What is Breakage Fusion Bridge or BFB cycle or Breakage Rejoining Bridge Cycle?

Breakage-fusion-bridge (BFB) cycle (also breakage-rejoining-bridge cycle) is a mechanism of chromosomal instability, discovered by Barbara McClintock in the late 1930's. This cycle involves breakage of chromosome followed by fusion and bridge formation.

Breakage-fusion-bridge cycle:  A type of chromosomal behavior in which a broken chromatid fuses to its sister, thus forming a "bridge." When the centromeres separate at mitosis, the chromosome breaks again (not necessarily at the bridge), thereby restarting the cycle.

Key things to remember:
  • A chromosome has a centromere and a telomere.
  • Centromere is the region where spindle fibers attach during cell division. Centromere breaks and sister chromatids are separated towards the poles during anaphase.
  • Telomere prevents the ends of chromosomes from fusing with other chromatids.
Steps in BFB cycle
Step 1: Breakage
Breakage Telomere breakage
Telomeric region of a chromosome breaks (breakage).
This Chromosome replicates forming sister chromatids that both lacks telomere at one end.

Step 2: Fusion
The lack of a telomere on these two sister chromatids causes them to fuse with one another.

Step-3: ‘Bridge’ formation
Bridge formation in BFB cycle

During anaphase, the centromere in one of the sister chromatids will be pulled in one direction of the dividing cell, while the centromere of the other will be pulled in the opposite direction forming a ‘bridge’ as the ends are fused
Ultimately, two sister chromatids break apart at any point or region.

Step 4: the cycle repeats

BFB cycle repeats
The two daughter cells receives an uneven chromosome without a telomere.
As the chromosome lacks a telomere, the cycle continues during next division. Therefore, called as BFB cycle.
breakage fusion bridge cycle 

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