What our genes are telling us
BY SRINATH PERUR
In December 2000, Kumarasamy Thangaraj flew to Port Blair, took an overnight ferry to Hut Bay on Little Andaman Island, and then travelled by road and a small motor-boat to Dugong Creek. “It was thrilling,” he says, mentioning crocodiles in the water and the rough sailing on the last leg of the journey, where a rivulet meets the sea.
At Dugong Creek, he was amazed to see the Onge people spearing fish with sharpened wooden sticks. This semi-nomadic aboriginal people, of whom fewer than a hundred were left, alternate between foraging and living in government-run camps. They were the reason K. Thangaraj was here. Armed with permissions that had taken much time and effort to secure, he planned to collect blood samples from them.
K. Thangaraj made himself understood with the help of a social worker who spoke the Onge language. “They were looking at us and laughing,” he recalls, but they indulged him, and he returned with blood drawn from around 40 members of the group. DNA was isolated from the samples and added to the DNA bank at the Centre for Cellular and Molecular Biology (CCMB), Hyderabad, where he has been a researcher for two decades.
CCMB’s DNA bank has been put together with the help of researchers like K. Thangaraj, and students who return from vacations carrying blood samples and cheek swabs from their communities. It is the largest such repository in India. Samples from around 25,000 people, including some from India’s most far-flung reaches, are stored in trays of tiny bar-coded vials kept refrigerated at -70ºC. Associated with each sample is the donor’s information: name, geographical coordinates, age, sex, language, caste or tribe or other ethnic grouping, and a signed (or often thumb-printed) consent form. Today, in addition to proving useful for research on medicine and health, this database is casting light on a contentious period of Indian history to reveal who we are and where we come from.
The human body—hair, skin, muscle, organs, blood, bone—is made of trillions of cells (37.2 trillion, according to one recent estimate). The genetic recipe for an individual is contained in the nucleus of most cells. This recipe is in strands of DNA packed into 23 pairs of chromosomes. The DNA from a single microscopically small cell’s nucleus would extend to several feet in length if—as science writers are always threatening—it is unwound and laid out end to end.
This paired filament would be thousands of times thinner than a human hair and shaped like a twisted ladder. Molecules that form the rungs of this ladder are the letters in which DNA’s information is written. There are four of these—named adenine, guanine, cytosine and thymine—and so DNA can be transcribed using only four characters: A, G, C, and T. For example, a small portion of the genetic code for haemoglobin, which carries oxygen and makes our blood red, reads:
Written like this, it would take more than three billion characters for all the DNA from a cell’s nucleus to be transcribed. A small amount of DNA—a little over 16,500 characters—is present inside cells but outside the nucleus, in bodies called mitochondria.
Mitochondrial DNA or mtDNA has a special property: it is passed on unchanged from mother to child, and so is an ancient inheritance down the maternal line. But occasionally there is a random mutation in mtDNA—a misprint in the recipe, say, an A turning to a T—that gets passed on and marks all of a woman’s descendants.
Since the rate at which mtDNA undergoes mutation is known, it also acts as a time-keeper of sorts. By finding out when and where and in which order mutations took place, studies of mtDNA have allowed the creation of entire family trees of human populations, along with a reconstruction of the geographical paths they took as they peopled the world.
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