Summarize the process by which the sequence of DNA bases in Human Genome Project was determined using the method developed by Frederick Sanger. Name a free living non-pathogenic nematode whose DNA has been completely sequenced.
Summarize the process by which the sequence of DNA bases in Human Genome Project was determined using the method developed by Frederick Sanger. Name a free living non-pathogenic nematode whose DNA has been completely sequenced.
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1. **Introduction to the Human Genome Project (HGP)**:
- The Human Genome Project was a large-scale research initiative aimed at sequencing the entire human genome, which consists of approximately 3 billion base pairs of DNA.
2. **Sequencing Method**:
- The sequencing of DNA bases was primarily accomplished using the Sanger sequencing method, developed by Frederick Sanger. This method is based on the principle of selective incorporation of chain-terminating dideoxynucleotides during DNA replication.
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The main cloning vectors used for DNA sequencing in the Human Genome Project are _________.
Match each term column-I with the definition in column-II, Column - I A) Bacterial enzymes used to cut DNA at defined sequences B) Sequences cut by restriction enzymes C) Ends left on DNA segments cut by DNA restriction enzymes D) Circular pieces of DNA found in bacteria E) Bacterial viruses F) Process by which bacteria take up pieces of DNA from the environment G) Plasmid DNA that has incorporated human DNA H) Term used to describe any vehicle that moves DNA from the organism to another Column - II I) Transformation II) Cloning vector III) Recombinant IV) Recognition sequences V) Plasmids VI) Sticky ends VII) Restriction enzymes VIII) Bacteriophages
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The chemical formula of deoxyribonucleic acid (DNA) is now well established. The molecule is a very long chain, the backbone of which consists of a regular alternation of sugar and phosphate groups. 5 To each sugar is attached a nitrogenous base, which can be of four different types. Two of the possible bases—adenine and guanine—are purines, and the other two—thymine and cytosine—are pyrimidines. So far as is known, the sequence of bases along the 10 chain is irregular. The monomer unit, consisting of phosphate, sugar and base, is known as a nucleotide. The first feature of our structure which is of biological interest is that it consists not of one chain, but of two. These two chains are both coiled around 15 a common fiber axis. It has often been assumed that since there was only one chain in the chemical formula there would only be one in the structural unit. However, the density, taken with the X-ray evidence, suggests very strongly that there are two. 20. The other biologically important feature is the manner in which the two chains are held together. This is done by hydrogen bonds between the bases. The bases are joined together in pairs, a single base from one chain being hydrogen-bonded to a single 25 base from the other. The important point is that only certain pairs of bases will fit into the structure. One member of a pair must be a purine and the other a pyrimidine in order to bridge between the two chains. If a pair consisted of two purines, for 30 example, there would not be room for it. We believe that the bases will be present almost entirely in their most probable forms. If this is true, the conditions for forming hydrogen bonds are more restrictive, and the only pairs of bases possible are: 35 adenine with thymine, and guanine with cytosine. Adenine, for example, can occur on either chain, but when it does, its partner on the other chain must always be thymine. The phosphate-sugar backbone of our model is 40 completely regular, but any sequence of the pairs of bases can fit into the structure. It follows that in a long molecule many different permutations are possible, and it therefore seems likely that the precise sequence of bases is the code which carries the 45 genetical information. If the actual order of the bases on one of the pair of chains were given, one could write down the exact order of the bases on the other one, because of the specific pairing. Thus one chain is, as it were, the complement of the other, and it is 50 this feature which suggests how the deoxyribonucleic acid molecule might duplicate itself. The table shows, for various organisms, the percentage of each of the four types of nitrogenous bases in that organism’s DNA. Adapted from Manju Bansal, “DNA Structure: Revisiting the Watson-Crick Double Helix.” ©2003 by Current Science Association, Bangalore 22. The authors use the word “backbone” in lines 3 and 39 to indicate that
The chemical formula of deoxyribonucleic acid (DNA) is now well established. The molecule is a very long chain, the backbone of which consists of a regular alternation of sugar and phosphate groups. 5 To each sugar is attached a nitrogenous base, which can be of four different types. Two of the possible bases—adenine and guanine—are purines, and the other two—thymine and cytosine—are pyrimidines. So far as is known, the sequence of bases along the 10 chain is irregular. The monomer unit, consisting of phosphate, sugar and base, is known as a nucleotide. The first feature of our structure which is of biological interest is that it consists not of one chain, but of two. These two chains are both coiled around 15 a common fiber axis. It has often been assumed that since there was only one chain in the chemical formula there would only be one in the structural unit. However, the density, taken with the X-ray evidence, suggests very strongly that there are two. 20. The other biologically important feature is the manner in which the two chains are held together. This is done by hydrogen bonds between the bases. The bases are joined together in pairs, a single base from one chain being hydrogen-bonded to a single 25 base from the other. The important point is that only certain pairs of bases will fit into the structure. One member of a pair must be a purine and the other a pyrimidine in order to bridge between the two chains. If a pair consisted of two purines, for 30 example, there would not be room for it. We believe that the bases will be present almost entirely in their most probable forms. If this is true, the conditions for forming hydrogen bonds are more restrictive, and the only pairs of bases possible are: 35 adenine with thymine, and guanine with cytosine. Adenine, for example, can occur on either chain, but when it does, its partner on the other chain must always be thymine. The phosphate-sugar backbone of our model is 40 completely regular, but any sequence of the pairs of bases can fit into the structure. It follows that in a long molecule many different permutations are possible, and it therefore seems likely that the precise sequence of bases is the code which carries the 45 genetical information. If the actual order of the bases on one of the pair of chains were given, one could write down the exact order of the bases on the other one, because of the specific pairing. Thus one chain is, as it were, the complement of the other, and it is 50 this feature which suggests how the deoxyribonucleic acid molecule might duplicate itself. The table shows, for various organisms, the percentage of each of the four types of nitrogenous bases in that organism’s DNA. Adapted from Manju Bansal, “DNA Structure: Revisiting the Watson-Crick Double Helix.” ©2003 by Current Science Association, Bangalore 22. The authors use the word “backbone” in lines 3 and 39 to indicate that
A
only very long chains of DNA can be taken from an organism with a spinal column.
B
the main structure of a chain in a DNA molecule is composed of repeating units.
C
a chain in a DNA molecule consists entirely of phosphate groups or of sugars.
D
nitrogenous bases form the main structural unit of DNA.
The chemical formula of deoxyribonucleic acid (DNA) is now well established. The molecule is a very long chain, the backbone of which consists of a regular alternation of sugar and phosphate groups. 5 To each sugar is attached a nitrogenous base, which can be of four different types. Two of the possible bases—adenine and guanine—are purines, and the other two—thymine and cytosine—are pyrimidines. So far as is known, the sequence of bases along the 10 chain is irregular. The monomer unit, consisting of phosphate, sugar and base, is known as a nucleotide. The first feature of our structure which is of biological interest is that it consists not of one chain, but of two. These two chains are both coiled around 15 a common fiber axis. It has often been assumed that since there was only one chain in the chemical formula there would only be one in the structural unit. However, the density, taken with the X-ray evidence, suggests very strongly that there are two. 20. The other biologically important feature is the manner in which the two chains are held together. This is done by hydrogen bonds between the bases. The bases are joined together in pairs, a single base from one chain being hydrogen-bonded to a single 25 base from the other. The important point is that only certain pairs of bases will fit into the structure. One member of a pair must be a purine and the other a pyrimidine in order to bridge between the two chains. If a pair consisted of two