Early 20th century biologists were plagued by many questions, one of which involved understanding how organisms inherit the molecular information required to grow and develop as unique species. Scientists knew that cells received molecules from their parental cells that encoded the details necessary to become that same type of cell. In 1952, Martha Chase and Alfred Hershey performed pioneering experiments to show that DNA was this hereditary molecule that is copied and passed down from cell to cell. Over 70 years later, scientists have uncovered fascinating new insights into our genomes, or the entire DNA content of an organism. Although we have learned much, there are still many wonders which remain less well understood.
These are two of the wonders of our genomes that continually fascinate me:
More complex organisms must have more DNA, right?
DNA is frequently referred to as the “blueprint” of life. Inside of our cells, our DNA contains many genes which encode for various protein products that make up a cell, like recipes in a cookbook. You may expect that complex organisms, such as animals, need more genes because their cells require more material to enact complicated processes. Indeed, humans have larger genomes and more genes than the bacteria E. coli. However, this paradigm does not hold up for all species. For example, the species with the largest genome sequenced thus far is the Japanese Canopy Plant. This counter-intuitive principle, where the complexity of an organism does not scale with the size of its genetic content, has been termed the C-value enigma. So why does a species of plant need more DNA than humans? This phenomenon still remains a mystery.
All of the human genome is human, right?
The first composite sequence of the human genome was completed in 2001 by the Human Genome Project. The human genome contains over 3 billion base pairs, or letters. Individual genes, which are like recipes in a cookbook, range from a few hundred to tens of thousands of base pairs. Thus, the scientists in the Human Genome Project expected to find close to 100,000 genes in the human genome. To their great surprise, the composite sequence of the genome revealed only 20,000 protein-coding genes. This observation begged the question: what is the rest of the DNA in the human genome? Further analysis revealed that almost half of the human genome is not strictly human but actually viral in origin. Indeed, when ancient viruses, known as transposable elements, infected early life forms, they left their DNA sequences behind. These pervasive transposable elements have expanded throughout evolution. In humans today, almost all of these viral-like elements have been silenced by various host defense systems. However, a very small number of these transposable elements are still active. Incredibly, some of these transposable elements have been integrated into human genes and serve important functions in our cells. This process of co-opting virus genes into the host genome is termed molecular domestication.
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