chromosomes structural changes

1. Variations in Chromosome Structure
Mutations involving changes in chromosome structure occur in four
common types:
a. Deletions.
b. Duplications.
c. Inversions (changing orientation of a DNA segment).
d. Translocations (moving a DNA segment).

2. Deletion
In a deletion, part of a chromosome is missing.
a. Deletions start with chromosomal breaks induced by:
i. Heat or radiation
ii. Viruses.
iii. Chemicals.
b. Deletions do not revert, because the DNA is missing.

3. A deletion of a chromosome segment

4. Examples of human disorders caused by large chromosomal deletions:
a. Cri-du-chat (“cry of the cat”) syndrome : result from deletion of part
of the short arm of chromosome 5 (5p)
The deletion results in severe mental retardation and physical abnormalities.
b. Prader-Willi syndrome : part of the long arm of one chromosome 15
homolog deleted.
The deletion results in poor development, behavioral problems, and
mental retardation.

5. Duplication
1. Duplications result from doubling of chromosomal
segments, and occur in a range of sizes and locations
a. Tandem duplications are adjacent to each other.
b. Reverse tandem duplications result in genes arranged in the
opposite order of the original.

6. Duplication, with a chromosome segment repeated

7. Forms of chromosome duplications are tandem, reverse tandem, and
terminal tandem duplications

8. Inversion
1. Inversion results when a chromosome segment excises and reintegrates
oriented 180° from the original orientation.
There are two types :
a. Pericentric inversions include the centromere.
b. Paracentric inversions do not include the centromere.
2. Inversions generally do not result in lost DNA .
3. A homozygote will have normal meiosis.
The effect in a heterozygote depends on whether crossing-over occurs.
i. If there is no crossing-over, no meiotic problems occur.
ii. If crossing-over occurs in the inversion, unequal crossover may
produce serious genetic consequences.

9. Inversions

10. Consequences of crossing-over in a paracentric inversion

11. Meiotic products resulting from a single crossover within a heterozygous,
pericentric inversion loop

12. Translocation
1. A change in location of a chromosome segment is a translocation. No
DNA is lost or gained. Simple translocations are of two types
a. Intrachromosomal: with a change of position within the same
chromosome.
b. Interchromosomal: with transfer of the segment to a nonhomologous
chromosome.
i. If a segment is transferred from one chromosome to another, it is
nonreciprocal.
ii. If segments are exchanged, it is reciprocal.
2. Gamete formation is affected by translocations.
Gametes produced with chromosomal translocations often have
unbalanced duplications and/or deletions and are inviable.
Eg. familial Down syndrome.

13. Translocations

14. Meiosis in a translocation heterozygote in which no crossover occurs

15. Robertsonian translocation (centric fusion)
produces three copies of the long arm of chromosome 21,
resulting in familial Down syndrome
In this nonreciprocal translocation, two nonhomologous
acrocentric (centromeres near end) chromosomes break at
centromeres.
(a) Both long arms become attached to the same centromere,
creating a chromosome with the long arm of chromosome
21 and the long arm of chromosome 14 (or 15).
(b) The short arms also fuse, usually lost within a few cell
divisions.

16. Robertsonian translocation