Nov 12, 2009
Eukaryotic organisms encompass a range of organisms, from humans to single-celled microorganisms such as protozoa. Eukaryotes are fundamentally different from prokaryotic microorganisms, such as bacteria, in their size, structure and functional organization.
The oldest known eukaryote fossil is about 1.5 billion years old. Prokaryote fossils that are over 3 billion years old are known. Thus, prokaryotic cells appeared first on Earth. The appearance of eukaryotic cells some 1.5 billion years ago became possible when cellular function was organized into regions within the cell called organelles.
The eukaryotes are organized into a division of life that is designated as the Eukaryota. The Eukaryota are one of the three branches of living organisms. The other two branches are the Prokaryota and the Archae.
The evolutionary divergence of life into these three groups has been deduced in the pasts several decades. Techniques of molecular analysis have been used, in particular the analysis of the sequence of a component of ribosomal ribonucleic acid (RNA), which is known as 16S RNA. This RNA species is highly conserved in life forms. Thus, great differences in the sequence of 16 S RNA between a eukaryotic and a prokaryotic microorganism, for example, indicate that the two organisms diverged evolutionarily a very long time ago. A similar 16 S RNA indicates the converse; that evolutionary branching is a relatively recent event.
Eukaryotic cells are about 10 times the size of all but a few prokaryotes. This translates to an internal volume which is very much larger, some 1000 times, that the internal volume of a bacterium. In order to survive, eukaryotes evolved a highly organized internal structure, in order that all the tasks necessary for life can be accomplished in the large internal volume. This internal structure is the fundamental distinguishing aspect of a eukaryote versus a prokaryote.
Functional specialization is the fundamental hallmark of eukaryotes. In larger organisms, such as humans, this specialization gives rise to organs such as the heart, lover, and brain, and to functional organizations such as the immune system. But organization is also evident in microscopic, even single-celled, eukaryotes.
In a eukaryote, the nuclear material is segregated within a specialized region called the nucleus. This feature is a key constituent of eukaryotic cells. Indeed, the word eukaryote means "true nuclei." The nucleus exists because of the presence of the so-called nuclear membrane, which encloses the nuclear material. The nuclear membrane contains pores, through which material can enter and leave the nuclear region. Prokaryotes lack an organized nucleus. Indeed, for many years the presence of a nucleus was the sole key feature that distinguished a eukaryote from a prokaryote.
Most of the eukaryotic DNA (deoxyribonucleic acid) is present in the nucleus. The remainder is contained within the energy-generating structures known as the mitochondria. The organization of the eukaryotic DNA is very different from bacterial DNA. In the latter, the genetic material is usually dispersed as a large circle throughout the interior of the bacterium, in a gel-like mixture termed the cytoplasm. In contrast, eukaryotic DNA is organized into discrete limb-like structures called chromosomes.
The replication of eukaryotic DNA is also different from that of prokaryotes. The latter is essentially an unwinding of the double helix of DNA, with ongoing complementary copies of daughter DNA strands made from each unwinding parental strand. The result is two double helices. The replication process in eukaryotes is more complex, involving several phases of chromosome replication, segregation to areas of the cell, collection together, and enclosure in a nuclear membrane.
Eukaryotic cells, including microorganisms, contain a specialized functional region known as the endoplasmic reticulum. This network of tubular structures is involved in the manufacture of protein from the template of RNA. In many eukaryotes a region called the Golgi apparatus or Golgi body is associated with the endoplasmic reticulum. The Golgi body is involved with the transport of compounds into and out of the cell.
Another distinctive feature of eukaryotic cells is the aforementioned mitochondria. These are the energy factories of the cell. Additionally, some eukaryotes possess structures called chloroplasts, which use the energy available in light to change carbon dioxide and water into carbohydrates. The carbohydrates provide a ready source of energy for cellular functions. This photosynthetic process is a feature of the microscopic eukaryotes called algae.
Other internal organization of eukaryotes includes lysosomes, which contain enzyme that digest food that is taken into the eukaryote. The lysosome represents a primitive stomach.
Eukaryotic cells such as amoeba possess an internal scaffolding that helps provide the shape and support to the cell. The scaffolding consists of filaments that are made of protein. Depending on the protein the filaments are designated as actin filaments, microtubules, and intermediate filaments.
Eukaryotes such as amoebae and algae are part of a group that is called Protista. More commonly, members of the group are referred to as protists.
The evolutionary branching of eukaryotes from prokaryotes involved the acquisition of regions specialized function within the eukaryotic cell. One of these regions, the mitochondria, was likely derived from the habitation of a eukaryote by a bacterium. Evidence from ultrastructural and molecular studies for a symbiosis between a bacterium and a eukaryote is convincing. Over time, the bacterium became truly a component of the eukaryotic cell. Today, however, the DNA of the mitochondria remains unique, with respect to eukaryotic nuclear DNA.
Likewise, chloroplasts may have had the origin in a symbiotic relationship between a cyanobacterium and a eukaryotic cell. Current evidence does not support the development of any other eukaryotic organelle from a prokaryotic ancestor.
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