Table of contents

1. Introduction to Genetics51m
2. Mendel's Laws of Inheritance3h 37m
3. Extensions to Mendelian Inheritance2h 41m
4. Genetic Mapping and Linkage2h 28m
5. Genetics of Bacteria and Viruses1h 21m
6. Chromosomal Variation1h 48m
7. DNA and Chromosome Structure56m
8. DNA Replication1h 10m
9. Mitosis and Meiosis1h 34m
10. Transcription1h 0m
11. Translation58m
12. Gene Regulation in Prokaryotes1h 19m
13. Gene Regulation in Eukaryotes44m
14. Genetic Control of Development44m
15. Genomes and Genomics1h 50m
16. Transposable Elements47m
17. Mutation, Repair, and Recombination1h 6m
18. Molecular Genetic Tools19m
- Genetic Cloning
  9m
- Methods for Analyzing DNA
  9m
19. Cancer Genetics29m
- Overview of Cancer
  21m
- Cancer Mutations
  7m
20. Quantitative Genetics1h 26m
21. Population Genetics50m
- Hardy Weinberg
  19m
- Allelic Frequency Changes
  31m
22. Evolutionary Genetics29m

7. DNA and Chromosome Structure

Bacterial and Viral Chromosome Structure

Genetics

7. DNA and Chromosome Structure

Bacterial and Viral Chromosome Structure

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1:30 minutes

Problem 24

Textbook Question

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.

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Understand the structure of the bacteriophage chromosome: Bacteriophage chromosomes are typically linear double-stranded DNA molecules with complementary single-stranded overhangs (also called cohesive ends or 'cos' sites) at their termini.

Recognize the role of cohesive ends: These cohesive ends are sequences of nucleotides that are complementary to each other. When the bacteriophage infects a host cell, these ends can anneal (bind) to form a circular DNA molecule.

Speculate on the mechanism of circularization: Upon infection, the host cell's environment provides the necessary conditions (e.g., enzymes like DNA ligase) for the cohesive ends to anneal and form a covalently closed circular DNA molecule.

Consider the role of DNA ligase: After the cohesive ends anneal, DNA ligase, an enzyme present in the host cell, seals the nicks in the sugar-phosphate backbone, completing the formation of the closed circular DNA.

Relate this to the bacteriophage life cycle: The circularization of the bacteriophage chromosome is a critical step for initiating replication and transcription within the host cell, enabling the phage to hijack the host's machinery for its own reproduction.

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Key Concepts

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

Bacteriophage Structure

Bacteriophages, or phages, are viruses that infect bacteria. They typically consist of a protein coat (capsid) that encases their genetic material, which can be either DNA or RNA. Understanding the structure of the bacteriophage is crucial, as it influences how the virus interacts with the host cell and how its genetic material is delivered during infection.

Circular DNA Formation

In many bacteriophages, the genetic material can exist in a linear form before infection. Upon entering the host cell, mechanisms such as ligation or recombination can facilitate the formation of a closed circular DNA molecule. This circularization is essential for the phage's replication and integration into the host's genome, allowing for efficient propagation.

Host Cell Mechanisms

The host cell's machinery plays a vital role in the lifecycle of a bacteriophage. Once the phage injects its genetic material, the host's enzymes and replication systems are hijacked to replicate the phage DNA. Understanding how the host cell responds to phage infection helps explain the process of circularization and the overall infection mechanism.

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Related Practice

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

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

To gauge the fidelity of DNA synthesis, Arthur Kornberg and colleagues devised a technique called nearest-neighbor analysis, which determines the frequency with which any two bases occur adjacent to each other along the polynucleotide chain (J. Biol. Chem. 236: 864–875). This test relies on the enzyme spleen phosphodiesterase (see the previous problem). DNA is synthesized by polymerization of 5'-nucleotides—that is, each nucleotide is added with the phosphate on the deoxyribose. However, as shown in the accompanying figure, the phosphodiesterase enzyme cleaves DNA between the phosphate and the C-5' atom, thereby producing 3'-nucleotides. In this test, the phosphates on only one of the four nucleotide precursors of DNA (cytidylic acid, for example) are made radioactive with ³²P, and DNA is synthesized. Then the DNA is subjected to enzymatic cleavage, in which the radioactive phosphate is transferred to the base that is the 'nearest neighbor' on the 5' side of all cytidylic acid nucleotides.

Following four separate experiments, in each of which a different one of the four nucleotide types is radioactive, the frequency of all 16 possible nearest neighbors can be calculated. When Kornberg applied the nearest-neighbor frequency test to the DNA template and resultant product from a variety of experiments, he found general agreement between the nearest-neighbor frequencies of the two. Analysis of nearest-neighbor data led Kornberg to conclude that the two strands of the double helix are in opposite polarity to one another. Demonstrate this approach by determining the outcome of such an analysis if the strands of DNA shown here are (a) antiparallel versus (b) parallel:

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Multiple Choice

Viral chromosomes exist in a variety of conformations and can be made up of ________.

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Multiple Choice

True or False:Bacteria and viruses can use RNA as their genetic material.

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Multiple Choice

What is the name of the bacterial chromosomal region where replication begins?

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