Chapter 8 : Gene Expression

• Gene


  A section of DNA containing genetic code to make polypeptides and functional RNA
• Genetic code


  Coded information in the form of a specific base sequence along the DNA
• Few amino acids are coded via a singular triplet where remaining are coded via 2-6 triplets, most amino acids are coded by more than one triplet so are called a degenerate code
• Stop codons


  Three triplets marking the end of the polypeptide chain
• Non-overlapping genetic code


  Read only once, e.g. 123 456
• DNA to Chromosome


  1. DNA is chromosomes held together via histones, highly coiled and folded looped coiled and folded into a chromosome

  2. DNA helix is wound around histones to fit into position

  3. DNA-Histone complex is coiled looped and coiled before packed into a chromosome therefore a lot of DNA is condensed into one chromosome
• Gene loci


  Single DNA molecule has a lot of genes where many genes occupy specific locations on the DNA molecule
• Homologous pair


  The 2 chromosomes that carry the same genes but different alleles
• Allele


  An alternative form of a gene containing coded information in the form of a sequence of bases
• Changing base sequence in a gene

  Produces a new allele of that gene = a mutation occurs
• Mutation in a gene


  Leading to new amino acids being coded for leading to production of new polypeptide
• But e.g. protein as an enzyme, different shape so cannot bind to a substrate: inadequate for enzyme activity
• Codon


  A sequence of 3 bases on mRNA that codes for single amino acids
• Genome


  Complete set of genes in a cell/organism
• Proteome


  Full range of proteins produced by genome
• RNA


  A polymer made up of repeating units of mono nucleotide subunits forming a single strand which each nucleotide is made up of pentose sugar ribose, organic base, and phosphate group
• mRNA


  Long stranded single helix, base sequence based on DNA length and sequence of bases. Produced during the process of transcription.
• tRNA


  Clover shaped, made up of 80 nucleotides one end extending beyond the other where the Amino Acids attaches too, each tRNA a specific amino acids binds to. Opposite end of tRNA are 3 other organic bases = anticodons.
• Protein synthesis


  1. Transcription - One complementary mRNA is copied from DNA

  2. Translation - mRNA joins with ribosomes and tRNA with the specific AA coding for complementary codons
• Transcription


  A complementary mRNA copy from DNA made in the nucleus. mRNA is shorter than DNA so can carry genetic code to the ribosomes in cytoplasm. DNA cannot carry the genetic code to ribosomes as the cytoplasm contains enzymes which may damage DNA.
• Transcription


  1. DNA helix is unwound and exposes bases to act as a template

  2. Only one chain acts like a template

  3. DNA helicase unzips and unwinds the double helix

  4. Free mRNA nucleotides align opposite exposed complementary DNA bases (A-U) (U-A)(G-C)(C-G)

  5. RNA polymerase bonds the RNA ADJACENT nucleotides forming a new RNA polymer chain = ONE GENE CREATED
• Splicing


  1. Pre mRNA must be modifiers to mRNA before leaving the nucleus ready for translation

  2. DNA molecules made up of 98% introns which DO NOT code for amino acids so much be removed before translation, introns spliced via a protein called spliceosome - attach to DNA and break the phosphodiester bonds which cut introns
• Translation


  1. Modified mRNA leaves nucleus and attaches to ribosomes in cytoplasm

  2. Ribosome attaches to start codon

  3. tRNA molecule with complementary anticodon to start codon oppositely align to mRNA, held together by ribosomes

  4. Ribosomes moves along mRNA enabling another complementary tRNA to attach to the next codon

  5. Two amino acids delivered by tRNA molecule are joined by a peptide bond and catalysed by an enzyme requiring ATP

  6. Continues until ribosome reaches stop codon as stop codon doesn't code for amino acids then the ribosome detaches

  7. Polypeptide chain created and enter the golgi for folding
• Describe how one amino acid is added to a poly peptide that is being formed at a ribosome during translation.

  1. tRNA brings specific amino acid to ribosome
  2. Anticodon on tRNA binds to codon on mRNA
  3. Amino acids join form a peptide bond (using ATP)
• Describe how mRNA is produced from an exposed template strand of DNA. (DONT INCLUDE DNA HELICASE OR SPLICING)

  1. Free mRNA nucleotides form complementary base pairs
  2. Phosphodiester bonds form
  3. By action of RNA polymerase