Mistakes in Meiosis
Including
Mutations &
Non-disjunction
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.
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
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.
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.
Mutations
• Can involve large regions of a
chromosome or just a single
nucleotide pair
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
Mutagens
•
•
•
•
•
X-rays
UV light
radioactivity
chemicals
cigarette smoke
Mutations
• Results of a mutation on the
organisms may be:
– Deleterious
– Positive
– no effect
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
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)
Alteration of Chromosome
Number
• Aneuploidy – incorrect number of
chromosomes
• Polyploidy – more than 2 pairs of
each chromosome
Deletions
• With a deletion, a segment of a
chromosome is lost
• Or a single base pair is lost
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
Inversions
• With an inversion, a segment of
DNA within a chromosome may
flip upside down but remain in
place
Translocation
• With a translocation, a stretch of
one chromosome’s DNA moves
to another location in the same
chromosome or a different one
Base-pair substitutions
• The replacement of one
nucleotide and its partner with
another pair of nucleotides
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
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
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
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
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
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
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
Animation of nondisjunction in meiosis I
Animation of nondisjunction in meiosis II
That’s all Folks