In addition to the size of the genome depends on the number of genes that are implicated, but also the structure and genomic characteristics of some relevance. In fact, the genome is often a considerable portion of the DNA non-gene DNA, which makes the number of genes in the genome and the genome size does not have a corresponding relationship. As the size of the genome previously used to indicate the content of DNA, known as the content of the specific values of C values [units generally pg (pg, that is 10-12g) DNA], so this gene and the C value indicates the number of The genome size of no correlation is called C value paradox. The structure of the genome in a more detailed understanding of the lack of prior, C value paradox that scientists have a big headache, but also makes some people even genes made of DNA that are suspect.
Although the nuclear genome size can be so huge, but the number of genes they contain, but a difference of but four or five times. Therefore, the prevalence of the nuclear genome of a large number of non-coding sequences, including non-coding repetitive sequences generally account for more than a third of the genome, the plant's nuclear genome, the sequence repeated even higher proportion in Some species even up to 90%. In addition, the nuclear genome in most of the genes have introns of split genes, introns are also accounted for a very important proportion of the genome. The length of a gene can have up to 90% of introns, some of the complex eukaryotic genome RNA transcription can be all out of half of the intron. In 1977, before, mainly because of fewer intron-containing bacterial genome, it has also been that the eukaryotic gene transcription is a continuous unit. Intron found in the last 20 years, molecular biology, one of the most significant discoveries, greatly contributed to the eukaryotic genome.
Repeat sequence and intron content directly affects the size of eukaryotic genomes, and even before the repeat sequence and intron presence as the characteristics of eukaryotic genomes.
Therefore, from a structural perspective, a prokaryotic genome and the genomes of eukaryotes far, so they can be naturally divided into two categories, a significant difference between these two categories is that the size of the genome sequence and internal repeated intron content. In general, class nuclear genome to genome smaller, contained no or only a small amount of repetitive sequences and introns for the characteristics that can be called "small genome" type; nuclear genome in large genome, containing a large number of repeat sequences and in intron of characteristics that can be called "large genomes" type.
Class nuclear genome and nuclear genome are in their cells in an unassailable position, therefore, the two are equivalent, and in the evolution of the two must be considered together. They divided into "small genome" type and "large genomes" type of evolutionary genomics approach is useful.
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