The following points highlights the five special types of chromosomes. The types are: 1. Polytene Chromosome or Giant Chromosome 2. B-Chromosome or Supernumerary Chromosome 3. Chimaera 4. SAT-Chromosome.
Type # 1. Polytene Chromosome or Giant Chromosome:
To prepare acetocarmine squash of giant chromosome or polytene chromosome from salivary glands of Drosophila.
Drosophila melanogaster larvae. NaCl, acetocarmine solution (1%), slides, coverslips. glacial acetic acid, rubber fitted glass rod. filter paper microscope, spirit lamp, dissection box
1. Pick out the largest larva of Drosophila with a fine forceps, place it on a clean slide and put drop of 0.78% saline water (NaCl).
2 Dissect out the salivary gland of the larva which is a Y-shaped, bilobed structure with thin strips of whitish fat.
3. Put the salivary gland in 1% acetocarmine stain for about 10-15 minutes, and transfer the stained gland on a slide.
4. Put a drop of glacial acetic acid (10%) on the stained salivary gland and then put the coverslip.
5. Press it by a rubber-fitted glass rod or by thumb, heat gently and squash.
6. Use a small piece of filter paper to absorb extra fluid. Observe the slide under microscope.
Giant chromosomes of salivary gland (Fig. 322B) are now clearly visible. These are also called polytene chromosomes.
Distinct transverse bands are present on these chromosomes which also show following characteristics:
1. These are commonly known as Giant chromosomes or salivary gland chromosomes or polytene chromosomes.
2. They were first observed by Balbiani in 1881.
3. Each chromosome is made up of two homologous chromosomes which are loosely twisted around each other.
4. These are much larger than the somatic chromosome at metaphase.
5. Each chromosome is made up of dark and light bands.
6. Dark bands are rich and DNA. RNA and proteins are, however, also found.
7. Inter bands show no staining with basic dyes. They contain small amount of nucleic acid and proteins.
8. Puffs and Balbiani rings are present in these chromosomes.
9. Drosophila salivary gland contains 4 haploid pair of giant chromosomes.
10. Functions of polytene chromosome include the following:
(i) They help in nucleic acid synthesis.
(ii) They help in the protein synthesis indirectly.
(iii) They help in the formation of nuclear material from heterochromatin.
Type # 2. B-Chromosome or Supernumerary Chromosome:
1. 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.
2. First described by Wilson in 1905 in Metapodius, an insect, these chromosomes have now been reported in several plants and animals.
3. These chromosomes are generally smaller than other members of the chromosomal complement.
4. Structurally, these chromosomes are mostly heterochromatic in nature. But in Tradeschantia B-chromosomes are completely euchromatic while in maize they are partly heterochromatic and partly euchromatic (Fig. 323).
5. In most of the grases, B-chromosomes are smaller than normal ones and they can be distinguished easily.
Type # 3. Chimaera:
1. This is a twig of Bougainvillea bearing flowers of two different colours (pink as well as white) on the same axis.
2. 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.
3. 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.
Type # 4. Inversion Bridge:
1. The slide shows a meiotically dividing cell at early telophase stage.
2. A dicentric chromosome bridge and an acentric fragment in the form of a dot in between two poles are also visible in the slide.
3. Because of the presence of acentric fragment the bridge is due to paracentric inversion.
4. In the formation of the inversion bridge,
(i) Crossing over is seen in between two chromosomes having heterozygous paracentric inversion (Fig. 324A);
(ii) Diplotene stage shows a chiasma within the inverted segment (Fig. 324B) and;
(iii) At anaphase I stage is seen a chromatidal bridge and an acentric fragment (Fig. 324C).
Type # 5. SAT-Chromosome:
1. This is a chromosome possessing a satellite, and hence called SAT-chromosome.
2. Heitz (1930) mentioned that the word ‘SAT stands for Sine-Acido-Thymonucleinico (without thymonucleic acid).
3. In these chromosomes, the secondary constriction marks the formation of a round or elongated body called satellite.
4. A thin chromatin filament separates the satellite from the remaining chromosome.
5. In diameter, the satellite may be smaller or similar to that of chromosome.
6. For each particular chromosome, the satellite and filament are always constant in form and size.
7. 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.