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Science 21B Presentation for Section 8.5 Translation Slide 1 Think back to an earlier discussion of codes and how a coded message compares to a computer code. How are these the same and how are they different? Both require translation of information. However, the coded message reproduces the same information, whereas a computer code produces actions. The translation of RNA into a protein is more like what happens with a computer code. The information encoded in nucleic acids becomes functioning proteins. Slide 2 The word transcription is not synonymous with translation. The root of transcribe is “to write,” whereas the root of translate is “to transfer.” What amino acid is coded by CUU? According to the diagram, you should see it is leucine. What amino acid is coded for by AUG? That would be methionine. Slide 3 What happens when a ribosome reads the codon AUG? It produces methionine, which is also a start codon. If it is UGA, the translation is going to stop. It is a common misconception that there are only twenty amino acids. The structure of an amino acid consists of a central carbon atom bonded to an amino group (NH4), a carboxyl group (COOH), a hydrogen atom (H), and a side group. An infinite number of side groups are possible, so is an infinite number of amino acids. But only twenty are commonly found in proteins, however. Slide 4 In 1961, Francis Crick and a group of colleagues performed an ingenious series of experiments to demonstrate that the genetic code was a triplet code, consisting of three consecutive nucleotides. They used a chemical to delete one, two, and three nucleotides from the DNA of a bacteriophage and then looked at a gene downstream from the deletions to see if it was transcribed correctly. They found that when one or two nucleotides were deleted, the reading frame of the gene was shifted. When three nucleotides were deleted, the downstream gene was read correctly. This also suggested that the reading of the code was continuous, without punctuation interrupting the reading. Slide 5 You might consider it to be odd to describe the genetic as a punctuation of stop and start codes. The Latin word puctum means “point” and is derived form an older form meaning “to pierce or puncture.” Punctuation, in a general sense, signifies an interruption. The word punctuate can also be used to describe the act of placing stress or emphasis on a point. Although most DNA is found in the nucleus, a small amount is located in the mitochondria and chloroplasts. Scientists were shocked in the early 1980s when they determined the base sequences of mitochondrial DNA and found a somewhat different genetic code than in nuclear DNA. Since then, the genetic code of some protists, such as Paramecium, has been found to differ. Thus, the genetic code is not quite universal. Slide 6 The process of translation requires the transfer of information from one chemical form (nitrogencontaining bases) to another (amino acids), that is, from the language of nucleic acids to the language of proteins. The translation is facilitated by an intermediate-transfer RNA. In contrast, the language of complementary base pairing of nucleotides used to copy information in both replication of DNA to DNA and transcription of DNA to RNA occurs directly. The language does not change. There are three binding sites in the ribosomes and the complementarity of the codon and anticodon. What is the function of messenger RNA? It provides a copy of the genetic code. Slide 7 What is the function of transfer RNA? It brings amino acids to the ribosome, in the proper order What then is the function of the ribosomes? The ribosomes position messenger RNA so that it can be read by transfer RNA. It breaks the bonds between the transfer RNAs and their attached amino acids, and forms peptide bonds between amino acids, assembling the protein coded for a gene. Slide 8 The end of a transfer RNA molecule opposite the anticodon binds to a specific amino acid with the help of the enzyme aminoacyl transfer RNA synthetase. There are 20 kinds of these enzymes in a cell, one for each kind of amino acid. Each enzyme ensures that the correct amino acid is attached to a transfer RNA having the anticodon corresponding to that amino acid. The enzymes have two specific binding sites, one for a particular amino acid and the other for a particular transfer RNA. This specificity is sometimes referred to as the second DNA code. Slide 9 A transfer RNA molecule is a large subunit of the ribosome that catalyzes the formation of peptide bonds between amino acids. In this role, it acts as an enzyme. This type of molecule is called a “ribozyme.” The small subunit holds onto the messenger RNA transcript. The large subunit has three sites where the transfer RNA molecules can dock. It helps form the peptide bond between the amino acids and helps break the bond between the amino acid and its carrier transfer RNA molecule.