Variations in Chromosome Structure

January 4, 2023 Agric4Profits

Variations in chromosome structure involve changes in parts of chromosomes rather than changes in the number of chromosomes or sets of chromosomes in a genome. There are four types of such mutations: deletions and duplications (both of which involve a change in the amounts of DNA on a chromosome), inversions(which involve a change in the arrangement of a chromosomal segment), and translocations (which involve a change in the location of a chromosomal segment).

Duplication, inversion, and translocation mutations can change back (revert) to the wild-type state by a reversal of the process by which they were formed. However, deletion mutations cannot revert because a whole segment of chromosome is missing, not simply changed in position or copy number.

All four classes of chromosomal structure mutations are initiated by one or more breaks in the chromosome. If a break occurs within a gene, then a gene mutation has been produced, the consequence of which depends on the function of the gene and the time of its expression.

Wherever the break occurs, the breakage process leaves broken ends without the usual specialized sequences found at the ends of chromosomes (the telomeres) that prevent degradation by exonucleases and “stickiness”.

As a result, the end of a chromosome that has broken is “sticky”, meaning that it may adhere to other broken chromosome ends or even to the normal ends of other chromosomes. This stickiness properly can help us understand the formation of the types of chromosomal structure mutations.

Types of Change

In discussions of chromosome rearrangements. It is convenient to use letters to represent different chromosome regions. These letters therefore represent large segments of DNA, each containing many genes.

The simple loss of a chromosomal segment is called a deletion or deficiency.

The presence of two copies of a chromosomal region is called a duplication.

A segment of a chromosome can rotate 180 degrees and rejoin the chromosome, resulting in a chromosomal mutation called an inversion.

Finally, two nonhomologous chromosomes can exchange parts to produce a chromosomal mutation called a translocation.

1. Deletion

The process of spontaneously occurring deletion must include two chromosome breaks to cut out the intervening segment. If the two ends join and one of them bears the contromere, a shortened chromosome results, which is said to carry a deletion.

The deleted fragment is acentric; consequently it is immobile and will be lost. An effective mutagen for inducing chromosomal rearrangmenets of all kinds is ionizing radiation. This kind of radiation, of which X rays and rays are examples, is highly energetic and causes chromosome breaks.

The way in which the breaks rejoin determines the kind of rearrangement produced. Two types of deletion are possible. Two breaks can produce an interstitial deletion.

In principle, a single break can cause a terminal deletion; but, because of the need for the special chromosome tips (telomeres), it is likely that apparently terminal deletions include two breaks, one close to the telomere.

The effects of deletions depend on their5 size. A small deletion within a gene, called an intragenic deletion, inactivates the gene and has the same effect as other null mutations of that gene.

If the homozygous null phenotype is viable (as, for example, in human albinism), then the homozygous deletion also will be viable. Intragenic deletions can be distinguished from single nucleotide changes because they are nonrevertible.

Consequences of Deletion

Deletion becomes very serious if it is multigenic. Multigenic deletions are those that remove two to several thousand genes. If multigenic deletion is made homozygous (that is, if both homologs have same deletion), then the combination is almost always lethal.

This outcome suggests that most regions of the chromosomes are essential for normal viability and that complete elimination of any segment from the genome is deleterious. Even individuals heterozygous for a multigenic deletion – those with one normal homolog and one that carries the deletion may not survive.

There are several possible reasons for this failure to survive. First, a genome has been “fine-tuned” during evolution to require a specific balance of genes, and the deletion upsets this balance.

We shall encounter this balance notion several times in this chapter and the next, because several different types of chromosome mutations upset the ratio, or balance, of genes in a genome.

Second, in many organisms there are recessive lethal and other deleterious mutations throughout the genome. If “covered” by wild-type alleles on the other homolog, these recessives are not expressed. However, a deletion can “uncover” recessives, allowing their expression at the phenotypic level.

Some heterozygous deletions are viable. In these cases the deletion can sometimes be identified by cyclogenetic analysis. During meiosis, homologous chromosomes attempts to maximize pairing such that corresponding segment on the normal homolog forms a deletion loop.

2. Duplication

The process of chromosome mutation sometimes produce an extra copy of some chromosome region. In considering a haploid organisms, which has one chromosome set, we can easily see why such a product is called duplication because the region is now present in duplicate.

The duplicate regions can be located adjacent to each other or one of the duplicate regions can be in its normal location and the other in a novel location on a different part of the same chromosome or even on another chromosome.

In a diploid organism, the chromosome set containing the duplication is generally present together with a standard chromosome set. The cells of such an organism will thus have three copies of the chromosome region in question, but nevertheless such duplication heterozygotes are generally referred to as duplications because they carry the product of one duplication event.

Cytologically, duplication heterozygotes also show interesting pairing structures at meiosis. The precise structure that forms depends on the type duplication. We should concern ourselves with adjacent duplications which can be:

Tandem: A B CD → A B C B C D or

Reverse: A B C D → A B C C B D

Evolutionary Significance of Duplication

Duplication is a very important process in gene evolution. The extra region of a duplication is free to undergo gene mutation because the necessary basic functions of the region will be provided by the other copy.

Mutation in the extra region provides an opportunity for divergence in the function of the duplicated genes, which could be advantageous in genome evolution.

Indeed, from situations in which different gene products with related functions can be compared, such as the globins, there is good evidence that these products arose as duplicates of one another.

In conclusion, the lethality of heterozygous deletions can be explained by genome imbalance and by unmasking of recessive lethal alleles. Deletions are recognized cytologically by deletion loop. Duplications supply additional genetic material capable of evolving new functions.

Variation in chromosome structure are exemplified by those mutations in which changes from the normal state occur in parts of individual chromosomes rather than number of chromosome. The four major types of structural mutations are:

Deletion, in which a chromosome segment is lost.
Duplication, in which more copies of a chromosome segment are present than in the normal state.
Inversion, in which the orientation of a chromosome segment is opposite that of the wild type and
Translocation, in which a chromosome segment has moved to a new location in the genome. The consequences of these structural mutations depend on the specific mutation involved.