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9.8Summary

  • On the mRNA strand transcribed from DNA, a codon is the sequence of three consecutive bases corresponding to one amino acid.
  • The first AUG codon on mRNA encodes methionine and is also the initiation codon for protein synthesis.
  • Protein synthesis occurs on granules called ribosomes.
  • A three-base set of mRNA (codon) and a three-base set (anticodon) of aminoacyl-tRNA (an amino acid bound to tRNA) form pairs on a ribosome through which amino acids are arranged by tRNA in the order of the mRNA codes.
  • During protein synthesis, the bond between amino acid and tRNA is cleaved and reactions proceed successively to link amino acids and form a polypeptide chain following the order of the mRNA code.
  • Three codons that do not correspond to any amino acid are called termination codons. Protein synthesis stops at termination codons on mRNA.
  • Transcription and translation occur in cooperation in prokaryotes.
  • Genes determine the primary structure of proteins. The primary structure contains information important for the formation of the higher-order structure; information affecting localization of proteins to organelles, extracellular secretion, degradation rate; and information on a range of modifications that can regulate protein functions.
  • Mutations in the gene sequence influence protein function by changing its primary structure.
  • Many individuals may exist in a population with altered sequences because of single base mutations in genes, and these alterations only have a slight effect on protein structure and function. This type of mutation is called SNP. SNPs have attracted attention as factors causing differences in genetic constitution, and determination of these SNPs helps realize whether an individual is susceptible to certain types of diseases.

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