How polytene chromosomes are formed?
Polytene chromosomes develop from the chromosomes of diploid nuclei by successive duplication of each chromosomal element (chromatid) without their segregation. The newly formed chromatids remain associated lengthwise and together form a cable-like structure, referred to as polytene chromosomes.
What is the function of polytene chromosomes?
Polytene chromosomes are large chromosomes which have thousands of DNA strands. They provide a high level of function in certain tissues such as salivary glands of insects.
Why are polytene chromosomes so large?
Secondly, at the end of each S-phase, DNA strands do not segregate; instead they remain paired to each other, forming polytene (“multi-stranded”) chromosomes. As a result, giant chromosomes is formed, and these giant chromosomes are 70–110 times longer than typical metaphase chromosomes (Fig.
What is the use of giant chromosome?
They increase the cell volume and DNA content so it has multiple copies of genes that allow a high level of gene expression. For example, in Drosophila melanogaster, the polytene chromosomes of the larval salivary glands help produce a large amount of adhesive mucoprotein, a type of glue before pupation.
How many chromosomes are in a polytene chromosome?
Polytene homologues are held together by somatic pairing. The centromeres of all four chromosomes aggregate to form a chromocentre that is seen amorphous and consists largely of heterochromatin.
What is polytene and lampbrush chromosome?
The main difference between polytene and lampbrush chromosome is that polytene chromosomes occur in the salivary glands and other tissues of insects whereas lampbrush chromosomes occur in the oocytes of vertebrates except for mammals and some invertebrates.
Do humans have polytene chromosomes?
The authors generalize that the main feature of nonclassic polytene chromosomes is that forces binding the sister chromatids are much weaker than in the Diptera classic polytene chromosomes. This concept is confirmed by comparative studies of human, mink, and fox polytene chromosomes.
Do polytene chromosomes show somatic pairing?
In salivary gland chromosomes/polytene chromosomes, pairing is formed between homologous chromosomes. A characteristics feature of these chromosomes is that somatic pairing occurs in them and hence their number appears half of normal somatic cells.
How are lampbrush chromosomes formed?
Chromosomes transform into the lampbrush form during the diplotene stage of meiotic prophase I due to an active transcription of many genes. They are highly extended meiotic half-bivalents, each consisting of 2 sister chromatids.
What is the function of lampbrush chromosome?
Lampbrush chromosomes are also involved in the production of “masked” mRNAs for early development. The giant granular loops could either be the sites where such mRNAs are packaged or they could be sites where specific alterations of the deoxyribonucleoprotein fiber take place.
What are polytene and Lampbrush chromosomes?
What are Balbiani rings?
Balbiani rings are exceptionally large puffs on the polytene chromosomes in the dipteran Chironomus tentans. These puffs are particularly well suited for studies of the structure of active genes and the synthesis and transport of specific RNA-protein (RNP) particles.