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PROTEIN SYNTHESIS Proteins are made by cellular organelles called ribosomes. Ribosomes can be attached to the endoplasmic reticulum or freely floating in the cytosol. Animal Cell with Organelles shown Proteins made on free ribosomes will be used within the cell. Proteins made on ribosomes attached to endoplasmic reticulum will be exported out of the cell. _______________________________________________________________________________________________________________________________________________ THREE TYPES OF RNA 1) mRNA: messenger RNA 2) tRNA: transfer RNA, transfers amino acids to the ribsome to build proteins. 3) rRNA: ribosomal RNA, makes up ribosomes Transfer RNA (tRNA) Recall that DNA is transcribed by mRNA in the nucleus. Transcription (in the nucleus) The mRNA leaves the nucleus and goes to the ribosome with the protein instructions. _______________________________________________________________________________ TRANSLATION A codon is a group of three mRNA nucleotides. AUG= start codon Each tRNA molecule has an anticodon, which also has three nucleotides. Translation (in the cytosol) Each tRNA molecule has a specific amino acid at one end and an anticodon at the other. The nucleotides in the tRNA anticodon pair up with the nucleotides in the mRNA transcript using the base pair rules we learned earlier. THE GENETIC CODE The genetic code specifies a correlation between an mRNA nucleotide sequence and an amino acid sequence (protein). mRNA codons bond with complementary tRNA anticodons, which carry amino acids. The Genetic Code UAA, UAG, UGA = Stop codons In this way, the amino acid sequence of a protein is specified. SUMMARY OF PROTEIN SYNTHESIS 1) mRNA makes a copy of DNA in nucleus. 2) mRNA leaves the nucleus and goes to a ribosome in the cytosol. 3) The ribosome (made of rRNA) is the site of protein synthesis. 4) The mRNA codons bond with complementary tRNA anticodons. 5) Each tRNA with a given anticodon always carries the same amino acid. 6) So the sequence of mRNA codons specifies the sequence of tRNA molecules carrying a certain amino acid. 7) The amino acids are bonded together with a peptide bond.