Prokaryotic and Eukaryotic Cells Revisited - Cell Structure and

Are Viruses eukaryotic or prokaryotic cells

Virus / May 4, 2018

Viruses are the simplest form of life but they are not cellular. They rely on entry into cells to reproduce. They enter and use the cell's metabolic machinery to replicate their nucleic acid and to produce their protein coats. All types of cells have their own specific viruses. In general, viruses can only infect a single species, however, there are exceptions to this rule.

A bacterium is a prokaryotic cell

The simplest and most primitive of cells are the prokaryotic cells (pro means before and caryon refers to the nucleus). Prokaryotic cells are much smaller than eukaryotic cells. They are between 1 micron and 10 microns (a micron is one millionth of a meter) while eukaryotic cells are usually minimally 10 microns but most are larger. Prokaryotic cells can be studied only with an electron microscope and that is why their unique cell structure was not known by the earlier light microscopists. Prokaryotic cells do not have a nucleus nor do they contain the other organelles characteristic of the more advanced eukaryotic cells. The ancestor of the first prokaryotic cells diverged into three lineages: the two main prokaryotic lineages are the eubacteria and the archaebacteria and the third was the prokaryotic ancestor of the present day eukaryotes. The archaebacteria are more closely related to the eukaryotes since the early ancestor of the archaebacteria also gave rise to the eukaryotic cell line.

The archaebacteria and eubacteria until very recently were put in the Kingdom Monera and we will use this terminology. Some scientists have awarded kingdom status to each of the two groups: Kingdom Archaebacteria and Kingdom Eubacteria. Both kingdoms are composed exclusively of organisms with prokaryotic cells. However, the archaebacteria have more characteristics in common with eukaryotic cells because they share a common ancestor. Archaebacteria include the methanogens, halophiles, and thermophiles, organisms that live in very harsh environments. They are believed to resemble the very first cells. The eubacteria include several groups including the common bacteria and cyanobacteria (blue-green algae). The cyanobacteria are photosynthetic. Most other bacteria are heterotrophs and are important recyclers along with the fungi. Some of them are pathogenic but many are not. There are three basic shapes: rods (bacilli); spheres (cocci) and spirals (spirilli).

All cells including the prokaryotes, have a cell (plasma) membrane. Prokaryotes with few exceptions also have a cell wall containing a unique peptidoglycan, a biomolecule made of amino acids and sugars. This molecule resembles a hairnet which encloses the cell. Some prokaryotic cells, such as pneumococcus, also have a capsule overlying the cell wall. The capsule helps them escape being captured and engulfed by the phagocytic white blood cells of your immune system. (The capsule was the difference between the "smooth" and "rough" strains of pneumococci mentioned in the lecture on the discovery of DNA as the genetic material.) Gram-negative and gram-positive are commonly used terms which refer to the different staining patterns among species of bacteria. The difference is due to the composition of the cell walls.

The interior of the cell contains the cytoplasm. The cytoplasm has a fluid base and is full of enzymes, metabolites and ribosomes. Ribosomes are structures composed of RNA and protein that are the protein factories of all cells. Ribosomes hold the messenger RNAs made from the DNA while the transfer RNAs read the codons and line of the amnio acids. All prokaryotic cells have a single, double-stranded (double helix), circular DNA molecule for their genetic material. This DNA is attached to the inner cell membrane where the DNA replicating machinery is located. The DNA replicates and forms two DNA molecules. (Cells may have more than one copy of the same DNA molecule.)

When the cell reproduces, new membrane is formed and the attached newly replicated DNA molecules move apart. New cell wall material is synthesized...