AP Biology · Unit 6 · Topic 6.1
Every claim about DNA's structure is a consequence of one pairing rule. Build the partner strand yourself and watch Chargaff's ratios fall out of it.
Learning objective 6.1.A Describe the structure and function of DNA and RNA.
Type a strand or pick one. The partner is built by applying the pairing rule to each base — and then written backwards, because the two strands run in opposite directions. The rungs show how many hydrogen bonds hold each pair.
Read the middle row of the drawing left to right and you get the complement in the order the bases pair. Read the bottom row — the same bases, labeled 5′ to 3′ from the other end — and you get the partner strand as it would be written down. They are the same strand. Only the reading direction differs.
Try a run of GCGCGC and then a run of ATATAT of the same length. Same number of base pairs, different number of hydrogen bonds — and the GC one takes more energy to pull apart. That is why a GC-rich region melts at a higher temperature, and it is a two-times-table calculation rather than a fact to memorise.
A large base always pairs with a small one, so every rung of the ladder is the same length. If two purines could pair, the helix would bulge; if two pyrimidines could, it would pinch. The constant width of the double helix is a consequence of the pairing rule, not a separate fact about it.
Try GAATTC. The partner strand, written 5′ to 3′, is GAATTC again — the same sequence. A DNA palindrome is not a sequence that reads the same backwards; it is one whose partner reads the same forwards.
This matters in Topic 6.8: restriction enzymes recognize palindromic sites, because that lets one enzyme cut both strands in the same way.
Chargaff's rules are usually given as something he observed. They are not an observation you have to take on trust — they are forced by the pairing. Count one strand and they fail; count both and they cannot fail.
If a double-stranded DNA sample is 20% adenine, then it is 20% thymine, because A = T. That leaves 60% to be shared equally between G and C, so each is 30%. One number, three deductions, no memorisation.
Work the slider below and check your own arithmetic against it.
It does not say A = G. Nothing pairs adenine with guanine, so nothing forces those amounts to match, and in most organisms they do not. The ratio of A-T pairs to G-C pairs varies from species to species, and that variation is real information about a genome.
It also does not apply to a single strand, or to RNA that is single-stranded. Both of those are checked on the next tab.
Given nothing but a table of base counts, you can say what a nucleic acid is. Ask which base is present — thymine or uracil — and then ask whether the complementary bases come in equal amounts. Neither question alone is enough.
| Base | Count |
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All four combinations occur, which is exactly why one question cannot settle it. Uracil does not imply single-stranded — some viruses have double-stranded RNA genomes. Equal amounts of complementary bases does not imply DNA.
The structural differences are not decorative. Deoxyribose is more stable than ribose, which suits a molecule that has to last the life of a cell; being double-stranded means each strand is a backup of the other, which is what makes accurate replication and repair possible at all. RNA is built to be made, used and destroyed.
Answer these in your own words before you open the answers.
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| What you are usually asked to remember | What it actually follows from |
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The right-hand column is one sentence repeated. If you can derive the left-hand column from it under exam conditions, there is nothing here left to memorise.