2.1 Essential ideas

2.1.6 Structure of DNA and RNA

The key understandings from this section are:

  • The nucleic acids – DNA, deoxyribonucleic acid, and RNA, ribonucleic acid – are polymers of nucleotides.
  • DNA differs from RNA in the number of strands present, the base composition and the type of pentose.
  • DNA is a double helix made of two anti-parallel strands of nucleotides linked by hydrogen bonding between complementary base pairs.


  • Each nucleotide consists of a 5-carbon sugar (pentose) covalently bonded to a phosphate in the C-5 position and a nitrogenous base at the C-1 position.

nucleotideFigure 2.1.6a – Structure of a nucleotide: chemical composition and schematic representation.

  • There are four bases in each type of nucleic acid – nucleotides differ only by their base structures.
  • Nucleotides can link in any combination, giving a wide range of possible DNA or RNA sequences.
  • Nucleic acids are long polymers of covalently bonded nucleotides. The sugars and phosphates alternate to form a strong backbone with the bases projecting inwards.

polymerisationFigure 2.1.6b – Linking of nucleotides to form RNA (left) and DNA (right)

Comparing DNA and RNA




Pentose sugar

DeoxyriboseFigure 2.1.6c – Deoxyribose

RiboseFigure 2.1.6d – Ribose


  • Adenine
  • Guanine
  • Cytosine
  • Thymine
  • (A, G, C, T)
  • Adenine
  • Guanine
  • Cytosine
  • Uracil
  • (A, G, C, U)


  • Two, antiparallel strands
  • Double helix structure
  • Single strand
  • Different structures, depending on function

Consequences of DNA structure

  • In DNA, two strands of nucleic acids are arranged in opposite orientations, so that the bases project inwards towards the centre of the structure. The two strands are antiparallel.
  • The two strands are held together by hydrogen bonds between bases.
  • The bases bond in specific pairs: A with T and C with G. This is called complementary base pairing (see Figure 2.1.6b, right).
  • The two strands spontaneously take on the configuration of a twisted ladder, or double helix.
  • DNA is a very stable structure because of the strong backbone and close association of strands. Its structure is well-suited to carry and conserve genetic information.

Skill: Drawing diagrams of DNA and RNA

1. Practise drawing schematic diagrams of DNA and RNA. Remember:

  • Use pentagons for sugars, circles for phosphates, and rectangles for bases.
  • The phosphate of one nucleotide and the third carbon on the pentose sugar of the second nucleotide are linked together with a solid line representing a covalent bond.
  • RNA should be drawn as a single strand with no base pairing.
  • DNA should be drawn as two antiparallel strands. Base pairs are A-T and C-G.

2. Identify what is wrong with these drawings. (Click on the images for answers.)

Figure 2.1.6e – RNA Figure 2.1.6f – DNA

double helixFigure 2.1.6g – Twisted ladder
The double helix. How does the structure of DNA allow efficient storage of genetic information?

watson crick Figure 2.1.6h – Watson and Crick

Nature of science

Using models: James Watson (left) and Francis Crick (right) built actual models to help them determine the structure of DNA.

Concept help

  • Draw one strand of DNA, then turn your paper over and draw the other strand. That way, you won’t have to think about how to orient the shapes upside down!
  • The bases in a single strand of RNA form hydrogen bonds with each other – this causes the strand to fold and take on different shapes.

Course links

  • DNA is a stable molecule but it can be affected by mutation – a process necessary for genetic novelty in evolution. See 3.1.4.
  • DNA is manipulated for new technologies. See 3.1.5.
  • HL students can learn more detail on DNA and RNA structure in Unit 7 Nucleic acids.