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Protein Synthesis

Protein Synthesis. Involves RNA (base change of uracil for thyamine, single small strand and less stable than DNA). Three types of RNA – messenger (mRNA), transfer (tRNA) and ribosomal (rRNA). It is the formation of a protein by the code on the DNA.

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Protein Synthesis

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  1. Protein Synthesis • Involves RNA (base change of uracil for thyamine, single small strand and less stable than DNA). Three types of RNA – messenger (mRNA), transfer (tRNA) and ribosomal (rRNA). • It is the formation of a protein by the code on the DNA.

  2. This stage is called transcription, it occurs in the nucleus. • DNA is unzipped by the enzyme RNA polymerase. It binds to part of the DNA called a promoter, this prises the two strands apart. • the leading strand is copied only to make a strand of mRNA. Introns: not transcribed, contain regulator genes and other genes for different parts of the body.

  3. The DNA rewinds once the section to be copied is done. • The start codon is always AUG, stop codon can vary. • Unlike DNA rep. one side is copied to make a single strand of RNA that contains the code for a protein. • mRNA moves by diffusion out the nuclear pore to the cytoplasm of the cell where protein synthesis occurs.

  4. Stage 2 is translation: mRNA moves to the ribosome into a slot which then readsthe message on the mRNA and translates it. • It triggers the tRNA to bring the appropriate amino acid to the ribosome. The anticodon on the tRNA matches the codon on the mRNA. The amino acid is added to the polypeptide chain and the tRNA is recycled. As an amino acid is joined the ribosome ‘nudges’ the mRNA along 3 notches.

  5. To summarise: • 2 stages • One in nucleus = transcription • One in cytoplasm = translation • tRNA and rRNA reused • mRNA breaks down • End result is a protein which is then moved to where it is needed

  6. Remember: The code of the DNA specifies the order of the amino acids in the polypeptide chain. The order determines the shape of the protein which determines the function of it. The 3 base code on the DNA is a triplet The 3 base code on the mRNA is a codon The 3 base code on the tRNA is the anticodon

  7. Reverse transcription RNA DNA Protein Translation Exceptions to usual Protein Synthesis • Viruses • -Some have RNA but no DNA so RNA is replicated by RNA • -Some have reversed the order of RNA and DNA. RNA is their permanent genetic code and is transcribed into DNA. Called reverse transcription and is done by enzymes.

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