A level Biology exam revision resources written by A level Examiners


RNA is a single-stranded polynucleotide. There are three different types of RNA in the cells, each with a particular structure and function

They are single-stranded and are made up of hundreds to several thousand nucleotides. mRNA is made in the nucleus from coded instructions in the DNA and then passes into the cytoplasm where it is involved in the process of protein synthesis on the ribosomes.

They are mainly single strands of 75-90 nucleotides wound up to form an overall "clover-leaf" shape due to the presence of some double stranded sections. All cells have at least twenty different kinds of tRNA. They interact with mRNA during protein synthesis on the ribosomes.


Two important features of the tRNA molecules are:

  • they possess an 'anticodon' loop through which they can interact with molecules of mRNA.
  • at the opposite end of the molecule they possess an amino acid binding site. The amino acids carried by tRNA molecules eventually form polypeptide chains during protein synthesis.

This is made inside the nucleus within the nucleoli and is a major component of ribosomes. The precise configuration of rRNA is unknown but they are very large molecules containing thousands of nucleotides.

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