A special type of chromosomes

1. Special types of chromosomes:
 By
 Dr. Harinatha Reddy A
 Christ University

2. 1. Polytene Chromsomes or Salivary Gland Chromosomes
2. Lampbrush Chromosomes

3. Polytene Chromsomes or Salivary Gland Chromosomes

4. 1. Polytene Chromsomes or Salivary Gland Chromosomes
 Polytene chromosomes are oversized chromosomes which have
developed from standard chromosomes.
 Commonly found in the salivary glands of Drosophila
melanogaster.

5.  Specialized cells undergo repeated rounds of DNA replication
without cell division (endomitosis), to increase cell volume,
forming a giant polytene chromosome.

6.  Polytene chromosomes form when multiple rounds of replication
produce many sister chromatids that remain fused together.
 In addition to increasing the volume of the cells' nuclei and
causing cell expansion.
 Polytene cells may also have a metabolic advantage as multiple
copies of genes permits a high level of gene expression.

7.  Polytene chromosomes were discovered by Balbiani in 1881.
 They are commonly found in the salivary glands of insects.
 Hence it is also called salivary gland chromosome.
 Polytene chromosomes is larger in size.

8. Lampbrush chromosomes:

9. Lampbrush chromosomes:
 Lampbrush chromosomes are a special form of chromosome found
in the growing oocytes (immature eggs) birds and some animals.
 They were first described by Walther Flemming in 1882.

11.  Chromosomes transform into the lampbrush form during the
diplotene stage of meiotic prophase I which results an active
transcription of many genes.

12.  The lampbrush chromosome is a bivalent (2 pairs of sister
chromatids held together by chiasmata).
 Where they are seen to be organized into a series of chromomeres
with large chromatin loops extended laterally.

13.  The lampbrush chromosomes are larger than even polytene
chromosomes.
 Lampbrush chromosomes occur in pairs.
 The pair consists of homologous chromosomes which are
joined at certain contact points called chiasmata.

14.  Both the chromatids bear rows of large number of chromomeres.
 Many of the chromomeres give out lateral projections or loops.
 The lateral loops provide a test tube or lampbrush-like
appearance to the chromosome pair.

15.  Usually a lateral loop has a thin take part in rapid
transcription of mRNA.
 RNA synthesis begins at the thinner end.
 Transcription genes synthesis of yolk and other substances
required for growth and development of oocytes.

16. THANK YOU……………………….