4. Genes and Gene expression

• Genetic code


  Universal triplet code that is degenerate
• Gene expression (protein synthesis)

  1. Transcription

  2. RNA processing in eukaryotic cells

  3. Translation by ribosomes
• Structure of genes


  • Exons
  • Introns
  • Promoter regions
  • Operator regions
• Regulatory genes


  • Determine if another gene is switched on (expressed) or off (not expressed)
  • Produce proteins (e.g. transcription factors) that control if the structural gene is switched on or off
  • Usually upstream (before) the gene they are controlling/regulating
• Structural genes


  Code for proteins that have a structural or functional purpose throughout a cell or organism (e.g. enzymes and antibodies)
• Flanking regions


  • The area either side of the gene
  • Upstream flanking region is before the start of the coding region
  • Downstream flanking region is after the ending of the coding region
• Promoter region


  • Controls the expression of a gene
  • Includes the TATA box which serves to define the direction of transcription
  • Where transcription factors and the enzyme RNA polymerase can bind
• Transcription factors


  • Proteins that help turn specific genes "on" or "off" by binding to nearby DNA called a promoter region
  • Activators turn on a gene and activate transcription
  • Repressors decrease/prevent transcription and thus turn off a gene
• Introns


  • Sequences of DNA that do not code for proteins or amino acids
  • Spliced out during RNA processing
• Exons


  Expressed sequences of DNA which code for amino acids, which are joined together
• Triplet


  A group of three adjacent DNA nucleotides
• Codon


  Three nucleotides that code for a specific amino acid in the final polypeptide chain
• Start codon


  AUG, codes for the amino acid methionine and signals the start of transcription
• Stop codons


  UAA, UAG, UGA, signal the termination of translation (don't code for an amino acid)
• The genetic code is almost universal, the same codons specifying the amino acids in eukaryotic cells are the same as those used by prokaryotic cells
• Degenerate code


  Most of the genetic code, where some amino acids are specified by more than one codon
• Transcription


  1. RNA polymerase attaches to the promoter region

  2. DNA unwinds, exposing the bases of the template strand

  3. RNA nucleotides complementary pair with the bases on the DNA template strand

  4. RNA nucleotides join to form pre-mRNA
• RNA processing (modification of pre-mRNA)

  1. Add a methyl cap to the 5' end

  2. Add a poly-A tail to the 3' end

  3. Splice/cut out introns by spliceosome

  4. Final product is mRNA
• Alternative splicing


  One gene can be regulated in different ways to produce more than one protein
• Translation


  1. Initiation

  2. Elongation

  3. Termination
• tRNA molecules


  • Made of RNA with a specific amino acid binding site at one end and a 3-nucleotide anticodon sequence at the other
  • Loading the tRNA with an amino acid requires enzymes and energy
• tRNA anticodon


  Complementary to mRNA codon
• Translation: Initiation


  1. Ribosome binds to the mRNA

  2. Ribosome reads the mRNA molecule

  3. tRNA with the anticodon to the start codon (AUG) will bind to the mRNA and the amino acid methionine will be delivered to the ribosome
• Translation: Elongation


  1. mRNA moves through the ribosome to the next codon

  2. Next amino acid is delivered by the specific tRNA

  3. Amino acid binds to methionine by a peptide bond

  4. Continues for each codon in the mRNA resulting in a long chain of amino acids
• Translation: Termination


  Once the ribosome reaches the stop codon on the mRNA, translation ends and the polypeptide (chain of amino acids) is released
• Protein processing


  • Amino acid sequence emerges from the ribosome as a primary structure protein and the folding process begins
  • Proteins called molecular chaperones help a newly-synthesized protein to begin folding into its final functional 3D shape
  • Most of this is done in the endoplasmic reticulum rough and golgi body
• The code is universal, the same DNA triplet in Prokaryotes and Eukaryotes will code for the same amino acid