What causes a duplicated chromosome
Like any physical structure, chromosomes and thus genes can be changed. The changes are noticeable as a change in heredity (mutation). Most of the time, the original state cannot be restored. If genetic information is lost, for example through the loss of a piece of chromosome, the loss remains irreversible. A change caused by mutation is retained in all subsequent generations, provided the damage does not result in lethality. A number of different chromosome changes (chromosome mutations) could be identified in giant chromosomes, which, as later showed, also apply to other chromosomes:
Deficiency. Loss of piece at one end. Since the loss of pieces almost always only occurs in one of the two homologous partners, pairings are obtained between a defective and an intact chromosome. Deficiencies are recognizable in the cytological picture on one step. The unpaired section of the intact partner is a good indication of the length of the lost piece.
Deletion. Loss of piece in the middle of a chromosome. A loss in the middle of the chromosome can be recognized by a protuberance. The defective partner lacks the section that the intact one has. The position of the deletion and its extent can in turn be inferred from the length of the protuberance.
Duplication. Individual chromosome sections are duplicated. Here, too, there is a protuberance during mating. In order to distinguish such a loop from a deletion loop, one must analyze the banding pattern. If there is a pattern in the unpaired section that is repeated elsewhere (usually adjacent), you are dealing with a duplication. If a section is missing, there is a deletion.
Inversion: This is a reversal of a specific part of the chromosome. The pairing of homologous chromosomes is possible with the formation of loops, which often appear to be quite complex.
Translocations. Translocation is the transfer of a chromosome segment (segment) from one chromosome to another (non-homologous). Translocations are common to a number of species (e.g. maize, Oenothera i.a.) has been proven.
How do chromosome mutations arise? The easiest way to explain the deficits is. They arise as a result of a simple chromosome break. Numerous mutation-inducing agents, e.g. certain chemicals or ionizing radiation, induce such damage, more precisely, they increase the likelihood of fractures occurring. Chromosome fragments without a centromere are lost in the subsequent mitosis. They are not integrated in either of the two daughter nuclei, usually remain in the plane of the equatorial plate and perish there.
The other chromosome mutations can be explained by two events that immediately follow one another: break, then fusion. This mechanism is also the basis of the crossing-over. In purely formal terms, deletion and duplication can be understood as an "illegitimate" crossing-over. In the "normal" crossing-over, two chromatids are created that are homologous to the original ones (i.e. of the same length and with the same number of loci). In the case of "illegitimate" crossing-over, on the other hand, two products of unequal length are created. As a result, one chromatid loses a piece (deletion), the second gains another, namely a section that it actually already has (duplication). Usually duplicated sections are next to each other (as a tandem).
The classic example of such deletion / duplication formation is the bar gene (the bar locus; "Bar" is capitalized because of the dominance over the wild type) from Drosophila. Drosophila usually has approximately oval shaped eyes. In the mutant Bar, if the gene is homozygous, they are bar-shaped (= bar). The appearance of Bar itself is based on a duplication of a chromosome segment. The predisposition for this trait can be assigned to a specific chromosome band. "Illegitimate" crossing-over in the area of this section can restore the normal state of the complex through loss of pieces (reverse mutation), and on the other hand, a section is created in the homologous chromatid in which the chromosome band is switched three times in a row (> double bar).
Duplications are by no means without consequences for the organism. The expression of the bar characteristic already shows that it is not the absence or the defect of a certain gene that causes the abnormal eye shape, but an excess chromosome segment.
In a normal diploid genome, there are two alleles for each gene. They are on separate (but homologous) chromosomes and are therefore said to be in the trans conformation. After duplicating a chromosome segment, the genes on it now appear four times, twice in trans and twice in cis (cis = on the same chromosome):
A B C D E F. . .
C and are now referred to as pseudo-alleles. They are not separated from one another during meiotic segregation (separation of homologous sets of chromosomes), unless by crossing-over. An allele and the associated pseudo allele can accumulate mutations that are independent of one another and thus undergo an evolution that is independent of one another. The effect of allele and pseudo-allele is only rarely additive. A pseudo allele (in cis) does not replace a second allele (in trans). This observation indicates that genes in a genome are not independent units, but that their activities are controlled by neighboring gene segments. This phenomenon was originally called the position effect. The molecular biological analysis begins to provide information about what the control mechanisms of individual genes might look like.
The development of an inversion was analyzed by B. McCLINTOCK (1938) using the example of the maize chromosomes. After breaking, homologous sections stick to one another, with the result that, in addition to centromeres-free pieces (which must be written off as a loss), those with two centromeres are formed. During the anaphase, these migrate to opposite poles so that the section between them forms a bridge that eventually breaks. Since the location of the break is left to chance, one of the daughter cells will receive an elongated piece of chromosome, the other a shortened one. In the extended one, the additional area is inverted to the existing one.
Translocations arise as a result of the pairing of non-homologous chromosomes during meiosis. Oenothera is the classic example of this. We have already seen that Oenothera lamarckiana is only viable as a hybrid. In contrast to most other animal and plant species, Oenothera the coupling groups and chromosomes cannot be correlated with one another. There are n = 7 chromosomes, but only one coupling group (= complex, alternative gaudens or velans). The reason for this lies in a series of regularly successive translocations (R. E. CLELAND, University of Indiana, Bloomington, 1949).
CLELAND symbolized the information content of each chromosome with two digits separated by a point. The maternal chromosome set would then be written as follows:
1.2 5.6 9.10 13.14
This contrasts with the paternal set of chromosomes:
4.5 8.9 12.13
During meiosis, the homologous chromosome segments pair. As a rule, they are identical to chromosomes that are homologous to one another. It is different here, because the chromosomes shown form a chromosome ring due to the regular (balanced) translocations. During anaphase I, the centromeres are distributed strictly alternately between one and the other daughter cell. The result: everything stays the same. The chromosomes of the once maternal and the once paternal chromosome set remain closed to each other, which explains the existence of only one linkage group.
This system usually works quite well, but it is more susceptible to failure than the usual meiosis mechanism. The disorders manifest themselves in an increased mutation rate within the genus Oenothera. This is striking enough and is probably the reason why H. de VRIES discovered the phenomenon of mutation in these species at the beginning of the century. Comparable situations were found at Rhoeo discolor, Paeonia californica as well as for species from the genus Datura proven.
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