M2a. Structural Rearrangements

What are chromosome structural repairs?

Chromosomal breakage caused by internal or external clastogens

• could be repaired most of the time

What causes chromosome structure rearrangement?

Incorrect DNA repair

What are the types of interchromosomal rearrangements?

• Translocations

• Dicentric chromosomes

• Insertion

What are the types of intrachromosomal rearrangements?

• Deletions

• Inversions

• Ring chromosomes

What are balanced translocations?

Reciprocal translocations where no genetic material is lost/gained, net is 0 between ≥2 chromosomes

What are unbalanced translocations?

Nonreciprocal translocations where material is either lost or gained between ≥2 chromosomes

What are Robertsonian translocations?

Fusion in the centromere region between two acrocentric chromosomes

What are dicentric chromosomes?

Chromosomes that possess 2 centromeres

• Unless 1 is inactivated, this will cause problems in mitosis (regarding anaphase bridge, lag, etc.)

What is insertion?

When an entire segment of DNA breaks off from one chromosome and inserts into a different region of another chromosome

What is a deletion?

The loss of chromosomal material, results in functional monosomy of the segment involved

• 2 Types: Terminal and Interstitial

What is terminal deletion?

Are the result of one break with the subsequent loss of the acentric fragment during cell division due to anaphase lag.

What is interstitial deletion?

Are the result from two breaks within the same chromosome arm. Again the acentric fragment is lost during the following cell divisions

What are inversions?

When two breaks occur in the same chromosome and the middle segment rotates 180^o before DNA repair

• results in no net loss of genetic material but will change the gene order

What are pericentric inversions?

Occur when one break is in the short arm of the chromosome and the other break is in the long arm of the chromosome

• Most constitutional pericentric inversions do not affect the phenotype of the carrier

What is paracentric inversions?

Occur when both breaks occur within the same arm of the chromosome, the centromere is not involved in this inversion.

• In meiosis an inverted chromosome must form a loop in the region of the inversion in order to pair with its normal homolog

What are duplications?

When a portion of chromosomal material is duplicated, results in a functional trisomy

• afterwards may be inserted in the same order as the original segment or may be inverted and then inserted in the reverse order.

What are isochromosomes?

Formed when there is a misdivision of the centromere

• Occasionally, the centromere will divide horizontally and the result will be one daughter cell with two sister chromatids of the short arm and the other daughter cell with two sister chromatids of the long arm

What are ring chromosomes?

Formed when there are two terminal breaks occurring on the same chromosome and form a union

• highly unstable because they either break or lag behind during anaphase (like dicentrics)