Numerical and structural variations in chromosomes and their significance

Changes in the number and structure of the chromosomes may occur spontaneously or experimentally by the action of radiation or chemicals. The number of chromosomes is generally constant for plant and animal species.

‘Chromosomal change’ in number are of two main kinds: (1) Euploids : a change in the number of chromosome sets, the set being kept balanced (2) Aneuploids: there is a loss or gain of one or more chromosomes. In exceptional cases there are haploid organisms. In plants, polyploids (triploid, tetraploid) are rather common. They originate by reduplication without cytokinesis.

In animals, polyploids are scarce, because sex is frequently determined by a pair of different chromosomes (XY). Polyploidy can be induced by colchicine, this substance in used in agriculture to improve certain plant species. Allopolyploidy consists of the formation of a hybrid with different sets of chromosomes. Sometimes a diploid gamete fertilizes a normal haploid gamete, producing a new species with a triploid number. Thus there are two main kinds of change in the number of chromosomes.

In euploids the change is in the haploid number but the entire set of chromosome is kept balanced, whereas, in aneuploids there is a loss or gain of one or more chromosome(s) causing the set to become unbalanced. In humans aneuploidy may cause severe alterations
of the phenotype. Polyploidy and allopolyploidy are expamples of euploidy.

Among the aneuploid organisms there are the trisomic (i.e. with three similar chromosomes instead of a normal pair) and the nionosomic (missing one member of a pair of homologous chromosomes) conditions. There two conditions are important in humans and may arise by nondisjunctional division.

Chromosomal aberrations:

Also called ‘structural alterations’ involve a change in the molecular organization of the chromosome. One or two breaks are produced at the level of the chromatin fiber, and the effect depends on whether it occurs in the G, phase (one chromatid) or in the G, (two chromatids).

Chromosomal aberration should be differentiated from gene or point mutations, which occur at molecular level. Ionizing radiations X-rays, y-rays, p-rays, fast neutrons, slow neutrons and ultraviolet light can produce point mutations or chromosomal aberrations.

The number of mutations increases proportionally with the dose of X-radiation. The effect of radiation is cumulative. An exposure of culture cells to 2DR (roentgen units) is sufficient to produce one chromosome break per cell. In contrast to mutations which increase proportionally with the dose, chromosomal aberrations increase exponentially with the dose. Breakage can be followed by “healing” of the broken end (the breaks may be at the chromosome or the chromatid level).

Chromatid breaks are produced in cells irradiated after the S- phase. If two breaks are produced, translocation, inversions, and large deletions may be induced. Dicentric chromosomes may be produced, which form a bridge at anaphase. In the human, heavy medical radiation, nuclear accidents or radioactive substances may produce chromosomal aberrations. In the production of new antibiotics and plants, irradiation is being used to economic advantage. Somatic mutations are not transmitted from one generation to another, whereas, germ cell mutations may be passed to the offsprings.

Some of the main chromosomal aberrations are:

(1) Deficiency or deletion, in which a part (either interstitial or terminal) of the chromosome is missing. The parts of the chromosome lacking the kinetochore are generally lost. Deficiency may be ‘heterogygous’ (one chromosome normal) or ‘homozygous’ (both chromosomes are deficient). The latter generally does not survive.

(2) Duplication, in which a chromosome segment is represent two or more times (tandem duplication).

(3) Translocation in which there is an exchange of segments between non-homologous chromosomes (reciprocal type) or between different parts of the same chromosome (simple type). Sometime centric fusion occurs when the two chromosomes are broken near the kinetochore and form a metacentric (v-shaped) chromosome.

(4) Inversion, in which there is breakage of a segment, followed by its fusion in a reverse position. It is ‘pericentric’ if the kinetochore is outside. Isochromosomes may arise from a break at kinetochore, resulting in two chromosomes with identical arms.

Sister chromatid’s exchange consists of the interchange of DNA segments between sister chromatids in a chromosome. First described in studies using ‘H-thymidine, it was interpreted as the result of radiation from ‘H. This aberration can be studied with bromodeoxyuridine (BrdU), which is incorporated instead of thymidine and which produces changes in fluorescence or in the Giemsa stain. This technique permits the detection of diseases, characterized by chromosome fragility and enables the study of the action of mutagenic drugs which increase the number of breaks.

Significance:

Cytogenetic studies have provided excellent methods for establishing taxonomic interrelationships and have, thereby, contributed to studies of evolution and systematics. One of the most frequent causes of evolution is a change in the order of genes as a result of chromosomal aberrations. In plants, aneuploids and polyploidy are frequent sources of variations, where as in mammals and birds, speciation depends more on chromosomal rearrangement and point mutations.

In the study of evolution, the number of chromosomes, the characteristics of the karyotype, the total chromosomal area, and the content of DNA are investigated. The presence of metacentric chromosomes may, in some cases, result from fusion of two acrocentric chromosomes.

The contrasting phenomenon (i.e. dissociation) may lead to an increase in chromosome number. The problem of evolution is very complex. Knowledge of cytogenetics is most fundamental to the understanding of this problem of evolution. One example of importance of cytogenetics in evolution is given in a comparison of the chromosome pairs of the primates and humans. Primates have 48 chromosomes (24 pairs) while human have 23 pairs instead of 24 pairs of primates. There comparative studies have demonstrated that

(i) 13 pairs of chromosome in humans are identical with 13 pairs in the chimpanzee.

(ii) Chromosome number 2 in humans has resulted from centric fusion (or Robertsonion translocation) of two chromosomes present in hominoid apes.

(iii) The other chromosomes differ in the occurrence of nine pericentric inversions and two additions of chromatin material.