Chapter 17 17.4-17.6
Molecular Components of Translation A cell interprets a genetic message and builds a polypeptide The message is a series of codons on mrna The interpreter is called transfer (trna) trna transfers amino acids from the cytoplasm to the ribosome All 20 amino acids can be found in the cytoplasm of a cell trna has an anticodon complementary sequence of bases and carries a specific amino acid The ribosome adds each amino acids to the growing end of a polypeptide chain.
The structure and function of Transfer RNA Transcribed from DNA templates It is made in the nucleus and must travel from the nucleus to the cytoplasm where translation occurs Each trna molecule is used repeatedly. trna folds due to hydrogen bonding and then looks like a cloverleaf in 2-D, but it s 3-D structure is roughly L-shaped
Translation of a genetic message trna that binds to an mrna codon specifying a particular amino acid must carry THAT amino acid no other Matching of trna and amino acid is carried out by a family of enzymes Aminoactyl-tRNA synthetases The active site of these enzyme fits only a specific combination of amino acid and trna There are 20 synthetases one for each amino acid Wobble of trna The ability of trna to base bair with more than one codon This occurs at the third position, usually NOT other locations
Ribosomes Ribosomes facilitate the specific coupling of trna anticodons with mrna Ribosomes are made of two submits the large and small subunit Ribosome subunits are composed of protein and rrna In eukaryotes, the ribosomes are made in the nucleolus Large and small subunits of a ribosome only join when binding to a mrna molecule Ribosomes differ in prokaryotes and eukaryotes, which enables drugs to be created that can inactivate prokaryotic ribosomes without affecting the eukaryote
Binding sites of Ribosomes There is a binding site for mrna Each ribosome has 3 binding site for trna P site holds the trna carrying the growing polypeptide A site holds the trna carrying the next amino acid to be added E site (exit) discharged trna molecules leave from this point
Building a polypeptide Broken down into three stages (same as transcription) Initiation Elongation Termination All three stages require protein factors that aid in the progress For certain aspects, energy is also required This is seen in elongation Energy comes from the hydrolysis of GTP (related to ATP)
Ribosome Association and Initiation of Translation Initiation stage brings together mrna, a trna with the first amino acid, and the two subunits of a ribosome The small unit of the ribosome binds both to mrna and a specific initiator trna which carries methionine (remember start codon AUG) This is important because it determines the reading frame The binding of these three parts is followed by the attachment of a large ribosomal subunit completing the translation initiation complex Proteins called initiation factors are required to bring all these together Polypeptide is always formed in one direction from the initial codon at the amino end (N-terminus) to toward the carboxyl end (C-terminus
Elongation of the Polypeptide Chain Amino acids are added one by one to the preceding amino acid Each addition involves several proteins called elongation factors Occurs in a three-step cycle Energy is used in the first and third step Codon recognition requires hydrolysis of one GTP molecule, which increases accuracy and efficiency One more GTP is hydrolyzed to provide energy for the translocation step mrna is moved through the ribosome in one direction 5 3 end Ribosome and mrna move relative to each other
Termination of Translation The final stage of translation is termination UAG, UAA, and UGA do not code for amino acids but instead act as signals to stop translation. Protein called arelease factor binds directly to the stop codon in the A site. Causes the additional of a water molecule instead of an amino acid This hydrolyzes the bond between the completed polypeptide and the trna in the P site releasing the polypeptide