Each living organism, including humans of course, store all the information in the DNA. In a non-technical way, we can define the DNA as the body’s instruction manual. We are what it’s written in our DNA.
More in detail, what is the DNA? I have already answered the question here, by comparing DNA to a long chain with specific features. Now I want to propose an alternative and more colored explanation. We can imagine the DNA of a person as a unique QR code, in which each module, instead of black or white, can be of one among four different colors.
So is each module of the QR code randomly colored? Are all combinations possible? Not really. In the vast majority of cases, a given module of this QR code exists only of one color. Only a small percentage of modules, scattered throughout the square, can vary. Anyway, keep in mind that the frequency of colors in these modules would not be equally distributed in the population, because some colors would be more frequent than others. This is because specific combinations of modules have a precise meaning, and evolution does not like the word “random”. People are physically and psychologically different thanks to the variation of colors in this subset of millions of modules (therefore billions of possible combinations are possible). This establishes the normal variability among people: height, eye color, physical fitness, propensity to obesity, etc.
Is the color of a module permanent? What happens if it suddenly changes? Theoretically each module can undergo a color switch. In biology, variations in the DNA sequence are caused by several factors (refer to this post for more info about it), and they can lead to different consequences. This depends both on which module changes color and which one of the other three colors replaces the original one. If you scan a QR code that contains a mistake, you may be directed to a completely different link, or you might still open the proper link if the mistake is non-relevant, or it may not open any link because the wrong QR code is not valid. The same happens with variations of the DNA sequence. Changing the color of a module in a given position may have no effect, or change the features of the individual, in other cases it may not be compatible with life, or it may predispose to genetic diseases such as tumors. It is important to consider that the color switch is not the only possible condition to develop a disease. Our QR code derives from the merge between our parents’ ones (with little changes), and the combination “position + color” for each module is pre-set at the conception. These combinations already determine the features that will characterize us during our life, including predisposition to diseases. If a module in a certain position is of one color rather than another, it can predispose to multiple sclerosis, or to an aggressive behavior, or to develop higher stamina during sport, or it can reduce the risk to get a disease and so on.
Got it. But why all these possibilities? You mentioned that the position of the module and the colors plays a role in this, right? Exactly. DNA is not just a linear sequence of colored modules. Some modules are more relevant than others, and modules that encode for genes are the most relevant. If the DNA is our instruction manual, a gene is a command that is read and subsequently executed. Therefore, a color switch inside a gene can have a high impact on the cell, compared to one away from any gene, because it alters the meaning of the command. Diseases that are not caused by viruses or bacteria are generally caused by color switches inside a gene, and include tumors, cystic fibrosis, muscular dystrophies, and so on. Now let’s see the role of the color. Consider a color switch of a module inside a gene. It can happen that a switch from blue to green has no effect on the command, but a switch from blue to red is deleterious. Why? Because evolution developed some sort of “tolerance mechanism” by reading colors in triplets, and not individually. The sequence of color triplets determine the words of the command. A specific word in the command can be encoded by more than one combination of three colors: in fact, 64 possible triplets exist, coding for 20 letters of the biological alphabet. Only if the new triplet (after the color switch) encodes for a different word compared to the original one, the switch has an effect on the command execution.
And what about the DNA that doesn’t encode for genes? Good question. For several years scientists called it “junk DNA”, because it was not possible to identify its function. Still, its role remains partially obscure, but we are getting close to the truth. Non-coding DNA has a regulatory function over the coding DNA, and this is the reason why genes are more or less interspersed into the genome, instead of being all clustered together. Different genes are regulated differently. Depending on stimuli received by the cell, regions of non-coding DNA interact with different factors to promote or inhibit the execution of one or more specific commands in response to the input received.
Few last considerations. I liked to present this new view of DNA as a colored QR code, because it becomes easy to explain how complex DNA is. In the non-coding DNA, modules are not read in triplets, but still specific color combinations determine many features of DNA: start and end sites of a gene, localization signal for some factors that regulate commands execution, terminals of chromosomes. Lastly, remember that also a color switch in a regulatory region like the aforementioned ones can have a massive effect, like switches inside a gene. If a gene is regulated by a factor that binds to a specific color sequence, and this color sequence is altered, the execution of the command encoded by the gene will have consequences.
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