Learning Objective

In this lesson we will learn about the structure of DNA – the molecule that carries genetic information in living things.

Learning Outcomes

By the end of this lesson you will be able to:

  • Describe the structure of nucleotides – the subunits of DNA molecules.

  • Describe the structure of DNA molecules, including:

    • The types of bonds that hold DNA molecules together.

    • The base-pairing rules in DNA.

    • DNA’s double helix structure.

  • Explain how DNA molecules carry a ‘genetic code’.

 

Introduction

  • All living things have genetic information stored in large molecules called DNA (deoxyribonucleic acid).
    DNA is stored in the nuclei of cells, where it is packaged into thread-like structures called chromosomes.
  • The order of the subunits that make up DNA molecules encodes the genetic information of an organism.

 
cell nucleus chromosome dna

Cells contain chromosomes, which carry genetic information in the form of DNA.

(Image: fancytapis, Adobe Stock)

 

Nucleotides – The Subunits of DNA

  • DNA molecules are made up of subunits called nucleotides.
  • Nucleotides consist of three parts – a sugar, a base and a phosphate.
    The sugar molecule is in the centre, with the phosphate attached to one side and the base attached to the other.
    The sugar in nucleotides is called deoxyribose, which is why the full name for DNA is deoxyribonucleic acid.

 
nucleotide dna subunit

DNA molecules are made up of subunits called nucleotides.

 

  • All nucleotides in DNA contain the same sugar and phosphate, but there are four different bases, and consequently four different nucleotides.
    The four bases are guanine, cytosine, adenine and thymine, but these are usually just referred to by their first letter – G, C, A and T respectively.
    Since nucleotides differ only in their type of base, they are also referred to by the letters G, C, A and T.

 
nucleotides bases dna

There are four different nucleotides in DNA – each with a different base.

 

DNA Molecules

  • DNA molecules consist of nucleotides that are joined in two ways – firstly, to form a single strand of DNA, and secondly, to form a double strand of DNA.

 

Single-Stranded DNA

  • Single-stranded DNA is formed when nucleotides are joined by a type of covalent bond known as a phosphodiester bond.
    These bonds form between the sugars and phosphates of adjacent nucleotides, creating what is referred to as the sugar-phosphate backbone of DNA.

     
    dna single strand

    Single strands of DNA are formed by covalent bonds between sugars and phosphates of adjacent nucleotides.

     

    Double-Stranded DNA

    • Double-stranded DNA is formed when nucleotides are joined by a type of bond called a hydrogen bond.
      These bonds form between the bases of nucleotides on each strand.
    • The two strands are aligned in opposite directions, which is referred to as antiparallel orientation.
    • Hydrogen bonds are not full chemical bonds, but are formed due to electrostatic attraction between slightly positive and slightly negative regions of DNA. Therefore, hydrogen bonds are weaker than covalent bonds, which allows the two strands of DNA to separate during processes such as DNA replication and gene expression.

     
    dna double strand

    Double strands of DNA are formed by hydrogen bonds between bases of opposite nucleotides.

     

    Base-Pairing in DNA

    • The hydrogen bonding between bases on different strands of DNA is known as base-pairing.
      Base-pairing only occurs between specific combinations of bases, hence it is often referred to as complementary base-pairing.
      • The bases G and C always pair together, forming a G-C pair.
      • The bases A and T always pair together, forming an A-T pair.
      These are known as the base-pairing rules.

     
    base-pairing dna

    Base-pairing in DNA only allows G-C pairs and A-T pairs.

     

    DNA Double Helix

    • Once joined, the two strands of DNA twist around each other, forming a structure known as a double helix.
      This resembles a twisted ladder, where the ‘rails’ consist of alternating sugars and phosphates, and the ‘rungs’ consist of base pairs.

     
    dna double helix

    A DNA molecule has a twisted structure known as a double helix.

     

    The Genetic Code

    • The order of the four different nucleotides in DNA molecules, known as a DNA sequence, determines the genetic make-up of an organism.
      This ‘genetic code’ is the same for all organisms. Therefore, DNA differentiates not only individuals of a species, but all living things on Earth.
      The genetic code works in much the same way as the 26 letters of the English alphabet, which code for all the English-language books that exist.

     
    genetic code dna sequence

    The sequence of nucleotides in DNA encodes the genetic make-up of all living things.

    (Image: schulergd, freeimages.com)

     

    Summary

    • Living things carry genetic information in large molecules called DNA (deoxyribonucleic acid ), which is packaged into chromosomes in the nuclei of cells.
    • DNA molecules are made up of subunits called nucleotides, which consist of a sugar, a base and a phosphate.
      There are four different nucleotides in DNA, each with a different base – G, C, A or T.
    • Nucleotides can join together in two ways.
      Firstly, nucleotides can join by phosphodiester bonds (covalent bonds) between sugars and phosphates, resulting in single-stranded DNA.
      Secondly, nucleotides can join by hydrogen bonds (weaker bonds) between bases, resulting in double-stranded DNA.
    • The hydrogen bonding between bases on different strands of DNA occurs according to the base-pairing rules – only G-C and A-T pairs can form.
    • Double-stranded DNA molecules have a twisted structure known as a double helix.
    • The order of the four different nucleotides in DNA molecules determines the genetic make-up of an organism.

     
    dna double helix animation

    (Image: brian0918, Wikimedia Commons)

     

    (Header image: svetlaborovko, Adobe Stock)