The Structure of DNA (AQA A Level Biology): Revision Note

Exam code: 7402

Lára Marie McIvor

Written by: Lára Marie McIvor

Reviewed by: Cara Head

Updated on

DNA: structure & function

Function

  • Deoxyribonucleic acid (DNA) is an important information-carrying molecule

  • The function of DNA is to hold or store genetic information

  • DNA is the molecule that contains the instructions for the growth and development of all organisms

Structure

Nucleic acid structure

  • The nucleic acid DNA is a polynucleotide – it is made up of many nucleotides bonded together in a long chain

Diagram of a DNA nucleotide showing a phosphate group, pentose sugar (deoxyribose), and nitrogenous base (A, C, G, T).
A DNA nucleotide

DNA molecule structure

  • DNA molecules are made up of two polynucleotide strands lying side by side, running in opposite directions – the strands are said to be antiparallel

  • Each DNA polynucleotide strand contains alternating deoxyribose sugars and phosphate groups bonded to form the sugar-phosphate backbone. These bonds are covalent bonds known as phosphodiester bonds

    • The phosphodiester bonds link the 5-carbon of one deoxyribose sugar molecule to the phosphate group from the same nucleotide, which is itself linked by another phosphodiester bond to the 3-carbon of the deoxyribose sugar molecule of the next nucleotide in the strand

    • Each DNA polynucleotide strand is said to have a 3’ end and a 5’ end (these numbers relate to which carbon on the pentose sugar could be bonded with another nucleotide)

    • As the strands run in opposite directions (they are antiparallel), one is known as the 5’ to 3’ strand, and the other is known as the 3’ to 5’ strand

  • The nitrogenous bases of each nucleotide project out from the backbone towards the interior of the double-stranded DNA molecule

Diagram of nucleotide structure showing phosphate group, pentose sugar, and nitrogenous bases: thymine, guanine, and adenine with bonds labelled.
A single DNA polynucleotide strand showing the positioning of the ester bonds

Hydrogen bonding

  • The two antiparallel DNA polynucleotide strands that make up the DNA molecule are held together by hydrogen bonds between the nitrogenous bases

  • These hydrogen bonds always occur between the same pairs of bases:

    • Adenine (A) always pairs with thymine (T) – two hydrogen bonds are formed between these bases

    • Guanine (G) always pairs with cytosine (C) – three hydrogen bonds are formed between these bases

    • This is known as complementary base pairing

    • These pairs are known as DNA base pairs

Diagram of DNA structure showing sugar-phosphate backbone and bases: thymine-adenine (2 bonds) and cytosine-guanine (3 bonds), with single nucleotide marked.
A section of DNA – two antiparallel DNA polynucleotide strands held together by hydrogen bonds

Double helix

  • DNA is not two-dimensional as seen in the diagram above

  • DNA is described as a double helix

  • This refers to the three-dimensional shape that DNA molecules form

Diagram of DNA structure shows a double helix with sugar-phosphate backbone, labelled base pairs, and antiparallel strands. Base key included.
DNA molecules form a three-dimensional structure known as a DNA double helix

Examiner Tips and Tricks

Be able to identify and label the components of a DNA molecule:

  • Sugar-phosphate backbone

  • Nucleotides

  • Complementary base pairs (A=T, C≡G)

  • Phosphodiester bonds (between nucleotides)

  • Hydrogen bonds (between bases)

You may be asked to calculate base numbers using base pairing rules if given the quantity of one base.

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Lára Marie McIvor

Author: Lára Marie McIvor

Expertise: Biology, Psychology & Sociology Subject Lead

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.

Cara Head

Reviewer: Cara Head

Expertise: Biology & Psychology Content Creator

Cara graduated from the University of Exeter in 2005 with a degree in Biological Sciences. She has fifteen years of experience teaching the Sciences at KS3 to KS5, and Psychology at A-Level. Cara has taught in a range of secondary schools across the South West of England before joining the team at SME. Cara is passionate about Biology and creating resources that bring the subject alive and deepen students' understanding

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