SPECIAL TYPES OF CHROMOSOMES SMG

A general account of special types of chromosomes - Giant chromosomes (Polytene chromosomes or salivary gland chromosomes and Lampbrush chromosomes ) & B chromosomes

1. Dr. Saji Mariam George Associate Professor (Retired) Assumption College Autonomous Changanacherry SPECIAL TYPES OF CHROMOSOMES

2. SPECIAL TYPES OF CHROMOSOMES 1. GIANT CHROMOSOMES i ).POLYTENE ( SALIVARY GLAND) CHROMOSOMES ii) . LAMPBRUSH CHROMOSOMES http://www.sliderbase.com https://www.dreamstime.com

3. i ) POLYTENE (SALIVARY GLAND) CHROMOSOMES First observed by Balbiani (1881) in the salivary glands of Chironomous and hence the name Salivary gland chromosome. Chironomous

4. Chironomous (Midges) – Salivary gland Chromosome.

5. POLYTENE (SALIVARY GLAND) CHROMOSOMES….. Laterally magnified, transcriptionally active, interphase chromosome. Remain in a despiralised state which is characteristic of interphase chromosomes. B - Bands IB - Interbands P - Puffs POLYTENE NATURE

6. Occur in salivary glands, malpighian tubules, fat body cells, gut, trachea etc. among the Dipteran larvae. Cell division is arrested in certain tissues like salivary glands, malpighian tubules etc. Cell continue to grow in size.

7. The chromosome undergo repeated duplication – Endomitosis – the products do not separate – result in a bundle of several hundreds or thousands of chromatids – provide additional genetic material needed to support to the high level of secretory activity of these enormous cells. In this process of polytenization , the chromosomes gain enormous increase in length and diameter – giant chromosomes. In fruit fly ( Drosophila melanogaster ) , total length of somatoidal chromosome - 7.5 µ salivary gland chromosome - 2000 µ.

8. Polytene chromosomes of Drosophila Credit   POWER AND SYRED / SCIENCE PHOTO LIBRARY https:// www.sciencephoto.com

9. POLYTENE (SALIVARY GLAND) CHROMOSOMES….. Image:https :// www.quora.com

10. CHARACTERISTICS OF POLYTENE(SALIVARY GLAND ) CHROMOSOMES Polytene nature : Depending on the number of replication cycles, the polyteny of the chromosome may be 8192, 16384 , …….. Enormous length : Extremly longer than somatic chromosome. Somatic pairing : The homologous pairs are closely associated and are considered to be in a permanent prophase – Paired units of homologous chromosomes – the number of elements in such polytene cells correspond to the haploid number of chromosomes. Transverse bands : Along the length of the chromosome is a series of dark bands alternating with clear zones called interbands .

11. Bands : Rich in DNA Stain intensively Represent the location of the genes in the chromosomes Formed as a result of enlargement, duplication and aggregation of homologous chromomeres .

12. At certain times, bands become enlarged to form swellings called Chromosome puffs or Balbiani rings Puffs represent regions where the tightly coiled chromosomal fibres open out to form many loops. The puffing is due to unfolding or uncoiling of individual chromomeres in a band. Puffs are active genes and represent sites of RNA synthesis.

13. Balbiani rings Image:https :// www.toppr.com

14. Interbands : Contain some amount of DNA, which probably connects DNA segments located in the adjoining chromomeres . They do not stain with basic stains .

15. BANDS Stain intensely with basic stains Fuelgen ( A DNA specific stain) positive Regions of high DNA concentration Chromonemata tightly packed by folding Absorb Ultraviolet light at 2600 A ° INTERBANDS Do not stain intensely Fuelgen negative Regions of low DNA concentration Chromonemata relatively extended Absorb little Ultraviolet light.

16. ii) LAMPBRUSH CHROMOSOMES Larger than polytene chromosomes. In some Salamander Oocytes , they may reach a length of 5900 µ. First observed by Flemming (1882) in amphibian oocytes .

17. Ruckert (1892) studied the chromosomes of the oocytes of Shark and coined the name Lampbrush chromosomes because of their resemblance to brushes used in those times to clean the chimneys of oil lamps – have many fine lateral projections giving them the characteristic hairy appearance. Occur at the diplotene stage of the meiotic prophase in oocytes of animal species like amphibians, fishes, reptiles, birds etc.

18. Composed of a main axis and lateral loops. The main axis is composed of four chromatids or two bivalent chromosomes. The chromonema of the chromatids gives out fine loops at the lateral sides .

19. Loops are rich in RNA (mRNA) and proteins. Synthesis of proteins and the yolk takes place near the loops and when the synthesized materials are given off, the loops are collapsed or reduced.

20. Lampbrush Chromosomes Image:http :// www.expertsmind.com

21. Lampbrush Chromosomes …. Image:https :// fineartamerica.com

22. Lampbrush Chromosomes ….

23. 2. B CHROMOSOMES (Accessory chromosomes or Supernumerary chromosomes) Discovered by Wilson (1905) in Metapodius terminalis ( a Hemipteran insect ) Metapodius terminalis

24. Reported in about 600 species of Flowering plants (e.g. Maize) and in more than 100 animal species (Jones, 1975) . Chromosomes over and above the normal zygotic complement.

25. B chromosomes in Maize Image:http :// www.funpecrp.com.br

26. Have a normal structure, smaller than the autosomes . Mostly heterochromatic (e.g. in many insects, Maize ). Some are euchromatic (e.g . in Rye, Tradescantia ( Rheo sps . ).

27. Number may vary – In maize, 0 – 30. Non – homologous group of chromosomes. Do not show any pairing affinity with the A chromosome.

28. Morphology is different from the normal chromosomes (A chromosomes ) – globular in appearance. The genetic constitution do not strongly influence the individual.

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