Replication

“ Mutations, sort of unexpected changes, can take place in either the chromosomes or the genes. You see, when this one cell starts dividing, every gene makes a copy of itself; but, sometimes, the copy is a little different from the original. ”

“ It is essential to distinguish the process of evolution toward the mutation–selection balance from an error catastrophe, as the two are completely different. We thus begin by briefly illustrating the dynamic approach to mutation–selection balance. For any combination of mutation rates and fitnesses in our model, a population will evolve until it reaches a unique equilibrium distribution of A1 and A2. Figure 2 illustrates the evolution of two populations—one that initially consisted entirely of A1 and another that consisted almost entirely of A2. ”

“ Thus all prokaryote evolution has been in the context of repeated growth and division and exclusively vertical inheritance of whole cells composed of membranes, genomes, cell skeleton, and a network of catalysts including ribosomes and enzymes. Against this historical dominance of universal verticality of cell lineages repeated instances of lateral transfer of genes have substituted foreign but broadly functionally equivalent versions of some enzymes and also added others that are novel to a lineage, having first evolved elsewhere. ”

“ Suddenly, instead of looking at a few species individually, we are able to study tens of thousands all together. Sequence data taken directly from the environment were dubbed the metagenome [12] , and the study of sequence data directly from the environment—metagenomics [13] .
However, environmental sequencing comes with its own information-restricting price tag. In single organism genomics practically all of the microbe's genome is sequenced, providing a complete picture of the genome. We know from which species the DNA or RNA originated. ”

“ Metagenomic Sample Coverage Coverage Coverage of a genome is defined as the mean number of times a nucleotide is being sequenced. Thus, 5× coverage means that each nucleotide in the genome is sequenced a mean number of five times. If we could sequence a genome in a single read, then 1× coverage would suffice for sequencing. Shorter read lengths (25–700, depending on sequencing technologies, see Table 1) , necessitate more coverage, to ensure all reads overlap, and that those overlaps are unique enough to reconstruct the genome by assembling the fragments. ”

“ When DNA is replicated, or copied, as the organism grows, the two nucleotide strands separate from each other by disjoining the rungs at the point where the bases meet, and each strand then makes a new and similarly complementary strand. The result is two double-stranded DNA molecules, each of which is identical to the parent molecule and contains the same genetic material. When the cell divides, each of the two daughter cells gets one of the new double strands; each new cell thus always has the same amount of DNA and the same genetic material as the parent cell. ”

“ Our recent knowledge of the cell cycle has therefore led to a shift in the focus of investigation from mitosis to DNA synthesis.
Another point to remember is that not all cells keep going through the cell cycle indefinitely. As shown in Figure 25, when a cell divides, the daughter cells have two alternatives, either to go through another cycle or to leave it altogether. Cells that leave the cycle are called differentiated cells and will eventually die without any further division. Many cells in an adult organism also have lost the capacity to make DNA and therefore the capacity to divide. ”

“ It is a basic fallacy to regard bacteria or eukaryotes as being simply the product of their genes and genomes. Genes can do nothing outside this structural framework. It is not clear even in theory how lateral gene transfer could cause a structural innovation like the origin of thylakoids or the addition or loss of an outer membrane. To use the word phylogeny to apply just to gene sequence trees is, as you candidly admit, a too narrow interpretation. One must consider structures and morphogenesis as well as genes in phylogeny. ”

“ Each of these modifications is a change of the molecule into equilibrium with its environment—an adaptation, as it were, to new surrounding conditions to which it is subjected; as through Evolution in general. Larger or more integrated aggregates are successively generated; as through Evolution in general. More complex or heterogeneous aggregates are so made to arise, one out of another; as through Evolution in general. A geometrically increasing multitude of these larger and more complex aggregates so produced, at the same time results; as through Evolution in general. ”

“ In short, gene losses and quantum evolution of sequences beyond recognition grossly distort such statistics. We must consider bacterial evolution as a problem of organismal evolution and not base conclusions only on the sequences of the non-representative minority of genes that evolved sufficiently trivially to still allow sequence alignment and tree making. ”

“ Some evolutionarily key organisms are greatly neglected. A worse problem with multiple-gene trees is genome-wide systematic biases that can give the wrong topology with increasing confidence as data are added [ [2] . Cladistic reasoning about unique or rare changes has a special role in formulating and testing relationships, having been decisive in eukaryotes, e.g. in creating and strongly corroborating the chromalveolate theory [ [23] - [24] and locating the root of the eukaryotic subtree [ [3] , [4] , [6] . ”

“ In seeking answers to these questions, we inquired whether there are common to both Homogenesis and Heterogenesis, any conditions under which alone sperm-cells and germ-cells arise and are united for the production of new organisms; and we reached the conclusion that, in all cases, they arise only when there is an approach to equilibrium between the forces which produce growth and the forces which oppose growth. This answer to the question—when does gamogenesis recur? ”

“ In Heterogenesis, on the other hand, we have the birth of dissimilar products, the beginning of a new branch from a "life-tree," in which the offspring have no tendency to assume the parental type. This occurs, for instance, where the protoplasmic matter of an animal or of a vegetal cell becomes modified and resolved into Bacteria. ”

“ Of two particles of protoplasm, between which we may defy all the power of the microscope, all the resources of the laboratory, to detect a difference, one can develop only to a jelly-fish, the other only to a man, and one conclusion alone is here possible—that deep within them there must be a fundamental difference which thus determines their inevitable destiny, but of which we know nothing, and can assert nothing beyond the statement that it must depend on their hidden molecular constitution. ”

