What are prokaryotes ?

All the living organisms on this planet are made up of cells. They can be divided into two groups : Prokaryotes and Eukaryotes.

Prokaryotes are single celled or colonial organisms, where the cell is surrounded by cell membrane or plasma membrane and lacks a defined nucleus (as found in eukaryotic cell). These cells are of simple organization and small (microscopic), measuring around 0.1-5 μm in diameter. The most distinctive feature of a prokaryotic cell is that the genetic material or DNA in not enclosed by a nuclear envelope and DNA is found coiled in the cytoplasm of the cell called Nucleoid. As found in eukaryotes, a prokaryotic cell also lacks all cells organelles, with the exception of ribosomes.

After comparing and analyzing the DNA sequences from a variety of prokaryotic organisms, scientists could classify prokaryotes into distinct types: Bacteria and Archaea.

Classification of prokaryotes:

Prokaryotes are divided into Bacteria and Archaea. Interestingly, members of the Archaea are more closely related to eukaryotes than they are to bacteria. Archean’s exhibit histones and ribosomes like those of eukaryotes but they also have many genes found in bacteria as well. So the exact relationship between bacteria, archaea and eukaryotes is still debatable.

rRNA sequence as markers for comparative studies / why nucleotide sequence coding for rRNA is best evolutinary marker for relatedness :

The pioneering work In the late 20^th century by Carl Woese and George Fox led to the idea that all life on earth evolved along three lineages or domains. They formulated an idea of classifying organisms based on differences in the structure of cell membranes and in rRNA.

The idea behind comparing sequences is based on the logic that organisms sharing similar DNA sequences will be more closely related (evolutionarily), than those which differ, thereby establishing evolutionary relationship between all organisms on our planet Earth. The greater the similarities in these DNA sequences, the more closer the organisms are thought to be.

For this purpose of comparing, it is important to take DNA sequences which are present in all species, quite stable or less prone to changes or mutations. These stable sequences usually are found in genes that code for molecules controlling important cellular processes such as protein synthesis (genetic pressure created on these important sequences will prevent them from any changes). As any changes in these sequences will lead to non functionality of cell and cause death of organism. One example of such sequence is the DNA that codes for ribosomal RNA (rRNA) , precisely the small subunit of ribosome is generally used for establishing evolutionary relationship between organisms. This type of study is called molecular phylogenetic. The rRNA sequences from two organisms can be aligned and compared for relatedness.

This led to proposal of three-domain classification (different from Whittaker’s five domain classification in 1969- Monera, Protista, Fungi, Plantae and Animalia) which divides all organisms into three domains : Bacteria, archaea and Eukaryotes. In three domain classification, Protista, Fungi, Plantae and Animalia are grouped under Eukaryotes and divides Monera into two: Bacteria and Archaea.

The domain Bacteria consists of all organisms in the kingdom Bacteria, the domain Archaea comprises the rest of the prokaryotes, and the domain Eukarya comprises all eukaryotes, including organisms in the kingdoms Animalia, Plantae, Fungi, and Protista.

Archaean’s usually grow in extreme conditions or environments:

Halophiles – Loves high salt concentrations, Thermoacidophiles – grow in hot (80°C) sulfur springs, where a pH of less than 2 is common.

Viruses doesn’t fall into any of the above domains as they lack a true cell and not considered as completely living entities. They are obligate parasites and rely on host cell for replication and reproduction.

Important features of prokaryotes

Peptidoglycan Cell wall: Most Prokaryotes are protected by a cell wall made up of peptidoglycan (amount differs from Gram +ve and Gram -ve bacteria). This peptidoglycan molecule is found only in cell walls of bacteria’s and no where else on Earth. It helps in providing protection to cell and also helps to maintain its shape, and prevents dehydration. Peptidoglycan is a polysaccharide made of two glucose derivatives, N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM), alternatively arranged in long chains with help of β-(1,4)-glycosidic bonds. The chains are connected to one another by a tetrapeptide made up of : L-alanine, D-glutamine, L-lysine or meso-diaminopimelic acid (DPA), and D-alanine. The tetrapeptide is extended from NAM sugar unit, which allows to form a lattice-like structure. Further, Cross-linking between amino acids of tetrapeptides with the help of the enzyme DD-transpeptidase results in a strong and rigid cell wall. The tetrapeptides can be either directly cross-linked to one another ( bond between L-alanine of one tetrapeptide and L-Lysine of another) or with the help of another five amino acids peptide.

Presence of peptidoglycan is restricted only to bacteria’s and absent in Archaea. The cell walls in Archaea is made up of four different types substances : pseudopeptidoglycan (somewhat similar to peptidoglycan but contains different sugars in the polysaccharide chain) polysaccharides, glycoproteins and pure protein.

Plasma membrane : The prokaryotic plasma membrane is a thin lipid bilayer that encloses cytoplasm and all it contents, hence separates the inside from the outside. The function of plasma membrane is similar to those found in eukaryotes, in being selectively permeable in nature.

Nucleoid: A central region of the cell that contains its DNA. 

Ribosomes : These are vital for synthesis of proteins.

Some bacteria have a polysaccharide capsule as outer covering an this enables the bacterium to attach to the surface of its environment. In some prokaryotes- flagella, pili, or fimbriae are observed and they play important functions. Flagella is used for locomotion, Pili for transfer of genetic material by direct transfer through a process called as conjugation and fimbriae is used to attach to a host cell.

Similarities – Prokaryotes and Eukaryote’s

Prokaryotes are primitive cells but still shares some important similarities with Eukaryotic cells. Such as

• DNA serves as source for genetic information and uses similar genetic code.
• Structure and function of Plasma membrane is also similar.
• Cytoplasm and Ribosomes are found in both kinds of cells.
• The mechanism behind Transcription and translation of genetic information is common.
• Shared metabolic pathways such as Glycolysis, Krebs’s Cycle etc.
• Photosynthesis observed in green plants (eukaryotic cell) and cyanobacteria is of similar mechanism.

Important Differences – Prokaryotes V/S Eukaryotes

Eukaryotic Cell	Prokaryotic Cell
Membrane bound nucleus is present	Absent
Different cell organelles are present in cytoplasm	Cell organelles are absent
Larger Ribosomes (80S)	Smaller Ribosomes (70S)
Mostly multicellular organisms, very few unicellular	Unicellular organisms
Cell wall is absent in many eukaryotes – Plant cells have cellulose cell wall, fungal cell wall has chitin but not made up of peptidoglycan	Peptidoglycan cell wall is present
DNA is organized into chromosomes and histones are present	No chromosomes and histones
Genes have introns and genome contains non coding regions also	Compact genes and lacks introns
Two copies of each gene (diploid)	Single copy
Examples : Animals, plants, fungi, protists	Bacteria, Archaea
Presence of three different RNA synthesizing enzymes (RNA polymerases)	Only one RNA polymerase for transcription
Specialized cytoplasmic organelles for aerobic respiration (mitochondria) and photosynthesis (chloroplasts)	Not found

Image Credit : Biology Otpenstax – A typical prokaryotic cell

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