You will identify parts of DNA, complete a partner strand and draw conclusions from base sequences or totals. The safest method is to separate structure, pairing and information before applying one rule at a time.
Use the pairing rule position by position
DNA has two strands wound around each other in a double helix. In the twisted-ladder model, the strands form the sides and each rung is a joined pair of bases. DNA uses A, T, C and G. The fixed partners are A with T and C with G; this is complementary base pairing. Complementary means that a base joins its partner, not another copy of itself.
| Task | Method |
|---|---|
| Complete the other strand | Keep every position in order and replace each letter with its partner. Check every position separately before reading the finished sequence. |
| Compare base totals | In double-stranded DNA, every A accounts for one T and every C accounts for one G. Match totals within each pair only. |
| Locate genetic information | Look to the order of bases. Two sequences can contain the same number of bases yet carry different instructions. |
| Compare different organisms | Keep the same four-letter DNA alphabet and look for differences in sequence rather than extra kinds of base. |
Within a gene, base order determines amino-acid order in the protein the gene codes for. Stay at that sequence relationship: base order is not a direct measure of how much protein is made, and one base does not specify an entire protein.
Where the reasoning goes wrong
| Tempting route | How to reject it |
|---|---|
| Putting U into the DNA alphabet | DNA uses four letters and includes T, not U. Keeping both would incorrectly create a fifth DNA base. |
| Copying a strand unchanged or pairing identical letters | Write the two partner rules first, then transform every position. Identical copying would place a base opposite itself. |
| Using total length or helix width as the message | Information depends on sequence. A count or a shape can stay the same while the order changes. |
| Calling one base or a whole gene a ladder rung | A rung crosses between the two strands, so it consists of two joined bases; a gene spans many such positions. |
| Treating base order as the amount of protein, or one base as a whole protein instruction | Base order sets amino-acid order in the coded protein. Protein quantity and the steps of protein production are separate from this sequence rule. |
Try it
What is the name of the shape formed by the two twisted strands of a DNA molecule?
- A single spiral chain
- A flat straight ladder
- A double helix
- A double circle
A sample of double-stranded DNA contains 300 A bases. What else must be true of the sample?
- It contains 300 T bases
- It contains 300 C bases
- It contains 600 T bases
- It contains 300 of each of the four bases
Which statements correctly compare the DNA of a buttercup with the DNA of a horse? Select all that apply.
- Each species has its own extra bases
- Both species use the same four bases
- The order of the bases differs between them
- The horse uses more kinds of base than the buttercup