Textbook Question
How do we know that viral and bacterial chromosomes most often consist of circular DNA molecules devoid of protein?
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Ch. 12 - DNA Organization in Chromosomes
Klug12th EditionConcepts of Genetics ISBN: 9780135564776
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Table of contents
- Ch. 1 - Introduction to Genetics17
- Ch. 2 - Mitosis and Meiosis36
- Ch. 3 - Mendelian Genetics51
- Ch. 4 - Extensions of Mendelian Genetics74
- Ch. 5 - Chromosome Mapping in Eukaryotes51
- Ch. 6 - Genetic Analysis and Mapping in Bacteria and Bacteriophages46
- Ch. 7 - Sex Determination and Sex Chromosomes33
- Ch. 8 - Chromosome Mutations: Variation in Number and Arrangement40
- Ch. 9 - Extranuclear Inheritance31
- Ch. 10 - DNA Structure and Analysis43
- Ch. 11 - DNA Replication and Recombination44
- Ch. 12 - DNA Organization in Chromosomes30
- Ch. 13 - The Genetic Code and Transcription54
- Ch. 14 - Translation and Proteins47
- Ch. 15 - Gene Mutation, DNA Repair, and Transposition41
- Ch. 16 - Regulation of Gene Expression in Bacteria29
- Ch. 17 - Transcriptional Regulation in Eukaryotes41
- Ch. 18 - Post-transcriptional Regulation in Eukaryotes33
- Ch. 19 - Epigenetics33
- Ch. 20 - Recombinant DNA Technology44
- Ch. 21 - Genomic Analysis36
- Ch. 22 - Applications of Genetic Engineering and Biotechnology36
- Ch. 23 - Developmental Genetics37
- Ch. 24 - Cancer Genetics34
- Ch. 25 - Quantitative Genetics and Multifactorial Traits54
- Ch. 26 - Population and Evolutionary Genetics45
Chapter 12, Problem 1b
What is the experimental basis for concluding that puffs in polytene chromosomes and loops in lampbrush chromosomes are areas of intense transcription of RNA?
Verified step by step guidance1
Understand that both polytene chromosome puffs and lampbrush chromosome loops are visibly distinct regions where the chromatin is less condensed, suggesting active processes occurring there.
Recognize that experimental techniques such as autoradiography have been used, where cells are incubated with radioactive RNA precursors (like tritiated uridine), allowing incorporation into newly synthesized RNA.
Note that after incubation, the chromosomes are fixed and exposed to photographic emulsion; the radioactive decay exposes the emulsion, revealing silver grains that indicate sites of RNA synthesis.
Observe that these silver grains are concentrated specifically over the puffs in polytene chromosomes and loops in lampbrush chromosomes, demonstrating that these regions are sites of active transcription.
Correlate these findings with biochemical assays showing increased RNA polymerase activity and the presence of nascent RNA transcripts in these regions, confirming that puffs and loops correspond to intense RNA transcription.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Polytene Chromosome Puffs
Polytene chromosomes are oversized chromosomes with many DNA strands aligned, found in certain cells like Drosophila salivary glands. Puffs are visibly swollen regions indicating active gene expression, where chromatin decondenses to allow RNA polymerase access for transcription.
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Lampbrush Chromosome Loops
Lampbrush chromosomes, seen in amphibian oocytes, have extended lateral loops representing regions of active transcription. These loops contain nascent RNA transcripts and associated proteins, demonstrating sites where genes are being actively transcribed.
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Experimental Evidence of Transcription Activity
Techniques like autoradiography using radioactive RNA precursors and electron microscopy have shown that puffs and loops incorporate labeled nucleotides, confirming RNA synthesis. Additionally, the presence of RNA polymerase and nascent RNA in these regions supports their role as transcriptionally active sites.
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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 did we learn that eukaryotic chromatin exists in the form of repeating nucleosomes, each consisting of about 200 base pairs and an octamer of histones?
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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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