Who am I? What made me so different from other species? Why I share similarities with my parents? Again, why I have dissimilarities with them? And so on … ???
May be these are the most preceding questions which are continuously striking us from the ancient time. Answers and explanations are still ambiguous but human are always curious. The level of curiosity that we have, is massive and that’s why we are special comparing with other species. So when a question strikes us, we become energized to draw an answer. The journey of finding those answers of what we are and why we are like this, was commenced many years ago but got a way in 1865 with the discovery of rules governing the inheritance of traits in organisms by Gregor Mendel. This journey got pace with the identification of Gene responsible for this inheritance and the elucidation of DNA structure by James Watson and Francis Crick in 1953 and found its destination with Human Genome project in 2001. The production of bacterial cell whose entire genomic material is synthetic, added new dimension in bioscience. Now there is no ambiguity that we are nothing but the composition of carbon, hydrogen, oxygen, nitrogen and some other elements. Our DNA is made up with all these elements where all the information of continuing a life is stored in the language of A, T, G, C. Now we know the basis of various diseases because we realized and detected the factors responsible for those diseases. We sequenced our whole genome and identified genes those are assumed to be responsible for everything. We are controlled by genes but how these genes are controlled? One specific gene act differently from time to time on different conditions with no alteration in DNA sequences. What are the reasons behind this?
Charles Darwin, in his “On the Origin of Species” made it clear to us that evolutionary changes take place over many generations and through millions of years of natural selection. So, environmental factor cannot alter genetic sequence so quickly. But surprisingly sometimes some short-time events can also leave an imprint to one’s genome that also can be transmitted to his offspring! These imprints are then can short-circuit evolution. Dr. Lars Olov Bygren, a preventive-health specialist from Karolinska Institute in Stockholm conducted a research on random sample of 99 individuals born in the Overkalix parish of Norrbotten in 1905. In 19th century Norrbotten was so isolated that if the harvest was bad, people starved. People of Norrbotten suffered from crop failure and starvation in the year of 1800, 1812, 1821, 1836, and 1856. But they had excess harvest in the year of 1801, 1822, 1828, 1844 and 1863. From historical records Bygren and his colleagues became able to trace the parents and grandparents of those 99 samples. From agricultural records, they determined how much food had been available to their parents and grandparents when they were young. They found that the grandsons of Overkalix boys who had overeaten died an average of six years earlier than the grandsons of those who had experienced poor harvest. Their later researches revealed that their findings of shorten life span due to a single winter of gluttony are also accurate for Norrbotten females. How could these be possible?
Molecular biologist Arturas Petronis and his colleagues at the Center for Addiction and Mental Health in Toronto, Canada wanted to understand why identical twins, who share virtually all the same genes, could still wind up being so different. One monozygotic twin could develop diabetes or cancer or arthritis, and their co-twin, the genetically identical one, could be perfectly healthy-why? Or why autism strikes boys four times as often as girls? Lots of questions but no clear explanation from genetic perspective.
May be the answers lies beyond both nature and nurture which already have given birth to a new science called Epigenetics. The word “epigenetic” literally means “in addition to changes in genetic sequence.” The term has evolved to include any process that alters gene activity without changing the DNA sequence, and leads to modifications that can be transmitted to daughter cells. These patterns of gene expression are governed by the cellular material — the epigenome — that sits on top of the genome, just outside it. Environmental factors like diet, stress and prenatal nutrition can make an imprint on genes that is passed from one generation to the next.
Many types of epigenetic processes have been identified so far—they include methylation, acetylation, phosphorylation, ubiquitylation, chromatin modification etc. Epigenetic processes are natural and essential to many organism functions, but if they occur improperly, there can be major adverse health and behavioral effects. One important point is that epigenetic changes cannot be treated as Evolution. It does not change DNA sequence. Epigenetic changes are occurred in response to an environmental stress. That response can be inherited through many generations via epigenetic marks, but if the environmental pressure is removed, the epigenetic marks will eventually fade, and the DNA code will begin to slip back to its original programming with time.
Only few years ago it was thought, once we sequence the entire genome, we can explain the mystery of our uniqueness and we can eliminate various diseases, but now we are realizing only genetic code unveils very little of us. Until we understand how genes are controlled through reversible chemical modification or epigenomic marks, we cannot diminish the obscurity of our individuality. So, the International Human Epigenome Consortium (IHEC) was formed last January and projects had been set to map 1,000 reference epigenomes within a decade. However, when we successfully decipher all the epigenetic marks, we may experience another factor that needs to be resolved and that’s why, may be our journey will never end. But until then, understanding of epigenetic marks are the most challenging tasks of today’s biologists!
[ The article has been collected and primarily reformed by-
Saadlee Shehreen (Undergraduate student, University of Dhaka)