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November  14, 2017

GROSS CHROMOSOME STRUCTURE 
REFERENCES:

Schulz-Schaeffer, J. 1980 Cytogenetics. Chapter 2.
Singh, R.J. 1993. Plant Cytogenetics CRC Press Inc. Chapter 5. Karyotype Analysis
Cytogenetics Gallery, Department of Pathology, University of Washington
http://www.pathology.washington.edu:80/Cytogallery/

Learning checklist:

1. Know the fundamental features of chromosomes:  including arms, euchromatin, heterochromatin, satellites and NORs.
2. Know the different ways of representing chromosomes: karyotypes, karyograms, idiograms.
3. Know chromosome nomenclature: regions bands, arm ratios.
4. Understand how chromosome painting works.



The ability to study eukaryotic genomes depends upon the fundamental ability to uniquely identify each chromosome, and to detect changes in chromosome number and structure. Even something as fundamental as the number of chromosomes (2N) can be crucial, but may be hard to determine. For example, until the 1950s there were many independent observations leading to a human diploid complement of 2N = 48. As banding methods improved, the correct number of 2N = 46 was finally determined, and, upon reexamination of previous cytology, found to be supported by the evidence. This caused some embarassment at the time.

This illustrates the point that before you can even count chromosomes, you have to be able to uniquely identify them.

A. Fundamental features of chromosomes

 
An idiogram of a metaphase chromosome.

The classification of chromosomes is based on physical characteristics such as telomere, position of kinetechore, secondary constrictions, size and position of heterochromatic knobs and relative lengths of the chromosomes.

B. Ways of representing chromosomes

Karyotype - the exact haploid chromosome set of the organism. The karyotype is particular to an individual or to related groups eg. species.

Karyogram - the physical measurement of the chromosomes from a photomicrograph where the chromosomes are arranged in the descending order- longest to shortest. Different groupings are also possible.

Idiogram - a diagrammatic sketch or interpretative drawing of the chromosomes based on physical characteristics visible in the karyogram.

Karyotype analysis is usually based on somatic metaphase chromosomes obtained after pretreatment.

mgs.jpg
G-banded chromosomes from a human male. 
From
Cytogenetics Gallery, Department of Pathology, University of Washington
http://www.pathology.washington.edu:80/Cytogallery

Banding makes it easier to uniquely identify specific chromosomes. However, since most genomes contain many chromosomes, it is most instructive to arrange them in a karyogram. In most organisms, the largest chromosome is numbered as 1, and successive numbers assigned to chromosomes in descending order of size.


Karyogram of  G-banded chromosomes from a human male.
From Cytogenetics Gallery, Department of Pathology, University of Washington
http://www.pathology.washington.edu:80/Cytogallery

The karyogram accomplishes several things. First,  it organizes the chromosomes into homologous groups, making it easier to detect nondisjunction.  Secondly, comparison between banding patterns of homologous chromosomes aids in the detection of chromosomal aberrations such as deletions, insertions, or translocations.

A uniform nomenclature makes it possible to compare chromosome structure among different individuals.  Idiograms allow the representation of most of the key features of chromosomes in a standardized way.

Ideogram of human chromosomes.
From Cytogenetics Gallery, Department of Pathology, University of Washington
http://www.pathology.washington.edu:80/Cytogallery

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