Eukaryotic genomes

Compare different eukaryotic genomes

One of the most important approaches in describing the content and the organization of the genetic material in living organisms is the genome sequencing. This is a laborious task, but scientists have already mapped completely the genomes of some representative eukaryotes. A short summary of the genome characteristics of 5 eukaryotic organisms is presented in the Table 1 below.

Table 1 Comparison of the features of some eukaryotic genomes that have been completely sequenced (Pierce, 2009)


Genome size (mbp)

No. of predicted genes

No. of protein-domain families

Saccharomyces cerevisiae (yeast)




Arabidopsis thaliana (thale cress)




Caenorhabditis elegans (roundworm)




Drosophila melanogaster (fruit fly)




Homo sapiens (human)




These eukaryotic organisms have been chosen because they help characterize all the other eukaryotes. In this sense, the S. cerevisiae provides valuable information in understanding the basic differences between prokaryotes and eukaryotes; the C. elegans is a fundamental model in describing a multi-cellular organism; A. thaliana proved to be the optimum model for studying the plants genome and the D. melanogaster is perfect for studying the principles of genetics.

A short inspection of the information in Table 1 shows that multi-cellular eukaryotes genomes contain more genes than the unicellular ones, but there is no apparent connection between the number of genes and the complexity of the phenotype. For instance, the number of genes in humans is much higher than the one in yeast, S. cerevisiae, but only double to the one of the fruit fly , D. melanogaster and almost equal to the genes number of A. thaliana. The same trend is observed for C. elegans which has less DNA than the plant A. thaliana, but it has a more complex structure.

When comparing the genomes of the yeast and the roundworm, it is observed that almost one fifth of the genes in both organisms are homologous- they code for basic eukaryotic functions. The rest of the genes code for typical multi-cellular eukaryotic functions, as for example: cell differentiation and intercellular communication. In this sense, almost 40% of the C. elegans genes are homologous with the other 3 eukaryotes presented above.

Another similarity between the eukaryotic genomes is the presence of transposable elements, which account for 10% of the thale cress genome, but they are much less frequent than in the human genome, for example.

From the Table 1, it is also obvious that C. Elegans, A. Thaliana and D. melanogaster genomes encode for a similar number of proteins, the difference in the genome size being due to the presence of duplicated copies of genes in the A. Thaliana. In comparison, the human genome encodes for only a slightly higher number of protein domains, but these domains are arranged into more combinations, which lead to a higher number of protein types.

As a specific feature, A. Thaliana has approximately 150 protein families that are only seen in plants and they are responsible for typical plant functions (photosynthesis, transport of water from roots, cell walls, etc).

Comparing the human genome with the roundworm and the fruit fly ones, it has been noticed that over 90% of the protein domains in the three organisms are similar, which means that the evolution of the vertebrate has required only a few new domains. The protein families in human that are distinctive from the other eukaryotes have roles in acquired immunity, neural development and intercellular and intracellular signalling.

Gene density is another major difference between the eukaryotic genome. The C. Elegans has the genes uniformly distributed across its chromosomes, whilst in humans, the chromosomes 17, 19 and 22 have the highest density and the sex chromosomes have the lowest gene density.

Some important conclusions can be reached following the comparison of different eukaryotic genomes. The most important is that all present day organisms have evolved from common ancestral genomes and that studying a gene in one organism provides information about the homologous gene in any other eukaryotic organism.
