All the DNA base pairs on Earth would weigh about 50 billion tonnes
Scientists estimate that Earth holds around 5 times ten to the 37th power DNA base pairs, together weighing some 50 billion tonnes. Each pair is just two small molecules gripping each other with hydrogen bonds. Yet those humble pairings give DNA its backup copy of every gene and the template for copying life.
The rules are simple. Adenine pairs with thymine, or with uracil in RNA, and guanine pairs with cytosine. The bigger bases, adenine and guanine, have two rings; the smaller ones have one. Only a big-small combination fits the helix: two small bases sit too far apart to bond, while two big ones crowd too close and repel. Among big-small pairings, only certain matches line up their hydrogen bond donors and acceptors correctly. Because each strand mirrors the other, the double helix carries a redundant copy of its information, which is exactly what enzymes use to replicate DNA and transcribe it into RNA.
Surprisingly, the pairing is not what mainly holds the helix together. Stacking forces between neighbouring bases supply most of the stability; the pairs matter chiefly for their precision. Still, guanine-cytosine pairs make DNA harder to pull apart, so heat-loving microbes such as Thermus thermophilus have genomes rich in them, while stretches of DNA that must open often, like the switches for busy genes, are comparatively poor in them.
Genomes are measured in base pairs. The haploid human genome runs to about 3.2 billion of them, containing 20,000 to 25,000 protein-coding genes, and each pair adds roughly 3.4 ångströms of length. RNA bends the rules more freely, forming unusual pairings such as guanine with uracil that let it fold into intricate shapes. Mistakes happen too: chemicals like 5-bromouracil mimic thymine but pair with guanine, and flat molecules such as ethidium bromide slip between bases and cause the copying machinery to skip or add letters.
Some chemists want to expand the alphabet. Steven Benner's team in Zurich put modified bases into DNA in 1989, and in 2006 Ichiro Hirao's group in Japan made an artificial third pair that works in replication and transcription.
Source: Base pair
