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Chapter 14: Problem 42

Discuss how the scientific community learned that DNA replication takes place in a semi conservative fashion. a. Meselson and Stahl experimented with E. coli. DNA grown in \(^{15} N\) was heavier than DNA grown in \(^{14} N.\) When DNA in \(^{15} N\) was switched to \(^{14} N\) media, DNA sedimented halfway between the \(^{15} N\) and \(^{14} N\) levels after one round of cell division, indicating fifty percent presence of \(^{14} N\) This supports the semi-conservative replication model. b. Meselson and Stahl experimented with S. pneumonia. DNA grown in \(^{15} N\) was heavier than DNA grown in \(^{14} N\) When DNA in \(^{15} N\) was switched to \(^{14} N\) media, DNA sedimented halfway between the 1 \(^{15} N\) and \(^{14} N\) levels after one round of cell division, indicating fifty percent presence of \(^{14} N\) This supports the semi-conservative replication model. c. Meselson and Stahl experimented with E. coli. DNA grown in \(^{14} N\) was heavier than DNA grown in \(^{15} N\) When DNA in \(^{15} N\) switched to \(^{14} N\) media, DNA sedimented halfway between the \(^{15} N\) and \(^{14} N\) levels after one round of cell division, indicating fifty percent presence of \(^{14} N.\) This supports the semi-conservative replication model. d. Meselson and Stahl experimented with S. pneumonia. DNA grown in \(^{15} N\) was heavier than DNA grown in \(^{14} N.\) When DNA in \(^{15} N\) was switched to \(^{14} N\) media, DNA sedimented halfway between the \(^{15} N\) and \(^{14} N\) levels after one round of cell division, indicating complete presence of \(^{14} N.\) This supports the semi conservative replication model.

Short Answer

Expert verified
a. Meselson and Stahl experimented with E. coli, showing semi-conservative replication.

Step by step solution

01

Understand the experiment setup

Meselson and Stahl conducted experiments to determine the method of DNA replication. They used isotopic nitrogen \(^{15} N\) and \(^{14} N\) to distinguish between old and new DNA strands.
02

Identify the experimental organism

They experimented with E. coli bacteria. This is identified in choices (a) and (c).
03

Observe the DNA in different isotopic conditions

E. coli was initially grown in \(^{15} N\), making its DNA heavier. It was then switched to a medium containing \(^{14} N\).
04

Analyze DNA after one round of replication

After one round of cell division in the \(^{14} N\) medium, the DNA sedimented halfway between the \(^{15} N\) and \(^{14} N\) levels, indicating that the DNA was of intermediate density.
05

Determine the method of replication

DNA sedimentation halfway between \(^{15} N\) and \(^{14} N\) after one replication round supports the semi-conservative model of DNA replication, where each new DNA molecule consists of one old \(^{15} N\) strand and one new \(^{14} N\) strand.
06

Correct choice identification

Considering all details, the correct choice is (a): E. coli, DNA grown in \(^{15} N\), shows intermediate density after being switched to \(^{14} N\) media for one cell division.

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Key Concepts

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

Meselson and Stahl experiment
The Meselson and Stahl experiment is a cornerstone in molecular biology that helped demonstrate how DNA replicates. In the late 1950s, scientists Matthew Meselson and Franklin Stahl used a clever experiment involving bacterial DNA and nitrogen isotopes to test different models of DNA replication.
They grew E. coli bacteria in a medium containing a heavy isotope of nitrogen, \(^{15}N\), which was incorporated into the bacterial DNA. This made the DNA denser. Subsequently, they transferred these bacteria into a medium with the lighter isotope, \(^{14}N\).

