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DNA: The Chemical Nature of the Gene

Greenland is the world’s largest island, consisting of over 830,000 square miles (2,200,000 square kilometers), but the vast majority of the land is permanently buried under hundreds of feet of ice. It is one of Earth’s most extreme environments. Temperatures along the coast rise a few degrees above freezing during summer days, but then drop to far below zero during much of the winter. The sun moves above the horizon for only a few hours on winter days, and hurricane-force winds, coupled with the extreme cold, create a dangerously inhospitable environment.

In spite of the severe conditions, Arctic peoples have continuously occupied Greenland for almost 5000 years. The earliest inhabitants were the Saqqaq people, who occupied small settlements on Greenland’s coast from around 4800 to 2500 years ago, living in small tents and hunting marine mammals and seabirds. The origin of the Saqqaq people had long been a mystery. Did they descend from Native Americans who migrated from Asia into the New World and later moved to Greenland? Or did they descend from the same group that gave rise to the Inuit people, who currently inhabit the New World Arctic? Or, perhaps they originated from yet another group that migrated independently from Asia to Greenland after the ancestors of both the Inuit and Native Americans entered the New World.

The mystery of the Saqqaq was solved in 2010, when geneticists determined the entire DNA sequence of a 4000-year-old Saqqaq male—nicknamed Inuk—whose remains were recovered from an archeological site on the western coast of Greenland. Scientists extracted DNA from four hair tufts found in the permafrost. Despite the great age of the sample, they were able to successfully determine Inuk’s entire genome sequence, consisting of over 3 billion base pairs of DNA.

By comparing Inuk’s DNA with sequences from known populations, the scientists were able to demonstrate that the Saqqaq are most closely related to the Chukchi, a present-day group of indigenous people from Russia. This finding indicates that the Saqqaq originated from hunters who trekked from Siberia eastward across Alaska and Canada to Greenland, arriving in the New World independently of the peoples who gave rise to Native Americans and the Inuit. Further analysis of Inuk’s DNA revealed that he was dark-skinned and brown-eyed, had blood type A+, and was probably going bald.

DNA, with its double-stranded spiral, is among the most elegant of all biological molecules. But the double helix is not just a beautiful structure; it also gives DNA incredible stability and permanence, as evidenced by the sequencing of Inuk’s 4000-year-old DNA. In an even more remarkable feat, geneticists in 2009 sequenced the entire Neanderthal genome from DNA extracted from 38,000-year-old bones.

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This chapter focuses on how DNA was identified as the source of genetic information and how it encodes the genetic instructions. We begin by considering the basic requirements of the genetic material and the history of the study of DNA—how its relation to genes was uncovered and its structure determined. The history of DNA illustrates several important points about the nature of scientific research. As with so many important scientific advances, the structure of DNA and its role as the genetic material were not discovered by any single person, but were gradually revealed over a period of almost 100 years, thanks to the work of many investigators. Our understanding of the relation between DNA and genes was enormously enhanced in 1953, when James Watson and Francis Crick, analyzing data provided by Rosalind Franklin and Maurice Wilkins, proposed a three-dimensional structure for DNA that brilliantly illuminated its role in genetics.

After reviewing the discoveries that led to our current understanding of DNA, we will examine DNA structure. The structure of DNA is important in its own right, but the key genetic concept is the relation between the structure and the function of DNA—how its structure allows it to serve as the genetic material.