CIE AS Biology

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First teaching 2020

Last exams 2024

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6.1.1 Nucleotides

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Nucleotide Structure

  • Nucleic acids such as DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are macromolecules (giant molecules)
  • Like proteins (polypeptides) and carbohydrates (polysaccharides), these nucleic acids are polymers (‘poly’ meaning ‘many’)
  • This means they are made up of many similar, smaller molecules (known as subunits or monomers) joined into a long chain
  • The subunits that make up DNA and RNA are known as nucleotides
  • Therefore DNA and RNA can also be known as polynucleotides

Nucleotides

  • Nucleotides are made up of three components:
    • A nitrogen-containing base (also known as a nitrogenous base)
    • A pentose sugar (containing 5 carbon atoms)
    • A phosphate group

Basic structure of a nucleotide, downloadable AS & A Level Biology revision notes

The basic structure of a nucleotide

Nucleotide structure table

Table 25 Nucleotide structure, downloadable AS & A Level Biology revision notes

Nucleotides in DNA and RNA, downloadable AS & A Level Biology revision notes

The nucleotides found in DNA and RNA

ATP

  • Adenosine triphosphate (ATP) is the energy-carrying molecule that provides the energy to drive many processes inside living cells
  • ATP is another type of nucleic acid and hence it is structurally very similar to the nucleotides that make up DNA and RNA
  • It is a phosphorylated nucleotide
  • Adenosine (a nucleoside) can be combined with one, two or three phosphate groups
    • One phosphate group = adenosine monophosphate (AMP)
    • Two phosphate groups = adenosine diphosphate (ADP)
    • Three phosphate groups = adenosine triphosphate (ATP)

Structure of ATP, downloadable AS & A Level Biology revision notes

The structure of AMP, ADP and ATP

Exam Tip

Don’t worry – you are not expected to know the structural formulae for the nucleotides that make up DNA and RNA or AMP, ADP and ATP (as in the diagram above)! You just need to learn the different groups that they are made up of (phosphate groups, pentose sugars and nitrogenous bases).Remember that adenine is a nitrogenous base whereas adenosine is a nucleoside (a base – adenine, attached to a pentose sugar).

Purines & Pyrimidines

  • The nitrogenous base molecules that are found in the nucleotides of DNA (A, T, C, G) and RNA (A, U, C, G) occur in two structural forms: purines and pyrimidines
  • The bases adenine and guanine are purines – they have a double ring structure
  • The bases cytosine, thymine and uracil are pyrimidines – they have a single ring structure

Purines and pyrimidines (1), downloadable AS & A Level Biology revision notes_Purines and pyrimidines (2), downloadable AS & A Level Biology revision notes

The molecular structures of purines and pyrimidines are slightly different

Exam Tip

You don’t need to know the structural formulae of these bases, just which are purines and which are pyrimidines.

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Lára

Author: Lára

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.