4.2.2 The Structure of RNA

• 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)

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

• 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)

Structure of an mRNA molecule

tRNA

• 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

Structure of a tRNA molecule