C12 Gene Mutation & DNA Repair

Learning objectives:

  • definition of mutation
  • causes of mutation
  • types and effects of gene mutations
  • types of DNA repair systems

Mutation

  • Heritable changes in the genetic material that provide the raw material for evolution.
  • Effects on human health depend on
    • The nature of the mutation
    • Where the mutation occurs
    • Whether it alters the function of essential proteins
  • Occur in any cell at any time
    • Spontaneous mutation
    • Induced mutation

Spontaneous & Induced Mutations

Spontaneous Mutations

  • Changes in the nucleotide sequence of genes that appear to occur naturally.
  • No specific agents are associated with their occurrence, and they are generally assumed to be accidental.
  • e.g. Copying errors during cell division
  • Types:
    • tautomeric
    • depurination
    • deamination
    • transposons

Induced Mutations

  • Caused by exogenous factors/mutagens
  • May be the result of either natural or artificial agents
  • Examples:
    • Base analogs
    • Alkylating agents
    • Intercalating agents
    • Ultraviolet radiation
    • Ionizing radiation

Spontaneous Mutations arise from Replication errors and Base Modifications

Mistakes during Replication

  • Occasionally, DNA polymerases insert incorrect nucleotides during replication of a strand of DNA.

  • Although DNA polymerase can correct most of these replication errors using their inherent 3’ to 5’ exonuclease proofreading capacity, misincorporated nucleotides may persist after replication.

  • If these errors are not detected and corrected by DNA repair mechanisms, they may lead to mutations. Replication errors due to mispairing predominantly lead to point mutations.

  • The fact that bases can take several forms, known as tautomers,

  • increases the chance of mispairing during DNA replication

  • Changed sequence is likely to be transmitted to all cells produced by subsequent cell divisions

Mistakes during transcription

  • errors in mRNA, potentially in a ‘one-off’ aberrant protein

  1. Tautomeric Shifts

    • purines and pyrimidines can exist in tautomeric forms
      • alternate chemical forms that differ by only a single proton shift in the molecule
      • keto-enol forms of thymine and guanine
      • amino-imino forms of cytosine and adenine
    • these shifts change the bonding structure of the molecule, allowing hydrogen bonding with noncomplementary bases
    • may lead to permanent base-pair changes and mutations

  2. Depurination

  • The loss of purine bases (Adenine or Guanine) in an intact double helical DNA molecule
  • Hydrolysis of the N-glycosidic bond that links it to deoxyribose C1, leading to apurinic site (AP site)
  • ~10,000purinebases/cell/day lost by depurination
  1. Deamination

  • Removal of an amine group (Adenine or Cytosine) from a molecule.

  • The most common type is the spontaneous deamination of cytosine to uracil – occurs at a rate of ~100 bases/cell/day

  1. Transposons (transposable element)

  • a.k.a jumping gene

  • These elements are present in the genomes of all organisms, from

  • bacteria to humans, and often comprise large portions of these

  • genomes.

  • Can move within, or between chromosomes, inserting themselves into various location within the genomes.

  • Act as naturally occurring mutagens.

Induced Mutations Arise from DNA Damage Caused by Chemicals and Radiation

  • increase the rate of mutation above the spontaneous background

  • all cells on Earth are exposed to a plethora of agents called mutagens

    • damage DNA and cause induced mutations

  • examples:

    1. base analogs
    2. alkylating agents
    3. intercalating agents
    4. ultraviolet radiations
    5. ionizing radiation

Base Analogs

  • Structure similarity with natural bases

  • Compounds that can substitute for purines or pyrimidines during ==nucleic acid biosynthesis.==

  • e.g. 5-bromouracil (5-BU), a derivative of uracil, behaves as a thymine analog but is halogenated at the number 5 position of the pyrimidine ring.

  • If 5-BU is chemically linked to deoxyribose, the nucleoside analog bromodeoxyuridine (BrdU) is formed

  • Increase frequency of mispairing

Alkylating Agents

  • chemicals that donate alkyl groups, such as CH3 or CH3CH2, to other molecules.
  • e.g. The sulfur-containing mustard gases, discovered during World War I, were some of the first chemical mutagens identified in chemical warfare studies.
    • Mustard gases are ==alkylating agents- donate an alkyl group to amino or keto groups in nucleotides.==
  • Induce transitions, transversions, frameshifts, and chromosome aberrations.
  • Alkylating agents of bases can ==change base-pairing properties==.
  • Can also ==activate error-prone DNA repair processes==.