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Biological Sciences

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DNA Structure and Function

Question
3 out of 11
 

Since only four types of nucleotides (adenine, guanine, cytosine and thymine) make up DNA, many scientists were skeptical that this was the hereditary molecule of life. Most believed proteins, with their building blocks of 20 amino acids, provided the complexity necessary to carry the genetic information.

In 1952, Alfred Hershey and Martha Chase designed an experiment to determine whether DNA or protein was the genetic material. They used a bacteriophage (a virus that infects bacteria) called T2. This bacteriophage was only composed of proteins and DNA. A short time after infecting a bacterial cell, the cell would lyse (break open) and release new T2 particles. T2 reprogrammed the cell to make more phage, but did it use DNA or proteins?

Hershey and Chase infected bacteria with T2 phage in growth medium containing radioactive sulfur (which labels proteins) and radioactive phosphate (which labels DNA). The resulting radioactive phages were then incubated with nonradioactive bacteria and allowed to infect these cells. After a short time, the mixture was placed in a blender to shake loose any phage particles remaining outside of or attached to the bacteria. The mixture was then centrifuged: bacterial cells would form a pellet at the bottom of the tube while any phage outside the cell would remain in the liquid portion (the supernatant). The radioactivity in each sample was measured.


A possible alternative to the blender technique used in this experiment would be:

A add a competitive inhibitor to the bacteria before adding phage.
B break apart the cell membranes with a strong detergent.
C incubate bacteria with a mutant phage that could not bind to the cells.
D shake the cell/phage mixture vigorously.
Ans. D

Answer D is the only choice that makes any sense. All the other answers defeat the purpose of the experiment: a competitive inhibitor would prevent the phage from entering the cells at all; breaking apart the cell membrane would cause the contents of the cell to be released, where they would be found in the supernatant, along with the components of the phage that did not infect the cell; a mutant phage that could not bind to the cell would probably not infect the cell.

