Mutation, mutagens and DNA repair

• Mutations

  • germ-line vs somatic
  • point mutations
  • spontaneous vs induced
• DNA damage and induced mutation:

  • chemical and physical mutagens
• Cancer:

  • disease of somatic mutation
  • link between mutagens and cancer
  • testing for carcinogens
• What is a mutation?

  A mutation is a permanent change in the DNA of cells
• What is a chromosomal mutation?

  change in gene position/number
• What is a point mutation?


  change in nucleotide sequence
• What is a source a variation?

  Mutation, usually detrimental
• Describe mutations in somatic tissues

  • not passed on to offspring
  • passed on to all cells descended from the orignial mutant
• Describe mutation in germ line tissue

  • cause of inherited genetic disease
  • raw material from which natural selection produces evolutionary change
  • passed on to offspring
• What is a null mutation?


  No protein is made at all
• what are the categories of point mutations?

  base pair substitution

  Base pair insertion/deletio
• What is an example of base pair substitution?

  sickle cell anaemia

  • T swapped for A, changing Glu to Val
• What is the effect of sickle cell anaemia?

  Gluatmic acid = large, charged, hydrophilic sidechain

  Valine = small, nonpolar, hydrophobic sidechain
• what are the causes of mutation?

  spontaneous

  induced
• what is a spontaneous mutation?

  Through DNA base replication (base tautomerism)
• what are the type are induced mutation?

  chemical

  physical
• what are the types of chemical mutation?

  Base analogues

  modifying agents

  intercalating agent
• What are type of physical mutation?

  Ionising radiation (X-rays)

  ultraviolet radiation
• what is an induce mutation?

  Occurs when an outside agent (mutagen) damages DNA
• What are the 3 classes chemicals agents act on DNA?

  • chemicals that resemble DNA bases but pair incorrectly when incorporated in DNA
  • Chemicals that remove the amino group from adenine or cytosine / chemicals that add hydrocarbon groups to nucleotides bases
  • Intercalating agents
• Describe chemicals that resemble DNA bases but pair incorrectly when incorporated in DNA:

  • 5 bromouracil is incorporated into DNA as though it were thymine
  • Once incorporated it tends to rearrange into a form that resembles cytosine
  • Upon DNA replication, can result in a point mutation converting AT to GC
• Describe chemicals that remove the amino group from adenine or cytosine:

  Example - nitrous acid/nitrite

  • Deamination of C to U
  • Replication of DNa containing deaminated C results in A being inserted
  • Daughter strand base pairing changes CG to TA
  • Occurs spontaneously at low rates
  • Uracil not usually found in DNA
  • Uracil DNA glycosylase has specific role in removing U from DNA to prevent mutations
• Describe chemicals that add hydrocarbon groups to nucleotide bases:

  Alkylating agents

  • many of these chemicals are mutagens.
  • Addition of an ethyl group at position O6 alters the base-pairing characteristics
  • Results in a point mutation converting a GC to an AT

  
• Describe intercalating agents:

  • insert between bases and distort DNA helix
  • Interfere with replication
  • Tend to cause frameshift mutation
  • E.g ethidium bromide
• Describe ionising radiation:

  1. directly ionises DNA (direct effect)
  2. Ionises water to produce free radicals (indirect effect)
• What does DNA damage lead to?

  Cell cycle arrest and DNA repair

  Apoptosis
• Describe the effect of ultraviolet radiation:

  • UV irradiation is absorbed specifically by the pyrimidine bases cytosine and thymine
  • Covalent bonds can form between adjacent T or C nucleotide - pyrimidine dimers
  • blocks DNA synthesis leaving a gap opposite the site of damage
• UV radiation leads to...

  pyrimidine dimers - a kink
• How are thymine dimers removed?

  By nucleotide excision repair
• Describe broad specificity repair system

  Broad specificity repair system recognises distortion in DNA
• what conditions are people deficient in NER

  Xeroderma pigmentosum
• what are xeroderma pigmentosum characterised by?


  Development of skin cancers at an early age but only on those parts of body exposed to the sun
• How is a thymine dimer repaired?

  Thymine distorts DNA molecule

  1. A nuclease enzyme cuts the damaged DNA strand at 2 points
  2. Repair synthesis by a DNA polymerase fills the gap
  3. DNA ligase seals the remaining nick
• What is cancer caused by?

  mutations
• What two specific genes mutation lead to cancer?

  oncogenes and tumour suppressors
• Do most cancers occur in somatic cells or are they inherited?


  Mutations in somatic cells
• How does inheritance affect cancer?

  gives a pre-disposition
• What is the Ames test?

  A test used to determine the mutagenic potential of chemicals.
• how to carry out the Ames test?

  salmonella strain that requires histidine is mixed with liver extract (contains enzyme that may convert non-mutagen into mutagens)

  High number of revertants to His+ suggests that the mutagens causes mutations