Mutations Mutations Mutations by huanghengdong


• Inheritable errors made in DNA during
• 3 broad groups:
  – Beneficial
  – Deleterious
  – Neutral
• Most mutations are neutral, either because
  they occur in introns, or because of the
  redundancy inherent in our genome
   Causes of Mutations

1. Spontaneous mutations
2. Induced mutations
     Spontaneous Mutations
• Caused by errors during DNA replication
  that are uncorrected
• Can also be caused by errors made during
  crossover in meiosis
         Induced Mutations
• Caused by mutagens (mutagenic agents)
• 2 kinds of mutagens:
  – Physical mutagens physically damage DNA
    strands (e.g. UV radiation)
  – Chemical mutagens alter the molecular
    structure of DNA without damaging it
    physically (e.g. pesticides)
• Note: mutagens that also cause cancer
  are called carcinogens
     Categories of Mutations
1. Point mutations
  – Mutations at a specific base pair on the
2. Chromosomal mutations
  – One DNA fragment is moved from one site on
    the genome to another
               1. Point mutations
Type              Result(s)
Substitution      1. Silent (neutral) mutation
-one or two         •   New codon codes for same or
bases are               similar amino acid (aa) as original;
                        protein is unchanged
                  2. Missense mutation
                    •   New codon codes for different aa;
                        protein is slightly altered
                  3. Nonsense mutation
                    •   New codon is a stop codon,
                        creating a non-functional protein
                        (truncated – many aa’s missing)
            1. Point mutations
Type        Result(s)
Insertion   Frameshift mutation
(an extra   • Change in reading frame of the
base) or       mRNA
Deletion    • Many aa’s changed
(a base
missing)    • Drastically altered protein
   2. Chromosomal Mutations
Type                      Result(s)
Translocation             •   Inactivation of genes
  – Usually occurs        •   Creation of fusion
  between two non-            protein with completely
                              altered function
  – A fragment of one
  switches places with
  a fragment of another
    2. Chromosomal Mutations
Type                      Result(s)
Inversion                 •   Inactivation of genes
  – chromosome section        OR altered gene
  that has reversed its
   2. Chromosomal Mutations
Type                 Result(s)
Duplication (repeats •   either dramatically
of gene sequence) or     increases or decreases
deletion (loss of part   amount of gene product
of the chromosome)
• Transposons, a.k.a transposable elements
  or “jumping genes,” are sections of DNA
  that naturally jump from one area of the
  genome to another
• They have effects similar to translocations
• They were discovered by Barbara
  McClintock, for which she was awarded the
  Nobel prize

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