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contents Principles of Biology 50 Translation Translation is the process by which a cell assembles proteins from the genetic code. The Rosetta stone. To translate from one language to another, you need a set of comparative rules that act as a template. The Rosetta stone is concrete evidence of how languages were first translated in early human cultures. By displaying the same story in different languages, it served as a template for comparison, a reference for how to get from one language to another. In cells, microscopic structures called ribosomes serve as key sites that support the translation of the mRNA language into the protein language. Archiv/Photo Researchers/Science Source. Topics Covered in this Module Translating DNA into Proteins Major Objectives of this Module Describe the molecular structures involved in translation. Explain the process of translation in detail. Explain how post-translational processes prepare proteins for their functions. page 256 of 989 3 pages left in this module contents Principles of Biology 50 Translation How does a genetic code with only four nucleotides provide the information needed to generate proteins containing up to 20 different amino acids? For this process to occur, many enzymes with specific structures and function are required. Translating DNA into Proteins Translation is the process of converting the information stored in mRNA into protein (Figure 1). Proteins are made up of a series of amino acids. With the help of adapter molecules called transfer RNAs (tRNAs) the appropriate amino acid is added for each set of three adjacent nucleotides in the mRNA, called a codon (Figure 2). Transfer RNAs transfer amino acids from a pool of cytoplasmic amino acids to the growing polypeptide. Figure 1: Translation of mRNA into protein. mRNA carries genetic information required for the synthesis of a specific protein as a series of three-nucleotide units called codons. In the process of translation, a sequence of nucleotides in a molecule of mRNA is converted to a sequence of amino acids in a polypeptide. A strand of mRNA is translated into a linear protein strand. Translation occurs in the 5′ to 3′ direction on the mRNA strand. © 2014 Nature Education All rights reserved. Figure Detail Figure 2: Transfer RNA (tRNA) links codons in mRNA to amino acids in proteins. tRNAs are adaptor molecules that base pair with codons in the mRNA. They bring the amino acids corresponding to each codon and facilitate their addition to the growing polypeptide. © 2014 Nature Education All rights reserved. Figure Detail Why is a codon made up of three nucleotides, and not one or two nucleotides? For gene expression to occur successfully, different arrangements of the four mRNA nucleotides A, G, C, and U must determine the sequence of the amino acids that make up a protein. How are 20 different amino acids encoded using only 4 nucleotides? Since there are 20 amino acids and only 4 possible nucleotides, the relationship between a nucleotide in the mRNA sequence and an amino acid in the protein sequence cannot be one-to-one. If two nucleotides coded for an amino acid, they would only encode a maximum of 16 amino acids (42=16). However, if three nucleotides code for an amino acid, 64 variations (43=64) are possible. Experiments with short stretches of synthetic mRNA demonstrated that a codon is made up of three adjacent nucleotides. Since there are 64 possible codons and only 20 amino acids, most amino acids are coded for by multiple codons (Figure 3). There are also three stop codons, UAA, UAG, and UGA, which do not code for any amino acid but instead signal the termination of translation. Just as there are stop codons, there is also a start codon. The codon AUG codes for the amino acid methionine (Met), but if encountered in conjunction with other signals, AUG also indicates the start of translation. Figure 3: Table of codons. Three-letter sequences of nucleotides, called codons, code for amino acids. To find a specific codon in the table, start by finding the first letter on the left side of the table. Then find the second letter along the top and the third letter along the right side of the table. © 2013 Nature Education All rights reserved. Figure Detail Test Yourself If a mutation changed the third nucleotide of a codon where the first nucleotide is U and the second nucleotide is C, what do you expect the result would be for the polypeptide generated? Submit The genetic code is nearly universal to all known species on Earth. There are a few exceptions such as mitochondria, chloroplasts and some prokaryotes. However, it is clear that the exceptions are very few and affect very few codons. Furthermore, all known genetic codes are more similar than