3.4.2 DNA and protein synthesis

• Transcription 


  Making a molecule of mRNA using the triplet codes on the gene of DNA
• Translation 


  Bonds the encoded amino acids together in the correct sequence using tRNA's and a ribosome
• Transcription:

  1. RNA polymerase binds to the start of the gene, moving along the DNA template strand while DNA helicase breaks the hydrogen bonds to expose the bases
  2. RNA nucleotides bind to exposed bases
  3. RNA polymerase joins the mRNA sugar-phosphate backbone together by forming phosphodiester bonds
  4. Stop codon reached, RNA polymerase detaches and DNA forms hydrogen bonds again
• Functional mRNA -

  In eukaryotes introns and exons are both copied into mRNA (this is pre mRNA). Splicing then occurs to remove introns and join exons together, forming mature mRNA. This happens in the nucleus.

  In prokaryotes this does not occur as their DNA doesn't contain introns
• Translation:

  1. mRNA binds to a ribosome exposing 3 codons
  2. 2 specific tRNA with specific amino acids attached bind to the codons
  3. Peptide bonds form between the two amino acids and a third tRNA enters
  4. The first tRNA is released which can now pick up another amino acid. The ribosome moves along 1 codon
  5. next 2 amino acids form peptide bonds etc, ribosome moves along, tRNA is released and the polypeptide chain grows
  6. Ribosome reaches the end and releases mRNA and the polypeptide chain
• Gene mutation


  The change in the base sequence of DNA - able to lead to a change in amino acid sequence of the encoded polypeptide
• Mutation may arise during DNA replication, but can also happen spontaneously with the mutation rate being increased by mutagenic agents. These include UV radiation, ionising radiation and chemical carcinogens
• Substitution 


  One or more bases are swapped for another - does not cause a frameshift
• Addition 


  One or more bases are added - causes a frameshift
• Deletion 


  One or more bases are removed - causes a frameshift
• Inversion 


  A sequence of bases is reversed - does not cause a frameshift
• Duplication 


  One or more bases are repeated - causes a frameshift
• Translocation between homologous chromosomes


  A sequence of bases is moved from one location in the genome to another
• Frameshift 


  All the triplets change downstream after the mutation causing all the encoded amino acids also change