purines, for 30 example, there would not be room for it. We believe that the bases will be present almost entirely in their most probable forms. If this is true, the conditions for forming hydrogen bonds are more restrictive, and the only pairs of bases possible are: 35 adenine with thymine, and guanine with cytosine. Adenine, for example, can occur on either chain, but when it does, its partner on the other chain must always be thymine. The phosphate-sugar backbone of our model is 40 completely regular, but any sequence of the pairs of bases can fit into the structure. It follows that in a long molecule many different permutations are possible, and it therefore seems likely that the precise sequence of bases is the code which carries the 45 genetical information. If the actual order of the bases on one of the pair of chains were given, one could write down the exact order of the bases on the other one, because of the specific pairing. Thus one chain is, as it were, the complement of the other, and it is 50 this feature which suggests how the deoxyribonucleic acid molecule might duplicate itself. The table shows, for various organisms, the percentage of each of the four types of nitrogenous bases in that organism’s DNA. Adapted from Manju Bansal, “DNA Structure: Revisiting the Watson-Crick Double Helix.” ©2003 by Current Science Association, Bangalore 25. The authors’ main purpose of including the information about X-ray evidence and density is to
The chemical formula of deoxyribonucleic acid (DNA) is now well established. The molecule is a very long chain, the backbone of which consists of a regular alternation of sugar and phosphate groups. 5 To each sugar is attached a nitrogenous base, which can be of four different types. Two of the possible bases—adenine and guanine—are purines, and the other two—thymine and cytosine—are pyrimidines. So far as is known, the sequence of bases along the 10 chain is irregular. The monomer unit, consisting of phosphate, sugar and base, is known as a nucleotide. The first feature of our structure which is of biological interest is that it consists not of one chain, but of two. These two chains are both coiled around 15 a common fiber axis. It has often been assumed that since there was only one chain in the chemical formula there would only be one in the structural unit. However, the density, taken with the X-ray evidence, suggests very strongly that there are two. 20. The other biologically important feature is the manner in which the two chains are held together. This is done by hydrogen bonds between the bases. The bases are joined together in pairs, a single base from one chain being hydrogen-bonded to a single 25 base from the other. The important point is that only certain pairs of bases will fit into the structure. One member of a pair must be a purine and the other a pyrimidine in order to bridge between the two chains. If a pair consisted of two purines, for 30 example, there would not be room for it. We believe that the bases will be present almost entirely in their most probable forms. If this is true, the conditions for forming hydrogen bonds are more restrictive, and the only pairs of bases possible are: 35 adenine with thymine, and guanine with cytosine. Adenine, for example, can occur on either chain, but when it does, its partner on the other chain must always be thymine. The phosphate-sugar backbone of our model is 40 completely regular, but any sequence of the pairs of bases can fit into the structure. It follows that in a long molecule many different permutations are possible, and it therefore seems likely that the precise sequence of bases is the code which carries the 45 genetical information. If the actual order of the bases on one of the pair of chains were given, one could write down the exact order of the bases on the other one, because of the specific pairing. Thus one chain is, as it were, the complement of the other, and it is 50 this feature which suggests how the deoxyribonucleic acid molecule might duplicate itself. The table shows, for various organisms, the percentage of each of the four types of nitrogenous bases in that organism’s DNA. Adapted from Manju Bansal, “DNA Structure: Revisiting the Watson-Crick Double Helix.” ©2003 by Current Science Association, Bangalore 25. The authors’ main purpose of including the information about X-ray evidence and density is to
A
establish that DNA is the molecule that carries the genetic information.
B
present an alternate hypothesis about the composition of a nucleotide.
C
provide support for the authors’ claim about the number of chains in a molecule of DNA.
D
confirm the relationship between the density of DNA and the known chemical formula of DNA.