different to each other, which supports the assertion that all life started from a common ancestor. Transfer RNA carries amino acids to the ribosome, the site of protein synthesis. How do transfer RNA molecules add the correct amino acid to the growing protein? The mRNA and tRNAs are brought together by a protein-RNA complex called a ribosome. Each tRNA molecule has a specific amino acid at one end and a nucleotide triplet, known as an anticodon, at the other (Figure 4). The nucleotides in the anticodon base pair with the complementary codons on the mRNA in an antiparallel orientation. For example, the codon 5′–GCU–3′ will pair with the anticodon 3′–CGA–5′ and bring the tRNA for alanine (Ala) into the ribosome. Figure 4: Schematic of tRNA. tRNAs are short molecules that are around 80 nucleotides long. In this schematic, the anticodon is highlighted at the bottom, and the amino acid corresponding to the tRNA appears at the top. © 2014 Nature Education All rights reserved. tRNA molecules are created by transcribing information stored in DNA. In eukaryotes, tRNA is generated in the nucleus and then exported to the cytoplasm to carry out its role in translation. In both prokaryotes and eukaryotes, a tRNA molecule is used over and over. It picks up an amino acid, adds it to a growing polypeptide chain, leaves the ribosome and is ready to pick up another amino acid and begin the process again. The process of matching a specific tRNA with the appropriate amino acid is carried out by a family of enzymes called aminoacyl-tRNA synthetases (Figure 5). There is a different synthetase for each of the 20 amino acids. Each synthetase contains an active site that fits only a specific combination of tRNA and amino acid. In addition, each synthetase is able to bind all of the tRNAs that code for its amino acid. The covalent attachment of the tRNA and the amino acid requires the hydrolysis of ATP. A tRNA to which an amino acid has been added is called an aminoacyl-tRNA or a charged tRNA. After charging, the aminoacyl-tRNA is released from the synthetase and is ready for translation. Figure 5: Formation of an aminoacyl-tRNA. Enzymes called aminoacyl-tRNA synthetases facilitate the covalent bonding of the appropriate amino acid to each tRNA molecule. In the first step (upper left), the enzyme catalyzes the linkage of ATP to an amino acid specific to each aminoacyl-tRNA synthetase. After the release of a pyrophosphate group, an amino acid-AMP intermediate remains bound to the enzyme (upper right). In the next step, the uncharged tRNA corresponding to the specific amino acid enters the active site of the enzyme (lower right), where it is linked to the amino acid, producing the charged tRNA and releasing AMP in the process (bottom). The charged tRNA leaves the active site, and the enzyme becomes available again (lower left). © 2014 Nature Education All rights reserved. Test Yourself What would you expect the result to be if a cell contains a mutation that knocks out a particular aminoacyl-tRNA synthetase? Submit A ribosome is made up of a small subunit and a large subunit. A ribosome has three sites that the tRNA moves through as translation occurs. The growing polypeptide chain is held by the tRNA in the P site (Peptidyl-tRNA binding site). The charged tRNA carrying the next amino acid to be added enters at the A site (Aminoacyl-tRNA binding site). A peptide bond is formed between the growing chain and the next amino acid to be added. This reaction moves the growing polypeptide to the tRNA in the A site. The tRNA in the A site now moves to the P site, and the tRNA in the P site moves to the E site (Exit site), where it is released from the ribosome (Figure 6). Figure 6: The three sites in the ribosome. Charged tRNA enters the ribosome in the A site, moves to the P site as a new peptide bond is formed between its amino acid and the growing polypeptide chain, and finally exits through the E site after its amino acid has been transferred. © 2014 Nature Education All rights reserved. Figure Detail The ribosome keeps the mRNA and tRNA close to each other and brings the next amino acid to the carboxyl end of the growing polypeptide. Without the ribosome, the hydrogen bonding between the tRNA and mRNA would be too weak to hold it there long enough for a peptide bond to form. The ribosome catalyzes the peptide bond formation that adds the amino acid to the polypeptide. Experiments have supported the hypothesis that it is the ribosomal RNA (rRNA) and not the ribosomal proteins that contains the catalytic site for the peptide bond formation function of ribosomes. While the function of ribosomes is the same in prokaryotes and eukaryotes, the proteins and RNAs that make up their ribosomes are different.
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