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Transcript
9.5 Translation: RNA to Protein • Translation converts genetic information carried by an mRNA into a new polypeptide chain • The order of the codons in the mRNA determines the order of the amino acids in the polypeptide chain Translation • Translation occurs in the cytoplasm of cells • Translation occurs in three stages: • Initiation • Elongation • Termination Initiation • An initiation complex is formed • A small ribosomal subunit binds to mRNA • The anticodon of initiator tRNA base-pairs with the start codon (AUG) of mRNA • A large ribosomal subunit joins the small ribosomal subunit Elongation • The ribosome assembles a polypeptide chain as it moves along the mRNA • Initiator tRNA carries methionine, the first amino acid of the chain • The ribosome joins each amino acid to the polypeptide chain with a peptide bond Termination • When the ribosome encounters a stop codon, polypeptide synthesis ends • Release factors bind to the ribosome • Enzymes detach the mRNA and polypeptide chain from the ribosome start codon (AUG) initiator tRNA first amino acid of polypeptide 1 Ribosome subunits and an initiator tRNA converge on an mRNA. A second tRNA binds to the second codon. 3 The first tRNA is released and the ribosome moves to the next codon. A third tRNA binds to the third codon. 5 The second tRNA is released and the ribosome moves to the next codon. A fourth tRNA binds the fourth codon. peptide bond 2 A peptide bond forms between the first two amino acids. 4 A peptide bond forms between the second and third amino acids. 6 A peptide bond forms between the third and fourth amino acids. The process repeats until the ribosome encounters a stop codon in the mRNA. Stepped Art Figure 9-11 p156 Transcription polysomes ribosome subunits tRNA Convergence of RNAs mRNA Translation polypeptide Figure 9-12a p157 ANIMATED FIGURE: Translation To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE Polysomes • Many ribosomes may simultaneously translate the same mRNA, forming polysomes mRNA polysomes newly forming polypeptide Take-Home Message: How is mRNA translated into protein? • Translation converts protein-building information carried by mRNA into a polypeptide • During initiation, an mRNA, an initiator tRNA, and two ribosome subunits join • During elongation, amino acids are delivered to the complex by tRNAs in the order dictated by successive mRNA codons; the ribosome joins each to the end of the polypeptide chain • Termination occurs when the ribosome reaches a stop codon in the mRNA; the mRNA and the polypeptide are released, and the ribosome disassembles 9.6 Mutated Genes and Their Protein Products • If the nucleotide sequence of a gene changes, it may result in an altered gene product, with harmful effects • Mutations • Small-scale changes in the nucleotide sequence of a cell’s DNA that alter the genetic code Mutations and Proteins • A mutation that changes a UCU codon to UCC is “silent” – it has no effect on the gene’s product because both codons specify the same amino acid • Other mutations may change an amino acid in a protein, or result in a premature stop codon that shortens it – both can have severe consequences for the organism Common Mutations • Base-pair-substitution • May result in a premature stop codon or a different amino acid in a protein product • Example: sickle-cell anemia • Deletion or insertion • Can cause the reading frame of mRNA codons to shift, changing the genetic message • Example: thalassemia Hemoglobin and Anemia • Hemoglobin is a protein that binds oxygen in the lungs and carries it to cells throughout the body • The hemoglobin molecule consists of four polypeptides (globins) folded around iron-containing hemes – oxygen molecules bind to the iron atoms • Defects in polypeptide chains can cause anemia, in which a person’s blood is deficient in red blood cells or in hemoglobin Mutations in the Beta Globin Gene Figure 9-13a p158 Figure 9-13b p158 Figure 9-13c p158 Figure 9-13d p158 Figure 9-13e p158 Sickle-Cell Anemia • Sickle-cell anemia is caused by a base-pair substitution which produces a beta globin molecule in which the sixth amino acid is valine instead of glutamic acid (sickle hemoglobin, HbS) • HbS molecules stick together and form clumps – red blood cells become distorted into a sickle shape, and clog blood vessels, disrupting blood circulation throughout the body • Over time, sickling damages organs and causes death sickled cell glutamic acid valine normal cell Figure 9-14 p159 ANIMATED FIGURE: Sickle-cell anemia To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE Thalassemia and Frameshifts • Another type of anemia, beta thalassemia, is caused by the deletion of the twentieth base pair in the beta globin gene • Deletions cause a frameshift, in which the reading frame of the mRNA codons shifts • Frameshifts garble the genetic message, just as incorrectly grouping a series of letters garbles the meaning of a sentence Thalassemia and Transposable Elements • Beta thalassemia can also be caused by insertion mutations, which also cause frameshifts • Insertion mutations are often caused by the activity of transposable elements, which are segments of DNA that can insert themselves anywhere in a chromosome Take-Home Message: What happens after a gene becomes mutated? • Mutations that result in an altered protein can have drastic consequences • A base-pair substitution may change an amino acid in a protein, or shorten it by introducing a premature stop codon • Frameshifts that occur after an insertion or deletion change an mRNA’s codon reading frame, so they garble its proteinbuilding instructions ANIMATED FIGURE: Base-pair substitution To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE ANIMATION: Frameshift mutation To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE