L10 2025 - Protein Synthesis - PDF

Biology 1B Semester 2, (28th Jan) Lecture 10 Lecture 8 DNA: a store of biological information Lecture 9 RNA: how genes are expressed Lecture 10 : How cells make proteins There are differences between the structures of DNA and RNA. Which one of these statements is false? A. DNA conta...

Biology 1B Semester 2, (28th Jan) Lecture 10 Lecture 8 DNA: a store of biological information Lecture 9 RNA: how genes are expressed Lecture 10 : How cells make proteins There are differences between the structures of DNA and RNA. Which one of these statements is false? A. DNA contains deoxyribose sugars, mRNA contains ribose sugars B. DNA uses thymine, mRNA uses uracil C. DNA has a double helix, mRNA is single stranded D. DNA and mRNA always reside in the nucleus of cells RNA is extensively modified before leaving the nucleus. Which one of these statements is NOT true. A. mRNA processing helps protect it from degradation B. mRNA processing helps it exit the nucleus C. mRNA processing helps to anchor it to ribosomes D. mRNA processing helps the spicing process by attracting snRNPs Ribosomes are the site of A. Translation B. Transcription C. RNA processing D. Splicing  Learning Objectives Lecture 14 (Protein) Learning objectives Explain that cells need to make polypeptides of defined length and sequence Describe the importance of base-pairing in ensuring the accuracy of protein synthesis Discuss the machinery (ribosome), raw materials (aminoacyl-tRNAs) and instruction tape (mRNA) for protein synthesis (translation) Explain the main characteristics of the genetic code which defines the association of codons with anti-codons Explain that, for proper cell function, proteins must be processed and targetted to their correct cellular locations From DNA to protein  Translation : the basic concept The cell interprets a genetic message and builds an appropriate polypeptide. The message is contained in the codons of the mRNA and deciphered by the * interpreter (codons in the tRNA). mRNA is moved through a ribosome, codons are translated into amino acids one by one. tRNA codons add amino acid cargo to growing polypeptide chain when anticodon recognises codon. Consider mRNA codon UUU  tRNA structure * tRNA * Aminoacyl-tRNA synthetase The Ribosome The Ribosome Note: in ribosome, RNA has structural and functional role. Consider it as one big ribozyme!  Initiation of translation * Initiation Initiation involves the binding of the 30 S subunit of the ribosome to the correct site on the mRNA (at the start codon). The binding site is specified by the upstream mRNA sequence (prokaryotes) or by the 5´-cap on the mRNA (eukaryotes). A special initiator tRNA carrying the amino acid methionine (Met) binds to the start codon AUG on the mRNA. The 50 S ribosome subunit binds to form the initiation complex. Protein initiation factors and energy (GTP) are required. Note 1st tRNA in P site! Elongation during translation - 3 steps In this phase, amino acids are added one at a time; the peptide chain grows from the amino towards the carboxyl end (NC). 3 steps are involved in the cycle:- 1.Codon recognition - to bring the correct aminoacyl-tRNA into the A site. This requires energy input (2xGTP). Increases accuracy and efficiency. 2.Peptide bond formation - the newly arrived amino acid is joined to the growing peptide chain anchored in the A site. This is catalysed by rRNA (large subunit). 3.Translocation - the ribosome moves the tRNA in the A site to the P site. The discharged tRNA in the P site is moved to the E site to leave the ribosome. This requires energy input (GTP). Various elongation factors (proteins) are required for the cycle. The A site is now vacant and can accept the incoming aminoacyltRNA specified by the next codon on the mRNA. The ribosome moves 5´ 3´ along the mRNA. Amino acids are added at a rate of 10-20 per second.  Termination of Translation * Elongation continues until a STOP codon appears in the A site of the ribosome (UAA, UAG or UGA). Instead of tRNA, a release factor protein binds and the completed peptide chain is freed by the action of release factor from the P site (adds a water instead of amino acid to end of chain). The components of the system come apart from each other and can be used for later cycles of translation. Microscopic analysis of Transcription A single ribosome can make an average sized polypeptide in a minute. However many copies can be made simultaneously by polyribosomes.  The Genetic code Marshall Nirenberg 1961  The Genetic code Reading frame! The dog ate the bone. Hed oga tet heb one  Post-translational modification Ribosomes can be “free” or 1. Polypeptide synthesis begins on a “free” ribosome attached to ER. Note synthesis ALWAYS starts in cytosol. 2. SRP binds signal peptide, halting process Proteins destined for export from the cell (e.g. digestive 3. SRP binds ER membrane translocation complex enzymes in the gut, antibodies etc.)or membrane have a 4. SRP leaves while polypeptide grows through pore "signal sequence" of about 20 5. Signal cleaving enzyme cuts of signal peptide amino acids at the N-terminus. 6. Final peptide leaves ribosome and is in final location Mutations Drake : each new human has approx. 64 novel mutations! It is entirely in line with the accidental nature of mutations that extensive tests have agreed in showing the vast majority of them detrimental to the organism in its job of surviving and reproducing, just as changes accidently introduced into any artificial mechanism are predominantly harmful to its useful operation.. Good ones are so rare that we can consider them all bad. -- H.J. Muller, Protein structure Mutations Ribosomes are made from A. rRNA and protein B. 100% rRNA C. DNA and hydrophobic amino acids D. protein The start codon in mRNA is A. CAT B. AUG C. FEB D. AAA Which statement is false for tRNA? A. There are 20 different tRNAs, one for each amino acid B. Some tRNA anticodons can pair with more than one mRNA codon. C. tRNA molecules are held together by disulphide bridges D. tRNA plays the role of transferring amino acids to the site of protein synthesis During elongation phase in Transcription, which process does not require energy from GTP hydrolysis. A. Codon recognition to bring correct tRNA into A-site B. Formation of initiation complex as 50S sub-unit binds C. Peptide bond formation between adjacent amino acids D. Release of tRNA from ribosome The triplet code is translated to amino acids on ribosomes. We know there are 20 amino acids that exist in organisms. Which statement is true? A. There are 3 sites on the ribosome and three possible codons, therefore only 9 possibilities exist but the other 11 are made by virtue of mRNA splicing. B. There are 3 codons and 7 possible configurations allowing for 21 amino acids…however one is a stop codon…making 20 amino acids. C. There are 3 codons, 4 different types of base and 3 positions on the ribosome giving (3x4x3=36) 36 possibilities. 20 code for specific amino acids the others are start and stop sequences. D. Codons show redundancy and most are encoded by more than one codon. Actually there are 64 possibilities of codon, 60 of which code for only 20 amino acids. Lecture 10 Translation Summary The concept of translation with three phases 1. Initiation, 2. Elongation, 3.Termination and know how each progresses Discover the ribosome, its composition, role in Translation and targeting a protein to its correct location within a cell. The genetic code and how rare errors lead to mutations with physiological consequences