The chemical formula of deoxyribonucleic acid (DNA) is now well established. The molecule is a very long chain, the backbone of which consists of a regular alternation of sugar and phosphate groups. 5 To each sugar is attached a nitrogenous base, which can be of four different types. Two of the possible bases—adenine and guanine—are purines, and the other two—thymine and cytosine—are pyrimidines. So far as is known, the sequence of bases along the 10 chain is irregular. The monomer unit, consisting of phosphate, sugar and base, is known as a nucleotide. The first feature of our structure which is of biological interest is that it consists not of one chain, but of two. These two chains are both coiled around 15 a common fiber axis. It has often been assumed that since there was only one chain in the chemical formula there would only be one in the structural unit. However, the density, taken with the X-ray evidence, suggests very strongly that there are two. 20. The other biologically important feature is the manner in which the two chains are held together. This is done by hydrogen bonds between the bases. The bases are joined together in pairs, a single base from one chain being hydrogen-bonded to a single 25 base from the other. The important point is that only certain pairs of bases will fit into the structure. One member of a pair must be a purine and the other a pyrimidine in order to bridge between the two chains. If a pair consisted of two purines, for 30 example, there would not be room for it. We believe that the bases will be present almost entirely in their most probable forms. If this is true, the conditions for forming hydrogen bonds are more restrictive, and the only pairs of bases possible are: 35 adenine with thymine, and guanine with cytosine. Adenine, for example, can occur on either chain, but when it does, its partner on the other chain must always be thymine. The phosphate-sugar backbone of our model is 40 completely regular, but any sequence of the pairs of bases can fit into the structure. It follows that in a long molecule many different permutations are possible, and it therefore seems likely that the precise sequence of bases is the code which carries the 45 genetical information. If the actual order of the bases on one of the pair of chains were given, one could write down the exact order of the bases on the other one, because of the specific pairing. Thus one chain is, as it were, the complement of the other, and it is 50 this feature which suggests how the deoxyribonucleic acid molecule might duplicate itself. The table shows, for various organisms, the percentage of each of the four types of nitrogenous bases in that organism’s DNA. Adapted from Manju Bansal, “DNA Structure: Revisiting the Watson-Crick Double Helix.” ©2003 by Current Science Association, Bangalore 31. Based on the table, is the percentage of adenine in each organism’s DNA the same or does it vary, and which statement made by the authors is most consistent with that data?
The chemical formula of deoxyribonucleic acid (DNA) is now well established. The molecule is a very long chain, the backbone of which consists of a regular alternation of sugar and phosphate groups. 5 To each sugar is attached a nitrogenous base, which can be of four different types. Two of the possible bases—adenine and guanine—are purines, and the other two—thymine and cytosine—are pyrimidines. So far as is known, the sequence of bases along the 10 chain is irregular. The monomer unit, consisting of phosphate, sugar and base, is known as a nucleotide. The first feature of our structure which is of biological interest is that it consists not of one chain, but of two. These two chains are both coiled around 15 a common fiber axis. It has often been assumed that since there was only one chain in the chemical formula there would only be one in the structural unit. However, the density, taken with the X-ray evidence, suggests very strongly that there are two. 20. The other biologically important feature is the manner in which the two chains are held together. This is done by hydrogen bonds between the bases. The bases are joined together in pairs, a single base from one chain being hydrogen-bonded to a single 25 base from the other. The important point is that only certain pairs of bases will fit into the structure. One member of a pair must be a purine and the other a pyrimidine in order to bridge between the two chains. If a pair consisted of two purines, for 30 example, there would not be room for it. We believe that the bases will be present almost entirely in their most probable forms. If this is true, the conditions for forming hydrogen bonds are more restrictive, and the only pairs of bases possible are: 35 adenine with thymine, and guanine with cytosine. Adenine, for example, can occur on either chain, but when it does, its partner on the other chain must always be thymine. The phosphate-sugar backbone of our model is 40 completely regular, but any sequence of the pairs of bases can fit into the structure. It follows that in a long molecule many different permutations are possible, and it therefore seems likely that the precise sequence of bases is the code which carries the 45 genetical information. If the actual order of the bases on one of the pair of chains were given, one could write down the exact order of the bases on the other one, because of the specific pairing. Thus one chain is, as it were, the complement of the other, and it is 50 this feature which suggests how the deoxyribonucleic acid molecule might duplicate itself. The table shows, for various organisms, the percentage of each of the four types of nitrogenous bases in that organism’s DNA. Adapted from Manju Bansal, “DNA Structure: Revisiting the Watson-Crick Double Helix.” ©2003 by Current Science Association, Bangalore 31. Based on the table, is the percentage of adenine in each organism’s DNA the same or does it vary, and which statement made by the authors is most consistent with that data?