After allowing the bacteria to divide and replicate their DNA in the new medium, Meselson and Stahl used a technique called density gradient centrifugation to distinguish between the heavy and light DNA. They found that after one round of cell division, the DNA had an intermediate density, indicating that each DNA molecule was composed of one old (heavy) strand and one new (light) strand. This crucial observation supported the semi-conservative model of DNA replication.
semi-conservative replication
Semi-conservative replication is the process by which DNA replicates in living cells. According to this model, each of the two strands of the original DNA molecule serves as a template for the production of a new complementary strand.
As replication progresses, the two DNA strands unwind, and new nucleotides are added to create two new strands. This results in two DNA molecules, each containing one original strand and one newly synthesized strand.
The Meselson and Stahl experiment confirmed this model by demonstrating that, after one round of replication in the presence of \(^{14}N\), the DNA molecules had an intermediate density. If DNA replication were conservative, they would have observed separate heavy and light DNA molecules. In a dispersive model, the DNA would have shown a more random pattern of densities.
This experiment's results unmistakably indicated that DNA replication is semi-conservative, with each new DNA strand containing one original strand and one new strand.
E. coli DNA
E. coli, short for Escherichia coli, is a widely studied bacterium in molecular biology. It served as the perfect organism for Meselson and Stahl's experiments for several reasons. First, E. coli has a relatively simple genome structure. Second, it grows quickly and can be easily manipulated.

In their experiment, Meselson and Stahl utilized E. coli's fast-growing nature to observe DNA replication over a short period. They initially grew the bacteria in a medium with \(^{15}N\) to label their DNA as 'heavy'. Once the DNA was fully integrated with \(^{15}N\), they transferred the bacteria to a medium containing \(^{14}N\), allowing them to track new DNA synthesis.

As the E. coli bacteria replicated their DNA in the medium with \(^{14}N\), the incorporation of this lighter isotope provided clear visual evidence through density gradient centrifugation. The intermediate density of DNA after one replication round directly pointed to the semi-conservative nature of DNA replication.

Overall, E. coli proved invaluable to this groundbreaking experiment, reinforcing its status as a model organism in genetic and cellular research.

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Most popular questions from this chapter

The mitochondria of eukaryote cells contain their own circular DNA (mtDNA), consistent with their origin according to the theory of endosymbiosis. The mitochondrial genome is highly conserved in Eukarya. In humans, the 50 to 100 mitochondria in each of the cells in most tissues have 5 to 10 copies of the genome. Each has 37 genes that primarily encode proteins of the electron transport chain. Point mutations in which a single nucleotide is incorrectly placed is not repaired because the error-checking provided by DNA polymerase is not present in the mitochondria. The mutation rate for mtDNA is approximately 100 times higher than the mutation rate for nuclear DNA. The simultaneous existence of multiple alleles in each cell is likely, a condition called heteroplasmy. In mammals, sperm mitochondria are destroyed prior to fertilization. A. Explain how point mutations in mtDNA can result in a loss of function in critical cellular components such as cytochrome c yet not be lethal to the cell. B. Oocyte mitochondria are randomly segregated during meiosis, resulting in variation in the frequency of mtDNA mutations in offspring relative to the parent. Explain how a loss of function does not accumulate, lowering the metabolic performance from generation to generation. As described in the Evolution Connection in this chapter of the text, a fossil fingertip found in a Siberian cave revealed an evolutionary link between Neanderthals and Denisovans. Fossils from 28 individuals were located in the 鈥減it of bones,鈥� Sima de los Huesos, in Spain, thousands of miles from the Siberian cave. In 2013, mtDNA from a femur of one of these individuals was compared with mtDNA of Denisovans, Neanderthals, and modern humans. It was found that the Sima fossil shared many more alleles with Denisovans than with either Neanderthals or modern humans. In 2016, the same group of scientists who sequenced the mtDNA from the femur of one of the Sima fossils partially sequenced the DNA from that fossil, showing a clear connection to Neanderthals. C. Analyze these data to draw alternative conclusions regarding the relatedness of the three fossils and support each with evidence. D. Design a plan to differentiate or resolve these alternative conclusions.

The ends of the linear chromosomes are maintained by what? a. DNA polymerase b. helicase c. primase d. telomerase

Which enzyme is most directly responsible for the main process of producing a new DNA strand? a. DNA pol I b. DNA pol II c. DNA pol III d. DNA pol I, DNA pol II, and DNA pol III

What is the difference in the rate of replication of nucleotides between prokaryotes and eukaryotes? a. Eukaryotes are 50 times slower. b. Eukaryotes are 20 times faster. c. Prokaryotes are 100 times slower. d. Prokaryotes are 10 times faster

What type of nucleic acid material is analyzed the most frequently in forensics cases? a. cytoplasmic rRNA b. mitochondrial DNA c. nuclear chromosomal DNA d. nuclear mRNA
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