chromosomal rearrangement

1. CHROMOSOMAL
REARRANGEMENT
Deepak arun

2. Chromosomal rearrangements:
gross changes in chromosomal morphology.
Chromosomal morphology

3. Rearrangements

4. Transversion and Transition Mutation
They refers to a point mutation
In transversion mutation ; a single purine is substituted for a
pyrimidine or vice versa.
In Transition mutation ; a purine nucleotide is substituted to
another purine (A ↔ G), or a pyrimidine nucleotide to another
pyrimidine (C ↔ T).

5. How do rearrangements occur?
Chromosome breakage.
Can occur via radiation, mutagens etc.
Repeated sequences, especially transposable elements, in
the DNA may frequently be involved, i.e. non-homologous
recombination
e.g. P- elements in Drosophila.
Alu elements probably do in mammals; perhaps in us?

6. Breakage leads to "sticky ends" (? something to do with the
function of a telomere?). Telomere – repeated motifs.
Telomere's function: to "cap" sticky ends, prevents
chromosomal mutation.
Grows to replace losses in DNA synthesis – telomerase.
Telomeric inversions are rare
⇒ Most (successful) rearrangements are
paracentric or pericentric inversion

7. Evolutionary effect of rearrangements
General rule:
Heterozygous rearrangements often lead to
the production, in meiosis, of:
UNBALANCED GAMETES
(duplications and deletions in progeny)

8. e.g. Paracentric inversions
Inversion heterozygotes; chromosomes pair in loops
No crossing over in inversion: gametes fine
Crossing over in inversion, problems

9. dicentric bridge (breaks at cell division)
acentric fragment (lacks centromere, becomes lost)
duplications and deletions of chromosomal material
→ heterozygote disadvantage
→ fixation
(except in many flies, where paracentric inversions act
crossover suppressors)

10. Similar repeated change in many chromosomes at
once. Not fully explained.
For example, the primitive chromosome number
of chromosomes in Musculus domesticus, the house
mouse, is 2n = 40, all acrocentrics.
However, by a series of Robertsonian fusions,
there are multiple chromosomal races with less,
some of which have as few as 2n = 22.
Nobody knows why!
"Karyotypic orthoselection"

11. Phylogeny from rearrangements
Banding patterns; can identify chromosomes and
chromosome parts.
In humans/apes, chromosome banding patterns first
showed that chimps are more closely related to humans
than gorillas
Humans differ from closest relatives by 9 pericentric
inversions and 1 centric fusion

12. Evolutionary significance
Heterozygous disadvantage may prevent evolution of
new chromosome rearrangements
Most populations should be fixed. In general, true.
But polymorphisms occur. e.g. Diptera.
Often, non-disjunction rates low; e.g Mus.
However, mostly some heterozygous disadvantage,
leading to fixation
Can cause a partial barrier between populations
fixed for different rearrangements (e.g. species).

13. Did chromosome change cause speciation? Or
did it occur since separation?
Most now think genic differences more important
than chromosomes in speciation (except
polyploidy).
Recent suggestions: Rearrangements trap groups
of genes effecting ecological differences? Due to
suppression of recombination by rearrangement.
e.g. in Drosophila pseudoobscura vs. D. persimilis