A
The same, “Two of . . . pyrimidines” (lines 6-8)
B
The same, “The important . . . structure” (lines 25-26)
C
It varies, “Adenine . . . thymine” (lines 36-38)
D
It varies, “It follows . . . information” (lines 41-45)
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The chemical formula of deoxyribonucleic acid (DNA) is now well established. The molecule is a very long chain, the backbone of which consists of a regular alternation of sugar and phosphate groups. 5 To each sugar is attached a nitrogenous base, which can be of four different types. Two of the possible bases—adenine and guanine—are purines, and the other two—thymine and cytosine—are pyrimidines. So far as is known, the sequence of bases along the 10 chain is irregular. The monomer unit, consisting of phosphate, sugar and base, is known as a nucleotide. The first feature of our structure which is of biological interest is that it consists not of one chain, but of two. These two chains are both coiled around 15 a common fiber axis. It has often been assumed that since there was only one chain in the chemical formula there would only be one in the structural unit. However, the density, taken with the X-ray evidence, suggests very strongly that there are two. 20. The other biologically important feature is the manner in which the two chains are held together. This is done by hydrogen bonds between the bases. The bases are joined together in pairs, a single base from one chain being hydrogen-bonded to a single 25 base from the other. The important point is that only certain pairs of bases will fit into the structure. One member of a pair must be a purine and the other a pyrimidine in order to bridge between the two chains. If a pair consisted of two purines, for 30 example, there would not be room for it. We believe that the bases will be present almost entirely in their most probable forms. If this is true, the conditions for forming hydrogen bonds are more restrictive, and the only pairs of bases possible are: 35 adenine with thymine, and guanine with cytosine. Adenine, for example, can occur on either chain, but when it does, its partner on the other chain must always be thymine. The phosphate-sugar backbone of our model is 40 completely regular, but any sequence of the pairs of bases can fit into the structure. It follows that in a long molecule many different permutations are possible, and it therefore seems likely that the precise sequence of bases is the code which carries the 45 genetical information. If the actual order of the bases on one of the pair of chains were given, one could write down the exact order of the bases on the other one, because of the specific pairing. Thus one chain is, as it were, the complement of the other, and it is 50 this feature which suggests how the deoxyribonucleic acid molecule might duplicate itself. The table shows, for various organisms, the percentage of each of the four types of nitrogenous bases in that organism’s DNA. Adapted from Manju Bansal, “DNA Structure: Revisiting the Watson-Crick Double Helix.” ©2003 by Current Science Association, Bangalore 26. Based on the passage, the authors’ statement “If a pair consisted of two purines, for example, there would not be room for it” (lines 29-30) implies that a pair
The chemical formula of deoxyribonucleic acid (DNA) is now well established. The molecule is a very long chain, the backbone of which consists of a regular alternation of sugar and phosphate groups. 5 To each sugar is attached a nitrogenous base, which can be of four different types. Two of the possible bases—adenine and guanine—are purines, and the other two—thymine and cytosine—are pyrimidines. So far as is known, the sequence of bases along the 10 chain is irregular. The monomer unit, consisting of phosphate, sugar and base, is known as a nucleotide. The first feature of our structure which is of biological interest is that it consists not of one chain, but of two. These two chains are both coiled around 15 a common fiber axis. It has often been assumed that since there was only one chain in the chemical formula there would only be one in the structural unit. However, the density, taken with the X-ray evidence, suggests very strongly that there are two. 20. The other biologically important feature is the manner in which the two chains are held together. This is done by hydrogen bonds between the bases. The bases are joined together in pairs, a single base from one chain being hydrogen-bonded to a single 25 base from the other. The important point is that only certain pairs of bases will fit into the structure. One member of a