DNA Structure and Base Pairing
DNA is two strands of nucleotides wound into a double helix, with A always pairing to T and C to G. That pairing rule is what makes an exact copy possible before a cell divides.
What a learner can do afterwards
- Names the three parts of a nucleotide and states the pairing rule between the bases.
- Writes the complementary strand for a short given sequence.
- Explains how base pairing lets one strand act as a template for its own copy.
1 · Read
DNA is two strands wound into a double helix, like a twisted ladder. Each strand is a chain of units called nucleotides. One nucleotide has three parts: a sugar, a phosphate group and a base. Four bases exist: A, T, C and G.
The rungs obey a fixed pairing rule: A always pairs with T, and C always pairs with G. The sequence GATTACA pairs with CTAATGT, rung by rung. Only one answer is possible because each base has exactly one partner.
That rule makes copying automatic. The helix unzips and each old strand attracts matching free nucleotides, so two identical ladders grow from one. Enzymes proofread as they build and fix most slips. Each new ladder keeps one original strand.
Write a complement left to right, pairing each base alone: G with C, A with T, T with A, C with G. Then check every rung backwards before you commit.
Fixed pairing turns each strand into a template for its own exact copy.
2 · Watch
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Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.