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Ch. 12 - DNA Organization in Chromosomes
Chapter 12, Problem 14

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

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Understand that the length of DNA is given in micrometers (µm) and needs to be converted to base pairs.
Recall that the length of one base pair in DNA is approximately 0.34 nanometers (nm).
Convert the length of the DNA from micrometers to nanometers by multiplying by 1,000 (since 1 µm = 1,000 nm).
Divide the total length of the DNA in nanometers by the length of one base pair (0.34 nm) to find the number of base pairs.
Perform the division to determine the number of base pairs in the DNA molecule.

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Key Concepts

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

Base Pair

A base pair consists of two nucleotides on opposite complementary DNA or RNA strands that are connected via hydrogen bonds. In DNA, the base pairs are adenine-thymine (A-T) and guanine-cytosine (G-C). Understanding base pairs is crucial for calculating the total number of pairs in a given length of DNA.
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DNA Length Measurement

DNA length can be measured in micrometers (µm) or base pairs. One base pair of DNA is approximately 0.34 nanometers long. To convert the length of DNA from micrometers to base pairs, one must use this conversion factor, which allows for the determination of the total number of base pairs in a given length of DNA.
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Phage T2

Phage T2 is a type of bacteriophage, a virus that infects bacteria, specifically Escherichia coli. Its DNA is a double-stranded molecule that serves as a model for studying viral genetics. Understanding the structure and properties of phage T2 DNA is essential for answering questions related to its length and base pair composition.
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Related Practice
Textbook Question
Provide a comprehensive definition of heterochromatin and list as many examples as you can.
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Textbook Question
Mammals contain a diploid genome consisting of at least 10⁹ bp. If this amount of DNA is present as chromatin fibers, where each group of 200 bp of DNA is combined with 9 histones into a nucleosome and each group of 6 nucleosomes is combined into a solenoid, achieving a final packing ratio of 50, determine (a) the total number of nucleosomes in all fibers, (b) the total number of histone molecules combined with DNA in the diploid genome, and (c) the combined length of all fibers.
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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
Examples of histone modifications are acetylation (by histone acetyltransferase, or HAT), which is often linked to gene activation, and deacetylation (by histone deacetylases, or HDACs), which often leads to gene silencing typical of heterochromatin. Such heterochromatinization is initiated from a nucleation site and spreads bidirectionally until encountering boundaries that delimit the silenced areas. Recall from earlier in the text (see Chapter 4) the brief discussion of position effect, where repositioning of the w⁺ allele in Drosophila by translocation or inversion near heterochromatin produces intermittent w⁺ activity. In the heterozygous state (w⁺/w) a variegated eye is produced, with white and red patches. How might one explain position-effect variegation in terms of histone acetylation and/or deacetylation?
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Textbook Question
In a study of Drosophila, two normally active genes, w⁺ (wild-type allele of the white-eye gene) and hsp26 (a heat-shock gene), were introduced (using a plasmid vector) into euchromatic and heterochromatic chromosomal regions, and the relative activity of each gene was assessed [Sun et al. (2002)]. An approximation of the resulting data is shown in the following table. Which characteristic or characteristics of heterochromatin are supported by the experimental data? Gene Activity (relative percentage) _ Euchromatin Heterochromatin hsp26 100% 31% w⁺ 100% 8%
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Textbook Question
While much remains to be learned about the role of nucleosomes and chromatin structure and function, recent research indicates that in vivo chemical modification of histones is associated with changes in gene activity. One study determined that acetylation of H3 and H4 is associated with 21.1 percent and 13.8 percent increases in yeast gene activity, respectively, and that histones associated with yeast heterochromatin are hypomethylated relative to the genome average [Bernstein et al. (2000)]. Speculate on the significance of these findings in terms of nucleosome–DNA interactions and gene activity.
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