Inheritance and variation

• Meiosis


  The process by which gametes (sperm and egg cells) are produced, with half the normal number of chromosomes
• Normal human cells


  • Contain 23 pairs of chromosomes
• Gametes (sperm and egg cells)

  • Contain 23 single chromosomes
• Meiosis


  1. Chromosomes are copied

  2. Cell divides into two

  3. Both cells divide one more time forming gametes
• Meiosis produces four gametes from one original cell, and each gamete is genetically different from the others
• Fertilization


  The joining of a male gamete (sperm) and a female gamete (egg) to form a new cell with the full number of chromosomes
• After fertilization, the new cell has 23 chromosome pairs
• After fertilization


  1. The new cell divides by mitosis, producing a clump of identical cells (an embryo)

  2. As the embryo develops, the cells differentiate, forming different cell types
• DNA


  Double-stranded polymer of molecules called nucleotides
• Nucleotides


  • A
  • T
  • G
  • C
• A always pairs with T on the opposite DNA strand, and C always pairs with G
• Proteins


  Polymers of amino acids
• Amino acids


  • 20 different amino acids in humans
• Order of amino acids in a protein

  Determines the shape of the protein
• Shape of a protein

  Determines its function
• Gene


  Sequence of bases that determines the order of amino acids in a protein
• Protein synthesis


  1. Transcription (in nucleus)

  2. Translation (in cytoplasm)
• Transcription


  DNA sequence is copied into a complementary template molecule called mRNA
• Translation


  mRNA attaches to a ribosome, amino acids are brought to the ribosome on tRNA molecules, ribosome reads mRNA triplets and joins amino acids in the correct order
• mRNA


  Single-stranded molecule that carries the DNA sequence information to the cytoplasm
• tRNA


  Carrier molecules that bring amino acids to the ribosome
• The protein chain folds into its unique shape after being synthesized
• Mutation


  A change to a base in the DNA sequence
• Mutations happen all the time
• A single base change in the DNA sequence

  Does not change the amino acid sequence of the protein
• A different mutation in the DNA sequence


  Changes the amino acid sequence and alters the shape of the protein
• Mutations that change the shape of a protein


  • Can have a dramatic effect on the function of the protein, e.g. an enzyme's active site may change shape so it can no longer bind to the substrate
  • A structural protein like collagen may lose its strength
• Non-coding regions of DNA


  Regions that switch genes on and off, telling genes when to produce proteins
• Mutations in non-coding regions


  Can affect how genes are switched on or off, e.g. a gene may be turned on when it should be turned off, leading to uncontrolled mitosis and cancer