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Lecture 19, part 1 of 4
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November 16, 2017

CHANGES IN CHROMOSOME STRUCTURE


REFERENCES: SINGH, R.J. 1993. Plant Cytogenetics. Chapter 6A
Schulz-Schaeffer, J. 1980 Cytogenetics. Chapters 11 & 12.

Learning checklist:

A. Deficiencies (Deletions)

B. Duplications

C.  Mechanisms That Change Gross Chromosomal Structure

D. Transposition



Mutations encompassing large regions of a chromosome can be visualized cytologically. There are four different classes of gross structural chromosome changes - deletions, duplications, inversions and translocations.

Critical points:

  1.  It's hard to talk about each of these classes of mutations separately because all are the result of the cell's machinery for rejoining broken pieces of chromosomes.
  2. Typically, a single event will produce both a duplication and a deletion.
  3. Breakage of chromosomes results in chromosome ends which lack telomeric sequences. Broken ends from either homologous or heterologous chromosomes can be fused together.

A. DEFICIENCIES (Deletions)

1. Types of deficiencies 


The loss of a chromosome segment from a normal chromosome is called a deletion or deficiency (Df). Deficiencies can be either intercalary or terminal.

Terminal deficiencies : deletion from the end of a chromosome, not including the centromere.

Intercalary deficiencies - the loss of intercalary or interstitial segments requires two chromosome breaks and rejoining of the flanking chromosomal fragments.


2. Detection of deficiencies

examples from Cytogenetics Gallery, Department of Pathology, University of Washington
http://www.pathology.washington.edu:80/Cytogallery/
 
DiGeorge Syndrome is characterized by cardiac and immune system defects and mental retardation due to a (cytologically) small deletion on chromosome 22.

The chromosome on the right in each pair is shorter than the one on the left. This is due to the deletion of a bright band in the long arm of the chromosome to the right, near the centromere.

FISH with a chromosome 22-specific probe shows that two loci hybridize in a complete copy of chromosome 22, but only one locus lights up in a DiGeorge chromosome 22.
digeorge.jpg

  3. Consequences of deficiencies

The size of the deficiency can vary from the loss of a nucleotide to loss of large chromosome segments. The genetic proof which can distinguish deficiencies from point mutations is the failure to mutate back to the original form. If the deletion contains a centromere, the resultant acentric fragment will be lost. 

B. CHROMOSOME DUPLICATIONS

Definition: An extra piece of chromosome segment attached to the same homologous chromosome (intra-chromosomal) or transposed to one of a nonhomologous chromosomes in the genome (inter-chromosomal). The size of the doubled segment can vary considerably. Duplications are generally more readily tolerated than deletions.
 

1. Types of duplications


 
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