Mistakes in Meiosis

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Mistakes in Meiosis

• Including
• Mutations
• Non-disjunction

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What is a Gene?

• A gene is the basic unit of heredity in a living
  organism
• Genes hold the information to build and maintain
  cells and pass genetic traits to offspring.

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What is a Gene?

• A gene is holds the information for the
  production of a particular protein.
• A gene is the basic instruction a sequence DNA

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What is a Gene?

• Remember that DNA is made up of four nucleotide
  bases A, T, G, C
• Each gene is a string of hundreds of base pairs
  in a particular sequence.
• An allele is one variant of that instruction.

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Mutations

• Mutations are changes in the genetic makeup of a
  cell
• They are changes in the sequence of base pairs
• Mutations can change the protein that the gene
  codes for.

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Mutations

• Can involve large regions of a chromosome or just
  a single nucleotide pair

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Causes of Gene Mutations

• Mutations can occur spontaneously during DNA
  replication
• Many mutations occur as a result of exposure to
  mutagens, or mutation causing agents in the
  environment

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Mutagens

• X-rays
• UV light
• radioactivity
• chemicals
• cigarette smoke

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Mutations

• Results of a mutation on the organisms may be
• Deleterious
• Positive
• no effect

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Mutations

• Species have evolved as a result of mutations
  that have given an organism a positive benefit
  that has been naturally selected
• Effects of a mutation may not surface immediately
  

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Alterations of Chromosome Structure

• There are several different mutations that alter
  the chromosome structure
• Deletions
• Duplication
• Substitutions
• Inversions
• Translocations
• Point mutations (insertion, deletions,
  substitutions of a single base pair)

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Alteration of Chromosome Number

• Aneuploidy incorrect number of chromosomes
• Polyploidy more than 2 pairs of each chromosome

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Deletions

• With a deletion, a segment of a chromosome is
  lost
• Or a single base pair is lost

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Duplications

• With a duplication, the same linear stretch of
  DNA within a chromosome is repeated, often
  several to many times in the same chromosome or
  in a different one

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Inversions

• With an inversion, a segment of DNA within a
  chromosome may flip upside down but remain in
  place

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Translocation

• With a translocation, a stretch of one
  chromosomes DNA moves to another location in the
  same chromosome or a different one

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Base-pair substitutions

• The replacement of one nucleotide and its partner
  with another pair of nucleotides

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Insertions and Deletions

• Additions or losses of one or more nucleotide
  pairs in a gene
• Usually have a more deleterious effect than
  substitutions

http//www.larasig.com/node/1806
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Insertions and Deletions

• Because mRNA is read as a series of nucleotide
  triplets during translation, the insertion or
  deletion may alter the reading frame (triplet
  grouping) of the gene
• All the nucleotides that are downstream of the
  deletion or insertion will be improperly grouped
  into codons

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Alteration of Chromosome Number

• Sometimes the movement of chromosomes during
  meiosis goes wrong
• When this happens the gamete may end up with an
  unusual number of chromosomes
• This is called nondisjunction

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Nondisjunction

• The members of a pair of homologous chromosomes
  do not move apart properly during meiosis I
• Or
• The sister chromatids fail to separate during
  meiosis II
• In these cases, one gamete receives two of a
  chromosome and another gamete receives no copy

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Nondisjunction

• If either of the aberrant gametes unites with a
  normal one, the offspring will have an abnormal
  chromosome number, known as aneuploidy
• If the chromosome is present in triplicate in the
  zygote, (the cell has a total of 2n 1
  chromosomes) then the cell is trisomic for that
  chromosome
• trisomy 21 Down syndrome

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Nondisjunction

• If a chromosome is missing
• (cell has 2n-1 chromosomes) the cell is
  monosomic for that chromosome
• Some organisms have more than two complete sets
  of chromosome sets and are termed polyploidy
• 3n triploidy
• 4n tetraploidy

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Syndromes Associated with Nondisjunction

• XXY Klinefelter syndrome
• have low levels of testosterone causing small
  testicles and the inability to make sperm. They
  also have learning disabilities and behavior
  problems such as shyness and immaturity
• XXX Triple X syndrome
• Individuals show no significant clinical
  manifestations, although there is usually some
  degree of mental retardation.
• X Turner syndrome
• is characterized by retarded growth that leads to
  a small stature and frequent infertility.
• Trisomy 21 Down syndrome
• Mental retardation and characteristic physical
  features

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Animation of nondisjunction in meiosis
I Animation of nondisjunction in meiosis II
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