Biol1110 2022 Lecture 7 - Translation PDF

Biol1110 Genes to Organisms Translation Lecture 7 – Translation  Lecture outline Codons – triplets of bases Translating information from nucleotides into amino acids (polypeptides) Roles of mRNA, tRNA and rRNA Ribosomes Initiation, elongation and termination...

Biol1110 Genes to Organisms Translation Lecture 7 – Translation  Lecture outline Codons – triplets of bases Translating information from nucleotides into amino acids (polypeptides) Roles of mRNA, tRNA and rRNA Ribosomes Initiation, elongation and termination stages of translation Mutations Lecture 7 – Translation  Short revision C G A T Pyrimidines: Cytosine & Thymine (in RNA: Uracil replaces Thymine) (one ring) Purines: Adenine & Guanine (two rings)  Flow of information transcription translation DNA RNA polypeptide Translation is the synthesis of a polypeptide, which occurs under the direction of mRNA Ribosomes are the sites of translation Lecture 7 – Translation  Flow of information transcription translation DNA RNA polypeptide Lecture 7 – Translation  Translation Conversion from nucleotides, the “language” of mRNA, to amino acids, the “language” of proteins Lecture 7 – Translation  Codons: triplets of bases There are 20 amino acids, but there are only four DNA bases The flow of information from gene to protein is based on a triplet code: a series of non- overlapping, three-nucleotide words (codons) Therefore, 43 (= 64) codons to specify amino acids Lecture 7 – Translation  Codons: triplets of bases All 64 codons were deciphered by the mid-1960s Of the 64 triplets, 61 code for amino acids; 3 triplets are “stop” signals to end translation One codon, AUG, is the universal “start” codon Lecture 7 – Translation  Codons: triplets of bases The genetic code is redundant but not ambiguous; no codon specifies more than one amino acid Flexible base pairing in the third codon position is called wobble Lecture 7 – Translation  DNA to protein DNA sequence of coding (sense, non-template) strand: 5’ – ATG TTA TAC CAC – 3’ RNA sequence: 5’ – AUG UUA UAC CAC – 3’ Amino acid sequence: Met Leu Tyr His Proteins are chains of amino acids that fold into a 3-D structure. Lecture 7 – Translation  Transcription & translation 5’ ATGTTCAGTCCTAA 3’ DNA (coding strand) 3’ TACAAGTCAGGATT 5’ DNA (template strand) exon intron exon 5’ AUGUUCAGUCCUAA 3’ pre-mRNA 5’ AUGUUCUAA 3’ mRNA (intron spliced out) Met – Phe - Stop(t (translated to polypeptide using the table on the right) right) Lecture 7 – Translation  Protein back to DNA Create one possible DNA sequence (both anti-parallel strands) for one combination of the mRNA, which makes the below protein sequence? 1) Create the mRNA sequences first using the table. 2) Include the 5’ and 3’ ends of each polynucleotide. 3) Create DNA sequence from the mRNA sequence Polypeptide chain of amino acids Met Leu Tyr His Trp Glu Cys Stop Lecture 7 – Translation  Polypeptide to DNA Met – Phe - Stop Can be any of the mRNA sequences: 5’AUGUUCUAA3’ 5’AUGUUCUAG3’ 5’AUGUUCUGA3’ 5’AUGUUUUAA3’ 5’AUGUUUUAG3’ 5’AUGUUUUGA3’ Introns? Leave out the step with inserting intron(s) as we have no information on this Can be any of the DNA sequences (coding strand): 5’ATGTTCTAA3’ 5’ATGTTCTAG3’ 5’ATGTTCTGA3’ 5’ATGTTTTAA3’ 5’ATGTTTTAG3’ 5’ATGTTTTGA3’ Lecture 7 – Translation  Evolution of the genetic code The genetic code is nearly universal, shared by the simplest bacteria to the most complex animals Genes can be transcribed and translated after being transplanted from one species to another Lecture 7 – Translation  RNA components of translation mRNA: A cell translates an mRNA message into protein with the help of transfer RNA (tRNA) tRNA: Molecules of tRNA are not identical: – Each carries a specific amino acid on one end – Each has an anticodon on the other end; the anticodon base-pairs with a complementary