alteration of chromosome structure

8/7/2019 Alteration of Chromosome Structure

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Alteration of Chromosome Structure.

Mutations in chromosomal structures result in chromosomal aberrations. Another type

of chromosomal mutation is the change in chromosome structure involving

rearrangement of whole blocks of genes on a chromosome. It may result in an alterationin the number of genes or in the sequence of whole sets of genes in chromosomes. In

addition, abnormalities of chromosome structure can be either unbalanced or balanced

rearrangements. If the rearrangement is unbalanced, there is additional or missing

genetic material, a situation that often results in a disease. On the other hand, if the

rearrangement is balanced, there is no extra or missing chromosome material.

Balanced rearrangements are not uncommon in "normal" individuals, although there is a

significant chance that they may pass an unbalanced rearrangement that could cause

birth defects to their children or, if the defect is severe enough, the balanced

rearrangement could cause infertility. Balanced rearrangements may themselves cause

problems in individuals who carry them. This occurs when the rearrangement interrupts

a gene or affects the expression of a gene. The different types of change in

chromosome structure are deletion, duplication, inversion, and translocation.

Deletion.

The first type of alteration of chromosome structure is deletion process. Deletion

involves the loss of a chromosome segment either from the end or internally. This

results in the loss of certain genes. If deletion of a small portion occurs only in one of a

homologous pair of chromosomes, the alleles present on the other homologous

chromosome will be expressed phenotopically skali, It will be expressed phenotypically

even if these are recessive. Below is the diagram show deletion process.


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If deletion affects a considerable number of genes on both homologous chromosomes,

the effects is usually harmful or lethal because of genetic imbalance. Cri du

Chat's ("cat's cry") syndrome individuals have a deletion of the short arm of

chromosome 5. Although they possess the usual signs of chromosomal anomalies,

such as mental retardation and low birth weight, their appearance is not extraordinarily

different from normal individuals. One peculiarity is that affected infants make an

unusual cry resembling that of a cat, hence the name of the syndrome. The other

symptoms are mental retardation, growth failure, round face and facial abnormalities

like small head, small receding chin, widely spaced eyes, and folds of skin over the

upper eyelid. Two other interesting diseases are Prader-Willi's syndrome and

Angelman's syndrome. In both cases, patients with these diseases possess a deletion

in the long arm of chromosome 15. Interestingly, the deletion is in the same location, but

the resulting syndrome depends on whether the deletion was in the maternal or paternal

chromosome.

Duplication.

Duplication process also known as addition process. Duplication occurs when a region

of chromosome replicates so that a set of genes is repeated. Duplications also result

from the reuniting of broken pieces of homologous chromosomes. In some cases the

chromosome pieces rejoin in such a way that there is a doubling, or redundancy, of a


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portion of the chromosome. This changes the number of genes present and may result

in a problem with health, development, or growth.

Large insertions and deletions prevent the production of useful proteins. The effect of

smaller insertions or deletions depends upon how many bases are involved. Sometimes

an entire gene can be inserted (in duplications) or deleted. The effect depends upon

where in the genome the changes occur and how many base pairs are involved. So, we

can conclude that duplication is generally not as deleterious to the organism as gene

deletion. Some duplicated genes may provide new material for evolution.

Inversion.

Besides, Inversion process occurs when a chromosome segment breaks and the

segment is reinserted in the opposite orientation with respect to te rest of the

chromosome. In other hand, inversion meanthe rotation of a broken chromosomesegment in such a way that it rejoins the chromosome in a reversed state, or is flipped,

end to end. Inversions are usually characterized by whether the centromere is included

in the inverted segment. Diagram below show the inversion process.


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Inversions containing the centromere are called pericentric. Those not containing the

centromere are called paracentric. Although an inversion does not change the overall

content of cellular DNA and can be considered a balanced translocation, it can affect a

gene at many levels because it alters the normal DNA sequence. The gene may not

produce its corresponding protein at all, or a nonfunctioning protein may result. There is

a common inversion seen in human chromosomes involving chromosome 9. A

small pericentric inversion is present in approximately 1 percent of tested individuals.

There appears to be no detrimental effect on the carrier, and it does not appear to

cause miscarriage or unbalanced off-spring.

Translocation.

There are two types of translocations that consist of reciprocal and robertsonian. A

reciprocal translocation occurs when two nonhomologous chromosomes break and then

switch genetic material. A robertsonian translocation occurs when two chromosomes

break on the p arm near the centromere and then the two q arms attach together while

the p arms are lost. Either of these types of translocation can occur at any point during

the cell cycle.


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In most cases of chronic myelogenous leukaemia (CML), the leukaemic cells share a

chromosomal abnormality known as Philadelphia chromosome. This abnormality is the

result of a reciprocal translocation between chromosomes 9 and 22. An abnormal hybrid

gene is created leading to the production of a novel protein that is not normally found in

the cell. This protein prevents normal growth and development, leading to leukaemia.

alteration of chromosome structure

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