2.3 – DNA Structure

2.3 – DNA Structure

2.3.1 – Outline DNA nucleotide structure in terms of sugar (deoxyribose), base and  phosphate

Nucleotides are formed from a pentose sugar, phosphate, and a base.

  • Phosphate links neighboring sugars together (PO43-)
  • The sugar is either ribose for RNA or deoxyribose for DNA, which has one less oxygen
  • Four types of bases, which comprise the coded genetic message
  •      Nitrogen-based ring structures
  •           Cytosine (C), Guanine (G), Adenine (A), Thymine (T)
  • Nucleotides form sequences, which are instructions for the organism
  • Changes to nucleotides cause mutations


2.3.2 – State the names of the four bases in DNA

Purines – Two Ringed Bases

  • Adenine (A)
  • Guanine (G)

Pyrimidines – Single Ringed Bases

  • Cytosine (C)
  • Thymine (T)
  • Uracil replaces Thymine in RNA (U)



2.3.3 – Outline how DNA nucleotides are linked together by covalent bonds into a single strand

  • DNA composed of two polynucleotide chains
  • Nucleotides are covalently bonded
  • Bond is a phosphodiester
  •           Two covalent bonds between the OH- and acidic phosphate group
  • Nucleotides bond at the 3l (three prime) end of the molecule
  • To form the polynucleotide, the nucleotides condense together one at a time, giving water in the reaction

2.3.4 – Explain how a DNA double helix is formed using complementary base pairing and hydrogen bonds

Complementary means matching

  • A double helix is made up of two anti parallel polynucleotide chains
  • Bases pair and are bonded with hydrogen bonds
  • Adenine and thymine are the same distance apart as guanine and cytosine


2.3.5 – Draw and label a simple diagram of the molecular structure of DNA

  • DNA is made up of two anti-parallel polynucleotide chains
  • They form the double helix (“spiral” structure)
  • The DNA has a sugar-phosphate backbone
  • Bonded with complementary base pairs
  •          Adenine to Thymine
  •          Cytosine to Guanine