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TRANSLATION: Ribosomes and Protein Synthesis TRANSLATION Ribosomes and Protein Synthesis Mrs. MacWilliams Academic Biology TRANSLATION: Ribosomes and Protein Synthesis I. The Genetic Code A. INTRODUCTION 1. First step in decoding genetic messages transcribe a nucleotide base sequence from DNA to RNA. 2. Transcribed information contains a code for making proteins. 3. Proteins are made by joining amino acids together into long chains, called polypeptides. 4. As many as 20 different amino acids are commonly found in polypeptides. TRANSLATION: Ribosomes and Protein Synthesis 5. The sequence of amino acids influences the shape of the protein, which in turn determines its function. 6. RNA contains four different bases: adenine, cytosine, guanine, and uracil. 7. These bases form a “language,” or genetic code 8. Each three-letter “word” in mRNA is known as a codon. TRANSLATION: Ribosomes and Protein Synthesis B. How to Read Codons 1. four different bases = 64 possible codons (4 × 4 × 4 = 64) 2. This circular table shows the amino acid to which each of the 64 codons corresponds. 3. To read a codon, start at the middle of the circle and move outward. TRANSLATION: Ribosomes and Protein Synthesis 4. Most amino acids can be specified by more than one codon. 5. For example, six different codons— UUA, UUG, CUU, CUC, CUA, and CUG—specify leucine. But only one codon— UGG—specifies the amino acid tryptophan. TRANSLATION: Ribosomes and Protein Synthesis C. Start and Stop Codons 1. The methionine codon AUG serves as the initiation, or “start,” codon for protein synthesis. 2. Following the start codon, mRNA is read, three bases at a time, until it reaches one of three different “stop” codons, which end translation. TRANSLATION: Ribosomes and Protein Synthesis II. Translation *The decoding of an mRNA message into a protein is a process known as translation. A. Steps in Translation 1. Messenger RNA is transcribed in the nucleus and then enters the cytoplasm for translation. TRANSLATION: Ribosomes and Protein Synthesis 2. Begins when a ribosome attaches to an mRNA molecule in the cytoplasm. 3. As the ribosome reads each codon of mRNA, it directs tRNA to bring the specified amino acid into the ribosome. 4. One at a time, the ribosome attaches each amino acid to the growing chain. TRANSLATION: Ribosomes and Protein Synthesis 5. Each tRNA molecule carries just one kind of amino acid. 6. Each tRNA molecule has three unpaired bases, collectively called the anticodon—which is complementary to one mRNA codon. 7.The tRNA molecule for methionine has the anticodon UAC, which pairs with the methionine codon, AUG. The ribosome has a second binding site for a tRNA molecule for the next codon. 8. If that next codon is UUC, a tRNA molecule with an AAG anticodon brings the amino acid phenylalanine into the ribosome. TRANSLATION: Ribosomes and Protein Synthesis 9. The ribosome helps form a peptide bond between the first and second amino acids— methionine and phenylalanine. 10. The bond holding the first tRNA molecule to its amino acid is broken. 11. tRNA then moves into a third binding site, from which it exits the ribosome. 12. The ribosome then moves to the third codon, where tRNA brings it the amino acid specified by the third codon. TRANSLATION: Ribosomes and Protein Synthesis 13. The polypeptide chain continues to grow until the ribosome reaches a “stop” codon on the mRNA molecule. 14. When the ribosome reaches a stop codon, it releases both the new polypeptide and the mRNA molecule, completing the process of translation. DNALC Tranlation Animation TRANSLATION: Ribosomes and Protein Synthesis B. Roles of tRNA and rRNA in Translation 1. Ribosomes are composed of roughly 80 proteins and 3-4 different rRNA molecules. 2. These rRNA molecules help hold ribosomal proteins in place and help locate the beginning of the mRNA message. TRANSLATION: Ribosomes and Protein Synthesis III. The Molecular Basis of Heredity A. Most genes contain instructions for assembling proteins. TRANSLATION: Ribosomes and Protein Synthesis 1. Many proteins are enzymes, which catalyze and regulate chemical reactions. 2. A gene that codes for an enzyme to produce pigment can control the color of a flower. Another gene produces proteins that regulate patterns of tissue growth in a leaf. 3. The central dogma of molecular biology is that information is transferred from DNA RNA protein. TRANSLATION: Ribosomes and Protein Synthesis 4. Gene expression is the way in which DNA, RNA, and proteins are involved in putting genetic information into action in living cells. 5. DNA carries information for specifying the traits of an organism. TRANSLATION: Ribosomes and Protein Synthesis 6. One of the most interesting discoveries of molecular biology is the universal nature of the genetic code. 7. Despite their enormous diversity in form and function, living organisms display remarkable unity at life’s most basic level, the molecular biology of the gene.