Previous Video 5. Then, the cell distributes a copy of each chromosome between the two daughter cells so that each daughter cell receives a complete set of chromosomes. Certain cells undergo multiple cycles of chromosome duplication without undergoing any cell division. Such cells, called polyploid cells, contain multiple sets of each chromosome. In the salivary gland cells of Drosophila larvae, this process is taken to an extreme. Here, hundreds to thousands of copies of each chromosome are generated without any cell division.
As thousands of identical DNA sequences lay side by side, like crayons in a box, certain features in the chromatin become visible.
These features exist in ordinary interphase chromosomes but are difficult to observe. When viewed under a microscope, an alternating pattern of dark bands and lighter interbands can be seen in the polytene chromosomes. Where the chromatin in the bands is more condensed and transcriptionally inactive, the chromatin in the interbands is less condensed and transcriptionally active. These puffs arise from the de-condensation of chromatin and are sites of active DNA transcription.
Polytene chromosomes are giant interphase chromosomes with several DNA strands placed side by side. They were discovered in the year by Balbiani in salivary glands, intestine, muscles, malpighian tubules, and hypoderm of larvae Chironomus plumosus. Hence, these are also called "Salivary gland chromosomes.
Polytene chromosomes are also regularly observed in cells of salivary gland in Drosophila. There are certain differences between cells with polytene chromosomes and mitotically dividing cells. First is the absence of cell division after DNA replication that results in the accumulation of a large number of chromatids. Second is the failure of DNA strands to segregate after each round of DNA replication, resulting in several thousand chromatids arranged side by side. Third is the intact nuclear membrane and nucleolus during consecutive DNA replication cycles.
Variation in the chromatin compaction can result in different concentrations of chromatin along the length of the polytene chromosome. Since the homologous chromosomes have identical chromatin compaction and are arranged side by side, it results in multiple compact dark bands called chromomeres.
The regions between the bands are called interband or inter-chromomere regions. Interbands are lightly stained and are made up of decondensed chromatin. The chromomere patterns are specific for a species; although the number and size of chromomeres can change during the organism's lifetime. In certain instances, the interband region forms expanded structures called puffs that are loosely coiled, to allow RNA synthesis. Therefore, the puffs are excellent models to study the process of transcription.
These have decondensed chromatin that supports a very high rate of transcription. To learn more about our GDPR policies click here. If you want more info regarding data storage, please contact gdpr jove. Your access has now expired. Provide feedback to your librarian. If you have any questions, please do not hesitate to reach out to our customer success team.
Login processing Chapter 2: Biochemistry of the Cell. Chapter 3: Protein Structure. Chapter 4: Protein Function. Chapter 6: DNA Replication. Chapter Gene Expression. Chapter Mendelian Genetics. Chapter Genomes and Evolution. Chapter Cell Signaling Pathways. Chapter Analyzing Gene Expression and Function.
Chapter Cell Proliferation. Sign in. Skip Nav Destination Article Navigation. Close mobile search navigation Article navigation. Volume 15, Issue 2. Previous Article. Article contents. Article Navigation. This site. Google Scholar. Author and article information. Received: 19 Oct Online Issn: J Cell Sci 15 2 : — Article history Received:.
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