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In this article, we will discuss the classification of organisms into three domains, i.e., Archaea, Bacteria, and Eukarya. Moreover, we will also describe that Archaea and Bacteria are prokaryotes and that there are differences between them. We will discuss the differences limited to membrane lipids, ribosomal RNA, and the composition of cell walls
Taxonomy and its Benefits
Taxonomy refers to the practice of biological classification which includes putting organisms into a series of taxa or categories
You may be wondering what benefits taxonomy offers. Well, we can easily understand and remember organisms by grouping them into taxa. In biology, several different levels or ranks within a hierarchical classification system are employed. The highest rank is referred to as a domain.
What are the two types of cells?
The type of cell plays a fundamental role in the classification of organisms into three domains. However, you should not confuse the types of cells and domains. The two types of cells are:
- Prokaryotic cells: They do not have a nucleus; hence we can easily differentiate them from eukaryotic cells
- Eukaryotic cells: These cells possess compartmentalized structures. At least their genetic material is separated from the remaining cell in the nucleus
The Three Domains
Depending on the molecular analysis of RNA genes, scientists have concluded that the type of cells is not enough to categorize organisms. Moreover, prokaryotes can be divided into two distinct domains. Hence, the three domains are:
- Archaea (prokaryotes)
- Bacteria (prokaryotes)
- Eukarya (eukaryotes)
In the next section of the article, we will discuss the archaea domain in detail.
Archaea
Organisms classified under this domain are sometimes called extremophile prokaryotes. This is because the archaea were initially discovered living in extreme environments. However, not all organisms classified under this domain live in extreme environments.
The cells of these organisms do not contain a nucleus, hence they are referred to as prokaryotic. First, they were classified as bacteria until scientists came to know about their many unique characteristics that separated them from known bacteria. These characteristics include:
- Presence of unique lipids in the membranes of their cells
- Absence of peptidoglycan in their cell walls
- Having ribosomal structure (especially that of a small subunit) that is quite similar to the ribosome of the eukaryotes as compared to the bacteria
- The size range of archaea is the same as that of the bacteria. Moreover, in many ways, the metabolism of archaea and bacteria is quite similar.
- DNA transcription of archaea is more similar to eukaryotes than bacteria. For instance, the species Halobacterium salinarum is classified under the Achaea domain that is found in environments that contain concentrations of salt such as the Dead Sea
In the next section of the article, we will discuss the bacteria domain.
Bacteria
Bacteria refer to organisms that have prokaryotic cells because they do not contain a nucleus. They differ in size over a wide range. The smallest of the bacteria are bigger as compared to the largest known viruses and the largest are smaller as compared to the smallest known single-celled eukaryotes. The cells of the bacteria divide through binary fission. For instance, the bacterial species Staphylococcus pneumoniae is known to cause pneumonia.
Eukarya
In this domain, those organisms are placed that have eukaryotic cells with nuclei and membrane-bound organelles. They differ greatly in size as they range from single-celled organisms many multiple micrometers across to large multicellular organisms that are several-meters in size. An example includes blue whales.
The eukaryotic cells undergo division through mitosis and they can reproduce either sexually or asexually. The example includes Canis lupus, also referred to as wolves.
In the next sections of the article, we will discuss the differences between archaea and bacteria in detail.
Differences Between Archaea and Bacteria
The highest known taxonomic ranks are known as domains. First, the scientists classified all the organisms within the archaea domain as bacteria. However, when they came to know many unique characteristics possessed by the Archaea, they made a separate domain. The fundamental differences between the archaea and bacteria are observed in the following structures:
- Membrane lipids
- Ribosomal RNA
- Cell wall composition
Membrane Lipids
The membrane lipids present in the archaeal cells are totally unique and they are not present in bacterial or eukaryotic cells. The membrane lipids of archaeal cells have branched chains of hydrocarbon that are bonded to glycerol through ether linkages. On the other hand, in bacteria, the membrane lipids consist of unbranched chains of hydrocarbons that are bonded to glycerol through ester linkages.
Ribosomal RNA
Both bacteria and archaea contain 70S ribosomes. The 70S ribosomes in Archaea contain a similar subunit that is quite similar to the subunit present in eukaryotic ribosomes as compared to the ribosomes in bacteria cells.
In archaea, the base sequences of ribosomal RNA exhibit more similarity to the rRNA of Eukarya as compared to that of bacteria. The fundamental structure of ribosome proteins in Archaea is more similar to the ribosome proteins in Eukarya as compared to that of bacteria.
Cell Walls Compositions
Organisms belonging to the bacterial domain contain cells that always have cell walls with peptidoglycan. Organisms from the archaea domain also contain cells that have cell walls, however, unlike bacteria, their cell walls do not contain peptidoglycan.
In the next section of the article, we will compare the features of all three domains.
Summary of the Features of Three Domains: Archaea, Bacteria, and Eukaryotes
Types of cell
Archaea: Prokaryotic
Bacteria: Prokaryotic
Eukaryotes: Eukaryotic
Cell membrane lipids
Archaea: Glycerol – ester lipids
Bacteria: Glycerol–ester lipids
Eukaryotes: Glycerol – ester lipids
Chromosome
Archaea: Circular
Bacteria: Circular
Eukaryotes: Linear chromosomes and circular mtDNA and cpDNA
Ribosomes
Archaea: 70S ribosomes, however, a small subunit resembles eukaryotic ribosomes a lot
Bacteria: 70S ribosomes
Eukaryotes: 80S ribosomes are present in the cytosol and 70S ribosomes are present in chloroplasts and mitochondria
Cell walls
Archaea: Cell wall is always there (without peptidoglycan)
Bacteria: Cell wall is always present (with peptidoglycan)
Eukaryotes: Cell walls are only present sometimes (without peptidoglycan)
Histones
Archaea: Yes
Bacteria: No
Eukaryotes: Yes
Introns
Archaea: Sometimes only
Bacteria: Rarely
Eukaryotes: Yes
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