The Structure of RNA (AQA A Level Biology): Revision Note

The structure of mRNA and tRNA

RNA nucleotides

• Like DNA, the nucleic acid RNA (ribonucleic acid) is a polynucleotide – it is made up of many nucleotides linked together in a long chain

• Both contain the nitrogenous bases adenine (A), guanine (G) and cytosine (C)

• RNA nucleotides never contain the nitrogenous base thymine (T); they contain the nitrogenous base uracil (U)

• RNA nucleotides contain the pentose sugar ribose (instead of deoxyribose)

RNA molecules

• RNA molecules are only made up of one polynucleotide strand (they are single-stranded)

• Each RNA polynucleotide strand is made up of a sugar-phosphate backbone and exposed unpaired bases

  • Alternating ribose sugars and phosphate groups link together, with the nitrogenous bases of each nucleotide projecting out sideways from the single-stranded RNA molecule

• Examples of RNA molecules are

  • messenger RNA (mRNA)

  • transfer RNA (tRNA)

  • ribosomal RNA (rRNA)

mRNA

• mRNA is a transcript copy of a gene that encodes a specific polypeptide

  • It carries the genetic code from DNA in the nucleus to the ribosomes, where it is used to synthesise proteins during translation

tRNA

• tRNA has a folded shape, despite looking like it is double-stranded it is single-stranded

  • There are hydrogen bonds between some of the complementary bases holding the single strand together in certain regions

• tRNA molecules have a role in protein synthesis

  • The specific anticodon found on the tRNA molecule is complementary to a specific triplet of bases on an mRNA molecule

  • This specificity allows amino acids to bind to a specific region of the tRNA molecule in their correct order