# DNA structure

Two strands of nucleotides twisted into a double helix. Each nucleotide has a sugar, a phosphate and a base: A, T, G or C. A pairs with T and G pairs with C. The strands run in opposite directions (antiparallel).

Biology (university) · Part 04: DNA, transcription and translation · Card 1 of 9 · Last updated 2026-10-19

## Analogy

A twisted rope ladder: sugar-phosphate rails and base-pair rungs.

## Example

**Base pairing**

A pairs with T (2 bonds).
G pairs with C (3 bonds).
Strand 1: A T G C
Strand 2: T A C G

## In practice

If one strand reads ATGC, the other reads TACG. That is how a damaged strand can be fixed using its partner.

## Practice

### Flashcards

- **Which bases pair in DNA?** A with T and G with C.

### Interview questions

**Describe the steps of DNA replication and explain why it is called semi-conservative.**

- Helicase unwinds and separates strands
- Primase starts a primer
- DNA polymerase adds nucleotides 5' to 3'
- Leading continuous, lagging in Okazaki fragments
- Each helix has one old and one new strand

## Questions people ask

### Why is DNA double-stranded?

Two strands give a built-in backup. Because A always pairs with T and G with C, each strand holds the information needed to rebuild the other. This lets the cell copy DNA accurately and repair damage by using the undamaged strand as the template. The paired helix is also more stable than single strands.

### What does 5' to 3' mean?

It describes the direction of a DNA strand. Each sugar has numbered carbons, and the 5' end has a free phosphate on carbon 5 while the 3' end has a free hydroxyl on carbon 3. Strands run in opposite directions, and enzymes always add new nucleotides to the 3' end.

## Related concepts

- [Semi-conservative replication](https://studyvisually.com/learn/science/biology-university/biology-university-part-4/biology-university-part-4-concept-2)
- [Lagging strand](https://studyvisually.com/learn/science/biology-university/biology-university-part-4/biology-university-part-4-concept-3)

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