Differentiate Between Paracentric and Pericentric Inversions

In nature, inversion can happen as a result of transposable components. This process is facilitated with the help of enzymes that cut and paste DNA sequences.

A recombination event may result in the loss of genes from one chromosome and the gain of genes on the other. If the orientation of genes on two homologs is wrong, loss of genes could be gained on the other. A result would be aneuploid gametes.

Chromosome rearrangement and chromosomal mutation can lead to inversion. It will be Paracentric inversion or Pericentric inversion. During this process, an element of the chromosome breaks and reinserts after turning 180 degrees. The arrangement occurs during a revert orientation. This process occurs within a single or individual chromosome. This might not affect the organism, but it’s going to have a negative impact if the rearrangement or inversion happens within the regulatory gene sequence, the gene moves away from its position and may cause disorders in an organism.

Inversion in Biology

Inversion simply alters the direction of genes, not the sequences. Inversion could be Paracentric that occurs outside of the centromere, or it is often Pericentric that involves the centromere. Inversions originated due to spontaneous mutation. We can induce artificially. It alters structural changes within the chromosomes. A bit chromosome might be deleted, duplicated, or inverted. Inversions can also be detected by artificially using X-rays or chemical mutagens. Mutagens have the capacity to interrupt the chromosomal segment.

Regulators of gene expression may be positioned differently in relation to their gene orientation. It may lead to abnormal quantities of gene products. Individuals with inversion have neither lost nor gained any genetic material. It results in the only change in the order of genes in a genetic map. Inversion may break into two parts, one is moving to a replacement location and destroying the function of that gene.

Chromosomal rearrangements are of 4 types

1. Deletion

2. Duplication

3. Inversion

4. Translocation

Types of Inversion

There are 2 sorts of Inversion:

1. Paracentric Inversion

2. Pericentric Inversion

Paracentric Inversion

• Para means “Next to”.

• In this type of Inversion, no Centromere is involved.

• It occurs in just only one arm of the chromosome.

• Inversion does not include the Centromere portion of chromosomes.

• Broken segments are rearranged in a reverse manner.

• Progenies have Acentric (no centromere) and Dicentric (two centromeres) segments of chromosomes.

• Acentric (Deletion of the segment) and Dicentrics (Duplication of Segment) are non-viable.

• With the Acentric fragment spindle fibres are not attached to it, so this fragment does not segregate into the nucleus in meiosis and is usually lost.

• Normal and Inverted are Viable and Monocentric.

• In an organism heterozygous for a Paracentric inversion, the chromosome from an inversion loop during pairing at Prophase 1.

• A single crossover within a paracentric inversion leads to 50% abnormal gametes, and 50% gametes will be viable.

(Image will be uploaded soon)

Fig A Paracentric inversion and Fig B pericentric inversion

Pericentric Inversion

• Peri means “ Around”.

• In this second type of inversion, the Centromere is involved.

• It is often found on both chromosomal arms.

• Results of crossing over of non-sister chromatids contain a monocentric segment of a chromosome.

• Normal and inverted are viable and survive.

• Single crossover within the inversion results in recombination within a Pericentric inversion.

• No dicentric bridge or acentric fragments are produced.

• Gametes receive recombinant chromosomes which cannot produce viable progeny.

• Double crossover results in functional, recombinant chromosomes.

• This inversion is considered a crossover suppressor because single crossover products are lethal.

• The evolution of Humans appears to have been influenced by a Pericentric inversion on Chromosome 18.

Disorders

• Disorders due to Deletion:

1. Cri du chat syndrome (Deletion of a small portion of the 5th Chromosome).

2. Prader-Willi syndrome ( Deletion in the long arm of the 7th chromosome).

3. Robertsonian translocation.

• Disorders due to Duplication:

1. Fragile X syndrome (Repeat CGS segment)

Position Effect

Inversion may cause a position effect. Numerous genes are controlled in a position-dependent manner; if their positions are altered by an inversion, they may be expressed at improper times or in inappropriate tissues. This result is referred to as a Position effect.

When an individual is homozygous for a particular inversion, no special problems arise in meiosis. Two homologous chromosomes can pair and separate normally. When an individual is heterozygous for a particular inversion, the gene order of the 2 homologs differs.

The presence of an inversion loop in meiosis indicates that an inversion is present. It also exhibits reduced recombination among genes located in the inverted region. When crossing over takes place, the result is a tendency to produce gametes that are not viable and thus no recombination progeny are observed.

Differentiate between Paracentric and Pericentric Inversion

Interesting Facts

• Approximately five million years ago, it is said that pericentric chromosome 18 inversion occurred in early ancestors, which diverted them from chimpanzees.

• Pericentric inversion in chromosome 1 is one of the reasons for male infertility.

Important Questions

1. Do pericentric inversions are inherited?

Ans: Similar to paracentric inversion, even a number of crossing-over events that occur during the inversion loop will result in normal gametes, with 50% of gametes inheriting the balanced pericentric inversion.

2. What is a mutation? Name different types of mutation.

Ans: Mutation is the sudden change in the DNA by different mutagens like radiations or chemicals. There are different types of mutations

• Missense mutation- Mutation in which an amino acid is replaced due to a change in a codon that alters the peptide change.

• Point mutation- The change in a single nucleotide, either deletion, addition, or replacement leading to a change in DNA sequence.

• Substitution- replacement of nucleotide with other base is called substitution

• Deletion- removal of base pairs within DNA segment is called a deletion.

Key Features of Inversion

• Paracentric and pericentric are the kinds of chromosomal abnormalities, during this process, either involvement of the centromere or no involvement of the centromere.

• Both inversions are found on an outsized scale of chromosomal mutations. This process occurs within a single chromosome.

• Inversions can be used to study the behaviour of chromosomes during meiosis.

• Chromosome pairing, cytological crossover, formation of dicentric and acentric fragments as seen in paracentric inversions, deletions and duplications in pericentric inversions of the chromosome breaks and reinserts after turning 180 degrees.

• The position impact could also be studied inversion since they alter the euchromatic region of a chromosome.