CHROMOSOME DUPLICATION: CHARACTERISTICS AND EXAMPLES - BIOLOGY - 2025

A chromosomal duplication describes a fraction of DNA that appears twice as a product of genetic recombination. Chromosomal duplication, gene duplication or amplification is one of the sources of generation of variability and evolution in living beings.

A chromosomal duplication is a type of mutation, since it involves a change in the normal sequence of DNA in a chromosomal region. Other mutations at the chromosomal level include chromosomal insertions, inversions, translocations, and deletions.

Chromosomal or chromosomal duplication. Courtesy: National Human Genome Research Institute, via Wikimedia Commons

Chromosomal duplications can occur at the same source site as the duplicated fragment. These are the batch duplications. Batch duplicates can be of two types: direct or inverted.

Direct duplicates are those that repeat both the information and the orientation of the repeated fragment. In batch inverted duplicate fragments the information is repeated, but the fragments are oriented in opposite directions.

In other cases, chromosomal duplication can occur at another site or even on another chromosome. This generates an ectopic copy of the sequence that can function as a substrate for crossover and be a source of aberrant recombinations. Depending on the size involved, the duplications can be macro- or micro-duplications.

Evolutionarily speaking, duplications generate variability and change. At the individual level, however, chromosomal duplications can lead to serious health problems.

Mechanism of chromosomal duplications

Duplications occur most frequently in regions of DNA that have repetitive sequences. These are substrates of recombination events, even if they occur between regions that are not perfectly homologous.

These recombinations are said to be illegitimate. Mechanistically they depend on sequence similarity, but genetically they can be carried out between non-homologous chromosomes.

In the human being we have several types of repetitive sequences. The highly repetitive ones include the so-called satellite DNA, limited to the centromeres (and some heterochromatic regions).

Others, moderately repetitive, include, for example, the batch repeats that code for ribosomal RNAs. These repeated or duplicated regions are located in very specific sites called nucleolus organizing regions (NORs).

NORs, in humans, are located in the subtelomeric regions of five different chromosomes. Each NOR, for its part, is made up of hundreds to thousands of copies of the same coding region in different organisms.

But we also have other repetitive regions scattered throughout the genome, with diverse composition and sizes. All can recombine and give rise to duplications. In fact, many of them are the product of their own duplication, in situ or ectopic. These include, among others, minisatellites and microsatellites.

Chromosomal duplications can also arise, more rarely, by the joining of non-homologous ends. This is a non-homologous recombination mechanism that is observed in some DNA double band break repair events.

Chromosomal duplications in the evolution of genes

When a gene is duplicated in the same place, or even in a different one, it creates a locus with sequence and meaning. That is, a meaningful sequence. If it stays that way, it will be a duplicate gene of, and from, its parent gene.

But it may not be subject to the same selective pressure as the parent gene and may mutate. The sum of these changes can sometimes lead to the appearance of a new function. Already the gene will also be a new gene.

Duplication of the ancestral globin locus, for example, led in evolution to the appearance of the globin family. Subsequent translocations and successive duplications made the family grow with new members performing the same function, but suitable for different conditions.

The globin family of genes. Yuhrt, Via Wikimedia Commons.

Chromosomal duplications in the evolution of species

In an organism the duplication of a gene leads to the generation of a copy called a paralogue gene. A well-studied case is that of the globin genes mentioned above. One of the best known globins is hemoglobin.

It is very difficult to imagine that only the coding region of a gene is duplicated. Therefore, every paralogue gene is associated with a paralogue region in the organism that undergoes duplication.

In the course of evolution, chromosomal duplications have played an important role in different ways. On the one hand, they duplicate the information that can give rise to new functions by changing genes with a previous function.

On the other hand, placing the duplication in another genomic context (another chromosome, for example) can generate a paralog with different regulation. In other words, it can generate greater adaptive capacity.

Finally, regions of exchange are also created by recombination that lead to large genomic rearrangements. This in turn could represent the origin of speciation events in particular macroevolutionary lineages.

The problems that microduplications can cause in an individual

Advances in next-generation sequencing technologies, as well as chromosome staining and hybridization, now allow us to see new associations. These associations include the manifestation of certain diseases due to the gain (duplication) or loss (deletion) of genetic information.

Genetic duplications are associated with a change in gene dosage and with aberrant crossovers. In any case, they lead to an imbalance of genetic information, which sometimes manifests itself as a disease or syndrome.

Charcot-Marie-Tooth syndrome type 1A, for example, is associated with microduplication of the region that includes the PMP22 gene. The syndrome is also known under the name of hereditary sensory and motor neuropathy.

There are chromosomal fragments prone to these changes. In fact, the 22q11 region carries numerous low copy number repeats specific to that portion of the genome.

That is, from the region of band 11 of the long arm of chromosome 22. These duplications are associated with numerous genetic disorders, including mental retardation, ocular malformations, microcephaly, etc.

In cases of more extensive duplications, partial trisomies can appear, with harmful effects on the health of the organism.

References

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