Ch. 11 - Chromosome Structure and DNA Sequence Organization

Klug - Essentials of Genetics 10th Edition

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Klug 10th Edition

Ch. 11 - Chromosome Structure and DNA Sequence Organization

Problem 4

Chapter 11, Problem 4

Describe the structure of giant polytene chromosomes and how they arise.

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Begin by explaining that giant polytene chromosomes are oversized chromosomes commonly found in the salivary glands of certain dipteran insects, such as Drosophila larvae, and are visible under a light microscope due to their large size.

Describe the structure of polytene chromosomes as consisting of many identical chromatids aligned side by side, resulting from repeated rounds of DNA replication without cell division, a process called endoreplication or endomitosis.

Explain that these chromosomes display characteristic banding patterns, with alternating dark and light bands representing regions of tightly packed (heterochromatin) and loosely packed (euchromatin) DNA, respectively.

Discuss how the multiple chromatids remain synapsed (paired) along their length, which allows for the visualization of specific gene loci and chromosomal puffs, where active transcription occurs.

Summarize that giant polytene chromosomes arise through repeated rounds of DNA replication without mitosis, leading to many sister chromatids aligned in parallel, creating their distinctive large, banded appearance.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Polytene Chromosome Structure

Polytene chromosomes are oversized chromosomes consisting of many identical chromatids aligned side-by-side, resulting from repeated rounds of DNA replication without cell division. They display characteristic banding patterns visible under a microscope, which correspond to regions of gene activity and chromatin organization.

Endoreplication (Endoreduplication)

Endoreplication is a process where DNA replicates multiple times without subsequent mitosis or cytokinesis, leading to polyploidy within a single nucleus. This mechanism produces the multiple chromatids that form the giant polytene chromosomes, commonly found in certain tissues like salivary glands of Drosophila.

Functional Significance of Polytene Chromosomes

Polytene chromosomes facilitate high levels of gene expression by providing multiple copies of genes in a single nucleus, aiding in rapid protein synthesis. Their distinct banding patterns also allow researchers to study gene activity, chromosomal rearrangements, and developmental regulation.

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Related Practice

Textbook Question

In the discussion, we focused on how DNA is organized at the chromosomal level. Along the way, we found many opportunities to consider the methods and reasoning by which much of this information was acquired. From the explanations given in the chapter, what answers would you propose to the following fundamental questions:

How do we know that satellite DNA consists of repetitive sequences and has been derived from regions of the centromere?

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Textbook Question

Write a short essay that contrasts the major differences between the organization of DNA in viruses and bacteria versus eukaryotes.

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Textbook Question

Contrast the size of the single chromosome in bacteriophage and T2 with that of E. coli. How does this relate to the relative size and complexity of phages and bacteria?

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Textbook Question

What genetic process is occurring in a puff of a polytene chromosome? How do we know this experimentally?

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Textbook Question

During what genetic process are lampbrush chromosomes present in vertebrates?

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Textbook Question

Why might we predict that the organization of eukaryotic genetic material will be more complex than that of viruses or bacteria?

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