Protein Synthesis

Protein Synthesis Overview

  • DNA’s Function: Acts as the blueprint for building proteins. It determines which proteins each cell produces.

  • Cell Specialization:

    • All cells contain the same DNA but use different parts based on their function.

    • Example:

      • Stomach cells secrete HCl and mucus.

      • Muscle cells contract and relax.

  • Protein Building:

    • Happens at ribosomes (in the cytoplasm or on rough ER).

    • DNA remains in the nucleus, so mRNA carries instructions to the ribosomes.

Types of RNA

  1. mRNA (Messenger RNA):

    • A single-stranded molecule.

    • Copies instructions (blueprints) from DNA.

    • Carries the instructions to specific ribosomes in the cytoplasm.

    • Specifies the starting point for protein synthesis with a start codon (AUG).

  2. tRNA (Transfer RNA):

    • Cloverleaf-shaped molecule.

    • Carries specific amino acids to ribosomes.

    • Matches amino acids to mRNA codons using its anticodon (complementary to mRNA codon).

  3. rRNA (Ribosomal RNA):

    • Found in ribosomes.

    • Combines with proteins to form ribosome subunits.

    • Facilitates the bonding of amino acids into a growing polypeptide chain.

Genetic Code

  • The genetic code is read in triplets (3 nucleotide bases) called codons:

    • Each codon codes for one amino acid.

    • Example: AUG codes for methionine (start codon).

  • Anticodons: Found on tRNA, these are complementary to mRNA codons.

  • The code is degenerate: Multiple codons can specify the same amino acid.

  • Total:

    • 20 amino acids.

    • 64 codons (including stop codons: UAA, UAG, UGA).

Protein Synthesis Steps

1. Transcription (DNA → mRNA)

  • Location: Nucleus.

  • Process:

    1. Unzipping: DNA unwinds and separates (enzyme: helicase).

    2. Base Pairing: RNA bases pair with the template strand of DNA.

      • A pairs with U (uracil replaces thymine).

    3. mRNA Formation:

      • The enzyme RNA polymerase bonds the RNA nucleotides.

      • mRNA detaches and exits the nucleus via nuclear pores.

    4. Only one side of DNA (the template strand) is copied.

    • The other side, the sense strand, matches mRNA (except U replaces T).

2. Translation (mRNA → Protein)

  • Location: Cytoplasm (at ribosomes).

  • Involves all 3 types of RNA:

    • mRNA: Carries instructions.

    • tRNA: Brings amino acids.

    • rRNA: Forms ribosomes that build the protein.

  • Stages:

    1. Initiation:

      • Ribosome binds to mRNA at the start codon (AUG).

      • tRNA brings the amino acid methionine to the ribosome.

    2. Elongation:

      • tRNA anticodons pair with mRNA codons.

      • Amino acids link via peptide bonds, forming a chain.

      • Ribosome shifts to the next codon, repeating the process.

    3. Termination:

      • A stop codon (UAA, UAG, UGA) signals the end.

      • The polypeptide chain (protein) is released.

      • Ribosome disassembles.