In this article we will discuss about some cytological slides and specimen.

1. B-Chromosome or Supernumerary Chromosome:

i. The nuclei of some animals or plants contain one or more chromosomes in addition to normal chromosomes. These additional chromosomes are called B-chromosomes or supernumerary chromosomes.

ii. First described by Wilson in 1905 in Metapodius, an insect, these chromosomes have now been reported in several plants and animals.

iii. These chromosomes are generally smaller than other members of the chromosomal complement.

iv. Structurally, these chromosomes are mostly heterochromatic in nature. But in Tradescbantia B-chromosomes are completely euchromatic while in maize they are partly heterochromatic and partly euchromatic (Fig. 41).

v. In most of the grases, B-chromosomes are smaller than normal ones and they can be distinguished easily.

Morphology of B-chromosomes in maize

2. Chimaera:

i. This is a twig of Bougainvillea bearing flowers of two different colours (pink as well as white) on the same axis.

ii. It is due to chimaera. Chimaera are a type of plants whose tissues are of more than one genetic kind. This can happen due to mutations in a cell of a very young plant, or can be caused by grafting.

iii. White flowers are produced by the tissues responsible for white flowers, and pink flowers on the same branch are produced by the tissues responsible for pink flowers.

3. Inversion Bridge:

i. The slide shows a meiotically dividing cell at early telophase stage.

ii. A dicentric chromosome bridge and an acentric fragment in the form of a dot in between two poles are also visible in the slide.

iii. Because of the presence of acentric fragment the bridge is due to paracentric inversion.

iv. In the formation of the inversion bridge, (i) crossing over is seen in between two chromosomes having heterozygous paracentric inversion (Fig. 42 A); (ii) diplotene stage shows a chiasma within the inverted segment (Fig. 42 B) and; (iii) at anaphase I stage is seen a chromatidal bridge and an acentric fragment (Fig. 42C).

A-C showing formation of the inversion bridge 

4. Salivary Gland Chromosome:

i. Because of multi-stranded nature, salivary gland chromosomes of Drosophila melanogaster are also called polytene chromosome (Fig. 40 B).

ii. In these chromosomes one small and five long threads are seen radiating out from a common point called chromocentre.

iii. Many chromomeres are present in the form of bands in each chromatid.

iv. Balbiani rings in the form of some large puffs are also seen on the chromosomes.

v. These chromosomes are in prophase stage.

5. SAT- Chromosome:

i. This is a chromosome possessing a satellite, and hence called SAT-chromosome.

ii. Heitz (1930) mentioned that the word ‘SAT’ stands for Sine-Acido-Thymonucleinico (without thymonucleic acid).

iii. In these chromosomes, the secondary constriction marks the formation of a round or elongated body called satellite.

iv. A thin chromatin filament separates the satellite from the remaining chromosome.

v. In diameter, the satellite may be smaller or similar to that of chromosome.

vi. For each particular chromosome, the satellite and filament are always constant in form and size.

vii. This is clearly seen in the mitotic metaphase stage because in such slides nuclear membrane and nucleolus are absent and the chromosomes are thick and short.

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