Protein Synthesis [The Central Dogma of Biology]

• • Gene expression is the process where the DNA directs the protein synthesis.
  • The unidirectional flow of genetic information is called the central dogma of molecular biology.

  ○ An exception to this rule is reverse transcription, mainly caused by retroviruses, in which they synthesize DNA from an RNA template.
• A genetic material in our body located inside the nucleus which carries genes and synthesizes certain proteins.
• Proteins

  • A polypeptide linked by peptide bond which plays a crucial role in building and maintaining cells.
  • ○ Polypeptide refers to a long chain of amino acids.
  • ○ There are 20 different amino acids where each has their own specific side chain, also known as R group.
  • Proteins have multiple functions such as building new body tissues, enzymes, and components of all cell membranes.
• Transcription

  • Initiation
  • Elongation
  • Termination
  • Modifications of Pre-mRNA
  • Mature mRNA
• Transcription

  • The process of synthesizing mRNA from DNA inside the:
  • ○ Nucleus - eukaryotes
  • ○ Cytoplasm - prokaryotes
  • Rate: 40 nucleotides per second (eukaryotes)
  • Final Product: Mature mRNA ready to export to cytoplasm
• Transcription

  • Promoters
  • Transcription Factors
  • RNA Polymerase
  • Transcription Initiation Complex
• Transcription

  • Stage for preparation of materials
• Transcription

  Promoters

  • Signals the transcription starting point, near the beginning of a gene. In eukaryotes, a promoter called TATA box is used.
• Transcription

  TRANSCRIPTION FACTORS

  • Bind to the DNA strand to mediate the binding of RNA Polymerase.
• Transcription

  RNA Polymerase

  • Pries the DNA strands apart and untwist around 10-20 bases at a time.
• Transcription

  Transcription Initiation Complex

  • A completed assembly made up of Transcription factor and RNA Polymerase bound to a promoter.
• Elongation

  • Copying of RNA nucleotides
  • RNA Polymerase moves along the DNA template strand.
  • Adds complementary RNA nucleotides one at a time based on the template strand.
  • Direction: 5’ - 3’ direction, downward
• Termination

  • To stop the elongation process
  • prokaryotes: Stops at the end of the terminator, which vary on the organisms
  • eukaryotes: RNA polymerase transcribes polyadenylation signal sequence, Released the RNA transcript after 10-35 nucleotides past the signal sequence
• Alteration of pre-mRNA

  • Exclusive only for eukaryotes
  • Functions:
  • ○ Facilitate the export of mRNA to cytoplasm
  • ○ Protect from hydrolytic enzymes
  • ○ Help ribosomes attach to the 5’ end
• Alteration of pre-mRNA

  • 5' cap
  • poly-A tail
  • mRNA processing: spliceosomes, introns, exons
• Alteration of pre-mRNA

  • 5' cap: A modified guanine (guanine + 3 phosphate) in the 5’ end
• Alteration of pre-mRNA

  • poly-A tail: 50-250 adenine in the 3’ end
• Alteration of pre-mRNA

  • mRNA processing: Editing ; to remove introns and joined exons through spliceosomes
  • exons: functional RNA that codes for proteins
  • introns: non functional RNA
• Translation

  • Synthesis of proteins using the information from mRNA which occurs in the ribosomes.
• Translation

  • matured mRNA
  • ribosomes: A site, P site, E site
  • transfer RNA
  • codons
• Translation

  • matured mRNA: Contains the codes for proteins
• Translation

  • ribosomes: Read mRNA three bases or 1 codon at a time and construct the polypeptide chain
  • Has large and small subunit which in between can find the sites:
  • A site - entrance; carries the next amino acid to be added to the chain
  • P site- holds the growing polypeptide chain
  • E site- exit
• Translation

  • tRNA: Carries amino acids to the ribosomes
  • Has specific anticodon, complementary to the codons in the mRNA
• Translation

  • codons: There are 64 codons decoded in the 1960’s in which: 61 codons - proteins; 1 start codon (AUG) 3 special codons (stop) - UAG, UGA, UAA
  • Note: More than one codons can specify for a particular amino acid
• Translation

  • Initiation
  • Elongation
  • Termination
• Translation

  Initiation

  • preparation of materials
  • mRNA will enter between the small and large subunits of the ribosomes.
• Translation

  Initiation

  • binding
  • moves along
  • hydrogen bond
  • translation initiation complex
• Translation

  Initiation

  • binding: A small ribosomal subunit binds with mRNA and a special initiator tRNA.
• Translation

  Initiation

  • moves along: The small subunit moves along the mRNA until it reaches the start codon (AUG).
• Translation

  Initiation

  • hydrogen bond: Initiator tRNA binds to AUG through hydrogen bond
• Translation

  Initiation

  • translation initiation complex: Once the initiation factors bring in the large ribosomal unit, a complete assembly made up of starting codon, tRNA, and ribosomes is made.
• Translation

  Elongation

  • A key step in protein synthesis that elongates the polypeptide chain by adding one amino acid per cycle.
• Translation

  Elongation

  • Codon Recognition
  • Peptide Bond Formation
  • Translocation
• Translation

  Elongation

  • Codon Recognition: The ribosomes will read the second codon in the mRNA after the start codon. The 2nd tRNA carrying amino acid 2 that corresponds to the codon in the mRNA will enter the A site with its anticodon. Once the codon recognizes the tRNA, a hydrogen bond will form between them.
• Translation

  Elongation

  • Peptide Bond Formation: After forming the hydrogen bond, amino acid 1 in the P site and amino acid 2 in the A site will bond through peptide bond. Once the bond is formed, amino acid 1 will detach from its tRNA due to the peptide bond formation.
• Translation

  Elongation

  • Translocation: After the detachment of the amino acid from the tRNA, the mRNA strand will move, and the first tRNA will move at the E site to exit. At the same time, the 3rd tRNA will enter the A site, carrying the 3rd amino acid which signals the start of the cycle to occur again.
• Translation

  Termination

  Stop Codon: Once the stop codon (UAG, UAA, UGA) reaches the A site, no tRNA will enter at the A site and the elongation process will come to an end.

  The final product of the translation process is a protein ready to be used for its specific function.
• RNA Polymerase II

  An enzyme that is responsible for prying the DNA strands apart and add complementary RNA nucleotides.
• False

  A gene cannot be transcribed simultaneously by RNA polymerases.
• Transcription

  It is the process of synthesizing mRNA from DNA.