Genetics Lecture 7 PDF

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