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