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11.2.2. Chromosomal or structural mutation

Chromosomal or structural mutation

The chromosomal mutations are changes in the normal structure of chromosomes without changing their number.

It changes the nucleotide sequence in DNA and therefore also the message transcribed to the mRNA . More or less large segments of the chromosomes are affected, causing changes in their structure , affecting several genes .

Types of chromosomal mutations:

Tipos de mutaciones cromosómicas: deleción, duplicación, inversión, inserción, translocación

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Alterations in the order of genes

Two types are distinguished:

Investments

It consists in that a chromosomal fragment has rotated 180º, so its gene sequence is inverted with respect to that of the rest of the chromosome.

As a result of inversions, loops can form during meiosis , rendering the gametes nonviable.

There are two types of inversions according to their relationship with the centromere:

  • Pericentric: They affect the centromere, and produce a change in the shape of the chromosome.
  • Paracentric: The centromere is not affected or the shape of the chromosome.

Example: hemophilia A, due to inversion affecting the factor VIII gene , on the X chromosome.

Video: Chromosome Inversions.

Video: Investment paracéntica I.

Translocations

It consists of the change of position of a fragment of the chromosome.

The translocation can occur on the same chromosome, on homologous or different chromosomes.

The mutation can be reciprocal , if fragments are exchanged, or not reciprocal, and is called transposition.

The translocations pose no deficiencies for the carrier but for their offspring, as you may inherit one chromosome incomplete or duplications.

An example: chronic myeloid leukemia, by reciprocal translocation between fragments of chromosomes 9 and 22.

Alterations due to the existence of an incorrect number of genes

They are produced as a consequence of an error in the meiotic pairing that can produce an erroneous crossover, leaving one chromosome with one more fragment and the other with one less fragment. They can also be the consequence of investments  or translocations  of the parents.

The gametes produced, after fertilization, could cause:

Deletion

The deletion is produced by the loss of a chromosomal fragment, and therefore, of the genes it contains. They are usually fatal in homozygous individuals.

An example of a deletion in humans on chromosome 5 is the one that produces the cat's meow syndrome , due to the characteristic cry of babies who suffer from it.

Video: Cat Meow Syndrome .

Duplication

Duplication occurs when a chromosomal fragment is repeated on the same or another chromosome.

The deletion and duplication are caused by faulty crossovers.

Consequences of chromosomal mutations

The translocations and investments, to not changing the number of genes, usually not greatly affect the carrier. The problem is greater when a gene ends up further away from other areas of the chromosome that control its expression or when they approach other regulatory areas that do not correspond to it.

The deletions and duplications can have more serious consequences, since it can transmit defective chromosomes to the offspring, being this unviable or with mutations.