Chemical Digestion and Gene Expression

Chemical Digestion and Gene Expression

• Chemical digestion uses proteins and enzymes to break down food particles into usable nutrients.
• Instructions for making proteins are located in DNA.
• DNA contains genes, which are strings of nucleotides.
• Genes contain regions that code for the production of RNA molecules.
• Gene expression refers to the process of using genes to synthesize proteins.
• Gene Expression occurs in two stages: transcription and translation.

Transcription

• Transcription occurs in the nucleus.
• Messenger RNA (mRNA) is created using DNA as a template.
• RNA polymerase is the enzyme that catalyzes transcription.
• Transcription consists of initiation, elongation, and termination.
• During initiation, RNA polymerase binds to the promoter region of a gene.
• Binding of RNA polymerase causes the DNA double helix to unwind and open.
• During elongation, RNA polymerase moves along the DNA template strand and links nucleotides to the 3' end of the growing RNA molecule.
• During termination, RNA polymerase reaches the terminator region of the gene, and the mRNA transcript is complete.
• RNA polymerase, DNA strand, and the mRNA transcript dissociate from each other.

Intron Splicing

• mRNA transcripts contain exons and introns.
• Exons code for proteins, while introns are non-coding.
• Introns must be removed to make a functional mRNA transcript.
• Intron splicing is performed by a spliceosome.
• The spliceosome is made up of proteins and RNA.
• The spliceosome removes introns and joins exons to form a mature mRNA transcript.
• The mature mRNA transcript leaves the nucleus through a nuclear pore and enters the cytoplasm for translation.

Translation

• Translation is the process of using mRNA to make polypeptides.
• Codons are three-letter codes that represent specific amino acids.
• The genetic code consists of 64 codons.
• Translation begins with the mRNA binding to the small ribosomal subunit upstream of the start codon.
• Each amino acid is delivered to the ribosome by a transfer RNA (tRNA) molecule.
• The type of amino acid delivered is determined by the anticodon sequence of the tRNA.
• There are three sites on the large ribosomal subunit: E, P, and A.
• Initiation begins with the tRNA molecule binding to the start codon.
• The large ribosomal subunit binds to the complex, completing initiation.
• During elongation, charged tRNA molecules bind to the A site and peptide bonds form between the amino acids in the P and A sites.
• The complex shifts one codon to the right.
• The uncharged tRNA exits from the E site, and the A site is open for the next tRNA molecule.
• Elongation continues until a stop codon is reached.
• A release factor binds to the A site at the stop codon.
• The polypeptide is released from the tRNA in the P site.
• The entire complex dissociates and can reassemble to begin translation again at initiation.

Protein Modification

• After translation, polypeptides may require modifications.
• Modifications can take place in different organelles depending on the protein.
• For example, digestive enzymes are translated in the endoplasmic reticulum and modified as they pass through the Golgi apparatus.
• They are then secreted using vesicles through the plasma membrane of the cell into the lumen of the digestive tract.

Protein Function

• Proteins are crucial for many physiological functions of the body.
• These functions include:
  • Breaking down food particles in digestion
  • Transcription
  • Translation