Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- DNA and RNA are polymers of nucleotides. Each nucleotide has a five-carbon sugar, a phosphate (on carbon 5′) and a nitrogenous base (on carbon 1′).
- Nucleotides are joined by covalent bonds between the 3′ –OH of one nucleotide and the phosphate of the next, forming a sugar–phosphate backbone. Hydrolysis breaks these links.
- A strand has a 5′ end (free phosphate) and a 3′ end (free –OH). New nucleotides are added only to the 3′ end.
- DNA is a double helix of two antiparallel strands. Bases pair by hydrogen bonds: A–T (two) and C–G (three). In RNA, A pairs with U.
- The backbone is held by strong covalent bonds; the strands are held together by weak hydrogen bonds, so they can be separated without breaking either strand.
Key relationships
| Feature | DNA | RNA |
|---|---|---|
| Sugar | deoxyribose (–H on 2′) | ribose (–OH on 2′) |
| Bases | A, G, C, T | A, G, C, U |
| Pairs | A–T, C–G | A–U, C–G |
| Strands | typically two | typically one |
| Rule | Use it to |
|---|---|
| In double-stranded DNA, %A = %T and %G = %C | find all four percentages from one |
| %A + %T + %G + %C = 100 | check your answer |
| H bonds = 2 × (A–T pairs) + 3 × (C–G pairs) | compare how strongly two segments hold together |
| Links in one strand = number of nucleotides − 1 | count covalent bonds in a backbone |
Assumptions behind the numbers
- The pairing percentages hold only for double-stranded molecules. Single strands need not follow them.
- “Typically” matters: some viruses have single-stranded DNA or double-stranded RNA.
- Counting hydrogen bonds is a model. Real strand separation also depends on other conditions.
Mistakes to avoid
- Writing a complement without reversing it, so the 5′ and 3′ labels are wrong.
- Saying hydrogen bonds link nucleotides within one strand.
- Pairing A with G, or putting T in RNA.
- Applying A = T to single-stranded DNA or RNA.
- Forgetting that DNA and RNA differ in sugar as well as base and strand number.
Quick self-check
- Complement of 5′-GGATC-3′, written 5′ to 3′? (5′-GATCC-3′)
- Double-stranded DNA is 40% C. Percentage of A? (G = 40%, so A + T = 20% and A = 10%)
- Why can a nucleotide not be added to the 5′ end? (There is no free 3′ –OH there for the new nucleotide’s phosphate to bond to.)
Next: practice questions.