Inheritance

• Semi-conservative DNA replication


  1. DNA helicase unwinds the double helix

  2. Complementary nucleotides attach to the template strand

  3. DNA polymerase catalyses the formation of phosphodiester bonds

  4. Two DNA molecules are formed
• DNA polymerase


  • Accurately copies the template strand to avoid placing the wrong DNA nucleotide in the incorrect position
  • Proofreads the complementary strand as it moves along the DNA
• DNA polymerase detects a mismatch


  It can identify the wrong nucleotide and replace it with the right one
• A mistake in DNA replication results in a change to the DNA base sequence, which is known as a mutation
• DNA mutations


  Can have detrimental effects to the organism, since an altered base sequence can change the sequence of amino acids in a protein, causing it to fold differently and possibly lose its function
• Conservative DNA replication


  The original DNA strands would remain intact and the newly synthesised DNA would consist of two freshly-made strands
• Semi-conservative DNA replication


  The new DNA molecule is made up of one strand of the original DNA whereas the other strand is made of freshly made DNA
• Meselson and Stahl's Experiment


  1. Grew bacteria in the presence of the heavy nitrogen isotope N-15

  2. Extracted and centrifuged the DNA to separate it by weight

  3. Took the N-15 bacteria and grew them in the light isotope N-14, then extracted and centrifuged the DNA again

  4. After the first round, saw one band of intermediate weight DNA

  5. After the second round, saw two bands of DNA, one intermediate and one light
• The evidence that DNA replication is semi-conservative comes from Meselson and Stahl's experiment
• If DNA replication was conservative, Meselson and Stahl would have seen two bands of DNA (one heavy and one light) after both the first and second rounds of replication
• DNA replication:

  
• Mutation


  A change to the base sequence of DNA occurring when the cell makes errors during DNA replication
• Mutation in DNA


  Can result in an altered primary structure of a protein
• Altered primary structure of a protein

  Determines later stages of folding, so can result in a change to the overall 3D structure (tertiary structure) of a protein
• Change to tertiary structure of a protein

  Could result in the protein losing its function
• Types of mutation


  • Substitution
  • Insertion
  • Deletion
  • Inversion
• Substitution


  One base is replaced for another
• Insertion


  One or more bases is added
• Deletion


  One or more bases is removed
• Inversion


  A sequence of bases is reversed
• Some mutations can have a neutral effect on a protein's function
• Reasons for neutral effect of mutations


  • The mutation changes a base in a triplet but the amino acid that the triplet codes for doesn't change
  • The mutation produces a triplet that codes for a different amino acid but the amino acid is chemically similar to the original so it functions like the original amino acid
  • The mutated triplet codes for an amino acid not involved with the protein's function
• Some mutations will have an effect and it could be beneficial or harmful to the organism
• Beneficial mutation


  • Antibiotic resistance in bacteria
• Harmful mutations


  • Mutations which cause cystic fibrosis or cancer
• Monohybrid inheritance


  Inheritance of characteristics controlled by a single gene
• Predicting offspring characteristics using Punnett squares


  1. Write parent gametes on the side

  2. Genotypes of possible offspring in the centre
• Dominant P allele


  Produces purple flowers
• Recessive p allele


  Produces white flowers
• Breeding two heterozygous individuals

  Produces offspring with a 3:1 ratio of purple flowers to white flowers
• The 3:1 ratio between the dominant phenotype and the recessive phenotype will always be seen in the offspring of two heterozygotes
• Genetic diagram:

  
• Cystic Fibrosis


  A genetic disorder that results from inheriting two recessive alleles
• CFTR protein


  Plays a role in transporting chloride ions out of the epithelial cells lining the trachea
• In healthy people, CFTR plays an important role in making mucus thin and watery, since the transport of chloride ions decreases the water potential of the mucus lining the trachea, stimulating the movement of water by osmosis
• Mutations in the CFTR protein mean that water doesn't move into the trachea, making the mucus thick and sticky
• The thick mucus can clog up tubes in our respiratory, digestive and reproductive systems
• Cystic Fibrosis:

  
• Respiratory system in people with cystic fibrosis


  • Cilia cannot move thick, sticky mucus
  • Mucus builds up and blocks airways
  • Reduced surface area for gas exchange
  • Harder to breathe
  • Bacteria inside mucus can reproduce, leading to lung infections
• Digestive system in people with cystic fibrosis


  • Mucus can build up and cause blockages in tube connecting pancreas to small intestine
  • Prevents digestive enzymes being secreted into small intestine
  • Makes food digestion less efficient
  • Thick, abnormal mucus can stimulate growth of cysts in pancreas, inhibiting production of digestive enzymes