Chapter 9 Ã Translation (2025)

Chapter 9 Ð Translation

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ÒThe synthesis of everyprotein molecule in a cell is directed by that cellÕs DNAÓ

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There are two aspects to how this is accomplished:

  • There is the information or coding problem: How does the sequence of nucleotides specify the sequence of amino acids?
  • There is the chemistry problem: What is the actual process by which the different amino acids are linked together in the correct sequence to form a functional protein?

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Introduce the Players and their roles:

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  • Ribosomes Ð These complex organelles are composed of several pieces of rRNA and many proteins. These organelles are the actual ÒfactoriesÓ where protein synthesis takes place.

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  • mRNA Ð this is the coded information for the protein which is about to be made. This information is copied from the DNA template. That template is called the gene for that protein.

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  • tRNA Ð This is the adapter molecule that links the language of mRNA to the language of proteins. These RNA molecules have an amino acid attachment site as well as an Òanti-codonÓ that can attach specifically to the three-base-long ÒcodonÓ found on the mRNA.

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See fig. 9-1 for an overview of how translation occurs. Note that there is a direction: themRNA is read 5Õ ˆ 3Õ.

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Also note that the new protein (nascent protein) is madeamino-terminus first.

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Please see figures 9-2 and 9-3 for a few more specificdetails:

  • the relationship of the template strand of DNA to the message
  • the definition of ÒcodonÓ

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The Genetic Code Ð Table 9-1

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<![if !vml]>Chapter 9 Ã Translation (1)<![endif]>

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  • AUG is the start codon (sometimes GUG)
  • Methionine and tryptophan have only one codon each. All the other amino acids have several codons Ð note that usually it is the third base that is different. This is often referred to as the ÒdegeneracyÓ of the genetic code
  • Stop codons donÕt code for amino acids. These are stop codons because there is no tRNA that matches them.
  • Note that this table depicts the mRNA codons written 5Õ ˆ 3Õ. We could just as appropriately devise a table of anti-codons written 3Õ ˆ

What is tRNA? Ð See fig 9-4 and 9-5.

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The tRNA molecule has several important characteristics:

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<![if !vml]>Chapter 9 Ã Translation (2)<![endif]>

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  • The tRNA molecule has an ÒanticodonÓ
  • The tRNA molecule has an Òamino acid attachment site at its 3Õ-end
    • An Òamino-acyl-tRNA-synthetaseÓ (specific for each tRNA/amino acid combination) matches the amino acid to the anti-codon and attaches the amino acid. (The enzyme charges or acylates the tRNA.)

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See a movie describing how tRNA getsacylated

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  • When the tRNA binds to the mRNA, the binding to the 3rd position of the codon is Òwobbly.Ó (See table 9-2. Note that the unusual purine base ÒinosineÓ is often present in the third position of the anticodon Ð this base can pair with U, C, and A.)
    • This explains why the code is degenerate.

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LetÕs review the general structure of mRNA

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<![if !vml]>Chapter 9 Ã Translation (3)<![endif]>

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LetÕs review polycistronic RNA again:

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  • See figure 9-6.
  • Note that each of the protein coding regions (cistrons) starts with an AUG start codon.
  • Note that each cistron also ends with a stop codon
  • Note that there are ÔspacerÓ sequences between each of the protein coding regions
  • Note that there is a ÒleaderÓ sequence at the beginning of the message, before the first start codon.

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What are ÒoverlappingÓ genes?

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The book refers to the phage fX174Ð a single-stranded DNA virus.

<![if !supportLists]>á <![endif]>Analysisrevealed that the genome of this virus was too small to account for all theproteins that it could make.

<![if !supportLists]>á <![endif]>Theproblem was solved by finding that some of fX174Õs genes areoverlapping:

<![if !supportLists]>o <![endif]>Thesame nucleotide sequence could be read in multiple reading frames:

<![if !supportLists]>o <![endif]>Thesequence below could be read differently depending on which AUG is used as astart codon:

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AUGNNNNAUGNNNNNNNAUGNNÉ É É

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AUG NNN NAU GNN NNN NNA UGNN.. or

AUG NNN NNN NAU GNN É or

AUG NN.

<![if !supportLists]>á <![endif]>Alsosee fig 9-7.

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Polypeptide Synthesis

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<![if !supportLists]>á<![endif]>Prokaryotic ribosome structure Ð 70s ribosomes

<![if !supportLists]>á <![endif]>30ssubunit Ð 16s rRNA plus 21 polypeptides

<![if !supportLists]>á <![endif]>50ssubunit Ð 23s rRNA, 5s rRNA and 32polypeptides

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<![if !supportLists]>á<![endif]>Eukaryotic ribosome structure Ð 80s ribosomes

<![if !supportLists]>á <![endif]>40ssubunit Ð 18s rRNA plus 30 proteins

<![if !supportLists]>á <![endif]>60ssubunit Ð 5s rRNA, 5.8s rRNA, 28s rRNA and 50 proteins

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<![if !supportLists]>á<![endif]>Initiation

<![if !supportLists]>á <![endif]>The16s rRNA of the 30s subunit binds to the ÒShine-DelgarnoÓ sequence.

<![if !supportLists]>¤<![endif]>The Shine-Delagarno sequence (AGGAGGU) is also known asthe Òtranslation initiation region.

<![if !supportLists]>¤<![endif]>The eukaryotic counterpart is the 5Õ-cap of theeucaryotic message.

<![if !supportLists]>¤<![endif]>(See fig 9-11) The Òpre-initiation complexÓ forms: the30s subunit + mRNA + f-met-tRNA + initiation factors (proteins) + GTP.

<![if !supportLists]>¤<![endif]>Then the 50s subunit binds and ÒelongationÓ begins.

<![if !supportLists]>á<![endif]>Note the A-site and the P-site in the figure

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<![if !vml]>Chapter 9 Ã Translation (4)<![endif]>

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<![if !supportLists]>á<![endif]>Elongation (fig. 9-13)

<![if !supportLists]>á <![endif]>Withf-met-tRNA in the P-site and the next acylated-tRNA in the A-site ---- apeptidyl transferase forms a peptide bond between the two amino acids.

<![if !supportLists]>á <![endif]>F-metis cleaved from its tRNA and the tRNA leaves, leaving the P-site open.

<![if !supportLists]>á <![endif]>Thesecond tRNA ÒtranslocatesÓ into the P-site from the A-site.

<![if !supportLists]>á <![endif]>Anew acylated tRNA moves into the empty A-site to base-pair with the codon inthat site.

<![if !supportLists]>á <![endif]>Thisprocess repeats over and over until a nonsense codon is presented in theA-site.

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<![if !supportLists]>á<![endif]>Termination

<![if !supportLists]>á <![endif]>Releasefactors cleave everything is the A-site is unoccupied for too long.

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<![if !supportLists]>á <![endif]>Inpolycistronic mRNA, the next AUG is not too far away and the ribosomereinitiates to synthesize the next protein.

<![if !vml]>Chapter 9 Ã Translation (5)<![endif]>

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What are polysomes?

  • Please study figures 9-15 and 9-16

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Read about Antibiotics and ribosomes on page 188.

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Chapter 9 Ã Translation (2025)

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