This is an interesting problem of biological evolution. The most accepted hypothesis claims that the simpler cell, the prokaryotic cell, appeared earlier in evolution than the more complex eukaryotic cell. The endosymbiotic hypothesis, for example, claims that aerobic eukaryotic cells appeared from the mutualistic ecological interaction between aerobic prokaryotes and primitive anaerobic eukaryotes.
Animal cells (the cells of organisms of the kingdom Animalia) have an interior membrane that encloses a cell nucleus and are therefore eukaryotic cells. In these cells, the genetic material is located within the nucleus. Bacterial cells (the cells of living organisms of the kingdom Monera) do not have organized cellular nuclei and are therefore prokaryotic cells. Their genetic material is found in the cytosol.
Eukaryotic cells can be divided into three main parts: the cell membrane that physically separates the intracellular space from the outer space by enclosing the cell; the cytoplasm, the interior portion filled with cytosol (the aqueous fluid inside the cell); and the nucleus, the membrane-enclosed internal region that contains genetic material.
Within the nucleus of a cell, the main structures are: the nucleolus, an optically dense region, sphere shaped region, which contains concentrated ribosomal RNA (rRNA) bound to proteins (there may be more than one nucleolus in a nucleus); the chromatin, made of DNA molecules released into the nuclear matrix during cell interphase; and the karyotheca, or nuclear membrane, which is the membrane that encloses the nucleus.
Chromatin, dispersed in the nucleus, is a set of filamentous DNA molecules attached to nuclear proteins called histones. Each DNA filament is a double helix of DNA and therefore a chromosome.
The aqueous fluid that fills the nuclear region is called karyolymph, or the nucleoplasm. This fluid contains proteins, enzymes and other important substances for nuclear metabolism.
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