The term “pseudoautosomal” means that they can act as autosomes being involved in recombination between X and Y chromosomes. There are several regions of homology between X and Y chromosomes, so-called pseudoautosomal regions (PARs) most probably arisen due to translocation of DNA from X to Y chromosome. Studying sex chromosomes also revealed several peculiar traits. As a result of homologous and ectopic recombinations chimpanzees demonstrate greater chromatin variability in their subtelomeric regions. The presence of such SCBs affects chimpanzees’ chromosomes behavior during meiosis causing persistent subtelomeric associations between homologous and non-homologous chromosomes. SCBs predominantly consist of the subterminal satellite (StSat) repeats, they are found in African great apes but not in humans. Additionally, the majority of chimpanzee’s chromosomes contain subterminal constitutive heterochromatin (C-band) blocks (SCBs) that are absent in human chromosomes. In addition, there are numerous differences in the chromosomal organization of pericentric, paracentric, intercalary and Y type heterochromatin for example, the chimpanzees have large additional telomeric heterochromatin region on chromosome 18. Two out of nine are thought to occur in human chromosomes 1 and 18, and the other seven – in chimpanzee chromosomes 4, 5, 9, 12, 15, 16 and 17. Also, significant pericentric inversions were found in nine other chromosomes. It has originated due to a fusion of two ancestral acrocentric chromosomes corresponding to chromosomes 2a and 2b in chimpanzee. However, there is a major difference corresponding to the human chromosome 2. In general, both karyotypes are very similar. Human karyotype is represented by 46 chromosomes, whereas chimpanzees have 48 chromosomes.
Here we tried to review the major known structural and regulatory genetic alterations that had or might have a functional impact on the human and chimpanzee speciation (Table 1). Moreover, even higher proportion was shaped by chromosomal inversions and translocations comprising several megabase-long chromosomal regions or even entire chromosomes, as for the chromosomal fusion that took place when the human chromosome 2 was formed. It was found that genome differences represented by single nucleotide alterations formed 1.23% of human DNA, whereas larger deletions and insertions constituted ~ 3% of our genome. However, the idea of ~ 99% similarity of genomes persisted for a long time, until 2005 when nearly complete initial sequencing results of both human and chimpanzee ( Pan troglodytes) genomes became available. This estimate was based on the comparison of protein-coding sequences and didn’t consider non-coding (major) part of DNA. In early works, divergence of human and chimpanzee genomes was estimated as roughly 1%. It’s a difficult task to quantitate the exact percentage of differences between human and chimpanzee genomes. It is still of a great interest to identify genetic elements that distinguish humans from chimpanzees and encode features of human physiological and mental identities. The divergence of human and chimpanzee ancestors dates back to approximately 6,5–7,5 million years ago or even earlier. In this review, we summarized the available information about genetic differences between humans and chimpanzees and their potential functional impacts on differential molecular, anatomical, physiological and cognitive peculiarities of these species. Most structural gene-influential changes happened at the level of expression regulation, which in turn provoked larger alterations of interactome gene regulation networks. However, despite of extensive knowledge of structural genomic changes accompanying human evolution we still cannot identify with certainty the causative genes of human identity. Moreover, much higher proportion is made by differential chromosomal inversions and translocations comprising several megabase-long regions or even whole chromosomes. Human-specific single nucleotide alterations constituted 1.23% of human DNA, whereas more extended deletions and insertions cover ~ 3% of our genome. After divergence of their ancestor lineages, human and chimpanzee genomes underwent multiple changes including single nucleotide substitutions, deletions and duplications of DNA fragments of different size, insertion of transposable elements and chromosomal rearrangements. Genetic features distinguishing us from chimpanzees and making us humans are still of a great interest. The divergence between human and chimpanzee ancestors dates to approximately 6,5–7,5 million years ago. Chimpanzees are the closest living relatives of humans.
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