“ When I look at a dividing cell I feel as an astronomer might do if he beheld the formation of a double star: that an original act of creation is taking place before me. Enigmatical as the phenomenon seems, I am not without hope that, if it were studied for its own sake, dissociated from the complications which obscure it when regarded as a mere incident in development, some hint as to the nature of division could be found. ”

“ To make clear the application of the terms chemical and mechanical to the problem of Genetics the nature of that problem must be more fully described. In its most concrete form this problem is expressed in the question, how does a cell divide? If the organism is unicellular, and the single cell is the whole body, then the process of heredity is accomplished in the single operation of cell-division. Similarly in animals and plants whose bodies are made up of many cells, the whole process of heredity is accomplished in the cell-divisions by which the germ-cells are formed. ”

“ Genes can survive only by coding for real physical structures of interacting macromolecules, whose evolution can and must be studied by transition analysis of their phenotypes, not just of their gene sequences. ”

“ Gene Calling Genes are the basic functional unit in the genome, which may constitute larger functional units such as operons, transcriptional units, and functional networks. Again, the incomplete and fragmentary nature of metagenomic data presents challenges to identifying genes. With Sanger random shotgun sequencing, whole genomes are rarely assembled, and in species-rich environments, many reads remain as singletons rather than being joined in contigs. In the Global Ocean Sampling (GOS) data, which were Sanger-sequenced, the mean number of whole reading frames per assembly is 4.7 [52] . ”

“ Those holding to the theory of descent, evolution, or transmutation, look upon varieties, species, subgenera, genera, families, orders, classes, and sub-kingdoms, as distinctions of merely transient importance, corresponding to the state of our information at the present time; it being assumed that by means of gradual transmutation during the course of ages all organisms have become evolved from a single primitive cell, or from a few primitive types. ”

“ Every living creature, as we now know these creatures, is a compound. Simplicity is nowhere. Even the simplest the microscope can show is probably already a compound of many removes from the beginning.
The law of compounding is not at all mysterious. If the first force of reproduction, the genesis of individuals, exhibits itself in the form of an evolution—a budding forth, a repetition of the form and structure of the parent cell—the law of compounding first presents itself as an arrestation of the law of gemmation. ”

“ After contemplating the several methods by which the multiplication of organisms is carried on—after ranging them under the two heads of Homogenesis, in which the successive generations are similarly produced, and Heterogenesis, in which they are dissimilarly produced—after observing that Homogenesis is nearly always sexual genesis, while Heterogenesis is asexual genesis with occasionally-recurring sexual genesis; we came to the questions—why is it that some organisms multiply in the one way and some in the other? ”

“ Speculation aside, however, we may say that the two functions of the chromatin do not exclude one another, but that the general activity which originates from it may be but a lower phase of that special activity caused by fertilization. [2]
§ 74g. Here we come unawares to the remaining topic embraced under the title Cell-Life and Cell-Multiplication. We pass naturally from asexual multiplication of cells to sexual multiplication—from cell-reproduction to cell-generation. The phenomena are so numerous and so varied that a large part of them must be passed over. ”

“ Hence, then, there is congruity between the above interpretation and certain general truths displayed by animal organization at large. We may infer that in a way parallel to that just indicated, cell-evolution was, under one of its aspects, a change from a stage in which the exciting substance and the substance excited were mingled with approximate uniformity, to a stage in which the exciting substance was gathered together into the nucleus and finally into the chromosomes: leaving behind the substance excited, now distinguished as cytoplasm. ”

“ Thus a synthesis of molecules produces the stage for Life, a synthesis of living particles produces the Cell, a synthesis of cells produces an organism, a synthesis of organisms is a species—for the evidence (most notably that derived from the consideration of bee and ant communities) seems to show that material discontinuity in the members does not preclude the existence of a true synthetic union.116 The characteristic power gained by a species is that of evolutionary development working in the obscure region of germinal combination and variation. ”

“ Environmental Shotgun Sequencing (ESS) . (A) Sampling from habitat; (B) filtering particles, typically by size; (C) DNA extraction and lysis; (D) cloning and library; (E) sequence the clones; (F) sequence assembly. Second generation sequencing methods have been rapidly gaining ground and are replacing Sanger sequencing for small sized genomes and environmental genomics. A common denominator among second generation methods is the generation of “polymerase colonies” or polonies [25] , [26] . Polonies are PCR amplicons derived from a single molecule of nucleic acid. ”

“ I said that many transfers seem of short-term significance, e.g. because many less-core genes seem to come and go. I did not say that no lateral gene transfers have long-term significance. Surely it would be ridiculous to claim that all lateral transfers shape long-term phylogeny. ”

“ This matter can be approached by considering certain biological analogues to the machine situation. In the known biological realm, all living organisms use the same underlying reproductive logic of protein synthesis and nucleotide sequence copying but employ vastly different broad strategies in producing more of their own kind. ”

“ If we treat DNA shearing and sequencing as random events, and our ability to detect and overlap between two truly overlapping reads does not vary between clones (when those are used) , then we can use a Poisson distribution model to estimate the number of reads required to sequence an entire genome. This model is given by the Lander-Waterman equation [34] :
Where L is the read length, N is the number of reads, G is the genome length, and C is coverage as described above. ”

“ We must, therefore, represent the possibilities of exchange between the nucleus, in its capacity of bearer of the hereditary mass, and the protoplasm as being such that all the rudiments are not simultaneously in activity, but that some of them can remain latent for a time. ”