Gene expression

• Types of RNA


  • mRNA
  • tRNA
  • rRNA
• mRNA


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


  • Composed of nucleotides
  • Single stranded
  • Found in nucleus and cytoplasm
• tRNA


  • Single stranded but folds due to complementary base pairing
  • Carries a specific amino acid to the ribosome
  • Has an exposed triplet of bases called an anti-codon at one end, and an attachment site for a specific amino acid at the other end
• Transcription


  1. Part of DNA strand unwinds and unzips

  2. mRNA synthesised along one DNA strand

  3. mRNA peels off and moves out of nucleus to cytoplasm
• Genes have non-coding regions called introns and coding regions which are called exons
• RNA splicing


  1. Introns are removed

  2. Exons are joined together to form the mature transcript of mRNA
• Different proteins can be expressed from one gene as a result of alternative RNA splicing
• Translation


  1. Mature mRNA molecule moves through nuclear pore to cytoplasm

  2. Sequence of 3 mRNA bases (codon) codes for one amino acid

  3. tRNA picks up specific amino acid and brings it to mRNA strand where anti-codon attaches to complementary codon

  4. Ribosome moves along mRNA and second tRNA brings another amino acid

  5. Peptide bonds form between adjacent amino acids to form polypeptide

  6. tRNA leaves ribosome as polypeptide is formed
• Start codon


  AUG - instructs the start point for translation of the mRNA molecule
• Stop codons


  UAA, UAG, UGA - do not code for amino acids but instead act to end the translation process
• Proteins are organic compounds containing carbon, hydrogen, oxygen and nitrogen
• Amino acids


  Linked together by peptide bonds to form long chains called polypeptides
• The number and sequence of amino acids helps determine a protein's structure and function
• Protein structure


  • Polypeptide chains fold to form 3D shape
  • Held in place by hydrogen bonds and other interactions between individual amino acids
• Protein functions


  • Hormones
  • Enzymes
  • Antibodies
  • Receptors
• Phenotype is determined by proteins produced as a result of gene expression but it can also be influenced by external factors