gene expression 3

• Gene expression

  Involves the transcription and translation of DNA sequences
• Only a fraction of genes in the cell are expressed
• RNA

  Single stranded and composed of RNA nucleotides
• RNA nucleotides

  • Phosphate group
  • Ribose sugar
  • Nitrogenous bases (Adenine, Uracil, Guanine, Cytosine)
• Adenine and Uracil


  Complementary base pair in RNA (as RNA does not contain Thymine)
• Types of RNA


  • mRNA
  • tRNA
  • rRNA
• mRNA


  Carries a complementary copy of the DNA code from the nucleus to the ribosome
• Transcription


  1. RNA polymerase moves along DNA, unwinding the double helix and breaking the hydrogen bonds between bases

  2. RNA polymerase synthesises a primary transcript of mRNA from RNA nucleotides by complementary base pairing
• RNA splicing


  1. Introns (non-coding regions) are removed from the primary transcript

  2. Exons (coding regions) are spliced together to form a mature mRNA transcript
• The order of the exons remains unchanged during splicing
• Translation


  1. mRNA mature transcript leaves the nucleus to the ribosome

  2. Translation starts at a START codon and ends at a STOP codon

  3. Ribosome exposes one codon on the mRNA allowing tRNA to bring a specific amino acid

  4. Correct anticodon on tRNA complementary base pairs with codon on mRNA

  5. Amino acids are joined by peptide bonds to form polypeptide chain
• Codon


  Triplet of bases on mRNA that codes for 1 specific amino acid
• tRNA


  • Folds due to complementary base pairing
  • Has a triplet of bases exposed as an anticodon
  • Carries a specific amino acid to the ribosome
• rRNA


  Together with proteins, forms the ribosome
• Polypeptide chain


  Sequence of amino acids that will eventually become part of a functioning protein
• Alternative RNA splicing

  Different proteins can be expressed from one gene as a result of alternative RNA splicing
• Protein folding

  Polypeptide chains fold to form the three-dimensional shape of a protein, held together by hydrogen bonds and other interactions between individual amino acids
• Proteins have a large variety of shapes which determines their functions
• Phenotype

  Physical expression of a gene/characteristic, determined by the protein produced as a result of gene expression