Prokaryotic Genomes (Circular Rna and Dna)
Life first appeared on Earth about 3.7 billion years ago in the form of unicellular organisms, i.e., single-cell organisms, termed prokaryotes. Bacteria and archaea are examples of ancient unicellular organisms that supposedly have evolved from a common primeval ancestral, through successive genetic variations, DNA recombination and gene divergence. Prokaryotes, (from the Greek, pro =before+ karya=nucleus) are therefore unicellular organisms without an organized nucleus separating chromosomes from the cytoplasm.
Archaea and bacteria were first considered as one sole species because of their similar anatomies. Recent genetic analysis, however, had established archaea as a species apart from bacteria. This analysis revealed that only 44% of archaea's genes are similar to those found either in bacteria or in unicellular eukaryotes known as yeast cells; and the remaining 56% are genes exclusive of archaea, completely different from those found in the other two species.
Prokaryotes found in nature today store their encoded genetic information (i.e., genes) in circular chromosomes, mainly formed by double-stranded DNA. Scientists assume that the first prokaryotes had simpler cellular structures, storing their genetic information in circular RNA strands that enabled them to make only rudimentary proteins. This hypothesis states that only later, through evolutionary changes and due to the appearance of new catalysts (enzymes), the first DNA molecules were developed and used as a more efficient way of storing genes, as do the prokaryotes known at present. Once DNA was available, prokaryotic organisms began using RNA molecules for complementary functions during protein synthesis and DNA replication, while DNA became the permanent repository of genes. DNA opened the opportunity for more complex cellular functions to be developed because it confers a better genomic stability than does RNA. DNA and RNA are presently found in every cell, whether prokaryotic or eukariotic cells.
A great number of crucial proteins such as structural and functional proteins, enzymes, hormones, growth factors, etc., and many cellular functions such as chemical signaling among cells, energy production, enzymatic systems for DNA damage repair, etc., were first developed by prokaryotes. These proteins were further conserved in new different species throughout the evolutionary chain, from yeast cells to mammals. This phenomenon, known as evolutionary conservation, implies that certain genes were first developed by prokaryotes, then further conserved in higher species.
A well-studied prokaryote is the bacterium Escherichia coli, because of its simple structure and fast proliferation. E. coli has a rod-like shape, and its plasmatic membrane is similar to that found in eukariotic cells, but the external face of the membrane is rigid and thicker than the internal face, thus sustaining the bacterial shape. The cytoplasm contains ribosomes bound to messenger RNA that form polyribosome complexes that take a crucial part in protein translation. DNA in E. coli is usually organized in two identical circular chromosomes, each located in different regions of cytoplasm, and attached to the internal face of the plasmatic membrane. The region where the chromosome is located is termed nucleoid, and although E. coli may have one, two, or sometimes even more chromosomes, they are always identical.
A peculiar trait of prokaryotes is the ability to transfer extra packages of genetic information, termed plasmids, among individuals of the same species and even among different groups. The plasmid is not an integral part of the prokaryotic genome and replicates independently of bacterial DNA. Plasmids are constituted by a short, ring-shaped double strand of extra genes. Individuals carrying one or more plasmids are better fitted to survive in stressful conditions, such as the presence of antibiotics, toxic compounds, high acidity medium, etc. Plasmids are also responsible for the well-known bacterial ability to acquire resistance against a number of antibiotics. This horizontal gene transfer among individuals, i.e., plasmid transfer, may occur through different approaches, such as transformation and conjugation.
During transformation, a bacterium finds free fragments of DNA delivered in the process of death of another bacterium. A protein complex, present on the outer membrane of bacteria, collects the free DNA pieces and digests one of the strands, releasing the nucleotides in the process, while the other strand is integrated to its chromosome. Transformation may also occur without digestion of one of the strands, when the particle taken is a plasmid, which is then partially integrated to the bacterial chromosome.
Conjugation may occur through two different ways. The donator cell shoots out extensions, known as pilli (pillus, in the singular), while the receptor cell presents one or more receptor docking sites. Sometimes, several donators conjugate with a single receptor, thus forming aggregates. Once the donators and the receptor aggregate, plasmids transfer through pilli to the receptor cell. The other way of plasmid transfer through conjugation is when the donator cells secretes chemical messengers that attract receptor cells and stimulate them to secrete adhesion proteins that bind both receptor and donator together. Once in contact, pores open in the membranes of both cells and the plasmids transfer.
Prokaryotic DNA also contains a class of genes termed transposons or "jumping genes" that can bind to other genes and transfer gene sequences from one site of the chromosome to another, or from the chromosome to a plasmid, also from a plasmid to the chromosome. There are three main types of transposons: insertion sequences of bases with length between 150-1500 bp (base pairs) that integrate to chromosome by homologous recombination; complex transposons, such as Tn5 and Tn3, that contain a central region of extra genes besides those necessary for transposition, and are mainly responsible for antibiotic resistance and other metabolic functions; and, phage-associated conjugative transposons. Phage-associated conjugative transposons are derived from the association of phage particles (proteins) and transposons elements. Phage proteins are resultant from bacteriophage transduction of viral proteins into the DNA of infected bacteria that are further integrated to bacterial genome through homologous recombination. Bacteriophages are viruses that infect bacteria and utilize their replicative system to synthesize its own viral genome and proteins.
Another kind of transposon gene, termed Type II transposons, do not have insertion sequences, but also carry extra genes, such as those that confer antibiotic resistance, and can be transferred horizontally among individuals. Instead of jumping, conjugative transposons perform replicative transposition; i.e., they replicate and transpose the copy to a plasmid that later is passed to another individual or will segregate in the daughter cells. This class of plasmid is known as resistance plasmids or R factors.
Although plasmid transfer is more commonly observed among bacterial individuals pertaining to the same group, such as gram-positive or gram-negative bacteria, genetic exchanges were also observed between individuals of both groups, as well as among different groups of archaea, or between bacteria and archaea. Even more surprising was the discovery that horizontal gene transfers also occur in nature among unrelated species, such as bacteria and unicellular eukaryotes (yeast cells and unicellular plants), and between archaea and yeast cells. This phenomenon is known as non-homologous or "illegitimate" DNA recombination because it may occur between chromosomes of species not related, and also between non-homologous DNA sequences. Evidence suggests that such horizontal gene transfers may also have contributed to foster the emergence of new species and biological diversity.
This is the complete article, containing 1,168 words
(approx. 4 pages at 300 words per page).
