Genome size
An organism's complexity is not directly proportional to its genome size; total DNA content is widely variable between biological taxa.
Since the 1950s, with the emergence of various molecular techniques, 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).
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).
[citation needed] Some free-living marine bacterioplanktons also shows signs of genome reduction, which are hypothesized to be driven by natural selection.
During this process the future parasite subjected to an environment rich of metabolite where somehow needs to hide within the host cell, those factors reduce the retention and increase the genetic drift leading to an acceleration of the loss of non-essential genes.
[22][23][24] or simply: In 1991, John W. Drake proposed a general rule: that the mutation rate within a genome and its size are inversely correlated.
[29][30] 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.
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)[33][34] 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.
[32] Cavalier-Smith also proposed that, as consequent reaction of a cell reduction, the nucleus will be more prone to a selection in favor for the deletion compared to the duplication.
The base question behind the process of genome miniaturization is whether it occurs through large steps or due to a constant erosion of the gene content.
[40] In Rickettsia prowazekii, as with other small genome bacteria, this mutualistic endosymbiont has experienced a vast reduction of functional activity with a major exception compared to other parasites still retain the bio-synthetic ability of production of amino acid needed by its host.
[41][42][37] The common effects of the genome shrinking between this endosymbiont and the other parasites are the reduction of the ability to produce phospholipids, repair and recombination and an overall conversion of the composition of the gene to a richer A-T[43] content due to mutation and substitutions.
[37][41] Same faith occurred uvrA, uvrB and uvrC, genes encoding for excision enzymes involved in the repair of damaged DNA due to UV exposure.
[35] One of the most plausible mechanisms for the explanation of the genome shrinking is the chromosomal rearrangement because insertion/deletion of larger portion of sequence are more easily to be seen in during homologous recombination compared to the illegitimate, therefore the spread of the transposable elements will positively affect the rate of deletion.
[35] The event of small local indels plays a marginal role on the genome reduction[44] especially in the early stages where a larger number of genes became superfluous.
[45][35] Single events instead occurred due to the lack of selection pressure for the retention of genes especially if part of a pathway that lost its function during a previous deletion.
During this process the mitosomes[49] 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.
Another example of miniaturization is represented by the presence of nucleomorphs, enslaved nuclei, inside of the cell of two different algae, cryptophytes and chlorarachneans.
[32] 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.
