1. Including Mutations & Non-disjunction
Mistakes in Meiosis
Including
Mutations &
Non-disjunction

2. 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.

3. 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

4. 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.

5. 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.

6. Mutations
Can involve large regions of a chromosome or just a single nucleotide pair

7. 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

8. Mutagens
X-rays
UV light
radioactivity
chemicals
cigarette smoke

9. Mutations Results of a mutation on the organisms may be: Deleterious
Positive
no effect

10. 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

11. 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)

12. Alteration of Chromosome Number
Aneuploidy – incorrect number of chromosomes
Polyploidy – more than 2 pairs of each chromosome

13. Deletions With a deletion, a segment of a chromosome is lost
Or a single base pair is lost

14. 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

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

16. Translocation
With a translocation, a stretch of one chromosome’s DNA moves to another location in the same chromosome or a different one

17. Base-pair substitutions
The replacement of one nucleotide and its partner with another pair of nucleotides

18. Insertions and Deletions
Additions or losses of one or more nucleotide pairs in a gene
Usually have a more deleterious effect than substitutions

19. 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

21. 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

22. 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

24. 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

25. 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

26. 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

27. Animation of nondisjunction in meiosis I
Animation of nondisjunction in meiosis II

28. That’s all Folks