Karyotype: The nature of eukaryotic nuclear chromosomes

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The set of chromosomes as viewed under the microscope is called a karyotype. The karyotype of an organism is defined by a number of visible characteristics. We also use the term genome to describe one unique set of chromosomes – the basic chromosome set “x”.

Visible chromosomal characteristics

Chromosomes can be viewed relatively easily under the microscope, but only just before, during, and immediately after cell division. During cell division chromosomes become condensed (through spiralizing) from their extended form found in non dividing cells into their shorter, fatter form that can easily be viewed under the microscope. Condensed chromosomes display characteristics that are used to describe a karyotype. It is presumed that these characteristics are in the same relative positions on the extended chromosomes of non dividing cells.  

Useful karyotypic characteristics:

• Chromosome size 
• Chromosome number
• Sex chromosomes
• Centromere position
• Nucleolar organizer position
• Chromomere pattern
• Heterochromatin pattern
• Secondary constriction
• Banding patterns

Chromosome size 

The chromosomes of a single genome may differ considerably in size. For example, chromosome 1, the largest chromosome of the human genome, is four times larger than chromosome 21, the smallest chromosome.

Chromosome number 

The number of chromosomes in a set is a fixed characteristic of the karyotype.

Sex chromosomes 

Sex chromosomes are a regular feature in animals, but also occur in a few plants. A wide range of different sex chromosome systems are known. For example, humans and Drosophila possess the XX/XY system where XX = female and XY = male. Most grasshoppers are XX/X0; XX = female and X0 = male. The plant species Melandrim album is dioecious and has a sex chromosome system similar to humans where XX = female and XY = male. Tiger beetle species displays a range of multiple sex chromosome systems; XnY, where n varies between 2 and 4. Avian species have a sex chromosome system similar to humans, but the sexes are reversed. The sex chromosomes have therefore been designated as ZW = females and ZZ = males.

Centromere position

During cell division the spindle fibers attach to a specialized region called the centromere, which has a fixed position. Under the microscope the centromeric region appears as a constriction, also called the primary constriction. The centromere divides the chromosomes into two “arms”. The shorter arm is called p and the longer arm q. The ratio between the lengths of the two chromosome arms, the arm ratio, is a useful characteristic for identifying individual chromosomes. Centromere positions allow chromosomes to be categorized as telocentric (centromere at one end of a chromosome), acrocentric (centromere close to one end of a chromosome), submetacentric (arms nearly the same length) or metacentric (arms the same length).

Nucleolar organizer position

Nucleoli are spherical, non-membrane bound structures found within the nucleus in which ribosomal RNA is transcribed. Nucleoli are formed around specific genetic loci called Nucleolar Organizing Regions (NORs) and are composed of protein and nucleic acids. Different organisms have different numbers of nucleoli, ranging from one to many per chromosome set. A NOR consists of tandem repeats of rRNA genes, which can be found in several different chromosomes. RNA is synthesized at the NOR and deposited into the nucleoli, and later exported to the cytoplasm to become incorporated into ribosomes. The positions of nucleoli are useful landmarks in cytogenetic analyses. In humans, more than 200 clustered copies of the rRNA genes occur on five different chromosomes.

Chromomere patterns

The chromomeres are beadlike, localized thickenings found along the chromosome during the early stages of nuclear division. The positions of chromomeres are the same in all homologous chromosomes. Although chromomeres can be useful as markers, their molecular nature is not known.  

Heterochromatin patterns

When chromosomes are treated with chemicals that react with DNA, such as Feulgen and Giemsa, distinct regions with different staining characteristics are revealed. Densely staining regions are called heterochromatin and reflect a high degree of compactness; poorly staining regions are called euchromatin and indicate less tightly packed regions. Most of the active genes are in euchromatin.   

Secondary constriction

Secondary constrictions are in addition to centromeres and are a narrowing of a chromosome. Secondary constrictions close to the telomere form a satellite structure at the tip of the chromosome, a knoblike structure. Some secondary constrictions are associated with NORs.

Banding patterns

Special chromosome staining procedures have revealed banding patterns in chromosomes. The positions and sizes of the bands are highly chromosome-specific; therefore, they represent useful landmarks. There are Q bands (produced by quinacrine hydrochloride), G bands (produced by Giemsa stain), and R bands (produced by reversed Giemsa). Chromosome banding is used to separate chromosomes that are similar is size and centromere position.

This chromosome set demonstrates many of the karyotypic landmarks