DNA Structure and Function Flashcard List

11 flashcards
1)
Since only four types of nucleotides (adenine, guanine, cytosine and thymine) make up DNA, many scientists were skeptical that this was the hereditary molecule of life. Most believed proteins, with their building blocks of 20 amino acids, provided the complexity necessary to carry the genetic information. In 1952, Alfred Hershey and Martha Chase designed an experiment to determine whether DNA or protein was the genetic material. They used a bacteriophage (a virus that infects bacteria) called T2. This bacteriophage was only composed of proteins and DNA. A short time after infecting a bacterial cell, the cell would lyse (break open) and release new T2 particles. T2 reprogrammed the cell to make more phage, but did it use DNA or proteins? Hershey and Chase infected bacteria with T2 phage in growth medium containing radioactive sulfur (which labels proteins) and radioactive phosphate (which labels DNA). The resulting radioactive phages were then incubated with nonradioactive bacteria and allowed to infect these cells. After a short time, the mixture was placed in a blender to shake loose any phage particles remaining outside of or attached to the bacteria. The mixture was then centrifuged: bacterial cells would form a pellet at the bottom of the tube while any phage outside the cell would remain in the liquid portion (the supernatant). The radioactivity in each sample was measured. Radioactive sulfur was found almost exclusively in the supernatant fraction, while the radioactive phosphate was found in the pellet fraction. This indicated:A phage protein entered the cell, but the DNA did not.B phage DNA entered the cell, but protein did not.C both protein and DNA entered the cell.D the phage did not infect the bacterial cell.
2)
Since only four types of nucleotides (adenine, guanine, cytosine and thymine) make up DNA, many scientists were skeptical that this was the hereditary molecule of life. Most believed proteins, with their building blocks of 20 amino acids, provided the complexity necessary to carry the genetic information. In 1952, Alfred Hershey and Martha Chase designed an experiment to determine whether DNA or protein was the genetic material. They used a bacteriophage (a virus that infects bacteria) called T2. This bacteriophage was only composed of proteins and DNA. A short time after infecting a bacterial cell, the cell would lyse (break open) and release new T2 particles. T2 reprogrammed the cell to make more phage, but did it use DNA or proteins? Hershey and Chase infected bacteria with T2 phage in growth medium containing radioactive sulfur (which labels proteins) and radioactive phosphate (which labels DNA). The resulting radioactive phages were then incubated with nonradioactive bacteria and allowed to infect these cells. After a short time, the mixture was placed in a blender to shake loose any phage particles remaining outside of or attached to the bacteria. The mixture was then centrifuged: bacterial cells would form a pellet at the bottom of the tube while any phage outside the cell would remain in the liquid portion (the supernatant). The radioactivity in each sample was measured. If the bacterial pellet were incubated in an appropriate growth medium, what would most likely occur?A No phage would be produced as the protein coat was removed.B Bacteria would survive as there is no proof they were infected with phage.C Phage should be produced and kill the cells.D The radioactive phosphate would kill the cells immediately.
3)
Since only four types of nucleotides (adenine, guanine, cytosine and thymine) make up DNA, many scientists were skeptical that this was the hereditary molecule of life. Most believed proteins, with their building blocks of 20 amino acids, provided the complexity necessary to carry the genetic information. In 1952, Alfred Hershey and Martha Chase designed an experiment to determine whether DNA or protein was the genetic material. They used a bacteriophage (a virus that infects bacteria) called T2. This bacteriophage was only composed of proteins and DNA. A short time after infecting a bacterial cell, the cell would lyse (break open) and release new T2 particles. T2 reprogrammed the cell to make more phage, but did it use DNA or proteins? Hershey and Chase infected bacteria with T2 phage in growth medium containing radioactive sulfur (which labels proteins) and radioactive phosphate (which labels DNA). The resulting radioactive phages were then incubated with nonradioactive bacteria and allowed to infect these cells. After a short time, the mixture was placed in a blender to shake loose any phage particles remaining outside of or attached to the bacteria. The mixture was then centrifuged: bacterial cells would form a pellet at the bottom of the tube while any phage outside the cell would remain in the liquid portion (the supernatant). The radioactivity in each sample was measured. A possible alternative to the blender technique used in this experiment would be:A add a competitive inhibitor to the bacteria before adding phage.B break apart the cell membranes with a strong detergent.C incubate bacteria with a mutant phage that could not bind to the cells.D shake the cell/phage mixture vigorously.
4)
Since only four types of nucleotides (adenine, guanine, cytosine and thymine) make up DNA, many scientists were skeptical that this was the hereditary molecule of life. Most believed proteins, with their building blocks of 20 amino acids, provided the complexity necessary to carry the genetic information. In 1952, Alfred Hershey and Martha Chase designed an experiment to determine whether DNA or protein was the genetic material. They used a bacteriophage (a virus that infects bacteria) called T2. This bacteriophage was only composed of proteins and DNA. A short time after infecting a bacterial cell, the cell would lyse (break open) and release new T2 particles. T2 reprogrammed the cell to make more phage, but did it use DNA or proteins? Hershey and Chase infected bacteria with T2 phage in growth medium containing radioactive sulfur (which labels proteins) and radioactive phosphate (which labels DNA). The resulting radioactive phages were then incubated with nonradioactive bacteria and allowed to infect these cells. After a short time, the mixture was placed in a blender to shake loose any phage particles remaining outside of or attached to the bacteria. The mixture was then centrifuged: bacterial cells would form a pellet at the bottom of the tube while any phage outside the cell would remain in the liquid portion (the supernatant). The radioactivity in each sample was measured. A modern experiment that confirms this experiment would be:A injecting DNA into a cell which causes the cell to produce an enzyme it never had before.B fertilizing an egg with a sperm in vitro.C prescribing enzyme pills to help cystic fibrosis patients with digestion.D consuming radioactive barium to view internal organs with X-rays.
5)
