**10. Conclusion**

*Cytogenetics - Classical and Molecular Strategies for Analysing Heredity Material*

appropriate in light of consideration [40].

eukaryotic cells as well as mammalian cells [41].

**9. Mechanism of formation of chromosome aberration**

DNA breaks (especially dsDNA) is a serious threat for cell when unrepaired or misrepaired, as they can result in genomic instability or later on may lead to chromosomal alterations and even cell death. The chromosomal aberrations formation is one of the major alteration formed during dsDNA breaks. Moreover, it has been reported that during each cell division approximately 5000 ssDNA breaks were generated per nucleus and approximately 1% of total ssDNA breaks converted in dsDNA breaks. There were two theories forwarded for explanation of how chromosomal aberrations and its formation take place i.e. breakage and reunion theory and exchange theory. The breakage and reunion theory explains that breaks in chromosome may rejoin and form the original structure through restitution and the exchange theory might lead to exchange the aberration by rejoining another type of breaks. dsDNA breaks could be repaired by possibly three pathways i.e. homologous recombination repairing (HRR) which restores the original sequences, non-homologous DNA end joining (NHEJ) which usually generates and repairs small alterations such as base pair substitution, insertions, deletions at break sites etc., and single strand annealing (SSA) which may lead to the formation of interstitial deletions. HRR (requires one break) and NHEJ (requires two breaks) and SSA are important pathways for repairing of dsDNA breaks with one or two breaks in

Chromosomal aberrations may be caused by various physical and chemical factors such as ionizing radiations, chemicals and spontaneous dsDNA breaks e.g. endogenous reactive oxygen species, topoisomerases and replication errors.

material missing or for incomplete interstitial deletions, third, observe for presence of any minutes and fourth, if any extra additional distal segment is present but not related to short arm with abnormal band sequences, it may be regarded as incomplete complex changes and look for the origin of additional segment, (iv) fragment simple but non terminal; perhaps represents incomplete acentric ring or large interstitial deletion]. Chromatid type structural aberrations could be detected and identified in the form of interchanges (involve the interchanges of chromatids), interarm intra changes (requires two breaks and causes pericentric inversions, double duplications-deletions, dicentric and centric rings), intra arm intra changes (two breaks involved and caused the formation of isochromatid deletion, duplication-deletion, chromatid minutes and paracentric inversions), and breaks [{causes chromatid terminal deletions and may be of various types; (i) a tandem duplication may be present in the complete chromatid, (ii) a tandem duplication may be present either in centric or acentric portion of the incomplete chromatid, (iii) the origin of acentric fragment may be of intercalary type and the sister chromatid has a normal pattern but it may show a bending opposite to the site of deletion, (iv) a paracentric inversion could be possible adjacent to the break in the acentric fragment but other sister chromatid may be normal, (v) a paracentric inversion could be possible adjacent to the break in the centric region or portion but other sister chromatid may be normal}].Additional chromatid type structural aberrations could be possible in the form of as isochromatid (isochromatid exchanges), insertions and additional dark bands in one of the chromatid at the exchange point. It is important to locate the position of aberration or break points at some stage in the production of breakage and rejoining of chromatid threads as this is the event that causes disruption and could be observed using different techniques. A detectable point of breakage is generally referred to as 'break point' but 'presumptive break point' would be more

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At present, most of the studies are surrounded towards the human health and disease caused by the interaction of genetic and environmental factors. Sometimes it is difficult to understand the genetic constitution or mechanism of chronic diseases. Therefore, it is very important to understand the mechanism behind the abnormal cells either genetic or environmental or both to solve the problems completely. The identification of a particular abnormality at the initial stage is crucial and banding techniques conventional or molecular provide such an opportunity. Banding and chromosome aberrations are played an important key role in the assessment of various risks faced by the genetic constitution of eukaryotic cells. Therefore, it may continue further to assess the risk of various kinds of ailments, diseases, and geno-toxicity induced by the radiations, pharmaceuticals, environmental and synthetic chemicals.
