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Ch. 11 - Chromosome Structure and DNA Sequence Organization
Klug - Essentials of Genetics 10th Edition
Klug10th EditionEssentials of GeneticsISBN: 9780135588789Not the one you use?Change textbook
All textbooksKlug 10th EditionCh. 11 - Chromosome Structure and DNA Sequence OrganizationProblem 19
Chapter 11, Problem 19

The following is a diagram of the general structure of the bacteriophage chromosome. Speculate on the mechanism by which it forms a closed ring upon infection of the host cell.
Diagram showing a bacteriophage chromosome with double-stranded DNA regions and complementary sequences forming a closed ring.

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1
Step 1: Identify the key features of the bacteriophage chromosome structure shown in the diagram, focusing on the double-stranded DNA region and the complementary single-stranded ends.
Step 2: Recognize that the complementary sequences at the ends of the linear DNA can base pair with each other, which is essential for forming a closed circular structure.
Step 3: Understand that upon infection, the single-stranded complementary ends can anneal through base pairing, bringing the two ends of the linear chromosome together.
Step 4: Consider the role of host or phage-encoded enzymes, such as DNA ligase, which can covalently join the sugar-phosphate backbones of the annealed ends, sealing the DNA into a closed circular ring.
Step 5: Summarize the mechanism as a process where complementary single-stranded ends hybridize to form a circular intermediate, followed by enzymatic ligation to create a stable closed ring chromosome inside the host cell.

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Key Concepts

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

Bacteriophage Chromosome Structure

Bacteriophage chromosomes often consist of double-stranded DNA with specific sequences that can interact to form unique structures. Understanding the arrangement of complementary sequences and double-stranded regions is essential to grasp how the DNA can circularize or form closed rings upon infection.
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DNA Complementarity and Base Pairing

DNA strands are complementary, meaning bases on one strand pair specifically with bases on the opposite strand (A with T, G with C). This complementarity allows single-stranded regions or sticky ends to anneal, facilitating the formation of circular DNA molecules through base pairing.
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Mechanism of DNA Circularization in Phages

Upon infection, linear phage DNA can circularize by annealing complementary single-stranded ends or through enzymatic ligation. This closed ring formation protects the DNA from degradation and is critical for replication and integration within the host.
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Related Practice
Textbook Question

Assume that a viral DNA molecule is a 50-µm-long circular strand with a uniform 20-Å diameter. If this molecule is contained in a viral head that is a 0.08-µm-diameter sphere, will the DNA molecule fit into the viral head, assuming complete flexibility of the molecule? Justify your answer mathematically.

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Textbook Question

How many base pairs are in a molecule of phage T2 DNA 52-µm long?

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Textbook Question

The human genome contains approximately 106 copies of an Alu sequence, one of the best-studied classes of short interspersed elements (SINEs), per haploid genome. Individual Alu units share a 282-nucleotide consensus sequence followed by a 3'-adenine-rich tail region [Schmid (1998)]. Given that there are approximately 3 x 109 base pairs per human haploid genome, about how many base pairs are spaced between each Alu sequence?

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