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Chapter 12: Problem 20

Phosphorus is required to synthesize the deoxyribonucleoside triphosphates used in DNA replication. A geneticist grows some E. coli in a medium containing nonradioactive phosphorus for many generations. A sample of the bacteria is then transferred to a medium that contains a radioactive isotope of phosphorus \((32 \mathrm{P}) .\) Samples of the bacteria are removed immediately after the transfer and after one and two rounds of replication. Assume that newly synthesized DNA contains \({ }^{32} \mathrm{P}\) and the original DNA contains nonradioactive phosphorus. What will be the distribution of radioactivity in the DNA of the bacteria in each sample? Will radioactivity be detected in neither, one, or both strands of the DNA?

Short Answer

Expert verified
Initially, no radioactivity is present; after one round, one strand per molecule is radioactive; after two rounds, some molecules are fully radioactive, others are half.

Step by step solution

01

Understand the Experimental Setup

The geneticist transfers E. coli from a medium with nonradioactive phosphorus to one with radioactive phosphorus, \({ }^{32} \mathrm{P}\). As DNA replicates, the new strands will incorporate \({ }^{32} \mathrm{P}\), while the original DNA will still have nonradioactive phosphorus.
02

First Sample - Immediate Transfer

Immediately after transfer, the sample is taken before DNA replication occurs. At this point, all DNA strands are still the original nonradioactive ones, as no new DNA synthesis has occurred. Thus, no radioactivity will be detected in the DNA.
03

Second Sample - After One Round of Replication

After one round of replication, each DNA molecule consists of one original strand (nonradioactive) and one new strand (radioactive). This results in the DNA being partially radioactive, with only one of the two strands containing the \({ }^{32} \mathrm{P}\).
04

Third Sample - After Two Rounds of Replication

After two rounds of replication, each DNA molecule consists of one old strand paired with a new strand, and completely new molecules. Essentially, half of the DNA molecules will be made entirely of new strands, both radioactive, while the rest will be half-radioactive. Thus, radioactivity will be detected in both strands of some molecules and only one strand of others.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Phosphorus
Phosphorus is a crucial element in the life sciences, especially when it comes to DNA replication. It forms a key component of deoxyribonucleoside triphosphates, the building blocks of DNA. These triphosphates are essential for synthesizing new DNA strands during replication. Without phosphate groups, DNA as a structurally stable molecule cannot exist.
In a laboratory setting, phosphorus can be introduced in various forms, either nonradioactive or radioactive, to study biological processes like DNA synthesis. When geneticists use radioactive phosphorus, it acts as a marker to follow the process of DNA replication.
  • Helps in labeling DNA strands
  • Makes it easy to track DNA synthesis over replication cycles
This is what makes radioactive isotopes, including phosphorus isotopes, a useful tool in molecular biology experiments.
Radioactive Isotopes
Radioactive isotopes are versions of elements that have unstable nuclei. This instability causes them to emit radiation, which can be useful in scientific experiments. An isotope of phosphorus, specifically phosphorus-32, is frequently used in studying processes like DNA synthesis, due to its radioactivity.
When bacteria like E. coli grow in a medium containing radioactive isotopes, these isotopes incorporate into the DNA during replication. This allows scientists to:
  • Identify newly synthesized DNA strands
  • Track the progression of DNA synthesis over multiple generations
In the exercise, radioactive isotopes are used to determine which strands in an E. coli population are newly formed following replication.
E. coli
E. coli, short for Escherichia coli, is a bacterium commonly used in genetics and molecular biology research. This microbe is favored due to its rapid growth rate and simple genome, making it ideal for experiments involving DNA replication and bacterial genetics.
In the context of the described experience, E. coli serves as the host organism for observing DNA replication dynamics when exposed to phosphorus-32. Scientists can:
  • Replicate generations quickly due to its fast division cycle
  • Analyze genetic changes in a controlled environment
This makes E. coli an efficient model organism in testing theories of DNA replication and phosphorus incorporation.
DNA Synthesis
DNA synthesis is the process through which new DNA strands are created. During replication, existing DNA strands serve as templates for creating new strands, ensuring genetic information is preserved. The process relies on enzymes that add deoxyribonucleotides, which contain phosphorus, to the growing DNA chain.
With each round of replication, one original and one new strand pair to make up DNA molecules, following the semi-conservative model of DNA replication. Recognizing how elements like phosphorus are utilized in synthesis helps in understanding these complex molecular events.
  • New strands incorporate radioactive phosphorus if available
  • Allows visualization of replication dynamics using isotope markers
In controlled experiments with labeling, scientists can trace how DNA evolves across generations by monitoring the incorporation of radioactive isotopes.

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Most popular questions from this chapter

What is the end-replication problem? Why, in the absence of telomerase, do the ends of linear chromosomes get progressively shorter each time the DNA is replicated?

Draw a molecule of DNA undergoing rolling-circle replication. On your drawing, identify the (a) origin, (b) template and newly synthesized strands, and (c) \(5^{\prime}\) and \(3^{\prime}\) ends of template and newly synthesized strands.

Eukaryotic licensing factors prevent DNA replication from being initiated at origins more than once in the cell cycle. After replication has begun at an origin, a protein called Geminin inhibits licensing factors that are required for MCM2-7 to bind to an origin and initiate replication. Thus, when Geminin is present, MCM2-7 will not bind to an origin. At the end of mitosis, Geminin is degraded, allowing MCM2-7 to bind once again to DNA and relicense the origin. Marina Melixetian and her colleagues suppressed the expression of Geminin protein in human cells by treating the cells with small interfering RNAs (siRNAs) complementary to Geminin messenger RNA (M. Melixetian et al. 2004. Journal of Cell Biology 165:473-482). (Small interfering RNAs form a complex with proteins and pair with complementary sequences on mRNAs; the complex then cleaves the mRNA, so there is no translation of the mRNA; see Chapter 14.) Forty-eight hours after treatment with siRNA, the Geminindepleted cells were enlarged and contained a single giant nucleus. Analysis of DNA content showed that many of these Geminin- depleted cells were 4 n or greater. Explain these results.

How does replication licensing ensure that DNA is replicated only once at each origin per eukaryotic cell cycle?

A line of mouse cells is grown for many generations in a medium with \({ }^{15} \mathrm{~N}\). Cells in \(\mathrm{G}_{1}\) are then switched to a new medium that contains \({ }^{14} \mathrm{~N}\). Draw a pair of homologous chromosomes from these cells at the following stages, showing the two strands of DNA molecules found in the chromosomes (see Chapter 2 for a review of chromosomes in different stages of the cell cycle). Use different colors to represent strands with \({ }^{14} \mathrm{~N}\) and \({ }^{15} \mathrm{~N}\). a. Cells in \(G_{1}\), before switching to medium with \({ }^{14} \mathrm{~N}\) b. Cells in \(G_{2}\), after switching to medium with \({ }^{14}\) N c. Cells in anaphase of mitosis, after switching to medium with \({ }^{14} \mathrm{~N}\) d. Cells in metaphase I of meiosis, after switching to medium with \({ }^{14} \mathrm{~N}\) e. Cells in anaphase II of meiosis, after switching to medium with \({ }^{14} \mathrm{~N}\)
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