have membranes of unbranched fatty acid chains connected to glycerol via ester bonds (Figure $$\PageIndex{3}$$).
3. The cell walls ofbacteria, not howyou archand Eukarya, contain peptidoglycan.
4. bacteriaare sensitive to conventional antibacterial antibiotics but resistant to most of the antibiotics that affect themeukaryotic.
5. bacteriacontain rRNA that is unique to thebacteriaas indicated by the presence of distinctly different molecular regions of the rRNAyou archjeukaryotic.

bacteriathese include mycoplasma, cyanobacteria, gram-positive bacteria, and gram-negative bacteria.

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## That oneeukaryotic(Eukaryotic)

That oneeukaryotic(also spelledeukaryotic) has the following properties:

1. eukaryoticThey have eukaryotic cells.
2. While thebacteria, have membranes of unbranched fatty acid chains connected to glycerol via ester bonds (Figure $$\PageIndex{3}$$).
3. No wayeukaryotichave cells with a cell wall, but for thateukaryoticwith a cell wall, this wall does not contain peptidoglycan.
4. eukaryoticThey are resistant to conventional antibacterial antibiotics but are sensitive to most antibiotics that attack eukaryotic cells.
5. eukaryoticcontain rRNA that is unique to theeukaryoticas indicated by the presence of distinctly different molecular regions of the rRNAyou archjbacteria.

That oneeukaryoticare divided into the following four Kingdoms:

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1. Kingdom Protista: Protists are simple, predominantly single-celled eukaryotic organisms. Examples are slime molds, euglenoids, algae, and protozoans.
2. Kingdom Fungi: Fungi are unicellular or multicellular organisms with eukaryotic cell types. Cells have cell walls but are not organized into tissues. They do not carry out photosynthesis and obtain nutrients through absorption. Examples are bag fungus, leg fungus, yeasts and molds.
3. Kingdom Plantae: Plants are multicellular organisms made up of eukaryotic cells. Cells are organized into tissues and have cell walls. They obtain nutrients through photosynthesis and absorption. Examples are mosses, ferns, conifers, and flowering plants.
4. Kingdom Animalia: Animals are multicellular organisms composed of eukaryotic cells. Cells are organized into tissues and do not have cell walls. They do not photosynthesize and get most of their nutrients from food. Examples are sponges, worms, insects, and vertebrates.

Previously, it was thought that the changes that allow microorganisms to adapt to new environments or alter their virulence abilities were a relatively slow process that occurred within an organism, primarily through genetic mutations, chromosomal rearrangements, deletions, and duplications. These changes would then be passed on to that microbe's descendants, and natural selection would take place. This transfer of genes from a parent organism to its offspring is called vertical gene transfer.

It is now known that microbial genes are not only transmitted vertically from a parent organism to its offspring, but also horizontally to distant relatives such as other species and other genera. This last process is known as horizontal gene transfer. Through mechanisms such as transformation, transduction and conjugation, genetic elements such as plasmids, transposons, integrins and even chromosomal DNA can easily spread from one microorganism to another. As a result, the former three-armed "tree of life" now appears more like a "web of life" in relation to microorganisms (Figure $$\PageIndex{1}$$).

Microbes are known to live in remarkably diverse environments, many of which are extremely hostile. This incredibly rapid adaptability is the result of their ability to quickly modify their repertoire of protein functions by changing, gaining or losing their genes. This gene expansion occurs predominantly by horizontal transfer.

## Continue

1. Phylogeny refers to the evolutionary relationships between organisms.
2. Organisms can be classified into one of three domains based on differences in the nucleotide sequences in the cell's ribosomal RNAs (rRNAs), the lipid structure of the cell membrane, and their sensitivity to antibiotics.
3. The three domains are Archaea, Bacteria and Eukarya.
4. Prokaryotic organisms belong to either the archaeal domain or the bacterial domain; Organisms with eukaryotic cells belong to the eukaryotic domain.
5. Microorganisms transfer genes to other microorganisms by horizontal gene transfer: the transfer of DNA to an organism that is not its progeny.

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