Characteristics that Distinguish Prokaryotic and Eukaryotic
The following brief comparison between prokaryotic and eukaryotic cells reveals many basic differences between the two types, as well as many similarities (see Figure 2.2). The similarities and differences between the two types of cells are listed in table 1.1. The shared properties reflect the fact that eukaryotic cells almost certainly evolve from prokaryotic ancestors. Because of their common ancestry, both types of cells share an identical genetic language, a common set of metabolic pathways, and many common structural features. For example, both types of cells are bounded by plasma membranes of similar construction that serve as a selectively permeable barrier between the living ad nonliving worlds. Both types of cells may be sorrounded by a rigid, nonliving cell wall that protects the delicate life form within. Although the cell walls of prokaryotes and eukaryotes may have similar functions, their chemical composition is very different.
Table 1.1 A Comparison of Prokaryotic and Eukaryotic Cells
Features held in common by the two types of cells:
Plasma membrane of similar construction
Genetic information encoded in DNA using identical genetic code
Similar mechanism for transcription and translation of genetic information, including similar ribosomes
Shared metabolic pathways (e.g., glycolysis and TCA cycle)
Similar apparatus for conservation of chemical energy as ATP (located in the plasma membrane of prokaryotes and the mitochondrial membrane of eukaryotes
Similar mechanism of photosynthesis (between cyanobacteria and green plants)
Similar mechanism of synthesizing and inserting membrane proteins.
Proteasomes (protein digesting structures) of similar constructions (between archaebacteria and eukaryotes)
Features of eukaryotic cells not found in prokaryotes:
Division of cell into nucleus and cytoplasm, separated by a nuclear envelope containing complex pore structure
Complex chromosomes composed of DNA and associated proteins that are capable of compacting into mitotic structures
Specialized cytoplasmic organelles for aerobic respiration (mitochondira) and photosynthesis (chloroplast)
Complex cytoskeletal system (including microfilaments, intermediate filaments, and microtubules) and associated motor proteins
Complex flagella and cilia
Ability to ingest fluid and particulate material by enclosure within plasma membrane vesicle (endocytosis and phagocytosis)
Cellulose-containing cell walls (in plants)
Cell division using a microtubule-containing mitotic spindle that separates chromosomes
presence of two copies of genes per cell (diploidy), one from each parent
Presence of three different RNA synthesizing enzymes (RNA polymerase)
Sexual reproduction requiring meosis and fertilization
Internally, eukaryotic cells are much more complex both structurally and functionally- than prokaryotic cells (Figure 2.1). The difference in structural complexity is evident in the electron micrographs of a bacterial and an animal cell shown in Figures 2.1 and 2.3, respectively. Both contains a nuclear region, which houses the cell's genetic material, surrounded by cytoplasm. The genetic material of a prokaryotic cell is present in a nucleoid: a poorly demarcated region of the cell that lacks a boundary membrane to separate ot from the surrounding cytoplasm. In contrast, eukaryotic cells posses a nucleus: a region bounded by a complex membranous structure called the nuclear envelop. This difference in nuclear structure is the basis for the terms prokaryotic (pro= before, karyon= nucleus) and eukaryotic (eu= true, karyon= nucleus). Prokaryotic cells contains relatively small amounts of DNA ; the DNA content of bacteria ranges from about 600,000 base pairs to nearly 8 million base pairs and encodes between about 500 and several thousand proteins. Although a "simple" baker's yeast cell has only slightly more DNA (12 million base pairs encoding about 6,200 proteins) than the most complex prokaryotes, most eukaryotic cells contain considerably more genetic information. Both prokaryotes and eukaryotes cells have DNA containing chromosomes. Eukaryotic cells possess a number of separate chromosomes, each containing a single linear molecule of DNA. In contrast, nearly all prokaryotes that have been studied contain a single, circular chromosome. More importantly, the chromosomal DNA of eukaryotes, unlike that of prokaryotes, is tightly associated with proteins to form a complex nucleoprotein material known as chromatin.
The cytoplasm of the two types of cells is also very different. The cytoplasm of the two types is also very different. The cytocplasm of eukaryotic cell is filled with a great diversity of structures, as is readily apparent by examining an electron micrograph of nearly any plant or animal cell (Figure 2.3). Even yeast, the simplest eukaryote, is much more complex structurally than an average bacterium, even though these two organism have a similar number of genes. Eukaryotic cells contains an array of membrane-bound organelles. Eukaryotic organelles include mitochondria, where chemical energy is made available to fuel cellular activities; an endoplasmic reticulum, where many of a cell's proteins and lipids are manufactured; Golgi complexes, where materials are stored, modified, and transported to specific cellular destinations; and a variety of simple membrane-bound vesicles of varying dimension. Plant cells contains additional membranous organelles, including chloroplast, which are the sites of photosynthesis, and often a single large vacuole that can occupy most of the volume of the cell. Taken as group, the membranes of the eukaryotic cell serve to divide the cytoplasm into compartments within which specialized activities can take place. In contrast, the cytoplasm of prokaryotic cells is essentially devoid of membranous structures. The complex photosynthetic membranes of the cyanobacteria are a major exception to this generalization (Figure 2.4).
