Translocation

Translocation

•   Mutual exchange (reciprocal) of the chromosome segments between non homologous chromosome. 

• An exchange of parts between two non homologous chromosomes is called reciprocal translocation. 
• In simple translocation a segment of one chromosome breaks and is transferred to another non-homologous chromosome.

Translocations in human beings

•    Certain types of cancer are associated with chromosome rearrangements.
•    Two examples of tumours associated with consistent chromosome translocations are Chronic Myelogenous Leukaemia (CML) and burkitt’s lymphoma.

(1) Chronic Myelogenous Leukaemia (CML)

•    Chronic myelogenous leukaemia in human beings is a fatal cancer involving uncontrolled replication of myeloblasts (stem cells of white blood cells).
•    Ninety percent of CML is associated with an aberration of chromosome 22.
•    This abnormal chromosome was originally discovered in the city of Philadelphia in 1959 and thus is called the ‘Philadelphia chromosome’.
•    Initially it was though to have a simple deletion in its long arm, however, subsequent analysis using molecular techniques has shown that the Philadelphia chromosome is actually the result of a reciprocal translocation between chromosomes 9 and 22.
•    In the Philadelphia translocation, the tip of the long arm of chromosome 9 has been joined to the body of chromosome 22 and the distal portion of the long arm of chromosome 22 has been joined to the body of chromosome 9.
•    CML is characterized by an excess of granular leucocytes in the blood.
•    With the increase in the number of leucocytes, there is a reduction in the number of RBCs resulting in severe anaemia.