chromosomal aberrations and changes in the structure of chromosomes

In the nucleus of each cell DNA is packed ino thread like str. called as chromosomes.
Each chromo has a constriction point called as CENTROMERE.
It divides the chromo. Into 2 arms : p & q.
Location of centromere gives characteristic shape & describes the location of spf genes.
Chromosomes hv spf str but sometimes they undergo structural modifications called CHROMO ABERATIONS

1. Changes in the Structure of Chromosome By D . REIYA BOSCO

2. Chromosomes In the nucleus of each cell DNA is packed ino thread like str. called as chromosomes . Each chromo has a constriction point called as CENTROMERE. It divides the chromo. Into 2 arms : p & q. Location of centromere gives characteristic shape & describes the location of spf genes. Chromosomes hv spf str but sometimes they undergo structural modifications called CHROMO ABERATIONS

3. Breakage & fusion bridge Structural changes are cause due to BREAKES in chromo. Mc.Clintok std on the broken ends of Zea maize. The broken ends behave as if they are STICKY . They undergo FUSION as ease. If breakes occur & r followed by duplication during prophase. Results in the fusion of 2 sticky ends of chromatids . Formation of long chromatid wit 2 centromeres . During anaphase spindle attach & separate the 2 centromeres.

4. It forms dicentric chromatid bridge. Bridge can be broken at diff points . It causes deficency / extra genes.

5. Types of chromosomal aberrations 2 TYPES Changes in NUMBER OF GENES : Deletion : Duplication Changes in ARRANGEMENTS OF GENES : Inversion : Translocation

6. 1) DELETION / DEFICIENCY Results from the loss of a part of chromo.

7. 2 types of deletion TERMINAL DELETION Deletion at the end of chromosome. Single break. Gives rise to small acentric fragment INTERCALARY DELETION Deletion at the centre of the arm. 2 breaks. Also forms Acentic fragment.

8. Terminal deletion

9. Intercalary deletion

10. Genetic effect of deletion If the deleted segment consist of genes physiologically important then organism will not survive . Pseudodominance Missing segment allows expression of recessive genes If dominant genes r deleted . Example in humans: Deficiency in segment of chromosome 18 – large ears , long fingers & low mental ability. Cri-du-chat was due to deletion of chromosome 5.

11. 2) duplication Occurs when section of chromo is presenton twice. Results in extra chromo material. TYPES Tandem dup Reverse tandem dup Displaced dup Transposition Dicentric chromosome

12. 1)Tandem duplication Section of chromo repeated immediately after its normal position in same direction.

13. 2) Reverse tandem duplication The repeated segment is a reversal of the duplicated segment. Arranged immediately after its normal position.

14. 3) Displacement duplication HOMOBRANCHIAL Repeated segment is some distance away from normal location. Occurs on the same arm of chromo HETEROBRANCHIAL Repeated segment is some distance away from its normal location. When repetation takes place on the other arm.

15. 4) Transposition When repeated segment is on a non-homologous

16. 5) Dicentric Chromosome When duplication occurs by taking the extra centromeres causing a dicentric chromosome.

17. 3)translocation Results from transfer of a segment to diffetent parts of same chromo/ diff chromo Like homologous/ non-homologous chromo. TYPES Reciprocal Non reciprocal interchromosomal Non reciprocal intrachromosomal Robertsonian

18. 1) Reciprocal translocation Exchange b/n non –homologous chromo. Meiotic behaviour of chromo is altered . Leads to rearrangement of genetic material. Tot gm is not altered thereby they are balanced . Causes potential genetic drift. Arise due to: chromo break Abnormal crossover

19. 2) non-reciprocal interchromosomal translocation A small segment of chromo is transferred to a non homologous chromo. Occurs between chromosomes.

20. 3) Non reciprocal intrachromosomal translocation A small segment is translocated from one region to another within Same chromo.

21. 4) Robertsonian translocation Transfer of genetic material occur only in 1 direction . Associated with phenotypic abnormalities /lethality. They are confined to chromosome 13,14,15,21. Eg: familial downs syndrome. In that majority of chromo 21 is attached to chromo14 . Individuals have 3 copies of genes on large segment of chromo 21.

22. 4) inversion Occurs when a segment of a chromo breakes & rotates by 180 degree. Genes are rearranged in reverse order . No change in the number of genes. Majority of inversions hv no phenotypic consequences. Minimum of 2 breakes is required. Normal pairing is not possible between 2 homologous chromo. 2 types Paracentric inversion Pericentric inversion

23. Paracentric inversion Occurs when there are 2 breaks on same chromosomal arm. Recombination occurs inside a large loop. Dicentric bridges form at the center where cross over does not occur. Acentric loop arise at the centromere. Pericentric inversion Occurs when there is a break on both sides of centromere . Recombination occur inside 2 loops. Chromo can be elongated / shortened. Deletion & duplication occur During cross over.

24. Paracentric

25. Pericentric