Identical twins are born from the same fertilized egg, so theoretically they both have the same sequence of DNA (deoxyribonucleic acid), which contains their genetic information. So why can such twins have different physical makeup, contract different illnesses, and even look slightly different?
Identical but different twins – artistic impression. Image credit: Bùi Thanh Tâm via Unsplash, free license
The most obvious reason is that their lifestyle habits differ. Differences in things like diet, exercise and sleep surely result in different types and volumes of substances generated within the body.
Another reason is connected to DNA: each twin’s DNA sequence changes slightly over time after they begin life. We know, for example, that people can become ill due to damage to the DNA of genes crucial to good health, as in the case of cancer caused by excessive ultraviolet light exposure.
These microscopic images from Professor Kanoh’s lab show a fission yeast cell dividing. You can see the dynamic movement of the telomere, shown in red. Image credit: Tokyo University
But these are not the only reasons for the differences between identical twins. A little-known fact is that in addition to the sequence of DNA, the way DNA is folded is important for living creatures. DNA strands are not straight lines: they are folded in many formations.
The volume of proteins genes synthesize depends on how tightly or loosely DNA is folded. Loose folds allow enzymes and other substances to enter easily, resulting in large volumes of protein synthesis. The types and volumes of proteins produced determine what organs the cells will form.
It is not a case of the more proteins the better: there are optimal volumes for each body part from eyes to legs. In other words, even if DNA sequences are identical, problems in how the DNA is folded can mean that proteins are not synthesized in the appropriate volumes, and this is a key cause of illness.
Differences in DNA folding can emerge in identical twins even after separating from the fertilized egg. This can occur for many reasons, including changes in the genetic sequences that determine the folding patterns.
DNA is composed of four bases: adenine (A), thymine (T), guanine (G) and cytosine (C). The arrangement of ATGC and the way DNA chains are folded are thought to help generate the differences among individual people. Image credit: Tokyo University
Moreover, females have two of the X chromosomes that determine sex (chromosomes are structures created when proteins and other substances combine in DNA), and one of these two can randomly become condensed and harden, impeding the synthesis of proteins. Your fate is determined by which of your X chromosomes hardens within which cells. This is one of the factors that leads to differences from person to person.
I’ve been doing research on a region at the end of the chromosome called the telomere, and the subtelomere adjacent to it. The telomere's roles include determining the cell's lifespan and passing on life to the next generation.
Last year, when doing research on a model organism known as fission yeast, I discovered that the DNA sequence of the subtelomere changes frequently.
Generally, DNA sequences don’t change very often. Frequent changes pose a threat to life, so there are multiple layers of mechanisms that function to prevent them. But in the subtelomere, I saw changes occurring frequently. Was this just a simple mistake? I believe that living organisms might have deliberately retained the scope for such changes.
Over the course of evolution on our planet there have been multiple mass extinctions, but organisms resistant to environmental changes have survived. The capacity to alter one’s DNA might be a strategy to respond to various environmental changes: a hallmark of evolution.
Organisms change, little by little, from the moment of their birth right through to their death. That is how some managed to survive even when the planet was covered in ice. All organisms with DNA have a common system comprising a sequence of four bases in their DNA, known as ATGC (some viruses have something similar to DNA, called RNA).
This tells us that all organisms evolved from the same original organism. This means that we are part of this long chain of life that began from a single cell. Doesn’t that give you a thrill?
Source: University of Tokyo