1.3

• Gene expression


  Due to cell differentiation some genes are switched off, only a fraction of the genes in a cell are expressed, the genes that are expressed are the ones characteristic of the particular cell type
• Gene expression

  1. Transcription

  2. Translation
• Types of RNA

  • mRNA
  • tRNA
  • rRNA
• mRNA

  Carries a copy of the DNA code from the nucleus to the ribosome, transcribed from DNA in the nucleus and translated into proteins by ribosomes in the cytoplasm, each triplet of bases on the mRNA molecule is called a codon and codes for a specific amino acid
• tRNA

  Folds due to complementary base pairing, each tRNA molecule carries its specific amino acid to the ribosome, has an anticodon (an exposed triplet of bases) at one end and an attachment site for a specific amino acid at the other end
• rRNA

  Forms the ribosome with proteins
• Transcription of DNA into primary mRNA transcripts

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

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

  3. Uracil in RNA is complimentary to Adenine

  4. Only one gene is transcribed/forms mRNA
• Introns

  Non-coding regions of genes
• Exons

  Coding regions of genes
• RNA splicing

  1. Introns of the primary transcript of mRNA are removed

  2. Exons are joined together to form mature RNA transcript

  3. The order of exons is unchanged during splicing

  4. The primary transcript is much shorter then the chromosomal DNA

  5. Mature mRNA transcripts are shorter than primary mRNA transcripts because Introns/non-coding regions of the gene are removed in RNA splicing

  6. The mature transcript only contains exons/coding regions of the gene

  7. Non-coding regions are removed from primary transcript before translation so no extra amino acids are placed in the polypeptide formed so the correct functional protein is formed
• The mature mRNA transcript leaves the nucleus and moves to the ribosome to be translated
• Translation of mRNA into a polypeptide

  1. tRNA is involved

  2. Translation begins at a start codon and ends at a stop codon

  3. Anticodons bond to codons by complementary base pairing, translating the genetic code into a sequence of amino acids

  4. Peptide bonds join the amino acids together

  5. Each tRNA then leaves the ribosome as the polypeptide is formed
• Role of mRNA and tRNA in protein synthesis

  • mRNA carries a copy of the DNA code from the nucleus to the ribosome
  • tRNA carries a specific amino acid to the ribosome
• Base sequencing

  • Given the sequence of bases on one type of molecule the base pairing allows the sequence of the others to be worked out
• Alternative RNA splicing


  Different proteins can be expressed from one gene, different mature mRNA transcripts are produced from the same primary transcript depending on which exons are retained
• Amino acids are linked by peptide bonds to form polypeptides
• Polypeptide chains


  Fold to form the three-dimensional shape of a protein, held together by hydrogen bonds and other interactions between individual amino acids
• Protein shape


  Determines their functions
• Phenotype


  Determined by proteins produced as the result of gene expression
• Environmental factors

  Also influence phenotype