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Lecture 14

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Biology 1202B
Brenda Murphy

1 mRNA to Protein (Translation) Translation  The assembly of amino acids into polypeptides on ribosomes  Prokaryotic organisms o Translation takes place throughout the cell  Eukaryotic organisms o Translation occurs in the cytoplasm  A few specialized genes are transcribed and translated in the mitochondria and chloroplasts Translation & RNA  mRNA o 100s of nucleotides long o Template for translation o Read in a 5’ to 3’ orientation; the polypeptide is assembled from the N-terminal to the C- terminal o In prokaryotes, it is not confined within a nucleus and therefore is immediately available for translation o For many eukaryotes, the mRNA produced by splicing of the pre-mRNA first exists the nucleus and is then translated in the cytoplasm o The sequence of the amino acids in a polypeptide chain is determined by the sequence of codons in the mRNA  tRNA o 75 – 90 nucleotides long o Brings amino acids to the complex to be joined, one by one, into the polypeptide chain o Has a highly distinctive structure that accomplishes their role in translation o Internal anticodon sequence is complimentary to mRNA codon o tRNA and mRNA bind in an antiparallel manner  rRNA- Protein Complexes o Ribosomal RNA o Small & large subunits  Each subunit is made up of rRNA and ribosomal proteins o Translates mRNA into amino acid o Amino acids are joined together to make a polypeptide chain tRNA Structure  All tRNAs can base-pair with themselves to wind into four double-helical segments, forming a cloverleaf pattern in two-dimensions 2 o At the tip of one end of the double-helical segments is the anticodon (3 nucleotide segments that base-pairs with a codon in mRNAs) o At the other end of the cloverleaf, opposite to the anticodon, is a free 3’ end of the molecule that links to the amino acid corresponding to the codon  The tRNA cloverleaf folds in three dimensions into an L-shape o If you know the anticodon, you can determine the amino acid o If you know the amino acid, you can predict the anticodon, but cannot predict for sure o Not always linear when tRNA binds up on itself due to internal binding o The anti-codon and the segment binding the amino acid are located at the opposite ends of the L structure  The correct amino acid must be present on a tRNA if translation is to be accurate  The process of adding an amino acid to a tRNA is called aminoacylation o This adds free energy as the amino acid—tRNA combinations are formed  The finished product of charging, a tRNA linked to its ―correct‖ amino acid, is called aminoacyl- tRNA  A collection of different enzymes called aminoacyl-tRNA synthetases catalyze aminoacylation  This energy in the aminoacyl-tRNA eventually drives the formation of the peptide bond linking amino acids during translation The Wobble Hypothesis  Proposed by Francis Crick  Proposed that the complete set of 61 sense codons can be read by fewer than 1 distinctive tRNAs because of the particular pairing properties of the bases in the anticodons o The pairing of the anticodon with the first two nucleotides of the codon is always precise, but the anticodon has more flexibility in pairing with the third nucleotide of the codon  In many cases, the same tRNA’s anticodon can read codons that have either U or C in the third position, or that have A or G in the third position  A special purine, called inosine allows for even more wobble by allowing the tRNA to pair with codons that have one of the U, C, and A in the third position Ribosome Structures  Ribosomes are ribonucleic particles that carry out protein synthesis by translating mRNA into chains of amino acids  The task of a ribosome is the join amino acids into ordered sequences to make a polypeptide chain  Prokaryotic Cells o Ribosomes carry out their assembly functions throughout the cells  Eukaryotic Cells o Ribosomes function only in the cytoplasm, either suspended freely in the cytoplasmic solution or attached to the membranes of the endoplasmic reticulum (ER) 3  A finished ribosome is made up of two parts, called the large and small ribosomal subunits o Each subunit is made up of a combination of ribosomal RNA (rRNA) and ribosomal proteins  Chloroplasts and mitochondria in eukaryotic cells have their own ―prokaryotic‖ ribosomes that are different from those in the cytoplasm  Prokaryotic and eukaryotic ribosomes are similar in structure and function  However, the differences in their molecular structure, particularly in the ribosomal proteins, give them distinct properties  To fulfill its role in translation, the ribosome has special binding sites active in bringing together mRNA with aminoacyl-tRNAs  Aminoacylation or charging: The addition of an amino acid to tRNA o The A site (aminoacyl site) – Where the incoming aminoacyl-tRNA (carrying the next amino acid to be added to the polypeptide chain) binds to the mRNA o The P site (peptidyl site) – Where the tRNA carrying the growing polypeptide chain is bound o The E site (exit site) – Where an exiting tRNA binds as it leaves the ribosome  Amino acids are added to a growing polypeptide chain between the subunits  The growing polypeptide chain exits the ribosome through the tunnel in the large subunit  Ribosome movement is from the 5’ to 3’ end Three Major Stages of Translation 1. Initiation  The translation components assemble at the start of the codon of the mRNA o AUG 2. Elongation  The assembled complex reads the string of codons in the mRNA one at a time while joining the specified amino acids into the polypeptide o Reading the string of codons is the single stranded mRNA and changing this information into a string of amino acids to a growing polypeptide chain 3. Termination  Completes the translation proc
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