Mutation

• A mutation is a change in the DNA sequence of an organism

  • Mutations in genes and chromosomes can result from errors in DNA replication, cell division or from damage caused by mutagens

  Size: 

  • Single DNA base pair (point mutation)
  • Multiple genes affected → structural change (chromosomal mutation)
• Mutagen: an environmental agent that induces mutations (change in the genetic material of a cell) 

  • Introducing a mutagen is known as mutagenesis
  • Alters sequence of nucleotides in the DNA
  • Alters the protein or RNA 
• Electromagnetic radiation


  Transfer of energy, includes heat and ionising radiation
• Ionising radiation


  • Can break chemical bonds in DNA and create free radicals (which break DNA strands)
  • Comes from visible light, UV radiation, X-rays and gamma rays
• Ionising radiation


  • UV radiation from sunlight damages the cell cycle by connecting adjacent base pairs so they cannot pair with the complementary base
• Mutagenic chemicals

  Usually similar to DNA bases and get incorporated into DNA, leading to incorrect nucleotides being paired
• Naturally occurring mutagens

  Chance of mutation increases with frequency and exposure
• Biological mutagens

  • Viruses (e.g. Hepatitis B and HPV)
  • Bacteria
  • Fungi
  • Metabolism end-products
  • Transposons (DNA fragments that spontaneously relocate/multiply/fragment, disrupting DNA functioning and triggering cancers)
• Non-Biological mutagens

  • Mercury
  • Cadmium
• Point mutation: 

  • Changes to the sequence of just ONE base
  • Occurs during transcription → changes the amino acid sequence → possible protein change
• Types of point mutation:

  • insertion
  • deletion
  • substitution
  • inversion
• Insertion / Deletion

  (eg, CAG TAT ACG to

  CAT GTA TAC G) <-- inserting a T in first codon

  • Adding or deleting a base
  • Frameshift mutation since all codons after the mutation will be incorrect → causes major protein function change
• Substitution 

  Replacing one base with another

  (eg, CAG to CGG)

  Nonsense mutation if new codon sequence codes for STOP amino acid

  Missense mutation if new codon causes continued but incorrect amino acid sequence → affects polypeptide and protein

  Silent mutation, eg if A was swapped with A → no effect
• Inversion 

  Two bases are swapped in order (eg, CAG to CGA)

  Nonsense mutation if new codon sequence codes for STOP amino acid

  Missense mutation if new codon causes continued but incorrect amino acid sequence → affects polypeptide and protein

  Silent mutation, eg if A was swapped with A → no effect
• Chromosomal mutation:

  • Change in overall structure of chromosome or number of chromosomes in a cell is altered 
  • 4 main types
  • Chromosomal deletion 
  • Part of a chromosome is lost 
  • Chromosomal insertion (duplication)
  • An extra piece of chromosome is added
  • Chromosomal inversion 
  • A piece of chromosome drops and rotates 180 and is rejoined 
  • Chromosomal translocation  
  • A piece of chromosome 1 breaks off and joins chromosome 2 breaks off and joins chromosome 1
  • Chromosomes are no longer homologous 
• Somatic mutation

  Location: somatic cells

  During: mitosis

  Inherited by offspring? no

  Does it directly change the allele frequency in the gene pool? no
• Germ-line mutation

  Location: sex cells

  During: meiosis and gametogenesis

  Inherited by offspring? yes

  Does it directly change the allele frequency in the gene pool? yes
• Germ-line mutation examples:

  Haemophilia, Huntington's disease (all inherited diseases and disorders)
• Somatic mutation examples:

  cancers, down-syndrome
• what is the significance of mutations on non-coding DNA segments?

  these sections of DNA are not transcribed or translated therefore the non-coding mutations will not affect any proteins and therefore will not affect the phenotype
• what is the significance of mutations on non-coding DNA segments?

  these sections of DNA are expressed as proteins, therefore any mutations (other than silent mutations) will be carried on, affecting the shape and function of proteins, hence affecting phenotype
• how does meiosis cause genetic variation?

  • crossing over creates new allele combinations
  • individual assortment shuffles alleles
  • random segregation shuffles alleles
• Mutations create new alleles

  • Caused by errors in DNA replication or exposure to harmful mutagens
  • Increases genetic variation
• Fertilisation (sexual reproduction) increases genetic variation by shuffling the alleles of each parent when creating the genetic information of the resulting offspring (half contributed by each parent)
• Gene pool


  Total of all alleles of all genes present within a population at a given time
• Allele frequency

  Frequency (as a fraction or percentage) of a particular allele in a gene pool
• Mutations


  Creation of NEW alleles, occurs by errors in DNA replication or exposure to mutagens
• Natural selection


  • Most important mechanism of evolution
  • Reduces gene pool by eliminating unfavourable alleles
• Genetic drift


  Population change due to chance

  Bottleneck effect

  Founder effect
• Founder effect


  New population is formed from a small group of individuals in a different environment that's gene pool does not accurately represent the parent population
• Bottleneck effect


  "Right place, right time" dramatically reduced population (brink of extinction) and small, not representative of parent gene pool, when they reproduce and the population grows again the gene pool will have significantly less genetic variation
• Gene flow


  Immigration and emigration between populations can introduce new alleles or change allele frequency within the gene pool
• Mutations occur by exposure to mutagens or what?


  errors in DNA replication