inheritance

• DNA Replication


  1. Before a (parent) cell divides, it needs to copy the DNA contained within it

  2. Doubling the DNA ensures that the two new (daughter) cells produced will both receive full copies of the parental DNA

  3. The DNA is copied via a process known as semi-conservative replication (semi = half)

  4. The new DNA molecule has conserved half of the original DNA and then used this to create a new strand
• Semi-conservative replication


  It ensures there is genetic continuity between generations of cells
• DNA replication


  1. Occurs in preparation for mitosis

  2. Occurs during the S phase of the cell cycle

  3. Helicase unwinds the DNA double helix

  4. Each single polynucleotide DNA strand acts as a template for the formation of a new strand

  5. DNA polymerase synthesises new DNA strands from the two template strands
• Nucleoside triphosphates


  Free nucleotides which contain three phosphate groups
• DNA polymerase


  Catalyses condensation reactions between the deoxyribose sugar and phosphate groups of adjacent nucleotides within the new strands, creating the sugar-phosphate backbone of the new DNA strands
• Leading & lagging strands


  1. DNA polymerase can only build the new strand in one direction (5' to 3' direction)

  2. The template strand that the DNA polymerase attaches to is known as the leading strand

  3. The other template strand is known as the lagging strand

  4. DNA polymerase moves away from the replication fork on the lagging strand, so it synthesises the lagging DNA strand in short segments (Okazaki fragments)

  5. DNA ligase joins the lagging strand segments together
• Meselson and Stahl's experiment showed that bacterial DNA replicated via semi-conservative DNA replication
• Mutation


  A change in the sequence of base pairs in a DNA molecule that may result in an altered polypeptide
• Types of mutations


  • Insertion
  • Deletion
  • Substitution
  • Duplication
  • Inversion
• Insertion mutation


  A mutation that occurs when a nucleotide is randomly inserted into the DNA sequence
• Deletion mutation


  A mutation that occurs when a nucleotide is randomly deleted from the DNA sequence
• Types of substitution mutations


  • Silent mutations
  • Missense mutations
  • Nonsense mutations
• Duplication


  A type of mutation that involves the production of one or more copies of a gene or a region of a chromosome
• Inversion


  A mutation where the DNA of a single gene is cut in two places, the cut portion is inverted 180° then rejoined to the same place within the gene
• Duplication mutation


  Entire genes can be duplicated
• Inversion mutation


  DNA of a single gene is cut in two places, the cut portion is inverted 180° then rejoined to the same place within the gene
• Inversion mutations usually occur during crossing-over in meiosis
• Inversion mutations frequently result in a non-functional protein
• In some cases an entirely different protein is produced
• The mutation is often harmful because the original gene can no longer be expressed from that chromosome
• If the other chromosome in the pair carries a working gene the effect of the mutation may be lessened
• Inversion mutations result in the reversal of sections of DNA
• Gene


  Codes for a single polypeptide
• Genetic disorder


  Caused by a mutation in a gene that results in a differently-functioning or non-functioning protein that alters the phenotype of the individual
• Cystic fibrosis


  Genetic disorder of cell membranes caused by a recessive allele of the CFTR gene located on chromosome 7
• CFTR gene


  Codes for the production of chloride ion channels required for secretion of sweat, mucus and digestive juices
• A mutation in the CFTR gene leads to production of non-functional chloride channels
• This reduces the movement of water by osmosis into the secretions
• The result is that the body produces large amounts of thick, sticky mucus in the air passages, the digestive tract and the reproductive system
• There are many mutations in the CFTR gene that can lead to cystic fibrosis
• All of the mutations that cause cystic fibrosis are recessive
• Heterozygous


  Won't be affected by the disorder but are carriers
• Homozygous recessive


  Must have the disorder
• If only one of the parents is a carrier with the other parent being homozygous dominant, there is no chance of producing a child with cystic fibrosis, as the recessive allele will always be masked by the dominant allele
• Mucus in the respiratory system is a necessary part of keeping the lungs healthy
• In people with cystic fibrosis, due to the faulty chloride ion channels, the cilia are unable to move as the mucus is so thick and sticky
• This means microorganisms are not efficiently removed from the lungs and lung infections occur more frequently
• Mucus builds up in the lungs and can block airways which limits gas exchange
• The surface area for gas exchange is reduced which can cause breathing difficulties
• Physiotherapy can support people with cystic fibrosis to loosen the mucus in the airways and improve gas exchange