Exons and introns

• what is an intron?

  non-coding nucleotide sequence in DNA and pre-mRNA, thats removed from pre-mRNA, to produce mature mRNA
• what is an exon?

  coding region in the nucleotide sequence of DNA and pre-mRNA that remains present in the final mature mRNA, after introns have been removed
• what does DNA contain?

  the information for making polypeptides
• where is the RNA version of the code first made?

  from the DNA
• in prokaryotes, what does the RNA version of the code do?

  this RNA is a messenger RNA (mRNA) and it directs the synthesis of the polypeptide
• in eukaryotes, what does the RNA version of the code do?

  the RNA has to be processed before it can be used to synthesise the polypeptide
• what is the RNA version of the code like in eukaryotes?

  • the initial RNA version of the code is much longer than the final mRNA and it contains sequences of bases that have to be removed
  • this RNA is sometimes called pre-messenger RNA (pre-mRNA) and the sequences to be removed are called introns - they aren’t translated into proteins
  • the introns are cut out of the pre-mRNA using endonucleases and the sequences left are the exons, which are joined together, or spliced, with ligases
• what have some introns been identified of containing?

  as containing DNA sequences incorporated from viruses in life’s evolutionary past
• what is this an image of?

  introns and exons