Genetics Chapter II: Protein Synthesis

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]
• Academic Year 2023-2024

Chapter II: Protein Synthesis

• DNA carries genetic information
• In eukaryotes, genomic DNA is exclusively in the nucleus
• DNA is not found in the cytoplasm due to cytoplasmic nucleases
• Expression involves transferring genetic information from DNA to mRNA, which moves out of the nucleus
• The process of transferring DNA information to RNA is called transcription

Transcription

• Transcription is a reaction involving polymerization of ribonucleotides based on a DNA template strand (3' → 5')
• The reaction is carried out by RNA polymerase
• Transcription direction is always 5' → 3'

Transcription Systems in Prokaryotes and Eukaryotes

• In prokaryotes, approximately 100% of the DNA is transcribed into RNA. High gene density.
• In eukaryotes, only about 3% of the DNA is transcribed. Low gene density.
• At least three different transcription systems exist, based on the type of RNA synthesized.
  • rRNA synthesized by RNA pol I. Gene Class I.
  • mRNA synthesized by RNA pol II. Gene Class II.
  • tRNA synthesized by RNA pol III. Gene Class III.

Roles of Different RNAs in Protein Synthesis

• rRNA functions as the factory for the formation of new proteins, forming polypeptide chains in association with ribosomal proteins, to create ribosomes.
• rRNA also functions as machinery, associating with ribosomal proteins to form ribosomes.
• mRNA carries genetic information from the nucleus to the cytoplasm, where it is translated at the ribosomes.
  • The message is specifically transported to the ribosomes where the message is processed.
• tRNA acts as both a transporter and a synthesis enzyme for polypeptides.
  • It is responsible for carrying amino acids to the ribosomes

Transcription

• The transcription process occurs in three main steps: initiation, elongation, and termination.
• Transcription machinery assembles and begins transcribing DNA into RNA during initiation.
• The RNA strand extends as nucleotides are added during elongation.
• Termination occurs when the process stops.
• Different regulatory sequences allow the initiation of transcription. E.g., a promoter sequence.
• The location of these sequences relative to the initiation site is characteristic for each RNA polymerase.
  • RNA polymerases I and II are located within 100 bp upstream of the transcription unit (TU). Located in the 5' region of the gene.
  • RNA polymerase III is present in the TU and is located in the coding region of the gene.
  • Each enzyme requires accessory proteins called transcription factors that bind specifically to promoter regions

1- Initiation

• Similar to other enzymes, each enzyme requires accessory proteins called transcription factors that bind specifically to promoter regions.
  • TFII
  • TFIII
  • TFI

1- Transcription by RNA polymerase I

• The first transcriptional complex is formed by transcription factors, and RNA polymerase I, UCE, core promoter, and TU.
• The complex works with a pre-rRNA molecule.

2- Transcription by RNA polymerase II

• The second transcriptional complex is formed by transcription factors, and RNA polymerase II, TATA, promoter region, and TU.
• The complex works with a pre-mRNA molecule.

3- Transcription by RNA polymerase III

• The third transcriptional complex is formed by transcription factors, and RNA polymerase III, promoter region, and TU.
• The complex works with pre-tRNA molecules..

2- Elongation

• Involves systematic addition of complementary ribonucleotides to deoxyribonucleotides.
• Replaces T with U in transcription.
• Begins with the decondesation of chromatin, and opening of the double helix to form the transcription loop.

3- Termination

• Transcription ends with a process called termination where the transcript ends.
• Termination depends on sequences within the RNA.
• Sequences that determine the end (3′) of each RNA functional are specific to each type of RNA polymerase

Translation

• Translation is a cyclical process that follows a specific order of events (initiation, elongation, and termination)
• Ribosome subunits are dissociated after termination, enabling a new cycle.
• Translation events need a large number of proteins called translation factors.

Translation - peptide synthesis stages

• Peptide synthesis
  • N-terminus to C-terminus of the polypeptide chain
  • Sequential amino acids add to the C-terminus of the chain.
• Decoding
• Formation of the peptide
• Translocation

Various stages of Translation

• Step 1: Decoding
• Step 2: Peptide Bond formation
• Step 3: Translocation

Termination of Translation

• Translation stops when a stop codon (UAG, UAA, or UGA) appears in the A site of the ribosome
• A release factor disassociates the ribosome subunits

Transcription by RNA polymerase I (RNA Pol I)

• The 3 transcription systems involve gene groups transcribed by different RNA polymerases
• RNA Pol I transcribes ribosomal RNA genes (rRNA).
• Transcribes long transcripts (45S) composed of rRNA sequences and spacer sequences
• Post-transcriptional cleavages generate mature rRNA. (e.g., 18S, 5.8S, 28S)

Transcription by RNA polymerase II (RNA Pol II)

• RNA Pol II transcribes mRNA genes producing long transcripts containing translated sequences (exons) and sequences needing removal (introns).
• mRNA maturation process includes:
  • Addition of a 5' cap
  • Splicing of introns
  • Addition of a poly-A tail at 3′
  • Production of mRNA (mature transcripts)

Transcription by RNA polymerase III (RNA Pol III)

• RNA Pol III transcribes tRNA genes (and other RNAs) creating long transcripts.
• Transcripts fold into a cloverleaf structure.
• One tRNA end binds to a specific amino acid, while the other end is specific to a codon on mRNA.