AS-LEVEL AQA BIOLOGY NOTES 
TOPIC 2: dna and protein synthesis

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Transcription and Translation of Genes
Transcription = the process of creating mRNA from DNA
  1. DNA strand unwinds and unzips by DNA helicase
  2. Instead of a new DNA strand being formed, RNA nucleotide bases pair up with the exposed DNA bases
  3. An enzyme called RNA polymerase forms covalent bonds between the nucleotide bases
  4. Unlike DNA, RNA is single-stranded
  5. A messenger RNA (mRNA) strand is formed and breaks away from the DNA which then re-zips itself up thanks to the natural formation of hydrogen bonds
  6. mRNA is now free to migrate out of the nucleus through nuclear pores
Translation = the process of creating polypeptides based off mRNA codons
  1. Once mRNA has left the nucleus, it attaches to a ribosome on the rough endoplasmic reticulum
  2. Transfer RNA (tRNA) carries the corresponding amino acid to each on the mRNA
  3. The anti-codon is a triplet of bases that form part of a tRNA molecule and ensure that the correct amino acid is joined onto the polypeptide chain
  4. This process is active and requires ATP
  5. The amino acid transported by the tRNA attaches to the ribosome
  6. Adjacent amino acids join together by peptide bonds, creating a polypeptide chain
  7. This process continues until the ribosome reaches a ‘stop’ codon (triplet) on the mRNA. At this point, the polypeptide breaks loose from the ribosome and is free.
Prokaryotic Transcription
  • mRNA directly produced from DNA
Eukaryotic Transcription
  • DNA is first copied into pre-mRNA
  • May contain pieces of non-useful polynucleotide
  • pre-mRNA is spliced into mRNA
    • Unneeded sections are removed
EXAM TIP
  • Several different triplets can code for the same amino acid. For example, CGG and CGU both code for Arginine.
  • This property of DNA is called degeneracy and DNA is known to be degenerate. It means that for instance, if there was an error in copying the CGU triplet and CGG was copied instead, there would actually be no difference in the primary structure of the polypeptide.