Figure 2.3 The Structure of a Eukaryotic cell. This epithelial cell lines the male reproductive tract in the rat. A number of different organelles are indicated depicted in schematic diagram around the border of the figure.
Figure 2.4 Cyanobacteria (a) Electron micrograph of a cyanobacterium showing the cytoplasmic membranes that carry out photosynthesis. These concentric membranes are very similar to the thykaloid membranes present within the chloroplast of plant cells, a reminder that chloroplast evolve from symbiotic cyanobacteria.
(b) Cyanobacteria living inside the hairs of these polar bear are responsible for the unsual greenish color of their coats
The cytoplasmic membranes of eukaryotic cells form a system of interconnecting channels and vesicles that function in the transport of substances from one part of a cell to another, as well as between the inside of the cell and its environment. Because of their small sizes, directed intracytoplasmic communication is less important in prokaryotic cells, where they necessary movement of materials can be accomplished by simple diffusion.
Eukaryotic cells also contain numerous structures lacking a surrounding membrane. Included in this group are the elongated tubules and filaments of the cytoskelton, which participate in cell contractility, movement, and support. It was thought until recently that prokaryotic cells lacked any trace of cytoskeleton, but primitive cytoskeletal filametns have been found in bacteria. It is still fair to say that prokaryotic cytoskeleton is much simpler, both structurally and functionally, than that of the eukaryotes. Both eukaryotic and prokaryotic cells possess ribosomes, which are non membranous particles that function as "workbench" on which the proteins of the cell are manufactured. Even though ribosomes of prokaryotic and eukaryotic cells have considerably different dimensions (thos of prokaryotes are smaller and contain fewer components). these structures participate in the assembly of proteins by a similar mechanism in both types of cells. Figure 2.5 is colorized electron micrograph of a portion of the cytoplasm near the thin edge of single-celled eukaryotic organism. This is a region of the cell where membrane-bound organelles tend to be absent. The micrograph shows individual filaments of the cytoskeleton (red) and other large macromolecular complexes of the cytoplasm (green). Most of these complexes are ribosomes. It is evident from this type of image that the cytoplasm of a eukaryotic cell is extremely crowded, leaving very little space for the soluble phase of the cytoplasm, which is called the cytosol.
Figure 2.6 The cytoplasm of a eukaryotic cell is crowded compartment. This colorized electron micrographic image shows a small region near the edge of a single-celled eukaryotic organism that had been quickly frozen prior to microscopic examination. The 3D appearance is made possible by capturing 2D images of the specimen at different angles and merging the individual frames using a computer. Cytoskeletal filaments are shown in red, macromolecular complexes (primarily ribosomes) are green, and portions of cell membranes are blue.
Other major differences between eukaryotic and prokaryotic cells can be noted. Eukaryotic cells divide by a complex process of mitosis in which duplicated chromosome condense into compact structures that are segregated by an elaborate microtubule-containing appratus (Figure 2.6). This apparatus, which is called a mitotic spindle, allows each daughter cell to receive an equivalent array of genetic material. In prokaryotes, there is no compaction of the chromosome and no mitotic spindle. The DNA is duplicated, and the two copies are separated accurately by the growth of an intervening cell membrane.
For the most part, prokaryotes are nonsexual organism. They contain only one copy of their single chromosome and have no processes comparable to meiosis, gamete formation, or true fertilization. Even though true sexual reproduction is lacking among prokaryotes, some are capable of conjugation, in which a piece of DNA is passed from one cell to another (Figure 2.7). However, the recipient almost never receives a whole chromosome from the donor, and the condition in which the recipient cell contains both its own and its partner's DNA is fleeting. The cell soon reverts back to possession of a single chromosome. Although prokaryotes may not be as efficient as eukaryotes in exchanging DNA with other members of their own species, they are more adept that eukaryotes at picking up and incorporating foreign DNA from their environment, which has had considerable impact on microbial evolution.
Figure 2.6 Cell division in eukaryotes requires the assembly of an elaborate-separating apparatus called mitotic spindle, which is constructed primarily of microtubules.
Figure 2.7 Bacterial conjugation, Electron micrograph showing a conjugation pair of bacteria joined by a structure of the donor cell, termed the F pilus, through which the DNA is though to be passed.