Protein Synthesis

DNA and Protein Synthesis Overview

  • DNA is long and complex, coding for numerous proteins.

  • Protein synthesis involves two main steps: transcription and translation.

Transcription

  • Definition: The process of copying a specific segment of DNA to create messenger RNA (mRNA).

  • Location: Takes place in the nucleus of the cell.

  • Process:

    • DNA strands unzip to allow mRNA synthesis using a template strand of DNA.

    • RNA polymerase enzyme facilitates this process by:

      • Unzipping DNA.

      • Adding RNA nucleotides as per DNA sequence.

        • C pairs with G, A pairs with U (uracil replaces thymine in RNA).

    • Only the portion of DNA relevant for the protein is copied.

      • Approximately 2% of DNA codes for proteins; coding sections have start and stop signals.

  • After formation, mRNA exits the nucleus through nuclear pores to enter the cytoplasm.

The Role of mRNA

  • Acts as a messenger to transmit protein synthesis instructions from the nucleus to ribosomes.

  • Types of RNA: There are three main types of RNA involved in protein synthesis: mRNA, tRNA, and rRNA.

Translation

  • Definition: The process of synthesizing proteins from the mRNA sequence.

  • Location: Occurs in the cytoplasm at ribosomes, which can be on the rough endoplasmic reticulum (ER) or free-floating.

  • Mechanism:

    • mRNA is read in sets of three bases, known as codons.

    • Ribosomes, composed of proteins and rRNA, facilitate this process by:

      • Reading the mRNA sequence.

      • Using transfer RNA (tRNA) to bring the correct amino acids.

  • tRNA:

    • Contains an anti-codon that pairs with corresponding mRNA codon.

    • Carries a specific amino acid for protein synthesis.

Steps in Translation

  1. mRNA binds to the ribosome; the start codon (AUG) is recognized.

  2. The first tRNA, with the anti-codon corresponding to AUG, brings in methionine (the first amino acid).

  3. Additional tRNA molecules bring their respective amino acids, forming peptide bonds between them.

  4. The mRNA shifts, allowing a new tRNA to enter, continuing until a stop codon is reached.

  5. The process ends upon reading stop codon (e.g., UAA), releasing the completed protein.

Example of Translating mRNA

  • Codon Sequence: "AUG CCC GGC UAA"

  1. AUG = Methionine (start)

  2. CCC = Proline

  3. GGC = Glycine

  4. UAA = STOP (termination of protein synthesis)

Conclusion

  • The entire process of transcription and translation leads to the formation of functional proteins that carry out essential roles in the cell.

  • Understanding the sequence of mRNA allows us to determine the sequence of amino acids in the resulting protein.