DNA Diagram Labeled – 14 Parts of the Double Helix

All 14 parts on the diagram

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Double helix: 4 parts 4

Sugar-phosphate backbone Alternating sugars and phosphates; the bases hang inward from it.
Base pair Two bases on opposite strands held together by hydrogen bonds.
Major groove The wide gap between the strands; proteins bind DNA there.
Minor groove The narrow gap between the strands, alternating with the major groove.

Flattened ladder: 10 parts 10

Phosphate group Links one sugar to the next; it makes DNA acidic.
Deoxyribose (sugar) The five-carbon sugar of DNA; RNA has ribose instead.
Adenine (A) A purine; it pairs with thymine by two hydrogen bonds.
Thymine (T) A pyrimidine; it pairs with adenine.
Guanine (G) A purine; it pairs with cytosine by three hydrogen bonds.
Cytosine (C) A pyrimidine; it pairs with guanine.
Hydrogen bonds Weak bonds holding the pairs: two for A-T, three for G-C.
Nucleotide One unit of DNA: a phosphate, a sugar and a base.
Five prime end The strand end with a free phosphate on the sugar's 5' carbon.
Three prime end The strand end with a free hydroxyl on the sugar's 3' carbon.

How this list was compiled: Read 2026-09-26. OpenStax Biology 2e, 14.2 DNA Structure and Sequencing, https://openstax.org/books/biology-2e/pages/14-2-dna-structure-and-sequencing : a nucleotide is a nitrogenous base, a five-carbon sugar and a phosphate group; the sugar is deoxyribose in DNA and ribose in RNA; purines adenine and guanine, pyrimidines cytosine and thymine (pyrimidines smaller, a single six-membered ring); A pairs with T and G with C; A and T form two hydrogen bonds, C and G three; the sugar and phosphate form the backbone with the bases stacked inside like ladder rungs; the strands are antiparallel; one end has a free 5' phosphate and the other a free 3'-OH; the twisting forms uniformly spaced major and minor grooves, binding sites for proteins; the phosphate groups make DNA negatively charged. Wikipedia, DNA, https://en.wikipedia.org/wiki/DNA , used as a check. Licence: own work

The drawing: Drawn for this site by script, 2026-09-26. Own work. Licence: own work

How it works

This labeled DNA diagram shows the double helix and the same molecule flattened into a ladder. Fourteen parts carry a ring, in sets: the shape of the helix, the nucleotide and strand ends, and the bases with their bonds.

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Reading the ladder

Each rail is a sugar-phosphate backbone: deoxyribose sugars and phosphate groups alternate. The bases point inward and pair across the middle, adenine with thymine by two hydrogen bonds, guanine with cytosine by three. The two strands run in opposite directions, so the left one reads 5' to 3' downward.

Why the shapes differ

Adenine and guanine, the purines, are drawn longer than thymine and cytosine, the pyrimidines. Each pair meets at a shape only its partner fits, a point for A-T and a round bump for G-C.

One nucleotide is a phosphate, a sugar and a base: the dashed outline shows exactly one.

Questions and answers

What are the major and minor grooves of DNA?

As the two strands twist, they leave a wide gap and a narrow gap between them, repeating along the helix. Proteins that read DNA bind in these grooves.

What holds the two strands of DNA together?

Hydrogen bonds between the paired bases: two between adenine and thymine, three between guanine and cytosine.

What do the 5' and 3' ends mean?

They name the two ends of a strand. The 5' end has a free phosphate on the sugar's fifth carbon, the 3' end a free hydroxyl on its third carbon.