Cell theory 

The original form of the cell theory consisted of three basic tenets: 

• All living things are composed of cells. 
• The cell is the basic functional unit of life. 
• Cells arise only from preexisting cells.

Through advances in molecular biology, a fourth tenet has been added to the theory:

Cells carry genetic information in the form of deoxyribonucleic acid (DNA). This genetic material is passed on from parent to daughter cell.

Cell theory has created an interesting dilemma with respect to viruses.Viruses are small structures that contain genetic material, but are unable to reproduce on their own. So, needs the structures that provide energy for reproduction and maintenance .This violates the third and fourth tenets of the cell theory because virions can only replicate by invading other organisms and because they may use ribonucleic acid (RNA) as their genetic information.
 

There are three overarching domains into which all life is classified: Archaea, Bacteria, and Eukaryote.    
Two of these—Archaea and Bacteria—contain prokaryotes. 

Archaea are single-celled organisms that are visually similar to bacteria, but contain genes and several metabolic pathways that are more similar to eukaryotes than to bacteria. Historically, Archaea were considered extremophiles, in that they were most commonly isolated from harsh environments with extremely high temperatures, high salinity, or no light. More recent research has demonstrated a greater variety of habitats for these organisms, including the human body. Archaea are notable for their ability to use alternative sources of energy. While some are photosynthetic, many are chemosynthetic and can generate energy from inorganic compounds, including sulfur- and nitrogen-based compounds such as ammonia.

Due to the similarities of this domain to eukaryotes, it is hypothesized that eukaryotes and the Archaea share a common origin. Both eukaryotes and Archaea start translation with methionine, contain similar RNA polymerases, and associate their DNA with histones. However, Archaea contain a single circular chromosome, divide by binary fission or budding, and share a similar overall structure to bacteria. Interestingly, Archaea are resistant to many antibiotics.

Characteristic features of archaeans:
• Cells with no nucleus 
• DNA exists as a circular 'chromosome' and does have histone proteins associated with it
• Smaller circular molecules of DNA called plasmids are often present
• Cells with no membrane-bound organelles
• Ribosomes (70S) are smaller than in eukaryotic cells, but the small subunit has features that are similar to those in eukaryotic ribosomes but not to bacterial ribosomes; the base sequences of rRNA and the primary structure of ribosomal proteins are more like those in eukaryotes rather than bacteria
• Membrane lipids are unique - they are not found in membranes of bacteria or eukarya 
• Cell walls are always present but do not contain peptidoglycans
• Cells divide by binary fission, not by mitosis 
• Usually exist as single cells or small groups of cells

Prokaryotes were the first organisms to evolve on Earth and they still have the simplest cell structure. They are mostly small in size and are found almost everywhere – in soil, in water, on our skin, in our intestines and even in pools of hot water in volcanic areas. Bacteria are all prokaryotic cells.

Prokaryotic cells are so called because they have no nucleus (‘prokaryote’ comes from the Greek, meaning ‘before the nucleus’). They also have no organelles (internal structures), so there is little compartmentalisation of function within them.
As no nucleus is present in a prokaryotic cell its interior is entirely filled with cytoplasm. The cytoplasm is not divided into compartments by membranes – it is one uninterrupted chamber.    
 

Prokaryotes do not have cytoplasmic organelles apart from ribosomes. ribosomes size, measured in Svedberg units (S) (a unit of time equal to 10⁻¹³ seconds, used in expressing sedimentation coefficients.) is 70S, which is smaller than those of eukaryotes (80S).

This region (nucleoid region) contains the DNA of the cell, usually in the form of one circular DNA molecule. The DNA is not associated with proteins, which explains the lighter appearance compared with other parts of the cytoplasm that contain enzymes and ribosomes. This lighter area of the cell is called the nucleoid – meaning nucleus-like as it contains DNA but is not a true nucleus.
In prokaryotes the cell wall contains peptidoglycan that is often referred to as being extracellular.