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What it means to be 98 pc chimpanzee

genetics
Last Updated : 05 August 2013, 15:35 IST
Last Updated : 05 August 2013, 15:35 IST

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What it means to be 98% chimpanzee is the title of a book by biologist Jonathan Marks. His aim? To make people aware of the fact that what we can learn from gene sequences is limited. The limitation is mostly in knowing what proteins can be made by an organism, and to some extent in estimating how genes are “networked”. In short, he exposes the fallacy of giving exaggerated importance to gene sequence information. No wonder that lay people are unaware of the fallacy: even the scientific community keeps falling into the trap.

As taught in school, the DNA is made up of sugars, phosphates and four different nitrogenous bases that are abbreviated as A,T, G and C. The sugars and phosphates provide a backbone to hold the bases but are irrelevant for understanding how DNA encodes information. It is the arrangement of the four bases in various combinations that decides what protein, if any, corresponds to a sequence of DNA. In the 1970s, scientist Frederick Sanger developed a technique by which DNA could be read, meaning that the precise sequence in which A, T, G and C occurred in a given sample could be determined.

Today, we have a host of techniques to sequence all of the DNA in an organism, which is known as its genome. The ability to do so has certainly expanded our understanding of the nature of living organisms. However, does this mean that until DNA sequencing came on the scene, we knew nothing about how living systems work at the molecular level? The answer is a clear no. In fact, the foundation for what is known as molecular biology today was largely laid by meticulous genetics done in the 1940s to 1960s, when there was no way of inferring the exact sequence of long stretches of DNA. However, in recent times, the hype surrounding gene sequencing has exaggerated the information one gets from it and the value of such information when we do have it at hand.

A DNA sequence that is called a gene is said to be a “coding sequence” because the sequence of bases carry coded information to make a protein or an RNA molecule that performs a specific function. The sequence can be compared to a meaningful sentence in a language. With some degree of success, biologists can identify where, i.e. with what base, a sentence begins and where it ends. The rest of the DNA that does not make up readable sentences is said to be “non-coding”. As far as we know, it does not code for a protein or RNA molecule. But, most of the time, we have no idea what it does. Some such sequences previously thought of as non coding have led to the discovery of several new kinds of RNA molecules that participate in regulating or co-ordinating the functions of genes. However, non-coding DNA can be quite a lot. For example, a staggering 95 per cent of the human genome belongs to this non-coding category. Just five per cent of our DNA seems to be making protein . Further investigations may raise the figure. But unless our ideas are fundamentally flawed — and as of today we do not see how that could be — it appears unlikely that more than 10 per cent of our genome could code for proteins or RNA.

Some genes are present across most life forms. They can then be used to look at how organisms have changed over time. However, when we compare two DNA sequences, it is important to compare the right bits, and to be conservative in drawing conclusions about similarity.

For example, if you wanted to compare a human genome with the genome of a fruit fly, the information is now available. A fruit fly genome is 60 per cent similar to that of a human. Does that mean a human being is 60 per cent fruit fly? It is clear that this is an absurd conclusion to draw. However, in case of the great apes, which includes chimpanzees, people frequently say “chimpanzees are 98 per cent human”. It did not take gene sequencing to see that they look more like humans than any other animal. However, it is scientifically inaccurate to translate genetic similarity to an all encompassing overall similarity.

As Marks points out, humans are not 98 per cent chimpanzees. We might add, nor are they 60 per cent fruit flies or 30 per cent daffodils. While data from gene sequences is undeniably useful, we are yet to fully understand how best to interpret or use it. More so, many of the promises held out by the knowledge of sequence information has not materialised.

Two major expectations have been disproved. The number of human genes expected was far more than the number actually found to exist (25,000 vs 100,000), indicating that gene numbers alone were unlikely to define a unique identity. Secondly, the expectation that knowing the sequences could lead to quick diagnosis and prevention of several diseases has not materialised. Even if we knew the function of every gene, it wouldn’t lead us to an instant understanding of how the organism is made. The idea that breaking down things to their most fundamental parts will reveal how the whole works does not necessarily work with complex systems. Their function depends on an intricate interplay between genes, physical environment, social inputs and so on. Even if it held true to all available comparison methods, the statement “Human DNA and chimpanzee DNA are 98 per cent similar” would tell us very little about what it means to be a human or a chimpanzee.

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Published 05 August 2013, 15:35 IST

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