pair must be a purine and the other a pyrimidine in order to bridge between the two chains. If a pair consisted of two purines, for 30 example, there would not be room for it. We believe that the bases will be present almost entirely in their most probable forms. If this is true, the conditions for forming hydrogen bonds are more restrictive, and the only pairs of bases possible are: 35 adenine with thymine, and guanine with cytosine. Adenine, for example, can occur on either chain, but when it does, its partner on the other chain must always be thymine. The phosphate-sugar backbone of our model is 40 completely regular, but any sequence of the pairs of bases can fit into the structure. It follows that in a long molecule many different permutations are possible, and it therefore seems likely that the precise sequence of bases is the code which carries the 45 genetical information. If the actual order of the bases on one of the pair of chains were given, one could write down the exact order of the bases on the other one, because of the specific pairing. Thus one chain is, as it were, the complement of the other, and it is 50 this feature which suggests how the deoxyribonucleic acid molecule might duplicate itself. The table shows, for various organisms, the percentage of each of the four types of nitrogenous bases in that organism’s DNA. Adapted from Manju Bansal, “DNA Structure: Revisiting the Watson-Crick Double Helix.” ©2003 by Current Science Association, Bangalor 27. The authors’ use of the words “exact,” “specific,” and “complement” in lines 47-49 in the final paragraph functions mainly to
The chemical formula of deoxyribonucleic acid (DNA) is now well established. The molecule is a very long chain, the backbone of which consists of a regular alternation of sugar and phosphate groups. 5 To each sugar is attached a nitrogenous base, which can be of four different types. Two of the possible bases—adenine and guanine—are purines, and the other two—thymine and cytosine—are pyrimidines. So far as is known, the sequence of bases along the 10 chain is irregular. The monomer unit, consisting of phosphate, sugar and base, is known as a nucleotide. The first feature of our structure which is of biological interest is that it consists not of one chain, but of two. These two chains are both coiled around 15 a common fiber axis. It has often been assumed that since there was only one chain in the chemical formula there would only be one in the structural unit. However, the density, taken with the X-ray evidence, suggests very strongly that there are two. 20. The other biologically important feature is the manner in which the two chains are held together. This is done by hydrogen bonds between the bases. The bases are joined together in pairs, a single base from one chain being hydrogen-bonded to a single 25 base from the other. The important point is that only certain pairs of bases will fit into the structure. One member of a pair must be a purine and the other a pyrimidine in order to bridge between the two chains. If a pair consisted of two purines, for 30 example, there would not be room for it. We believe that the bases will be present almost entirely in their most probable forms. If this is true, the conditions for forming hydrogen bonds are more restrictive, and the only pairs of bases possible are: 35 adenine with thymine, and guanine with cytosine. Adenine, for example, can occur on either chain, but when it does, its partner on the other chain must always be thymine. The phosphate-sugar backbone of our model is 40 completely regular, but any sequence of the pairs of bases can fit into the structure. It follows that in a long molecule many different permutations are possible, and it therefore seems likely that the precise sequence of bases is the code which carries the 45 genetical information. If the actual order of the bases on one of the pair of chains were given, one could write down the exact order of the bases on the other one, because of the specific pairing. Thus one chain is, as it were, the complement of the other, and it is 50 this feature which suggests how the deoxyribonucleic acid molecule might duplicate itself. The table shows, for various organisms, the percentage of each of the four types of nitrogenous bases in that organism’s DNA. Adapted from Manju Bansal, “DNA Structure: Revisiting the Watson-Crick Double Helix.” ©2003 by Current Science Association, Bangalore 30. According to the table, which of the following pairs of base percentages in sea urchin DNA provides evidence in support of the answer to the previous question?
