Chromosomal aberrations (structural)

These slides provide you the simplest explanation about structural aberrations in chromosomes with beautiful illustrations.

1. Structural abberetions in chromosomes. Chromosomal Abberetions

2. Created by : Gurunath A.M B.Sc in Biotechnology 3 rd semester

3. Index : Deletion Terminal Interstitial Duplication Tandem Reverse Tandem Displaced Translocation Simple Isochrome Reciprocal Shift Inversions Pericentric paracentric 01 02 03 04

4. Definition. The chromosomes of each species has characteristic morphology and number. But, sometimes due to some accidents or irregularities at the time of cell division, crossing over or fertilization some alterations in morphology and number of chromosomes takes place. The changes in the genome involving chromosome parts, whole chromosomes or whole chromosome sets are called “ Chromosomal aberrations” or “Chromosomal mutations”. These mutations can be inherited.

5. Structural changes in chromosomes may be of following types : 1. Deletion – Terminal, Interstitial. 2. Duplication – Tandem, reverse tandem & displaced. 3. Translocation – Simple, isochrome, reciprocal, Shift. 4. Inversion – Pericentric & Paracentric. Types of structural changes in chromosomes

6. Deletion . Portions of chromosome without a centromere (acentric fragments) lag in anaphase movement and are lost form reorganizing the nuclei or digested by nucleases. Such loss of a portion of chromosome and the genes associated with it is termed deletion . The chromosomes with deleted parts can never revert back to normal condition. If the deletion takes place in allosomes, then the deletion is transmitted to the next generation as well. When a part of chromosome of the dominant allele is deleted, the recessive allele starts showing dominance, this phenomenon is called “pseudodominance” . There are two type of deletions : Terminal Interstitial

7. Terminal Deletion : This is when a part of the chromosome is deleted at the terminal end of the chromosome. It results by only one break in the chromosome. A B C D E G F A B C D E G F Deletion - Aberrated chromosome - A B C E D Normal chromosome -

8. 2. Interstitial : This type of aberration results when a portion is lost in the middle of the chromosome. This is caused by two breaks in the chromosome, one at the starting and another at the end of the deleted region. A B C D E G F A B C D E G F Deletion - Aberrated chromosome - Normal chromosome - A B C G F

9. Examples : Cri du chat syndrome Deletion of some chromosome regions produce their own unique phenotypes such as “cri du chat” syndrome in human babies. Human babies missing a portion of the short arm of chromosome 5 (autosome) have a distinctive cat-like cry, hence the name. They are also mentally retarded (IQ below 20), have saddle nose and exhibit microcephally (small head). The karyotype of cri du chat patient is called 5  

10. Duplication. The presence of a part of chromosome in excess of the normal chromosome is known as duplication. Due to this, even though the chromosome number is the same, there is extra genetic material. These extra genes present in the duplication may cause those genes to not function properly, these extra instructions leads errors while producing the phenotype. There are three major types of duplication : Tandem. Reverse tandem. Displaced.

11. 1. Tandem : In case of tandem duplication the duplicated section of the chromosome sits right beside the normal chromosome part which it duplicated form. Normal - Duplicated - A B C D E G F A B C D E G F D E F

12. 1. Reverse Tandem : In this case the duplicated section of the chromosome sits right beside the normal chromosome part which it duplicated form but in a reversed sequence. Normal - Duplicated - A B C D E G F A B C D E G F F E D

13. 1. Displaced : In Displaced duplication the duplicated section does not sit adjacent to the section which it duplicated from. Depending on weather the duplicated portion is on the same side of the centromere as the section which it duplicated from or on the opposite side, displaced duplication can be termed either homobranchial or heterobranchial. Normal - Homobranchial - A B C D E G F A B C D E F F G D E Homobranchial - A B C D E G F D E F

14. Example : Bar eyed Drosophila. The bar phenotype of Drosophila is characterized by narrower, oblong, bar shaped eye with fewer number of facets. There are few genes which are responsible to give the narrow shape to the eye of the Drosophila , when the region of the chromosome containing these genes are duplicates, the expression of genes will become higher and the eye of the fly becomes narrower, like a bar.

15. Translocation. The shifting or transfer of a part of chromosome or setoff genes to a non-homologous chromosome is called translocation. There is no addition or deletion of genes in translocation, only the sequence and the position of the genes are altered. There are four types of translocation : Simple. Isochrome. Reciprocal. Shift.

16. 7 Simple Translocation : When a terminal segment of the chromosome breaks and re-attaches to one end of a non-homologous chromosome then it is called simple translocation. A B C D E G F G A B C D E F 1 2 3 4 5 6 Aberrated chromosome - Non-homologous chromosomes - 1 2 3 4 5 7 6

17. 2. Isochrome Translocation : This type of translocation is caused by the simultaneous deletion and duplication of chromosome parts. In this type of abnormality the chromosomes arms are mirror images if each other. This type of chromosome is also called isochromosome. Aberrated chromosome - Normal chromosome - A B C D E H F G A B C D D A C B

18. 3. Reciprocal Translocation : In this type of translocation a segment of one chromosome is exchanged with a segment from another non-homologous chromosome. Hence, two translocations are simultaneously achieved. A B C D E G F G F 1 2 3 4 5 7 A B C D E 6 Aberrated chromosome - Normal chromosomes - 1 2 3 4 5 7 6

19. 4. Shift Translocation : In this type of translocation an intercalary part of one chromosome breaks and gets itself inserted into intercalary position of a non-homologous chromosome. A B C D E G F G A B C D 1 2 3 4 5 7 F E 6 Aberrated chromosome - Normal chromosomes - 1 2 3 4 5 7 6

20. Inversion. This type of chromosomal aberration involves rotation of a part of a chromosome by on its own axis. Just like in translocation, there is no addition or deletion of genes but simply a re-arrangement of the gene sequence. It takes place in a intercalary position. The location of the inverted segment can be detected in meiotic nuclei by the presence of an inversion loop. Accordingly there are two types of inversion : Pericentric Paracentric  

21. Paracentric Inversion : In this type of inversion the segment involved is present wholly within one chromosome, that means no centromere is involved. Normal - A B C D E H F G Paracentric inversion- A B C D H E F G Aberrated chromosome - A B C D H E F G G

22. 2. Pericentric Inversion : In this type of inversion the break points are located on both the arms of the chromosome are involved including the centromere. Normal - A B C D E H F G A B E H F C D G Pericentric inversion - A B E H F C D G Aberrated chromosome -

23. Dr. P.S Verma, D r. V.K Agarwal C ell biology. Bibliography.

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