Operations on Chromosomes

What kind of genome rearrangement events (also called operations) take place?

1.

Operations on a single chromosome:

• Deletions (a certain part is lost, for example abc $\rightarrow$ ac )
• Insertions (a part is added, for example ac $\rightarrow$ abc)
• Duplications (can be tandem, for example abc $\rightarrow$ abbc, or not, for example
  abc $\rightarrow$ abcb)
• Reversals, or inversions (a part is turned around, head to tail, for example
  abc₁c₂c₃c₄de $\rightarrow$ abc₄c₃c₂c₁de) - see figure 10.1.
• Transpositions (two parts change places, for example abcd $\rightarrow$ acbd) - this operation is believed to be the most rare since it requires 3 points of leverage along the choromosome.

How do these operations take place?

If two regions along a chromosome are very similar, they might hybridize just like two different strands of the double helix. Once they are attached, a loop forms. This loop might be discarded (deletion), or its direction might switch (inversion).

2.

Operations on two chromosomes:

• Translocation: two chromosomes swap their "tails". It is important to note that not all translocations are possible. A chromosome contains a part called a centromere which is crucial to cell division; the centromere usually lies somewhere in the middle of the chromosome, and if upon translocation it will be lost from one of the chromosomes, the cell will surely die.
• Fusion: two chromosomes merge.
• Fission: one chromosome splits up into two chromosomes.

  It is not known what exactly happens to the centromere in these cases.

  

Figure 10.1: Illustration of a reversal operation on a DNA strand