Botany NOU Presentation

CHROMOSOMAL ABERRATION: 

CHROMOSOMAL ABERRATION BY PADMINI DUTTA PG 1 ST SEM

CONTENTS: 

CONTENTS What is chromosomal aberration Types of chromosomal aberration Deficiency Duplication Translocation inversion Conclusion

WHAT IS CHROMOSOMAL ABERRATION?: 

WHAT IS CHROMOSOMAL ABERRATION? The chromosomal aberration is the visible changes in the structure of chromosome involving changes either in the total number of genes in a chromosome or their re-arrangement. So chromosomal aberration causes a single deviation from wild type.

TYPES OF CHROMOSOMAL ABERRATION: 

TYPES OF CHROMOSOMAL ABERRATION Chromosomal aberration is mainly of four types:- Deficiency or deletion Duplication Translocation inversion

DEFICIENCY OR DELETION: 

DEFICIENCY OR DELETION

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C.Bridges in 1917 first observed a deficiency in the bar locous of a Drosophila. A deficiency involves a loss of a chromosome section from the total chromosome. After breaking the part without the centromere is lost, while the part having the centromere functioning as a deficient chromosome. The break may be caused by several agent like chemical, drugs and radiation. The break may occur at any time during cell cycle, either in the somatic or germ cell.

TYPES OF DELETION: 

TYPES OF DELETION Deletion is mainly of two types. Terminal deletion - it is the single break occurs near the end of the chromosome. It is relatively rare. Intercalary deletion - it is the break down of chromosome taken place in the middle portion and followed by reunion of broken end.

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Example - Deletion may be observed in both animals and plants. In drosophila a mutant produce notched margin of the wings and known as notched character. Cri-du-chat or cat-cry-syndrome caused by deletion in the short arm of chromosome. In plant deletion are not easily transmitted to the offspring because pollens with deficiency chromosome are found to be sterile. Significance of deletion- If a segment of chromosome is missing then the genes present on that section are absent. If the missing part is physiologically important then the organism is not survive. The recessive gene also express themselves phenotypically. The expression of recessive gene is called Pseudo-dominance.

DUPLICATION: 

DUPLICATION

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Bridges in 1919 observed duplication in some individual of Drosophila. The presence of some block of genes more than one in a haploid complement is known as duplication. It is due to some gene added to original chromosome as a result some part of the chromosome is doubled.

TYPES OF DUPLICATION: 

TYPES OF DUPLICATION Duplication is mainly of four types- Tandem Duplication -In this type of duplication the added segment has the same genetic sequences as it is present in the original state in the chromosome. Reverse Tandem Duplication- in this type of duplication the sequence of gene aligned in the attached chromosome piece is just the reverse of the original one. Displaced Duplication- In some cases the extra segment may be located in same chromosome but away from the normal segment that’s termed as displaced duplication. Translocation Duplication- The additional chromosome segment is located into a non-homologous chromosome. The extra segment is translocated into a non-homologous chromosome.

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Example-Bar eye of Drosophila Bar eye is a character where the eyes are narrow as compared to normal eye shape. It is due to duplication for a part of the chromosome. Bar eyed individuals give rise to ultra bar condition due to unequal crossing over. Significance Of Duplication- Duplication play significance role in evolution because duplication increases number of genes in the chromosome complement.

TRANSLOCATION: 

TRANSLOCATION

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It is a kind of chromosomal rearrangement in which a block of genes from one linkage group is transferred to another linkage group. Translocation occurs as a result of interchange of chromosome segment in non-homologous chromosome.

TYPES OF TRANSLOCATION: 

TYPES OF TRANSLOCATION Translocation is mainly of four types- Simple translocation -In such cases a small segment of a chromosome is added to the end of other non-homologous chromosome. It is very rare in nature and is caused by a single break in one chromosome only. Shift translocation -In this type of translocation an interstitial segment of one chromosome is broken off and is inserted within the break in another non-homologous chromosome. Thus it involves three breaks, two in one chromosome and one in non-homologous chromosome.

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Reciprocal translocation- It is the exchange of parts between two homologous chromosome. If both the homologous chromosome exchange parts with two homologous of another pair is called homozygous reciprocal. If only one number of each of the two homologous pair exchange parts are called heterozygous reciprocal. Multiple Translocation -In this more than two pair of non-homologous chromosome exchange their parts.

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Example -Translocation have been studied in maize. In maize translocation between 15 and 21 chromosome results in 21 trisomy causes Down’s syndrome. Significance of translocation- It plays important role in the introduction of genetic polymorphogin in a population and origin of new species.

INVERSION: 

INVERSION

Slide 24: 

In inversion the sequence of gene is alternative by the rotation of gene block within a chromosome by 180 degree. It arises by breakage of chromosome followed by the joining of pieces in reverse order. Inversion arises by the formation of loop on a chromosome. Breaks may occur at the point of intersection of the loop. The reunion of the broken end takes place in a new combination.

Types of inversion: 

Types of inversion It is mainly of two type- Paracentric Inversion -When both the breaks on the chromosome during inversion occurs in same side of the centromere, the inversion is known as the Paracentric. The inverted segment of chromosome is without centromere. If Paracentric inversion occurs singly on a side of the centromere alone it is known as homobracial. When two Paracentric inversion occur one on either side of the centromere i.e. bracial inversion.

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Pericentric inversion -Here the inverted segment contains the centromere. It involves one break on either side of the centromere. Inversion occurring in a single chromosome is called chromosomal, whereas that occurring in the member of a homologous pair is called allelosomal. If allelosomal occur in homologous chromosome is called allelobracial but occurring in non-homologous chromosome are called heterosomal.

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Significance of inversion It helps in the origin of new species. Inversion provides proof for the occurance of crossing over. Inversion, similar to translocation help in establishing species relation in Drosophila. As a result of inversion chances of crossing over are reduced and therefore the combination of genetic loci is restricted

CONCLUSION: 

CONCLUSION We know chromosomal aberration have so many changes in the structure of chromosome but it also have beneficial activity as it helps in evolution as well as hybrid vigour in plants. Though it is called as abnormality of chromosome structure but this abnormality is also useful to us in some way.

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Thank you