Protein Synthesis Notes (2nd Year LMD, Ferhat Abbas University)

FERHAT ABBAS UNIVERSITY, SETIF 1- SETIF Faculty of Natural and Life Sciences Department of TC/SNV 2nd Year LMD Module: Genetics Module Coordinators: Dr. BOUZID. [email protected] Dr. OUARET-G...

FERHAT ABBAS UNIVERSITY, SETIF 1- SETIF Faculty of Natural and Life Sciences Department of TC/SNV 2nd Year LMD Module: Genetics Module Coordinators: Dr. BOUZID. [email protected] Dr. OUARET-GUIDOUM. ([email protected]) 13-25/10/2024 Academic Year 2023-2024 1  Chapter II: Protein Synthesis The DNA molecule carries genetic information  In eukaryotes, genomic DNA is exclusively located in the nucleus.  It cannot be found in the cytoplasm due to degradation by cytoplasmic nucleases.  The expression of genetic information involves transferring it from DNA to another type of nucleic acid capable of moving oTU of the nuclear envelope: mRNA.  The process of transferring DNA information to RNA is called 2 transcription  Transcription Transcription is a reaction involving the polymerization of ribonucleotides based on a DNA template strand (3' → 5') his reaction is carried oTU by a specific enzyme: RNA polymerase. The direction of transcription is always 5' → 3 3  Transcription Systems in Prokaryotes and Eukaryotes  In prokaryotes, approximately 100% of the DNA is transcribed into RNA (r,m,t) High gene density  In eukaryotes, only about 3% of the DNA is transcribed Low gene density There are at least three different transcription systems based on the type of RNA being synthesized RNA synthesized Enzyme Gene rRNA RNA pol I Classe I mRNA RNA pol II Classe II tRNA RNA pol III Classe III At least 3 transcription systems 4  Roles of Different RNAs in Protein Synthesis rRNA  rRNA Functions as the factory for the formation of new proteins,  forming polypeptide chains in association with ribosomal proteins to create ribosomes  It functions as machinery by associating with ribosomal proteins to form ribosomes. rRNA Protéines 02-06/10/2022 5  Roles of Different RNAs in Protein Synthesis mRNA  mRNA: Carries genetic information from the nucleus to the cytoplasm, where it is translated at the ribosomes.  The genetic information is transported in the form of a message, specifically to the ribosomes, where the message will be processed. 6  tRNA  tRNA acts as both a transporter and a synthesis enzyme for polypeptides.  It is responsible for carrying amino acids to the ribosomes. amino acids tRNA mRNA 7  transcription The first step, where the transcription machinery assembles and begins to transcribe the DNA into RNA. → initiation The next step → Elongation where the RNA strand is extended as nucleotides are added → and finally stopped by termination at least three transcription systems Transcription RNA DNA factors polymerase matrice 8  1- initiation The 3 RNA polymerases require: Different regulatory sequences to allow the initiation of transcription → promoter sequence. The location of these sequences relative to the initiation site is characteristic for each of the enzymes. → RNA polymerases I and II (located within 100 bp upstream of the transcription unit (TU)) => in the 5’ region of the gene →RNA polymerase III (present in the TU)=> in the coding region of the gene 10  1- initiation  Similarly, each enzyme requires accessory proteins, called transcription factors, that bind specifically to promoter regions. →TFI: → TFII: → TFIII: 11 12  First transcriptional complex classe I Gene Transcription factors RNA POLYMERASE I UCE Core Pre- RNAr promoter region TU 13  second transcriptional complex Class II gene Transcription factors RNA POLYMERASE II TATA Pre- RNAm promoter region TU 14  third transcriptional complex classe III GENE Transcription factors RNA POLYMERASE III TU tRNA promoter region 15  2- Elongation It involves a systematic addition of complementary ribonucleotides to the deoxyribonucleotides present at the gene level (DNA) (T is replaced by U: Uracil) It always begins with two essential phenomena: The decondensation of chromatin The opening of the double helix to form the transcription loop 16  2- Elongation the decondensation of chromatin the opening of the double helix 17 transcription by RNA poly I Classe I groupe of genes RNAr (18S) (5.8S) et (28S) 1 Pre-rRNA Long transcripts (45S) containing a sequence of each rRNA + spacer sequences 2 Post-transcriptional cleavages will produce mature rRNAs. 18  Transcription by RNA polymerase I (RNA Pol I) 5’ ETS 5’ ITS 3’ ITS IGS 5’ 3’ Pre-RNAr 45S 19 transcription by RNA poly I mature rRNA 20 transcription by RNA poly II Classe II  Transcription by RNA polymerase II of mRNA genes produces long transcripts containing sequences that will be translated (exons) and sequences that will be removed (introns) Pre-mRNA  maturation of mRNA is done by : →The addition of a 5' cap → the splicing of introns → the addition of a poly-A tail at 3' 21 transcription by RNA poly II Pre-mRNA Mature mRNA 22 alternative splicing Mature mRNA proteins 23 transcription by RNA poly III Classe III  Transcription by RNA polymerase III of tRNA genes (and other RNAs) produces long transcripts  that will fold onto themselves to form a specific cloverleaf structure. 24 transcription by RNA poly III 25 transcription by RNA poly III 26  3- termination Transcription ends in a process called termination. Termination depends on sequences in the RNA, which signal that the transcript is finished. the sequences that determine the end 3' of each RNA functional are also specific to each type of RNA poly. 27  Translation The translation follows a specific order of events: initiation, elongation and terminaison  The translation is a process cyclic: (the termination follows the elongation, which itself follows the initiation) The ribosome subunits are dissociated at the end of the termination before a new cycle takes place. These events require a very large number of proteins:  traduction factors 28  translation  Peptide synthesis is carried out:  End N-ter → end C-ter of the polypeptide chain  => elongation is then performed by the addition of sequential amino acid to the C-terminus of the polypeptide chain bound to the ribosome.  Peptide synthesis is carried out on 3 steps:  Decoding  Formation of the peptide bond  Translocation 29  step 1 : decoding The tRNA must have the correct anticodon to interact with the codon of the mRNA positioned at the A site to establish a base pair with the proper geometry 30 02-06/10/2022 31 02-06/10/2022 32  Step 2 formation of the peptide bond The A site or Aminoacyl site is the 'acceptor' site occupied by the aminoacyl-tRNA. The P site or Peptidyl site is the site occupied by the peptidyl-tRNA: that is, the tRNA that carries the growing polypeptide chain The E site or Exit site is the ribosomal site occupied by the tRNA that 33 exits the ribosome. Transpeptidation, or the formation of the peptide bond, involves an attack by the amino group of the amino acid at the A site on the carbonyl group of the amino acid at the P site. 34  Step 3 : translocation  The ribosome moves one codon forward on the mRNA.  The next codon of the mRNA is available for interaction with a new aminoacyl-tRNA at the A site.  Simultaneously, the 'empty' tRNA is shifted from the P site to the E site, and the peptidyl-tRNA is translocated from the A site to the P site.  Termination of translation.  These steps are repeated until the ribosome encounters a STOP codon in the reading frame: UAG, UAA, or UGA.  Translation ends when one of these codons occupies the A site. 35 36 37 38 39 40