CH12 Molecular Genetics

• Deoxyribonucleic acid (DNA)


  Biological macromolecule that contains genetic information to produce all the proteins an organism needs for survival and growth

  
• DNA


  • Double helix structure made of basic units called nucleotides
  • Each nucleotide contains a deoxyribose sugar, a phosphate group and one of four nitrogenous bases: Adenine (A), Thymine (T), Cytosine (C) and Guanine (G)
  • Genetic information is encoded in sequences formed by different arrangements of the four bases

  
• Relationships among DNA, genes and chromosomes

  1. A chromosome is made of a single molecule of deoxyribonucleic acid (DNA) highly coiled with packing proteins to form a condensed structure

  2. Each DNA molecule contains many genes

  3. A gene is a unit of inheritance made of a sequence of nucleotides as part of a DNA molecule and codes for a single polypeptide

  
• DNA


  • Double helix comprising two strands of nucleotides
  • Each strand of polynucleotide is made of monomers known as nucleotides
  • Each nucleotide molecule is made of three components: a phosphate group, a deoxyribose sugar, and one of four different nitrogenous bases

  
• Complementary base pairing


  Adenine always pairs with thymine while cytosine always pairs with guanine, in a ratio of 1:1
• Genetic code


  Carried by DNA and used to synthesise specific polypeptides

  
• Genetic code to synthesise polypeptides

  1. DNA undergoes transcription to produce a mRNA molecule in the nucleus

  2. The mRNA molecule diffuses out of the nucleus into the cytoplasm

  3. The mRNA undergoes translation by ribosomes to synthesise a specific polypeptide

  
• Genetic engineering


  Technique used to transfer genes from one organism to another
• Transgenic organism


  An organism, which possesses genes from at least two different species, produced using an artificial process such as genetic engineering

  
• Transgenic plants


  • Bt-corn: genetically engineered to produce its own insecticide using a gene from a bacterium
  • Golden rice: genetically engineered to produce beta-carotene in the endosperm of the grain using a gene from a bacterium
• Producing human insulin in bacteria

  1. Lyse human cells and cut out insulin gene from its chromosome using a restriction enzyme

  2. Lyse bacteria cells to obtain bacteria plasmid. Cut bacteria plasmid with the same restriction enzyme to obtain complementary sticky ends

  3. Mix the extracted insulin gene with the cut plasmid and add DNA ligase to join the insulin gene to the plasmid

  4. Mix the recombinant DNA with E.coli bacteria and apply temporary heat shock or electric shock for the bacteria to take in the recombinant DNA

  5. Transgenic bacteria are isolated and grown in fermenters for mass production of insulin

  6. Bacteria cells are lysed to obtain insulin which is extracted and purified for use
• Possible benefits and ethical considerations of genetic engineering

  • Costs and affordability
  • Local availability and ease of access by masses (including lower SES members / LDCs)
  • Inclusivity in term of SES / race / religion / dietary preference / nationality (does the method of production exclude any groups from benefitting?)
  • Risk of contamination of the gene pool of wild organisms
  • May result in genetic diseases in the transgenic organisms, which may affect physiological functioning of organism or result in co-production of toxic substances
  • Animal exploitation