Summary

This document is a lecture on genetics, specifically focusing on protein synthesis and the central dogma of molecular biology. The lecture notes detail the process of translation, highlighting aspects such as genetic codons, tRNA, and ribosomes. It provides an overview of the fundamental concepts and mechanisms involved in this critical biological process.

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Genetics Translation LECTURE (7) DR. El-Sawy 0 Genetics Translation Central Dogma of mol...

Genetics Translation LECTURE (7) DR. El-Sawy 0 Genetics Translation Central Dogma of molecular biology: Definition: Two-step process, by which the information in genes flows into proteins: 1. Transcription: DNA → RNA 2. Translation: RNA → protein DNA → RNA → protein DR. El-Sawy 1 Genetics Translation Genetic codon (triplet codon) :  Three adjacent nucleotides in 5´-3´direction on mRNA. Def  One genetic codon codes for One amino acid.  There are 20 aa.  Each amino acid has specific codons (1 or more).  Each codon consists of 3 nucleotides.  There are (4)3 64 codons : Characters 61 = sense codons codes for 20 aa 3 = non sense codons or termination codons (UAA, UAG, UGA)  AUG (which encode for methionine) act as initiation codon for translation. Codons are recognized by tRNA molecules (containing anti-codons complementary to codons). Each tRNA carries a specific amino acid. Colinear Read from 5' to 3' end. ‫منظم‬ Specific Codes for specific a.a. ‫متخصص‬ Triplet Triplet sequence specify certain a.a. Characteristics ‫ثالثي‬ Universal Same for all species. ‫عالمي‬ Redundant A given a.a may have > one codon. (degenerate) They are different in 3rd base. ‫فضفاض‬ Read from a fixed starting point, taken 3 at a time Non-overlapping & commaless without punctuation ( ) codons. ‫التداخل & الفواصل‬ Read continuously without spacing or overlapping. DR. El-Sawy 2 Genetics Translation Def:  Synthesis of protein using mRNA as the template.  In other words, to translate the nucleotide sequence of mRNA into the amino acid sequence of protein according to the genetic code. Requirements: Ribosome  Protein synthesis machine. mRNA  Carries message from DNA to ribosome. tRNA  Carries amino acids. Amino acids  Building units of the protein. Aminoacyl-tRNA  Connects amino acids to the specific carrier tRNA. synthetase enzyme  Initiation factors (IF). Protein factors  Elongation factors (EF).  Releasing factors (RF). Source of energy  ATP & GTP. DR. El-Sawy 3 Genetics Translation Prokaryotic 50S and 30S ribosomal subunits form  70S ribosome. Eukaryotic 60S and 40S ribosomal subunits form  80S ribosome. Binding sites: A site  Binding site for aminoacyl-tRNA. P site  Binding site for peptidyl-tRNA. E site  Binding site for uncharged tRNA. Carries amino acids during translation. There is at least one specific tRNA for each amino acid. Some amino acids (having > one codon) may be carried by > one tRNA type. When tRNA carries amino acid it is said to be charged. When amino acid carried by tRNA it is said to be activated. DR. El-Sawy 4 Genetics Translation Activation of amino acids (Synthesis of aminoacyl-tRNA):  Amino acids must be at first activated [It binds to specific tRNA by ester bond].  The enzyme responsible for charging of tRNA by its specific A.A: Aminoacyl tRNA synthetase  There are at least 20 different aminoacyl-tRNA synthetases.  2 high energy bonds (from 1 ATP) are needed for a.a activation. Active form Linkage type Activation energy 2 high-energy bonds aminoacyl-tRNA ester bond from 1 ATP Initiation  Ribosome assembles with other 2 types of RNAs forming initiation complex: 1. The 2 subunits of ribosome binds to mRNA strand. 2. 1st tRNA binds to mRNA. DR. El-Sawy 5 Genetics Translation Elongation  Cyclic process , catalyzed by elongation factors. Binding of new Entry of a new amino acyl-tRNA to empty A site. amino acyl-tRNA 1 GTP is needed for binding of amino acyl tRNA to A site. to A site COOH of aminoacyl-tRNA in P site binds with Peptide bond NH2 of new aminoacyl-tRNA in the A site. formation This reaction is catalyzed by peptidyl transferase enzyme present inside 60s subunit. After peptide bond formation, ribosome moves 3 Translocation nucleotides towards 3ʹ end of mRNA. This requires eEF2 and 1 GTP. NB: The following events occur as a result of translocation 1. Release of uncharged tRNA from E (exit) site. 2. Transfer of the newly formed peptidyl-tRNA from A site to occupy P site. 3. The A site becomes free (can be occupied by another new aminoacyl-tRNA). DR. El-Sawy 6 Genetics Translation Termination:  Elongation of polypeptide chain continues until A site is occupied by one of the 3 non-sense codon (UAA, UAG, UGA), stop signal of translation.  Releasing factors (RF) appear in A site and can recognize all three termination codons.  RF + GTP + peptidyl transferase promotes the hydrolysis of bond between peptide chain and tRNA occupying the P site.  This hydrolysis leads to : 1. Release of both peptide and tRNA. 2. Dissociation of 80s ribosomes into 40s and 60s subunit. Q: Energy required for formation of one peptide bond  For each new peptide bond formed 4 high energy phosphate bond are cleaved: 2 (from 1 ATP) for activation 1 GTP for binding of aminoacyl-tRNA to A site 1 GTP for translocation DR. El-Sawy 7 Genetics Translation Q: Polyribosome (polysomes):  Several ribosomes can translate an mRNA at the same time, forming polysome producing many polypeptides simultaneously from a single mRNA. DR. El-Sawy 8

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