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explosion, whereas the second and next doubling occurred in the early Devonian period. In the evolution of fish, a further doubling of the genome occurred with up to eight copies of the original Deuterostomia (1‐2‐4‐8 hypothesis) in the late Devonian period. This took place after the Actinopterygii and Sarcopterygii had already divided. Among the Sarcopterygii are the famous Coelacanthus and lungfishes. All land vertebrates (amphibians, reptiles, birds, and mammals) have apparently descended from them. Within the eukaryotes, the maximum genome size has only a small relationship to the developmental level. This is because many plants and amphibians have genomes with up to 1011 bases, and the genomes are therefore one to two orders of magnitude higher than the genome of humans – it is obvious that many genome duplications must have taken place in these groups.
Figure 4.1 Number of nucleotides in the haploid genomes of important groups of organisms.
When the human genome is considered, it is obvious that a massive amount of information is present. If the DNA in an individual human cell was stretched out, it would be 2 m long. With around 1013 cells in our body, the total length of DNA in all cells is 2 × 1010 km. This length would be a distance that runs many times from the earth to the sun and back again!
Of the 3.2 million bases that are present in human haploid chromosomes, about 25% of the DNA defines genes, but only 1.5% of the DNA codes directly for proteins (Table 4.2 and Figure 4.2). The rest of the DNA is made up of RNA genes and noncoding sequences, which often either serve no function or their function is still unknown. In recent years microRNAs have been detected encoded in the “functionless” DNA, which are important for gene regulation (see Chapters 3 and 21).
Table 4.2 Relation between genome size and the number of genes of a few selected species whose genomes have been sequenced.
| Organisms | Genome size (bp)a) | Approximate number of genesb) |
|---|---|---|
| Archaea | ||
| Archaeoglobus fulgidus | 2.18 × 106 | 2405 |
| Methanothermobacter thermautotrophicus | 1.75 × 106 | 1866 |
| Pyrococcus furiosus (Archaea) | 1.91 × 106 | 2057 |
| Sulfolobus acidocaldarius (Archaea) | 2.99 × 106 | 2221 |
| Bacteria | ||
| Clostridium tetani | 2.8 × 106 | 2373 |
| Escherichia coli | 4.67 × 106 | 4288 |
| Haemophilus influenzae | 1.83 × 106 | 1702 |
| Mycoplasma genitalium | 0.58 × 106 | 476 |
| Rhodospirillum rubrum | 4.35 × 106 | 3791 |
| Fungi | ||
| Aspergillus fumigatus | 2.9 × 107 | 9920 |
| Saccharomyces cerevisiae | 1.3 × 107 | 6600 |
| Candida glabrata | 1.4 × 107 | 5180 |
| Sporozoa | ||
| Plasmodium falciparum (causes malaria) | 2.3 × 107 | 5300 |
| Plants | ||
| Arabidopsis thaliana | 2.2 × 108 | 29000 |
| Animals | ||
| Caenorhabditis elegans (nematode) | 1.3 × 108 | 21 000 |
| Drosophila melanogaster (fruit fly) | 2.0 × 108 | 32 000 |
| Danio rerio (zebra fish) | 1.4 × 109 | 21 000 |
| Mus musculus (mouse) | 2.8 × 109 | 30 000 |
| Homo sapiens (human) | 3.2 × 109 | 30 000 |
((done))
a Haploid genome.
b Including protein‐coding and noncoding RNA genes.
Source: www.ebi.ac.uk/genomes.
Figure 4.2 Composition of eukaryotic genomes and a fraction of a few DNA elements of the entire human genome.
Possibly the largest part of the genome (over 50% with higher eukaryotes) is not transcribed and according to our present knowledge is partially functionless. Important elements are pseudogenes and repetitive DNA sequences (Table 4.3 and Figure 4.2).
Table 4.3 A few characteristics of the