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Nucleotides

Biology: Cambridge International A Level, topic 6.1.2

Nucleic Acids

Nucleic acids carry the cell's genetic code. These are the instructions for the function of the cell. Nucleic acids are important for passing on information from generation to generation.

Components of nucleic acids

Components of nucleic acids

  • DNA and RNA are made of monomers called nucleotides.
  • Nucleotides join together to form polynucleotides.
  • Polynucleotides make up nucleic acids such as DNA or RNA.
Structure of nucleotides

Structure of nucleotides

  • Each nucleotide is formed from a pentose sugar, a nitrogen-containing organic base and a phosphate group.
  • A condensation reaction between two nucleotides forms a phosphodiester bond.
  • These condensation reactions happen between a sugar group on one nucleotide and a phosphate group on a different nucleotide.
DNA nucleotide

DNA nucleotide

  • The components of a DNA nucleotide are:
  • A deoxyribose sugar.
  • A phosphate group.
  • An organic base - adenine (A), cytosine (C), guanine (G), thymine (T).
  • The components of an RNA nucleotide are:
  • A ribose sugar.
  • A phosphate group.
  • An organic base - adenine (A), cytosine (C), guanine (G), uracil (U).
Purines and pyrimidines

Purines and pyrimidines

  • Adenine and guanine are classified as purines.

  • The primary structure of a purine is two carbon-nitrogen rings.

  • Cytosine, thymine, and uracil are classified as pyrimidines.

  • Pyrimidines have a single carbon-nitrogen ring as their primary structure

Deoxyribose vs ribose

Deoxyribose vs ribose

  • The pentose sugar in DNA is deoxyribose, and in RNA, the sugar is ribose.
  • The difference between the sugars is the presence of the hydroxyl group on the second carbon of the ribose and hydrogen on the second carbon of the deoxyribose.
  • The carbon atoms of the sugar molecule are numbered as 1′, 2′, 3′, 4′, and 5′ (1′ is read as “one prime”).
  • The phosphate residue is attached to the hydroxyl group of the 5′ carbon of one sugar and the hydroxyl group of the 3′ carbon of the sugar of the next nucleotide, which forms a 5′–3′ phosphodiester linkage.

Test yourself on Nucleotides

  • Which of the following is an example of a nucleic acid?

  • Which of the following organic bases are found in DNA nucleotides but NOT in RNA nucleotides?

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