Variations in Chromosomal Structure

Variations in chromosomal structure can be external physical stress, ionizing radiations and chemically synthesized compounds. There is a chance for chromosome to break before duplicating DNA during S phase of cell division cycle. Broken DNA replicates on its own. When chromosomes break after entering in to S phase it ultimately damages single chromatids. Damaged chromatids produce two ends which are sticky. This stickiness helps in reunion. If broken ends are separate then they remain opened. When they are brought near it helps in reunion of ends.  Thus single chromatid is formed back. Now when this chromatid end breaks it can attach to the other chromatid.  Obviously this results in transformation of information forming new alleles or genes. When there is multiple breaks it can lead to genetic mutation called chromosomal aberrations. Thus variation in chromosomal structure has major consequence in medical, genetic or evolutionary type.

Classification of variations in chromosomes

Depending on genetic linkage variations they are of two types namely noncentrometric variation and centerometric variations. Non centrometric variation can produce single break or two break of same and different chromosomes.

• Non centrometric variations
  • Single breaks
  • Homologous chromosomes breaks
  • Non homologous breaks
• Centrometric variations
  • Fission
  • Fusion

Noncenterometric single break

When a chromosome breaks it rejoins. It is of three types which are as follows.

• Restitution: When broken end rejoins without any consequence it is called restitution. They may be acentric or centric fragments depending on presence of centromeres.
• Deletion: Centric fragment generally moves with the help of centromere in to cell division. When this centric fragment is subjected to variation, it is lost resulting in deletion.
• Dicentric bridge:  When centric fragment of same chromosomes join together forming two centromeres is referred as dicentric bridge.

Noncenterometric homologous chromosome variation

Homologous chromosome break can result in deletion or insertion. Reunion of two breaks can avoid acentric fragment resulting in deletion. During the reunion of chromosomes it revert its configuration resulting in insertion.

Noncenterometric non homologous chromosomal variation

Two nonhomologous chromosomes can reunite in many ways. When translocation occurs in between two nonhomologous chromosomes it results in reciprocal translocation.

Centerometric chromosomal variations

Two types of centerometric variation are fission and fusion. Robertsonian fusion results in decreased chromosome number and same genetic material informations. Here chromosomes arms and not chromosome number. Fundamental number is count of chromosome arms. Centerometric fission results in increased chromosome number and not fundamental number.