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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
• 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
Duplications
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