Protein Synthesis and the Role of DNA and RNA

Importance of DNA and Proteins

• DNA as the Code: DNA carries the genetic information essential for the production of proteins.
• Similarity between DNA Replication and Protein Synthesis: Both processes involve the use of the DNA template but serve different purposes.
• Proteins as Macromolecules: Focus on internally synthesized proteins exclusive to bodily functions, distinct from dietary proteins.

Role of Genes

• Gene Function: Genes direct the production of proteins, specifying amino acid sequences through the genetic code.
• Common Functions of Proteins:
  • Production of antibodies.
  • Creation of hormones regulating physiological processes.
• Genetic Disorders: Result from anomalies in protein synthesis and function, underscoring the link between structure (form) and function.

One Gene - One Polypeptide Hypothesis

• Polypeptide Chains: Some proteins consist of multiple polypeptide chains (example: hemoglobin).
• Specific Gene for Each Chain: Each polypeptide requires a dedicated gene that codes for it.

Overview of Protein Synthesis

• Two Main Processes:
  • Transcription: DNA is transcribed into mRNA in the nucleus.
  • Translation: mRNA is translated into a polypeptide chain at ribosomes in the cytoplasm.

Importance of RNA

• Role of RNA: Critical for protein synthesis, RNA acts as a messenger carrying genetic instructions from DNA to the ribosomes.
• Key Differences Between RNA and DNA:
  • RNA contains ribose vs. DNA's deoxyribose.
  • RNA uses uracil (U) rather than thymine (T).
  • RNA is generally single-stranded while DNA is double-stranded.

Steps in Protein Synthesis

Transcription

1. Location: Occurs in the nucleus.
2. Process:
   • RNA polymerase binds to the promoter region of DNA (TATA box).
   • Transcription occurs as RNA nucleotides pair with the DNA template strand, forming mRNA from 5’ to 3’ direction.
   • mRNA undergoes editing (removal of introns, splicing of exons, addition of a 5’ cap and 3’ poly-A tail) before exiting the nucleus.

Translation

1. Location: Takes place in the cytoplasm at the ribosomes.
2. Process:
   • mRNA binds to ribosomes, where tRNA molecules carry specific amino acids.
   • tRNA reads mRNA codons in triplets (codons) to determine which amino acids to deliver.
   • Example: The mRNA codon AUG pairs with tRNA anticodon UAC, which carries methionine (the start codon).
   • Amino acids are linked via peptide bonds, forming a polypeptide chain until a stop codon is reached, which signals the completion of translation.

Genetic Code

• Codons: Groups of three bases that code for amino acids.
  • Example: AUG is the start codon; multiple codons can correspond to the same amino acid (redundancy).
• Codon Chart: A tool for translating mRNA sequences into their corresponding amino acids.

Key Terms

• Central Dogma: The flow of genetic information from DNA to RNA to protein.
• mRNA: Messenger RNA that carries the genetic instructions.
• tRNA: Transfer RNA that brings amino acids to ribosomes.
• rRNA: Ribosomal RNA that makes up ribosomes.

Final Notes

• Transcription Phases: Initiation, Elongation, Termination, and Post-transcriptional modifications.
• No proofreading in transcription: Errors can lead to faulty proteins, but multiple mRNA copies mitigate potential issues.
• Protein Folding: Essential for proper function and occurs after the polypeptide chain is synthesized.
• Understanding the Flow: Essential to grasp how proteins are synthesized from genetic information, a fundamental concept in biology and genetics.