Polytene chromosome

• Polytene chromosome was discovered by Balbiani in 1881 in dipteran larval salivary glands (also known as salivary gland chromosomes).
• Found also in other tissues including gut epithelium, Malpighian tubules and some fat bodies of Diptera (Drosophila, Sciara, Rhyncosciara)
• These chromosomes are very long and thick (upto 200 times their size during mitotic metaphase in the case of Drosophila)
• They are formed due to multiple round of replication without separation of sister chromatids. In these cases, the duplicated chromosomes pile up next to each other, forming a bundle of strands that are aligned in parallel. The resulting chromosomes are said to be polytene from the Greek words meaning “many threads.”.
• The giant chromosomes of Drosophila consist of about 1,024 strands after 10 rounds of replication (210).
• In the case of Chironomus may have about 4,096 strands.

Lampbrush Chromosome

• Lampbrush chromosomes are the giant chromosomes found in oocyte nuclei of vertebrates (sharks, amphibians, reptiles and birds) as well as in invertebrates (Sagitta, Sepia, Echinaster and several species of insects).
• It was given this name because it is similar in appearance to the brushes used to clean lamp chimneys in centuries past.
• First observed by Flemming in 1882 and the name lampbrush was given by Ruckert in 1892.
• Lampbrush chromosomes are up to 800 µm long; thus they provide very favorable material for cytological studies.
• The homologous chromosomes are paired and each has duplicated to produce two chromatids. Each lampbrush chromosome contains a central axial region, where the two chromatids are highly condensed.

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