Prokaryotic Cells and in Simple Eukaryotes

The main difference between prokaryotic and eukaryotic cells is the existence of organelles, especially the nucleus, in eukaryotes. An organelle is a part of the cell that has a distinct function it is surroimded by its own membrane within the cell. In contrast, the structure of a prokaryotic cell is relatively simple, lacking membrane-enclosed organelles. Like a eukaryotic cell, however, a prokaryotic cell has a cell membrane, or plasma membrane, separating it from the outside world. The plasma membrane is the only membrane found in the prokaryotic cell. In both prokaryotes and eukaryotes, the cell membrane consists of a double layer (bilayer) of lipid molecules with a variety of proteins embedded in it. 

In terms of evolutionary biology, the complex mitotic process of higher animals and plants has evolved through a progression of steps from simple prokaryotic fission sequences. In prokaryotic cells, the two copies of replicated chromosomes become attached to specialized regions of the cell membrane and are separated by the slow intrusion of the membrane between them. In many primitive eukaryotes, the nuclear membrane participates in a similar process and remains intact the spindle microtubules are extranuclear but may indent the nuclear membrane to form parallel channels. In yeasts and diatoms, the nuclear membrane also remains intact, an intranuclear polar spindle forms and attaches at each pole to the nuclear envelope, and a single kinetochore microtubule moves each chromosome to a pole. In the cells of higher animals and plants, the mitotic spindle starts to form outside of the nucleus, the nuclear envelope breaks down, and the spindle microtubules are captured by chromosomes (Kubai, 1975 Heath, 1980 Alberts et al., 1989). 

Cells are organized in a variety of ways in different living forms. Prokaryotes of a given type produce cells that are very similar in appearance. A bacterial cell replicates by a process in which two identical daughter cells arise from an identical parent cell. Simple eukaryotes can also exist as single nonassociating cells. Eukaryotes of increasing complexity can contain many cells with specialized structures and functions. For example, humans contain about 1014... 

We have many times emphasized that all life has very similar chemistry. Indeed, in terms of biochemistry there is little need for the classifications of mammals, plants, and so on. There is only one important division—into prokaryotes and eukaryotes. Prokaryotes, which include bacteria, evolved first and have simple cells with no nucleus. Eukaryotes, which include plants, mammals, and all... 

In simple situations encountered in prokaryotes such as bacteria, a stretch of DNA is copied into a collinear RNA molecule, and is immediately available to the ribosomes for translation. Every base in the RNA transcribed from the DNA arrives at the translation machinery. However, this is not the case in higher eukaryotic cells, where transcription occurs within the nucleus and the resultant mRNA is transported to the cytoplasm to be translated by ribosomes. In the process of synthesis and transport, the primary transcription product undergoes maturation and evolves as the message suitable for translation. 

Cells are the fundamental units of life. They are functional entities, each of which is enclosed in a semipermeable membrane that varies in composition and function both over a single cell surface and between different cell types. There are two basic forms of cell prokaryotic and eukaryotic. Prokaryotes are most noted for their small sizes and relatively simple structures. Presumably because of these traits, in addition to their remarkably rapid reproduction rates and biochemical diversity, various prokaryotic species occupy virtually every ecological niche in the biosphere. In contrast, the most conspicuous feature of the eukaryotes is their extraordinarily complex internal structure. Because eukaryotes carry out their various metabolic functions in a variety of membrane-bound organelles, they are capable of a more sophisticated intracellular metabolism. The diverse metabolic regulatory mechanisms made possible by this complexity promote two important lifestyle features required by multicellular organisms cell specialization and intercellular cooperation. Consequently, it is not surprising that the majority of eukaryotes are multicellular organisms composed of numerous types of specialized cells. 

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