The Watson and Crick model of the structure of DNA eloquently suggested the method of DNA replication. The double helical nature, and the complementary base pairing, implied that one strand of DNA provided the information for making the other strand. Once the strands of DNA separated, each would be used as a template to make a new, complementary strand. Therefore, the two resulting strands of DNA would contain both an old strand and a new strand. This is known as the semiconservative theory of replication. The conservative theory stated that one double helix would contain only the newly synthesized DNA while the other would contain only the original strands. In 1958, Matthew Meselson and Franklin Stahl confirmed the semiconcervative model. They grew the bacterium E. coli for many generations in a heavy isotope of nitrogen, 15N. The isotope was incorporated into the DNA. The bacteria were then placed in medium that contained only the light isotope of nitrogen, 14N. The bacteria were sampled over a period of time. Their DNA was extracted and subjected to centrifugation techniques, which would separate the DNA based on density. DNA containing solely 15N was “heavy” whereas DNA containing solely 14N was “light”. After one round of division in the medium containing 14N, all the DNA was found to be intermediate in size, between the light and heavy types. This “intermediate” DNA was further analyzed. The hydrogen bonds between the base pairs were broken so the DNA was single stranded. These strands were then centrifuged. Half of the single stranded DNA was in the heavy form, and half was in the light form. If the replication theory suggested by Watson and Crick’s model was correct, then the density of the double stranded DNA after 2 rounds of replication in 14N medium would be:A half heavy, half lightB half heavy, half intermediateC half intermediate, half lightD all intermediate
6)
The Watson and Crick model of the structure of DNA eloquently suggested the method of DNA replication. The double helical nature, and the complementary base pairing, implied that one strand of DNA provided the information for making the other strand. Once the strands of DNA separated, each would be used as a template to make a new, complementary strand. Therefore, the two resulting strands of DNA would contain both an old strand and a new strand. This is known as the semiconservative theory of replication. The conservative theory stated that one double helix would contain only the newly synthesized DNA while the other would contain only the original strands. In 1958, Matthew Meselson and Franklin Stahl confirmed the semiconcervative model. They grew the bacterium E. coli for many generations in a heavy isotope of nitrogen, 15N. The isotope was incorporated into the DNA. The bacteria were then placed in medium that contained only the light isotope of nitrogen, 14N. The bacteria were sampled over a period of time. Their DNA was extracted and subjected to centrifugation techniques, which would separate the DNA based on density. DNA containing solely 15N was “heavy” whereas DNA containing solely 14N was “light”. After one round of division in the medium containing 14N, all the DNA was found to be intermediate in size, between the light and heavy types. This “intermediate” DNA was further analyzed. The hydrogen bonds between the base pairs were broken so the DNA was single stranded. These strands were then centrifuged. Half of the single stranded DNA was in the heavy form, and half was in the light form. 14N and 15N were good choices as isotopes because they would be incorporated into the DNA via: I. thedeoxyribose sugar II. the phosphate group III. the basesA I onlyB II onlyC III onlyD I and III
7)
The Watson and Crick model of the structure of DNA eloquently suggested the method of DNA replication. The double helical nature, and the complementary base pairing, implied that one strand of DNA provided the information for making the other strand. Once the strands of DNA separated, each would be used as a template to make a new, complementary strand. Therefore, the two resulting strands of DNA would contain both an old strand and a new strand. This is known as the semiconservative theory of replication. The conservative theory stated that one double helix would contain only the newly synthesized DNA while the other would contain only the original strands. In 1958, Matthew Meselson and Franklin Stahl confirmed the semiconcervative model. They grew the bacterium E. coli for many generations in a heavy isotope of nitrogen, 15N. The isotope was incorporated into the DNA. The bacteria were then placed in medium that contained only the light isotope of nitrogen, 14N. The bacteria were sampled over a period of time. Their DNA was extracted and subjected to centrifugation techniques, which would separate the DNA based on density. DNA containing solely 15N was “heavy” whereas DNA containing solely 14N was “light”. After one round of division in the medium containing 14N, all the DNA was found to be intermediate in size, between the light and heavy types. This “intermediate” DNA was further analyzed. The hydrogen bonds between the base pairs were broken so the DNA was single stranded. These strands were then centrifuged. Half of the single stranded DNA was in the heavy form, and half was in the light form. If DNA replication was conservative, after 1 round of replication in 14N, the density of the double stranded DNA would be:A all intermediateB all heavyC half intermediate, half heavyD half heavy, half light
8)
The Watson and Crick model of the structure of DNA eloquently suggested the method of DNA replication. The double helical nature, and the complementary base pairing, implied that one strand of DNA provided the information for making the other strand. Once the strands of DNA separated, each would be used as a template to make a new, complementary strand. Therefore, the two resulting strands of DNA would contain both an old strand and a new strand. This is known as the semiconservative theory of replication. The conservative theory stated that one double helix would contain only the newly synthesized DNA while the other would contain only the original strands. In 1958, Matthew Meselson and Franklin Stahl confirmed the semiconcervative model. They grew the bacterium E. coli for many generations in a heavy isotope of nitrogen, 15N. The isotope was incorporated into the DNA. The bacteria were then placed in medium that contained only the light isotope of nitrogen, 14N. The bacteria were sampled over a period of time. Their DNA was extracted and subjected to centrifugation techniques, which would separate the DNA based on density. DNA containing solely 15N was “heavy” whereas DNA containing solely 14N was “light”. After one round of division in the medium containing 14N, all the DNA was found to be intermediate in size, between the light and heavy types. This “intermediate” DNA was further analyzed. The hydrogen bonds between the base pairs were broken so the DNA was single stranded. These strands were then centrifuged. Half of the single stranded DNA was in the heavy form, and half was in the light form. In this experiment, the Okazaki fragments would contain:A only14NB only15NC neither14N nor 15ND both14N and 15N
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