Study Guide: Protein Synthesis (Transcription & Translation)

Overview of Protein Synthesis

• Protein synthesis follows the Central Dogma: DNA → RNA → Protein

• The two main processes are transcription (DNA to RNA) and translation (RNA to protein).

Transcription: DNA to RNA

• Location: Occurs in the nucleus.
  
  

• Purpose: Converts genetic information from DNA into messenger RNA (mRNA).
  
  

• Steps of Transcription:
  
  

  • Initiation: RNA polymerase binds to the promoter region of DNA and unwinds the double helix.

  • Elongation: RNA polymerase moves along the template strand, adding complementary RNA nucleotides (A pairs with U, G pairs with C).

  • Termination: RNA polymerase reaches a terminator sequence, stopping transcription and releasing the mRNA.

• Post-Transcriptional Modifications:
  
  

  • 5’ Cap: A modified guanine is added to the 5' end for stability and ribosome binding.

  • Poly-A Tail: A string of adenine nucleotides is added to the 3' end for mRNA protection.

  • Splicing: Introns (non-coding regions) are removed, and exons (coding regions) are joined to form mature mRNA.

Translation: RNA to Protein

• Location: Occurs at the ribosome in the cytoplasm or rough endoplasmic reticulum.
  
  

• Purpose: Converts the mRNA sequence into a chain of amino acids, forming a protein.
  
  

• Key Players:
  
  

  • mRNA: Carries the genetic code in triplets (codons).

  • tRNA: Matches amino acids to codons using its anticodon.

  • Ribosome: Facilitates the assembly of the protein.

• Codons:
  
  

  • A codon is a sequence of three nucleotides in mRNA that corresponds to a specific amino acid or a stop signal.

  • There are 64 codons in total, coding for 20 different amino acids.

  • Start codon: AUG (Methionine) signals the start of translation.

  • Stop codons: UAA, UAG, and UGA signal the termination of translation.

  • Multiple codons can code for the same amino acid due to redundancy in the genetic code.

• Steps of Translation:
  
  

  • Initiation: The ribosome assembles at the start codon (AUG) and the first tRNA brings methionine.

  • Elongation: Ribosome moves along mRNA, adding amino acids as tRNAs bring them in sequence.

  • Termination: A stop codon (UAA, UAG, or UGA) is reached, releasing the completed polypeptide.

• Post-Translational Modifications:
  
  

  • Folding into a functional protein.

  • Addition of chemical groups (e.g., phosphate, carbohydrate, lipid) for specific functions.

Mutations in Protein Synthesis

• Silent Mutation: No change in protein function.

• Missense Mutation: Changes one amino acid, potentially altering function.

• Nonsense Mutation: Creates a premature stop codon, leading to incomplete proteins.

• Frameshift Mutation: Insertion or deletion shifts the reading frame, disrupting the entire sequence.