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DNA

DNA double helix.
DNA double helix, by Tumisu via Pixabay, licensed under CC0.

DNA carries the genetic instructions needed for the growth, development and functioning of all living organisms. You must understand the structure of DNA to grasp how genetic information is passed down and used in biological processes. This is crucial in fields like medicine, where it aids in developing treatments and in biotechnology, where it drives innovation. Use this resource to learn about the structure of DNA.

DNA is composed of two long strands of nucleotides twisted around each other. Go back to Macromolecules if you need to review nucleic acids and nucleotides.

The nucleotides in a strand of DNA are connected by phosphodiester bonds between the third carbon (3’ or three prime) of a sugar of one nucleotide and the phosphate attached to the fifth carbon (5’ or five prime) of the sugar on the next nucleotide. Nucleotides are only added to the 3’ end, so the direction of DNA is 5’ to 3’. DNA replication and transcription also occur in this direction.

Two nucleotides joined together by a phosphodiester bond.
Phosphodiester bond
Specific sequences of nucleotides are called genes, which code for specific proteins. You will learn about how DNA is used to make proteins in Protein synthesis.

The double helix

DNA has a double helix structure. This is formed through complementary base pairing. Adenine (A) pairs specifically with thymine (T), and cytosine (C) pairs specifically with guanine (G). DNA is often referred to as a twisted ladder as these bases form hydrogen bonds with each other to create "rungs". The two strands run in opposite directions causing the sugar-phosphate backbone to alternate.

Guanine and cytosine pair via three hydrogen bonds, whereas there are only two hydrogen bonds between adenine and thymine. The stronger association between guanine and cytosine gives DNA its twisted structure.

A segment of double-stranded DNA. The cytosine nucleotide pairs with a guanine nucleotide via three hydrogen bonds. The adenine nucleotide pairs with a thymine nucleotide via two hydrogen bonds.
Complementary base pairing links the two DNA strands.

You can use this interactive to see a 3D model of the DNA structure.

DNA by Sketchfab

A three-dimensional model of a segment of DNA with atoms shown as coloured sticks.

DNA double helix model

Phosphate is shown is gold, oxygen is red, nitrogen is blue, carbon is teal and hydrogen is white. The atoms are arranged to form two strands of nucleotides. The strands form a twisted ladder structure called the double helix.

Chromosomes

In the nucleus, the DNA is coiled up into a condensed structure which protects the DNA during cell division.

Packaging proteins called histones bind to the DNA molecule. When a section of DNA is wrapped around a cluster of eight histones, it is called a nucleosome. Chains of nucleosomes are further coiled into a spiral called a solenoid. The solenoids coil to form the fibres of chromatin.

When the cell is not dividing, DNA is present in the nucleus as chromatin. When it is ready to divide, it further condenses to form chromosomes.

eukaryotic chromosome by shylajashrikanth on Sketchfab

A three-dimensional model of DNA structure, showing the different levels of organisation.

DNA structure model

  • Strands of DNA are organised into a double helix structure.
  • The double helix is coiled around clusters of four histone proteins, shown in pink and yellow.
  • The histones are further coiled to form solenoids.
  • Solenoids are coiled even further to eventually form chromatin.
  • Chromatin condenses into chromosomes.

A chromosome consists of a chromatid, which has a short arm and a long arm. Between the arms is a constricted region called a centromere. One chromosome is one DNA molecule.

When DNA is being replicated, the chromosome duplicates and consists of two sister chromatids joined at the centromere.

Chromatid and sister chromatids. Both have short arms, a centromere and long arms. The sister chromatids consist of two chromatids joined together at the centromere.
Chromosomes

Exercise

See how well you understand the structure of DNA with a quick quiz.

Ready for some more? Practise your skills by determining the sequence of the DNA strand that is complementary to the following strands:

  1. TAC–CGA–CCA–ATT–TAG–TTA–CCC
  2. CCG–AAG–CTG–ATG–GGA–CAG–ACG
  3. GTT–AGA–CTA–TAA–GCG–ACT–TTT
  4. AAT–ACC–CAG–TTG–CGG–AAA–TCA
  5. AGA–CTC–TGA–CGG–TAC–GAA–CTG
  6. TCT–GAG–ACT–GCC–ATG–CTT–GAC
  7. TGC–AAG–TTC–GGA–CCA–ATG–TAC

  1. ATG–GCT–GGT–TAA–ATC–AAT–GGG
  2. GGC–TTC–GAC–TAC–CCT–GTC–TGC
  3. CAA–TCT–GAT–ATT–CGC–TGA–AAA
  4. TTA–TGG–GTC–AAC–GCC–TTT–AGT
  5. TCT–GAG–ACT–GCC–ATG–CTT–GAC
  6. GCT–ATG–CCA–TTG–AAT–GGC–CTA
  7. ACG–TTC–AAG–CCT–GGT–TAC–ATG

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