Rearrangements of chromosomes include deletions of DNA sequences and duplication

Question

Rearrangements of chromosomes include deletions of DNA sequences and duplications of segments, both of which can encompass thousands to hundreds of thousands of bases. Why do these large rearrangements occur? For one, certain structural features of the genome, also referred to as genome architecture, can render various regions fragile and thus prone to events such as chromosome breakage, which often result in translocations, deletions, and duplications. Often, these alterations happen due to errors during cell division when chromosomes align (Figure 1). Homologous recombination between areas of concentrated repeated sequences frequently creates deletions and duplications. Because they commonly involve more than one gene, the disorders caused by these large deletion and duplication mutations are often severe Chromosomal Duplications In chromosomal duplications, extra copies of a chromosomal region are formed, resulting in different copy numbers of genes within that area of the chromosome. If the duplicated sections are adjacent to the original, the process is known as tandem duplication, whereas if they are separated by nonduplicated regions, the duplication is said to be displaced. Duplications may affect phenotype by altering gene dosage. For example, the amount of protein synthesized is often proportional to the number of gene copies present, so extra genes can lead to excess proteins. Because most embryonic developmental processes are heavily dependent on carefully balanced levels of proteins, duplications resulting in extra gene copies (Figure 1) can therefore lead to developmental defects such as those seen in the Drosophila Bar eye mutation (Figure 2) -::::::: Figure 2 Figure Detail Figure 1 Similarly to the effect of other rearrangements, however, duplications can also provide raw material for evolution by producing new copies of genes that are free to mutate and take on other functions. Gene families such as the human globin gene family attest to the role of duplication in evolution. Several globin genes have arisen from a single ancestral precursor, thus making individual genes available to take on specialized roles, with some genes becoming active during embryonic and fetal development, and others becoming active in the adult organism (Figure 3) Chromosomal Deletions Deletions involve the loss of DNA sequences. Phenotypic effects of deletions depend on the size and location of deleted sequences on the genome. For instance, deletions that span a centromere result in an acentric chromosome that will most likely be lost during cell division. As with duplications, deletions can affect gene dosage and thus the resulting phenotype. Also, the larger the deletion, the more genes are likely to be involved, and the more drastic the resulting defect is likely to be The copy number of specific genes needs to be tightly regulated to ensure that when a gene is expressed, the functional product is produced at the correct level, or dosage. Some genes require two copies for normal expression levels to be produced. If one copy is lacking and only one allele remains, a mutant phenotype results; this is called haploinsufficiency. For example, a group of genetic conditions known as short stature Figure 3 Figure Detail

Explanation / Answer

In eukaryotes genetic recombination occurs during meiosis and Mitosis which involves the pairing of homologous chromosomes. it occurs by transfer of information between the chromosomes. This may occur without any physical change or by breaking or joining of DNA strands which may result in forming of new DNA molecule.
It may occur During mitosis in eukaryotes where it involves the formation of two sister chromosomes after chromosomal replication. New combination of alleles are not produced since the 2 sister chromosomes are identical.

Genetic duplication leads to mutation, drift and selection to act upon by providing source of genetic material making new evolutionary opportunities. Genome duplication is a main factor in the evolution of complexity and diversity.

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