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How big is DNA?

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DNA

DNA, short for deoxyribonucleic acid, is the molecule that stores genetic information in living cells and passes it on. It is shaped like a ladder twisted into a spiral, called a double helix, and that helix is about 2 nanometers wide. A nanometer is a billionth of a meter.

Size

The size of DNA usually means the width of its double helix, measured across both twisted strands. Each strand is a chain of small units called nucleotides. The two strands are held together by pairs of chemical bases, which form the rungs of the ladder.

The common form of DNA in cells, called B-DNA, is 1.9–2 nanometers wide. Neighboring base pairs sit 0.34 nanometers apart along the helix, and the helix makes one full turn about every 10 base pairs, or 3.4 nanometers. DNA also comes in two much rarer forms, called A-DNA and Z-DNA, which are twisted differently.

The length of DNA depends on the molecule. Human chromosome 22, one of the smallest, is a single DNA molecule of about 48 million base pairs. Stretched out, it would be about 1.5 centimeters long.

Size comparisons

  • About 40,000–50,000 DNA double helices laid side by side would span the width of a human hair.
  • The lipid bilayer that forms cell membranes is about 5 nanometers thick, two and a half times the width of DNA.
  • A gold atom is about a third of a nanometer across, so DNA is about six gold atoms wide.
  • If DNA were enlarged to the width of a human hair, the DNA in a single human cell would stretch about 100 kilometers (60 miles). It would be packed into a nucleus about 30 centimeters (12 inches) across.

How much DNA a cell holds

A typical human cell contains about 2 meters (6.6 feet) of DNA, if it were stretched out end to end. Yet the nucleus that holds it is only about 6 micrometers across. A micrometer is a millionth of a meter. The DNA is more than 300,000 times as long as the nucleus is wide.

The human genome, the full set of human DNA, is about 3.2 billion nucleotides long. It is split among 24 different chromosomes. Each chromosome is one very long DNA molecule, bound to proteins that fold and pack it into a compact shape.

How the double helix was found

James Watson and Francis Crick, working at the University of Cambridge in England, described the double helix in 1953. Their model showed how the bases pair up: adenine with thymine, and cytosine with guanine. It also suggested how DNA could be copied.

X-ray diffraction helped reveal the structure. The method works out a molecule's shape from the pattern X-rays make when they scatter off it. Rosalind Franklin was studying DNA this way, and Maurice Wilkins was also working on its structure. The X-ray pattern showed the regular 0.34-nanometer spacing of the bases.

Watson, Crick and Wilkins shared a Nobel Prize in 1962. Franklin had died by then, and Nobel Prizes are not awarded after death.

Sources

宇宙的刻度 > DNA
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Carbon nanotubes are the strongest material yet discovered. As another bonus, they're lightweight. As a result, nanotubes have a lot of potential applications, including buildings, batteries, solar panels, and possibly even a space elevator.
Phospholipid
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Phospholipid Bilayer
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Porcine Circovirus
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