The chemical formula of deoxyribonucleic acid (DNA) is now well established. The molecule is a very long chain, the backbone of which consists of a regular alternation of sugar and phosphate groups. 5 To each sugar is attached a nitrogenous base, which can be of four different types. Two of the possible bases—adenine and guanine—are purines, and the other two—thymine and cytosine—are pyrimidines. So far as is known, the sequence of bases along the 10 chain is irregular. The monomer unit, consisting of phosphate, sugar and base, is known as a nucleotide. The first feature of our structure which is of biological interest is that it consists not of one chain, but of two. These two chains are both coiled around 15 a common fiber axis. It has often been assumed that since there was only one chain in the chemical formula there would only be one in the structural unit. However, the density, taken with the X-ray evidence, suggests very strongly that there are two. 20. The other biologically important feature is the manner in which the two chains are held together. This is done by hydrogen bonds between the bases. The bases are joined together in pairs, a single base from one chain being hydrogen-bonded to a single 25 base from the other. The important point is that only certain pairs of bases will fit into the structure. One member of a pair must be a purine and the other a pyrimidine in order to bridge between the two chains. If a pair consisted of two purines, for 30 example, there would not be room for it. We believe that the bases will be present almost entirely in their most probable forms. If this is true, the conditions for forming hydrogen bonds are more restrictive, and the only pairs of bases possible are: 35 adenine with thymine, and guanine with cytosine. Adenine, for example, can occur on either chain, but when it does, its partner on the other chain must always be thymine. The phosphate-sugar backbone of our model is 40 completely regular, but any sequence of the pairs of bases can fit into the structure. It follows that in a long molecule many different permutations are possible, and it therefore seems likely that the precise sequence of bases is the code which carries the 45 genetical information. If the actual order of the bases on one of the pair of chains were given, one could write down the exact order of the bases on the other one, because of the specific pairing. Thus one chain is, as it were, the complement of the other, and it is 50 this feature which suggests how the deoxyribonucleic acid molecule might duplicate itself. The table shows, for various organisms, the percentage of each of the four types of nitrogenous bases in that organism’s DNA. Adapted from Manju Bansal, “DNA Structure: Revisiting the Watson-Crick Double Helix.” ©2003 by Current Science Association, Bangalore 23. A student claims that nitrogenous bases pair randomly with one another. Which of the following statements in the passage contradicts the student’s claim?
The chemical formula of deoxyribonucleic acid (DNA) is now well established. The molecule is a very long chain, the backbone of which consists of a regular alternation of sugar and phosphate groups. 5 To each sugar is attached a nitrogenous base, which can be of four different types. Two of the possible bases—adenine and guanine—are purines, and the other two—thymine and cytosine—are pyrimidines. So far as is known, the sequence of bases along the 10 chain is irregular. The monomer unit, consisting of phosphate, sugar and base, is known as a nucleotide. The first feature of our structure which is of biological interest is that it consists not of one chain, but of two. These two chains are both coiled around 15 a common fiber axis. It has often been assumed that since there was only one chain in the chemical formula there would only be one in the structural unit. However, the density, taken with the X-ray evidence, suggests very strongly that there are two. 20. The other biologically important feature is the manner in which the two chains are held together. This is done by hydrogen bonds between the bases. The bases are joined together in pairs, a single base from one chain being hydrogen-bonded to a single 25 base from the other. The important point is that only certain pairs of bases will fit into the structure. One member of a pair must be a purine and the other a pyrimidine in order to bridge between the two chains. If a pair consisted of two purines, for 30 example, there would not be room for it. We believe that the bases will be present almost entirely in their most probable forms. If this is true, the conditions for forming hydrogen bonds are more restrictive, and the only pairs of bases possible are: 35 adenine with thymine, and guanine with cytosine. Adenine, for example, can occur on either chain, but when it does, its partner on the other chain must always be thymine. The phosphate-sugar backbone of our model is 40 completely regular, but any sequence of the pairs of bases can fit into the structure. It follows that in a long molecule many different permutations are possible, and it therefore seems likely that the precise sequence of bases is the code which carries the 45 genetical information. If the actual order of the bases on one of the pair of chains were given, one could write down the exact order of the bases on the other one, because of the specific pairing. Thus one chain is, as it were, the complement of the other, and it is 50 this feature which suggests how the deoxyribonucleic acid molecule might duplicate itself. The table shows, for various organisms, the percentage of each of the four types of nitrogenous bases in that organism’s DNA. Adapted from Manju Bansal, “DNA Structure: Revisiting the Watson-Crick Double Helix.” ©2003 by Current Science Association, Bangalore 28. Based on the table and passage, which choice gives the correct percentages of the purines in yeast DNA?