codon on mRNA rRNA: ribosomal RNAs combine with proteins to form the site of translation - ribosome Lecture 7 – Translation  Transfer RNA A tRNA molecule consists of a single RNA strand that is about 80 nucleotides long Contains an amino acid attachment site and an anticodon that recognizes the complementary codon on the mRNA Lecture 7 – Translation  Binding of amino acids Matching the correct amino acid with a tRNA is performed by proteins called aminoacyl-tRNA synthetases There are 20 different synthetases, one for each amino acid Lecture 7 – Translation  rRNA Like tRNA, rRNA is a structural molecule that folds on itself rRNA is a component of the ribosome Lecture 7 – Translation  Ribosome Ribosomes facilitate specific coupling of tRNA anticodons with mRNA codons in protein synthesis The two ribosomal subunits (large and small) are made of proteins and ribosomal RNA (rRNA) Lecture 7 – Translation  Ribosome http://bass.bio.uci.edu/~hudel/bs9 9a/lecture22/ribosome70s_a.gif http://bass.bio.uci.edu/~hudel/bs99a/lecture22/exit_tunnel.jpg Lecture 7 – Translation  Ribosome A ribosome has three binding sites for tRNA: – The A site binds the tRNA that carries the next amino acid to be added to the chain – The P site holds the tRNA that carries the growing polypeptide chain – The E site is the exit site, where discharged tRNAs leave the ribosome tRNAs move through these sites in the order A-P-E Lecture 7 – Translation Gene expression Lecture 7 – Translation Gene expression Lecture 7 – Translation  Translation The three stages of translation: – Initiation – Elongation – Termination All three stages require protein “factors” that aid in the translation process Lecture 7 – Translation  Initiation The initiation stage of translation brings together mRNA, an initiator tRNA with the first amino acid, and the two ribosomal subunits – forms the initiation complex Lecture 7 – Translation  During the Elongation elongation stage, amino acids are added one by one to the preceding amino acid Each addition involves proteins called elongation factors and occurs in three steps: codon recognition, peptide bond formation, and translocation Lecture 7 – Translation  Termination Termination occurs when a stop codon in the mRNA reaches the A site of the ribosome The release factor causes the addition of a water molecule instead of an amino acid Lecture 7 – Translation  Polyribosomes A number of ribosomes can translate a single mRNA simultaneously, forming a polyribosome (or polysome) Polyribosomes enable a cell to make many copies of a polypeptide very quickly Lecture 7 – Translation  Transcription & translation Prokaryotes e.g. in bacteria Transcription and translation both in the cytoplasm (transcription in convoy and polyribosomes) Lecture 7 – Translation  Transcription & translation Eukaryotes Transcription and mRNA modification in nucleus, translation in cytoplasm Lecture 7 – Translation  Functional protein Folds – amino acid sequence – has a 3D shape Post-translational modification Lecture 7 – Translation Mutations Lecture 7 – Translation  Lecture summary Flow of information goes from DNA ➔ RNA ➔ Protein Translation refers to the information in mRNA converted to amino acids (protein) Codons are triplets of nucleotides The genetic code is redundant, but not ambiguous Binding of a tRNA with the correct amino acid is performed by aminoacyl-tRNA synthetases The ribosome is a large complex of multiple proteins and rRNA Lecture 7 – Translation  Lecture summary Translation involves the addition of amino acids via tRNAs that move through the A, P and E sites of ribosomes Amino acids are added to the carboxyl-end of the amino acid chain (the amino end is at the start of the polypeptide chain) Transcription and translation can occur at the same time in bacteria Variety of mutations can occur Lecture 7 – Translation