Genetics and Cellular Function: Protein & Gene Regulation

Protein Synthesis

• Background:
  • Activated gene leads to messenger RNA (mRNA) formation.
  • mRNA, a gene's mirror image, migrates to cytoplasm.
  • Ribosomes read mRNA code.
  • Ribosomes consist of ribosomal RNA (rRNA) and enzymes.
  • Transfer RNA (tRNA) carries amino acids to ribosome.
  • Ribosomes assemble amino acids per mRNA codons.

Protein Synthesis Process

• DNA\rightarrowmRNA\rightarrowprotein.
• Transcription: DNA to mRNA (occurs in nucleus).
• Translation: mRNA to protein (mostly in cytoplasm).
• 10-15% of proteins synthesized in the nucleus.

Genetic Code and Protein Synthesis

• Enzymes assemble nucleotides into mRNA.
• Complementary base pairing: A-U, G-C.
• Codon: Three RNA nucleotides coding for a specific amino acid.
• mRNA exits nucleus through nuclear pores.
• mRNA codons bind to tRNA anticodons at ribosome.
• tRNA carries specific amino acid.

Transcription

• DNA is too large to leave the nucleus.
• mRNA is a small copy that can migrate.
• Transcription: Copying DNA instructions to RNA.
• RNA polymerase binds to DNA and assembles mRNA.
• TATATA or TATAAA sequences indicate where to begin.
• RNA polymerase opens DNA helix.
• Reads base from one DNA strand and makes mRNA (C on DNA becomes G on mRNA; A on DNA becomes U on mRNA).

Transcription Details

• RNA polymerase rewinds DNA helix.
• Multiple polymerase molecules can transcribe a gene at once.
• Terminator: Base sequence signaling polymerase to stop.
• Pre-mRNA: Immature RNA.
  • Exons: "Sense" portions that will be translated.
  • Introns: "Nonsense" portions that must be removed.
• Alternative splicing: Intron removal and exon splicing for functional RNA.
• One gene can code for multiple proteins.

Translation

• Translation: Converts nucleotides to amino acids.
• Ribosomes translate nucleotide sequence into amino acid sequence.
• Occurs in cytosol, rough ER, and nuclear envelope.
• Ribosomes have large and small subunits made of rRNA and enzymes.
• Three phases: Initiation, Elongation, Termination.

Translation - Initiation

• mRNA has a leader sequence for small ribosomal subunit binding.
• Large subunit attaches to small subunit.
• Ribosome pulls mRNA, reading bases.
• Protein synthesis starts at start codon (AUG); all proteins begin with methionine.

Transfer RNA (tRNA)

• tRNA is a small RNA molecule with an L shape.
• Anticodon: Three nucleotides on one end of L.
• Other end binds to a specific amino acid.
• tRNA picks up amino acids from cytosol using ATP for peptide bond formation.

Translation Process

• Some codon-anticodon pairing imprecision exists.
• 48 tRNAs pair with 61 codons.
• Ribosome binds and holds tRNA with specific amino acid.
• Large subunit forms peptide bond linking amino acids.
• First tRNA is released, second tRNA anchors peptide chain.
• Ribosome shifts, third tRNA brings its amino acid.

Protein Processing and Secretion

• Protein synthesis isn't complete after amino acid sequence assembly.
• Proteins must coil/fold into secondary and tertiary structures.
• Chaperone proteins guide new proteins in folding and prevent improper associations.
• Also called stress or heat-shock proteins; produced under heat or stress to refold damaged proteins.

Protein Processing and Secretion Details

• Proteins for cytosol made on free ribosomes.
• Proteins for lysosomes or secretion assembled on rough ER, sent to Golgi for packaging.
• Polyribosome migrates to rough ER and docks.
• Amino acid chain completed on rough ER, sent to Golgi for modification.
• Proteins thread through ER membrane into cisterna.
• ER modifies protein: removing segments, folding, stabilizing with disulfide bridges, adding carbohydrates.
• Rough ER pinches off transport vesicle with clathrin.
• Clathrin selects proteins and molds vesicle.
• Vesicles carry protein to Golgi complex.

Protein Processing in Golgi

• Vesicles fuse, unloading proteins into Golgi cisterna.
• Golgi modifies protein further, moving from cisterna closest to ER to farthest.
• New Golgi vesicles bud off with finished protein.
• Some vesicles become lysosomes; others become secretory vesicles that release product by exocytosis.

Gene Regulation

• Genes are turned on/off based on need.
• Many genes permanently off in a given cell.
• Example: Hemoglobin or digestive enzyme genes.

Gene Regulation Mechanisms

• Hormones can trigger gene activation.
• Example: Prolactin stimulates mammary cells to produce breast milk.
• Prolactin binds to receptors, activating regulatory protein (transcription activator).
• Regulatory protein moves to nucleus and binds to DNA near casein gene.
• RNA polymerase binds, transcribing casein mRNA.
• mRNA moves to cytoplasm, translated by ribosomes on rough ER.
• Golgi packages casein into secretory vesicles.
• Secretory vesicles release casein by exocytosis.

Synthesizing Compounds Other Than Proteins

• Cells synthesize glycogen, fat, steroids, phospholipids, pigments, etc.
• No direct genes for these; synthesis under indirect genetic control.
• Produced by enzymatic reactions; enzymes are proteins encoded by genes.
• Example: Testosterone production.
  • Testes cell takes in cholesterol, converts it to testosterone only when enzyme genes are active.
• Genes affect complex outcomes like behavior, aggression, sex drive.