Science · Honors Biology
Chapter 1: Biochemistry
A molecule shaped for copying.
Each human cell holds about 2 meters of DNA and copies all of it before dividing. Its two strands are held together only by hydrogen bonds, about a twentieth as strong as its backbone bonds.
Why would the molecule that stores hereditary information use a weak bond at all?
Make your guess to turn the page.
Still to come: 11 pages · Lab · Words to Know · What We Learned · 4 questions · Explain It in Writing · Open Questions
The idea this lesson is working on — Students assume a molecule that lasts must be held together by strong bonds throughout. The book separates the covalent backbone from the hydrogen bonds across, and Meselson and Stahl’s bands show the strands separating every generation. Ask: what would happen to copying if the bonds across were as strong as the backbone? The idea to reach is strong along, weak across, by design.
A molecule shaped for copying — The base percentages on the table are the commonly cited values for human DNA from Chargaff’s era. Franklin’s Photo 51 dates from 1952; Watson and Crick’s paper appeared in April 1953. Meselson and Stahl used a cesium chloride density gradient in an ultracentrifuge; their densities were about 1.724 and 1.710 g/cm³. Error rates: polymerase selection about 10⁻⁵, proofreading brings it near 10⁻⁷, mismatch repair near 10⁻⁹.
Three differences from RNA — Deoxyribose rather than ribose, thymine rather than uracil, double-stranded rather than single. The 2′-hydroxyl of ribose can attack the adjacent phosphodiester bond, which is why RNA hydrolyzes readily, especially in base, and DNA does not.