Textbook Question
In eukaryotic DNA, along a 6000-bp segment of DNA, approximately how many molecules of each kind of histone protein do you expect to find? Explain your answer.
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7. DNA and Chromosome Structure
Eukaryotic Chromosome Structure
0:55 minutesProblem 4
Textbook Question
Describe the structure of giant polytene chromosomes and how they arise.
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Polytene chromosomes are oversized chromosomes that are formed by repeated rounds of DNA replication without cell division, a process known as endoreplication or endomitosis.
These chromosomes are commonly found in certain tissues of organisms like Drosophila (fruit flies), particularly in the salivary glands, where they facilitate high levels of gene expression.
The structure of polytene chromosomes is characterized by their banding pattern, which consists of alternating dark and light bands. These bands correspond to regions of chromatin that are either tightly packed (heterochromatin) or loosely packed (euchromatin).
The chromosomes are formed by the alignment of multiple sister chromatids in parallel, creating a thick, visible structure under a microscope. This alignment is due to the lack of separation of chromatids during the replication cycles.
Polytene chromosomes also exhibit 'puffs,' which are regions of decondensed chromatin where active transcription is occurring. These puffs indicate areas of high gene activity and are often used to study gene expression.
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Key Concepts
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Polytene Chromosomes
Polytene chromosomes are oversized chromosomes that are formed by the replication of DNA without subsequent cell division, resulting in many identical sister chromatids aligned side by side. They are typically found in certain tissues of organisms like Drosophila (fruit flies) and are characterized by their distinct banding patterns, which are useful for genetic mapping and studying gene expression.
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Chromosome Structure
Chromosome Structure
The structure of chromosomes includes a DNA molecule wrapped around histone proteins, forming nucleosomes that further coil and fold to create a compact structure. In polytene chromosomes, the repeated DNA strands create a thick, banded appearance, where each band corresponds to a specific region of the genome, allowing for detailed analysis of chromosomal architecture and gene localization.
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Formation of Polytene Chromosomes
Giant polytene chromosomes arise through a process called endoreduplication, where the DNA replicates multiple times without mitosis, leading to a high number of chromatid strands. This process is often observed in the salivary glands of Drosophila, where the extensive replication allows for increased gene expression necessary for the production of saliva, demonstrating a unique adaptation in certain tissues.
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