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Amount of DNA contained in a genome

Genome size is the total amount of DNA contained within one copy of a single complete genome. It is typically measured in terms of mass in picograms (trillionths (10−12) of a gram, abbreviated pg) or less frequently in daltons, or as the total number of nucleotide base pairs, usually in megabases (millions of base pairs, abbreviated Mb or Mbp). One picogram is equal to 978 megabases. In diploid organisms, genome size is often used interchangeably with the term C value. An organism's complexity is not directly proportional to its genome size; total DNA content is widely variable between biological taxa. Some single celled organisms have much more DNA than humans, for reasons that remain unclear (see non coding DNA and C value enigma).

The term "genome size" is often erroneously attributed to a 1976 paper by Ralph Hinegardner, even in discussions dealing specifically with terminology in this area of research (e. g., Greilhuber 2005). Notably, Hinegardner In this context, "genome size" was being used in the sense of genotype to mean the number of genes. In a paper submitted only two months later, Wolf et al. (1969) used the term "genome size" throughout and in its present usage; therefore these authors should probably be credited with originating the term in its modern sense. By the early 1970s, "genome size" was in common usage with its present definition, probably as a result of its inclusion in Susumu Ohno's influential book Evolution by Gene Duplication, published in 1970.

With the emergence of various molecular techniques in the past 50 years, the genome sizes of thousands of eukaryotes have been analyzed, and these data are available in online databases for animals, plants, and fungi (see external links). Nuclear genome size is typically measured in eukaryotes using either densitometric measurements of Feulgen stained nuclei (previously using specialized densitometers, now more commonly using computerized image analysis) or flow cytometry. In prokaryotes, pulsed field gel electrophoresis and complete genome sequencing are the predominant methods of genome size determination. Nuclear genome sizes are well known to vary enormously among eukaryotic species. In animals they range more than 3,300 fold, and in land plants they differ by a factor of about 1,000. Protist genomes have been reported to vary more than 300,000 fold in size, but the high end of this range (Amoeba) has been called into question. In eukaryotes (but not prokaryotes), genome size is not proportional to the number of genes present in the genome, an observation that was deemed wholly counter intuitive before the discovery of non coding DNA and which became known as the "C value paradox" as a result. However, although there is no longer any paradoxical aspect to the discrepancy between genome size and gene number, the term remains in common usage. For reasons of conceptual clarification, the various puzzles that remain with regard to genome size variation instead have been suggested by one author to more accurately comprise a puzzle or an enigma (the so called "C value enigma"). Genome size correlates with a range of measurable characteristics at the cell and organism levels, including cell size, cell division rate, and, depending on the taxon, body size, metabolic rate, developmental rate, organ complexity, geographical distribution, or extinction risk. Although the latter contrasts with the previous view that no correlation exists for the eukaryotes, the observed nonlinear correlation for eukaryotes may reflect disproportionately fast increasing non coding DNA in increasingly large eukaryotic genomes. Although sequenced genome data are practically biased toward small genomes, which may compromise the accuracy of the empirically derived correlation, and ultimate proof of the correlation remains to be obtained by sequencing some of the largest eukaryotic genomes, current data do not seem to rule out a possible correlation.

In humans, the total female diploid nuclear genome per cell extends for 6.37 Gigabase pairs (Gbp), is 208.23 cm long and weighs 6.51 picograms (pg). Male values are 6.27 Gbp, 205.00 cm, 6.41 pg. In eukaryotes, in addition to nuclear DNA, there is also mitochondrial DNA (mtDNA) which encodes certain proteins used by the mitochondria. The mtDNA is usually relatively small in comparison to the nuclear DNA. For example, the human mitochondrial DNA forms closed circular molecules, each of which contains 16,569 DNA base pairs, with each such molecule normally containing a full set of the mitochondrial genes. Each human mitochondrion contains, on average, approximately 5 such mtDNA molecules.

Genome reduction, also known as genome degradation, is the process by which an organism's genome shrinks relative to that of its ancestors. Genomes fluctuate in size regularly, and genome size reduction is most significant in bacteria. The most evolutionarily significant cases of genome reduction may be observed in the eukaryotic organelles known to be derived from bacteria: mitochondria and plastids. These organelles are descended from primordial endosymbionts, which were capable of surviving within the host cell and which the host cell likewise needed for survival. Many present day mitochondria have less than 20 genes in their entire genome, whereas a modern free living bacterium generally has at least 1,000 genes. Many genes have apparently been transferred to the host nucleus, while others have simply been lost and their function replaced by host processes. Other bacteria have become endosymbionts or obligate intracellular pathogens and experienced extensive genome reduction as a result. This process seems to be dominated by genetic drift resulting from small population size, low recombination rates, and high mutation rates, as opposed to selection for smaller genomes. Some free living marine bacterioplanktons also shows signs of genome reduction, which are hypothesized to be driven by natural selection.

In 1972 Michael David Bennett hypothesized that there was a correlation with the DNA content and the nuclear volume while Commoner and van’t Hoff and Sparrow before him postulated that even cell size and cell cycle length were controlled by the amount of DNA. More recent theories have brought us to discuss about the possibility of the presence of a mechanism that constrains physically the development of the genome to an optimal size. Those explanations have been disputed by Cavalier Smith’s article  where the author pointed that the way to understand the relation between genome size and cell volume was related to the skeletal theory. The nucleus of this theory is related to the cell volume, determined by an adaptation balance between advantages and disadvantages of bigger cell size, the optimization of the ratio nucleus:cytoplasm (karyoplasmatic ratio) and the concept that larger genomes provides are more prone to the accumulation of duplicative transposons as consequences of higher content of non coding skeletal DNA. The genome of the endosymbiont B. aphidicola is characterized by a genome size that is seven times smaller than E. coli (643 kb compared to 4.6 Mb) and can be view as a subset of the enteric bacteria gene inventory. This hypothesis is confirmed by the analysis of the pseudogenes of Buchnera where the number of deletions was more than ten times higher compared to the insertion. content due to mutation and substitutions. especially in the early stages where a larger number of genes became superfluous. One of the consequences of the elimination of such amount of sequences affected even the regulation of the remaining genes. The loss of large section of genomes could in fact lead to a loss in promotor sequences. This could in fact pushed the selection for the evolution of polycistronic regions with a positive effect for both size reduction and transcription efficiency.

One example of the miniaturization of the genome occurred in the microsporidia, an anaerobic intracellular parasite of arthropods evolved from aerobic fungi. During this process the mitosomes was formed consequent to the reduction of the mitochondria to a relic voided of genomes and metabolic activity except to the production of iron sulfur centers and the capacity to enter into the host cells. Except for the ribosomes, miniaturized as well, many other organelles have been almost lost during the process of the formation of the smallest genome found in the eukaryotes. This extreme process was possible thanks to the advantageous selection for a smaller cell size imposed by the parasitism. Another example of miniaturization is represented by the presence of nucleomorphs, enslaved nuclei, inside of the cell of two different algae, cryptophytes and chlorarachneans. Nucleomorphs are characterized by one of the smallest genomes known (551 and 380 kb) and as noticed for microsporidia, some genomes are noticeable reduced in length compared to other eukaryotes due to a virtual lack of non coding DNA. The most interesting factor is represented by the coexistence of those small nuclei inside of a cell that contains another nucleus that never experienced such genome reduction. Moreover, even if the host cells have different volumes from species to species and a consequent variability in genome size, the nucleomorph remain invariant denoting a double effect of selection within the same cell.