Width: always purine + pyrimidine. Melting temperature: percent G-C. Heat breaks hydrogen bonds, not the backbone.
DNA is a twisted ladder with a direction. The rails are sugar and phosphate, joined by strong covalent bonds. The rungs are base pairs, held by weak hydrogen bonds. Tell those two apart and this topic falls into place.
Key terms
- Nucleotide — one unit of DNA or RNA: a phosphate, a five-carbon sugar, and a nitrogenous base.
- Purine vs pyrimidine — purines (A, G) have two rings. Pyrimidines (C, T, U) have one.
- Antiparallel — the two strands point opposite ways. Where one runs 5'→3', its partner runs 3'→5'.
- Phosphodiester bond — the covalent link from one sugar's 3' carbon to the next nucleotide's phosphate. It builds the backbone.
- RNA — like DNA, but with ribose (an extra –OH on carbon 2'), uracil in place of thymine, and usually one strand.
How it works
- The sugar is the hub of each nucleotide. The base attaches to its 1' carbon, and the phosphate to its 5' carbon.
- Phosphodiester bonds chain nucleotides into a strand. The 5' end has a free phosphate on carbon 5'. The 3' end has a free –OH on carbon 3'.
- New nucleotides join only at the 3' end, so strands grow 5'→3'. Each arrives as a triphosphate. Two phosphates break off, and that release powers the new bond.
- Two strands pair antiparallel, bases facing inward. Hydrogen bonds hold each pair: two for A-T, three for G-C.
- Each pair joins a two-ring purine to a one-ring pyrimidine. Every rung is three rings wide, so the helix keeps a constant width.
- Heat breaks the hydrogen bonds, so the strands separate while each backbone stays whole. A higher percent of G-C pairs means more heat is needed.
Why it matters
The rest of this unit runs on these rules. Replication and transcription both build strands 5'→3' by base pairing, so direction questions start here. Know where DNA lives, too. Bacteria usually have a circular chromosome, often plus small extra circles called plasmids. Eukaryotes keep their DNA as linear chromosomes in a nucleus. And RNA's extra –OH is part of why it breaks down more easily than DNA.
Exam traps
| Trap | Actually |
|---|
| The helix keeps a constant width because every pair has the same number of hydrogen bonds. | A-T has two and G-C has three. The width holds because every pair is one purine plus one pyrimidine. |
| Heating DNA until the strands separate breaks its backbone. | Heat breaks only the hydrogen bonds between bases. The covalent backbone survives, which is why cooled strands can pair up again. |
| DNA with more purines is harder to melt. | In double-stranded DNA, purines always equal pyrimidines. What raises the melting temperature is a higher share of G-C pairs. |
| Adding each nucleotide costs a separate ATP. | The incoming nucleotide is its own fuel. It arrives with three phosphates, and releasing two of them powers the bond. |
Worked example
Researchers analyze the genome of a newly discovered virus. Its bases are 24% adenine, 31% uracil, 19% guanine, and 26% cytosine. No thymine is found. What kind of genome does the virus most likely have?
- Sort DNA from RNA first. Uracil is present and thymine is absent, so the genome is RNA.
- Test for a second strand. In a double-stranded molecule every A sits across from a U (or T) and every G across from a C, so partner amounts must match.
- They don't. A is 24% but U is 31%; G is 19% but C is 26%. Purines total 43%, not the 50% that pairing would force.
- Unmatched partners mean there is no partner strand. The genome is a single strand.
Single-stranded RNA.
Equal amounts of A and U (or T), and of G and C, come from pairing between two complementary strands. A single strand can hold any mix of bases. The near-miss is double-stranded RNA: uracil settles RNA versus DNA, but the base ratios are what reveal the number of strands.