Mutations: Heritable Changes in Genes

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Mutations Heritable Changes in Genes

• Mutations are heritable changes in DNAchanges
  that are passed on to daughter cells.
• Multicellular organisms have two types of
  mutations
• Somatic mutations are passed on during mitosis,
  but not to subsequent generations.
• Germ-line mutations are mutations that occur in
  cells that give rise to gametes.

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Mutations Heritable Changes in Genes

• Mutations are rare events and most of them are
  point mutations involving one nucleotide.
• Different organisms vary in mutation frequency.

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Mutations Heritable Changes in Genes
Mutations can be detrimental, neutral, or
occasionally beneficial. Random accumulation of
mutations in the extra copies of genes can lead
to the production of new useful proteins and new
functions.
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Point mutations

• Involve only one or a few nucleotides
• can occur spontaneously also as a result of
  mistakes during DNA replication
• or
• can be caused by environmental mutagens

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Point mutations
Base substitutions are point mutations which
alter one DNA base pair without adding or
deleting any base pairs
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Types of mutations (DNA level)

• -Transition mutations- a purine is substituted
  for a purine or a pyrimidine is substituted for a
  pyrimidine

-Transversion mutations- a purine replaces a
pyrimidine or vice versa
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Types of mutations (DNA level)

• Transitions
• Transversions

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Consequences of Point mutations

• Types (Protein level)
• Silent mutations are base substitutions which do
  not alter the amino acid composition of the
  protein encoded by a gene
• -silent mutations may affect the noncoding
  portion of a gene or may occur in the coding
  portion but may not alter codon usage

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Silent Mutation
GGC
GGA
CCU
CCG
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Consequences of Point mutations

• Synonymous or neutral mutation codon specifies
  a different but functionally equivalent amino
  acid
• E.g. AAA Lys gt AGA Arg both positively charged
  (basic)

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Point Mutations

• Missense mutations change a single amino acid as
  a result of a change in codon specification
• -missense mutations can have serious
  consequences on the biological properties of a
  protein

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Missense mutation
CTA
CAA
GAU
GUU
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Single base change in hemoglobin gene causes
sickle cell anemia
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Point Mutations

• Nonsense mutations change a codon to a stop codon
  which results in a premature termination of
  translation

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Nonsense mutation
ACC
ATC
UGG
UAG
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Non senso
missense
silente
Sinonima o neutra
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Point Mutations

• Point mutations can also alter signals used to
  regulate gene expression
• Promoter mutations may block transcription
• Splice site mutations may block splicing or
  create new splice signals

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Insertions and Deletions

• Insertions add one or more nucleotide pairs to
  DNA sequence
• Deletions remove one or more nucleotide pairs
  from DNA sequence

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Insertions and Deletions

• Insertions or deletions involving a multiple of 3
  DNA base pairs in-frame since they do not alter
  the reading frame of the genetic code

• Insertions or deletions which involve a
  non-multiple of 3 DNA base pairs frameshift
  mutations since they alter the codon translation
  reading frame

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Frameshift mutations change the reading frame
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Frame-shift mutation
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Insertions and Deletions

• These shifts almost always lead to the production
  of nonfunctional proteins

• Insertion and deletion mutations may result from
  unequal crossing-over during recombination

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• Base substitutions (point mutations) ,insertions,
  and deletions