Notes on Protein Synthesis

1. Introduction to Cells and Nucleus

• Cells are the basic unit of all living tissue.
• The nucleus contains the genome, which is organized into 23 pairs of chromosomes in humans.
• Each chromosome consists of a long strand of DNA tightly packaged around proteins known as histones.
• Genes within DNA contain instructions for making proteins.

2. Transcription Process

• When a gene is activated, RNA polymerase attaches to the gene's start.
• It moves along DNA to create messenger RNA (mRNA) from free bases in the nucleus.
• Transcription: DNA code determines the order of bases in mRNA.
• Processing of mRNA: Involves removing non-coding sections called introns and adding regions called exons.
• Process is facilitated by a complex known as spliceosome.
• The mature mRNA exits the nucleus into the cytoplasm.

3. Translation Process

3.1 Overview of Translation

• Ribosomes, located in the cytoplasm, read mRNA to synthesize proteins.
• Transfer RNA (tRNA) carries amino acids to ribosomes.
• Amino acids: 20 types; linked together to form proteins.

3.2 Stages of Translation

3.2.1 Initiation

• The small subunit of the ribosome binds to the mRNA.
• Ribosome scans mRNA in the 5' to 3' direction to find the AUG codon.
• Met-tRNA pairs with the AUG codon.
• The large ribosomal subunit combines with the small subunit to form the initiation complex.

3.2.2 Elongation

• Another tRNA with an amino acid enters the A site of the ribosome.
• A peptide bond forms between the new amino acid and the one in the P site.
• The ribosome moves one codon forward, shifting tRNA molecules.
• This process continues until a stop codon (UAG) is reached.

3.2.3 Termination

• At the stop codon, elongation ceases.
• The newly formed polypeptide is released from the ribosome.
• The ribosomal subunits dissociate, and tRNA detaches from mRNA.

4. Importance of Protein Synthesis

• Proteins are essential for various physiological functions, including digestion and metabolism.
• The process of protein synthesis involves transcription in the nucleus and translation in the cytoplasm.

5. Genetic Code and Translation Mechanics

• The genetic code consists of 64 codons, including:
  • Start codon: AUG
  • Stop codons: UAA, UAG, UGA
• Codons are read in groups of three nitrogenous bases.
• A, P, and E sites in ribosomes facilitate tRNA binding and amino acid addition.

6. Summary of Protein Synthesis Stages

1. Initiation: Binding of Met-tRNA, mRNA, and small ribosomal subunit.
2. Elongation: Addition of amino acids, forming a growing chain.
3. Termination: Release of the completed polypeptide when a stop codon is reached.

7. Role of Nutrients in Protein Synthesis

• Dietary nutrients are essential for the body to produce proteins needed for survival and function.
• The breakdown of food into usable nutrients occurs through chemical digestion using proteins and enzymes.