Tech UPTechnologyThis is how little separates us (genetically) from chimpanzees

This is how little separates us (genetically) from chimpanzees

In 1982 Jorge Yunis and Om Prakash published an article in Science that is now considered a cornerstone of evolutionary biology. Their work confirmed what had been suspected for some time: the chromosomes of humans, gorillas, chimpanzees and orangutans are so similar that an almost perfect correlation can be established between them. Later work expanded on this idea, showing that the human and chimpanzee genomes are 98% identical. The two sets of chromosomes are so similar that every human chromosome has a chimpanzee counterpart… with one exception: human chromosome 2.

In this case, there are two chimpanzee chromosomes that fit perfectly with the human : the so-called 2A and 2B. Nine years after Yunis and Prakash’s article, scientists at Yale University sequenced the site on human chromosome 2 that would correspond to a fusion of the telomeres (the ends of each chromosome) of chimpanzee chromosomes 2A and 2B. And there they found a DNA sequence that shows that at some point this is what happened: the telomere of one chromosome fused with that of another. The exact site of fusion has been preserved showing 158 copies of the sequence marking the site; is the X on the treasure plans.

And not only that, the centromere (the part that, as its name suggests, is in the center) of human chromosome 2 coincides with that of chimpanzee 2A. And the centromere of 2B? In 1992, remains of the characteristic sequences of this place, now inactive, were found. And precisely for this reason, because they no longer serve their initial function, the number of mutations they present is relatively high. This is clearly an evolutionary process: for those DNA sequences that are useful, there are correction mechanisms against mutations; if it is not useful or does not perform any function, there is no reason to correct the errors.

jumping genes

In 1950 Barbara McClintock made what can be called a fundamental discovery in the field of genetics. As often happens in these cases, and despite the fact that their experiments and their logic were completely unappealable, most scientists dismissed the conclusions as too revolutionary : “This woman is either crazy or a genius,” wrote the biologist EF Keller . “I was so shocked that I thought they would ridicule me, or tell me I was really crazy,” McClintock herself said years later. “Only” 36 years later the entire scientific community celebrated her discovery when she was awarded the Nobel Prize for Medicine, the only person who has not shared the prize in this category. And all because his theories about certain elements in the chromosomes of corn have been fundamental to understanding genetics, evolution, disease and cancer.

What was it that he discovered? In the 1940s he realized that there was something strange about the heritability of certain genetic elements, since they did not fit the expected pattern. After years of experimentation, McClintock discovered why: There were segments of DNA on the chromosomes that moved from one place to another in the genome. They are called transposons (previously “jumping genes”) or mobile genetic elements. Thus, if the transposable element is inserted inside a gene, it may happen that when it moves, the gene recovers the function it had and was “short-circuited” by the transposon. In the same way, if when changing position it is inserted into a gene, it causes the loss of function of that gene. Ultimately, transposons create instability in the genome by moving freely through it. And ours contains around 3 million of them, almost half of all our DNA! It is not uncommon then that scientists, every time they work with genes, do not fail to come across all kinds of transposons: you kick a stone in the genome and a transposon skips.

In subsequent years it was discovered that there were two types of these elements: McClintock transposons (which make up 2.8% of the human genome) and retrotransposons (which make up 42.8%). These elements make a copy of themselves into RNA that is “backcopied” into DNA and inserted into the genome. This behavior is very similar to that of retroviruses, such as AIDS or the flu.

Viruses that “live” in the genome

All virus-like retroelements of humans originated tens of millions of years ago, in most cases. We have no evidence of modern retroviruses settling comfortably into our genome and becoming retrotransposons . However, it has happened in the rest of the mammals. For example, chimpanzees and gorillas have many copies of a retroelement, descendants of a retrovirus that independently infected their genomes but did not infect humans and orangutans. Another type of retroelements are opportunistic pieces of DNA that, thanks to mutations, have acquired the ability to move freely through the genome.

The most common of all is a relatively small one called Alu : about 10% of the human genome consists of more than a million Alu elements , one of the few types of retrotransposons still active in our DNA. Its importance when it comes to understanding how evolution has worked is obvious: if two individuals have the same retroelement inserted in the identical part of their genome, it means that they must have inherited it from a common ancestor. This is what was discovered almost by accident in the mid-1980s.

2% difference

In 1985 scientists from the University of California at Davis and Berkeley conducted the first study of the DNA surrounding the genes that code for hemoglobin, comparing it to that of chimpanzees. As expected they found many Alu elements. But what was striking was that all of them, without exception, were found in the same places and with the same directions in both species. Comparing the DNA sequences of 7 Alu elements in chimpanzees and humans, it was found that the similarity between them ranged from 94.7% to 98.9%.

Another example is HERV-K, a retrotransposon that was introduced into the common ancestor of humans, apes, and monkeys tens of millions of years ago, and unlike most of these virus-like elements, is still active in our genome . In the year 2000, scientists from the Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry in Russia discovered that 11 of the 14 elements present in humans are found in the same position in chimpanzees and gorillas, which tells us that they are the closest species to the our. Next up is the orangutan, with whom we share 9 HERV-Ks, and the gibbon, with 7.

In addition to transposons, we also have pseudogenes in our genome, genes that have lost their functional capacity, that is, they do not code for any protein. There are about 20,000 of these pseudogenes in the human and chimpanzee genomes, and, like the transposons, virtually all of them are found in the same places. There are only a few that set us apart: 163 in humans and 246 in chimpanzees, according to the latest count from the Chimpanzee Genome Sequencing and Analysis Consortium. It was this group of more than 60 scientists who in 2005 published the first draft of the complete sequencing of the genome of our first cousin, which has confirmed what Darwin pointed out in his day: that humans shared recent common ancestors with the great apes Africans.

All this leads us to a key idea in biology: Darwin’s theory of evolution tells us about the survival of the one that best adapts to the environment; molecular biology tells us how that best-fit was built.

References:

Fairbanks, D. J. (2007) Relics of eden, Prometheus Books

Slaves and Disabled: Forced Medical Test Volunteers

The main problem to carry out medical research is to have willing volunteers for it. And if they come out for free, much better. This is the story of unethical behavior in medical research.

How are lightning created?

Summer is synonymous with sun, but also with storms. Who has not contemplated one from the protection that the home gives that electrical display that is lightning?

How global warming will affect astronomy

Astronomical observations around the world will worsen in quality as a result of climate change, according to a new study.

New images of Saturn's rings in stunning detail

New images of Saturn's rings in stunning detail

NASA discovers more than 50 areas that emit exorbitant levels of greenhouse gases

NASA's 'EMIT' spectrometer locates has targeted Central Asia, the Middle East and the US among others.

More