AQA AS Biology

Revision Notes

4.2.2 The Structure of RNA

The Structure of mRNA and tRNA

  • Like DNA, the nucleic acid RNA (ribonucleic acid) is a polynucleotide – it is made up of many nucleotides linked together in a long chain
  • Like DNA, RNA nucleotides contain the nitrogenous bases adenine (A), guanine (G) and cytosine (C)
  • Unlike DNA, RNA nucleotides never contain the nitrogenous base thymine (T) – in place of this they contain the nitrogenous base uracil (U)
  • Unlike DNA, RNA nucleotides contain the pentose sugar ribose (instead of deoxyribose)

Comparison between RNA nucleotide and DNA nucleotide, downloadable AS & A Level Biology revision notes

An RNA nucleotide compared with a DNA nucleotide

  • Unlike DNA, RNA molecules are only made up of one polynucleotide strand (they are single-stranded)
  • Each RNA polynucleotide strand is made up of alternating ribose sugars and phosphate groups linked together, with the nitrogenous bases of each nucleotide projecting out sideways from the single-stranded RNA molecule
  • The sugar-phosphate bonds (between different nucleotides in the same strand) are covalent bonds known as phosphodiester bonds
    • These bonds form what is known as the sugar-phosphate backbone of the RNA polynucleotide strand
    • The phosphodiester bonds link the 5-carbon of one ribose sugar molecule to the phosphate group from the same nucleotide, which is itself linked by another phosphodiester bond to the 3-carbon of the ribose sugar molecule of the next nucleotide in the strand
  • An example of an RNA molecule is messenger RNA (mRNA), which is the transcript copy of a gene that encodes a specific polypeptide. Two other examples are transfer RNA (tRNA) and ribosomal RNA (rRNA)


  • mRNA is a single-stranded molecule
  • It is made up of a sugar-phosphate backbone and exposed unpaired bases
  • Uracil bases are present instead of thymine bases (which are found in DNA)

mRNA as an example of RNA structure, downloadable AS & A Level Biology revision notes

Structure of an mRNA molecule


  • tRNA is a single-stranded molecule
  • It has a sugar-phosphate backbone
  • It has a folded shape
    • There are hydrogen bonds between some of the complementary bases
  • Amino acids bind to a specific region of the molecule
  • The specific anticodon found on the tRNA molecule is complementary to a specific codon on an mRNA molecule

tRNA structure, downloadable AS & A Level Biology revision notes

Structure of a tRNA molecule

Exam Tip

You need to know the difference between DNA and RNA molecules (bases, number of strands, pentose sugar present).

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.

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