1.6 Nucleic Acids

1. AA five-carbon sugar, ribose in RNA or deoxyribose in DNA Correct
   Each nucleotide has three components: a five-carbon sugar (ribose in RNA, deoxyribose in DNA), a phosphate group and one nitrogenous base.
2. BAn amino acid, the same monomer that builds a protein
   A student who confuses nucleic acids with proteins picks this. Amino acids are the monomers of proteins; nucleotides contain none.
3. CGlucose, a sugar whose molecules contain six carbons
   A student who thinks the sugar in every biological molecule is glucose picks this. Glucose has six carbons; the sugar of a nucleotide has five.
4. DA second base, paired with the first by hydrogen bonds
   A student who confuses a nucleotide with a base pair picks this. A nucleotide has one base; a base pair is two bases on different nucleotides held together by hydrogen bonds.

CED 1.6.A.1 · Read this in Fix

1. AA covalent bond between its sugar and the phosphate group at the strand's 5′ end
   A student who thinks nucleotides are added at the 5′ end picks this. New nucleotides are joined at the 3′ end of the strand, to the hydroxyl group of its last sugar.
2. BA covalent bond between its base and the base of the last nucleotide in the strand
   A student who thinks the bases form the strand's backbone picks this. Bases project from the backbone and are not bonded to neighboring bases in the same strand; the backbone is formed by bonds between sugars and phosphates.
3. CA covalent bond between its phosphate group and the sugar at the 3′ end of the strand Correct
   Nucleotides are added to the 3′ end of a growing strand: the phosphate of the incoming nucleotide forms a covalent bond with the 3′ hydroxyl of the sugar of the last nucleotide. Repeated, this builds the sugar–phosphate backbone.
4. DA hydrogen bond between its base and the base of the strand's last nucleotide
   A student who thinks the nucleotides of a strand are held together by hydrogen bonds picks this. Nucleotides along a strand are joined by covalent bonds between phosphate and sugar; hydrogen bonds hold paired bases together, one on each strand.

CED 1.6.A.2 · Read this in Fix

1. ABy hydrogen bonds between complementary bases, one base on each strand Correct
   The two strands are held together by hydrogen bonds between complementary bases, one from each strand: adenine pairs with thymine and cytosine with guanine. The nucleotides within each strand are joined by covalent bonds.
2. BBy covalent bonds between complementary bases, with one base on each strand
   A student who has the bonds in DNA the wrong way round picks this. Paired bases are held by hydrogen bonds, which is why the strands can be separated without breaking either strand; covalent bonds join the nucleotides within each strand.
3. CBy hydrogen bonds between identical bases, such as A with A, one on each strand
   A student who thinks the two strands are identical copies picks this. The strands are complementary: adenine pairs with thymine and cytosine with guanine, not each base with an identical one.
4. DBy covalent bonds joining the sugar–phosphate backbones of the two strands
   A student who thinks the sugar–phosphate parts form the 'rungs' of the DNA ladder picks this. The backbones form the outside of the helix; the strands are joined in the middle by hydrogen bonds between paired bases.

CED 1.6.A.3 · Read this in Fix

1. ARibose and uracil are present; deoxyribose and thymine are absent. Correct
   RNA contains the sugar ribose and the base uracil, whereas DNA contains deoxyribose and thymine. If the virus's nucleic acid is RNA, its components will include ribose and uracil but no deoxyribose or thymine.
2. BRibose and thymine are present; deoxyribose and uracil are absent.
   A student who has the bases of DNA and RNA the wrong way round picks this. RNA contains uracil, not thymine; finding thymine and no uracil would point to DNA.
3. CDeoxyribose and uracil are present; ribose and thymine are absent.
   A student who mixes up the sugars of DNA and RNA picks this. RNA (ribonucleic acid) contains ribose; deoxyribose is the sugar of DNA (deoxyribonucleic acid).
4. DDeoxyribose and thymine are present; ribose and uracil are absent.
   A student who thinks RNA is built from the same components as DNA picks this. These are the components of DNA; RNA differs from DNA in its sugar (ribose) and in one of its bases (uracil).

CED 1.6.A.4.i · Read this in Fix

1. AThe amino acids attached along their strands are arranged in a different order in each.
   A student who confuses nucleic acids with proteins picks this. DNA is made of nucleotides and has no amino acids along its strands; its information lies in the order of its nucleotides.
2. BThe nucleotides of each species are built around a different five-carbon sugar.
   A student who thinks different species have chemically different DNA picks this. The DNA of every species contains the same sugar, deoxyribose; what differs is the order of the nucleotides.
3. CTheir nucleotides are arranged in a different order along each molecule. Correct
   In nucleic acids, information is encoded in the sequence of nucleotides. Two molecules of the same length and the same base composition can have their nucleotides in different orders, as the same letters can spell different words, so they can carry different information.
4. DEach molecule is folded into a different three-dimensional shape that stores information.
   A student who carries over the idea that a protein's function depends on its shape picks this. Both molecules are double helices of the same general shape; their information is in the order of their nucleotides.

CED 1.6.A.1 · Read this in Fix

1. A21%
   A student who thinks adenine pairs with cytosine picks this, setting cytosine equal to adenine. In DNA, adenine pairs with thymine and cytosine with guanine, so C = (100 − 42)/2 = 29%.
2. B29% Correct
   In double-stranded DNA, A pairs with T and C pairs with G, so T = A = 21% and A + T = 42%. That leaves 100% − 42% = 58% for C and G together, and because C = G, cytosine is 58%/2 = 29%.
3. C25%
   A student who thinks the four bases are always present in equal amounts picks this. Base proportions differ among DNA molecules; only the amounts of paired bases (A and T, C and G) are equal in double-stranded DNA.
4. D58%
   A student who counts a C–G base pair as if it were a single base picks this: 100% − 42% = 58% is the percentage of bases that are C or G. Cytosine is half of this, 29%.

Working In double-stranded DNA, A pairs with T and C pairs with G, so %A = %T and %C = %G. A = 21%, so T = 21% and A + T = 42%. C + G = 100% − 42% = 58%, so C = 58%/2 = 29%. Distractors: pairing A with C gives C = A = 21%; four equal bases gives 100%/4 = 25%; counting C and G together as one gives 58%.

CED 1.6.A.3 · Read this in Fix

1. AAt end P, joined to the free phosphate group of the first nucleotide
   A student who thinks nucleotides are added at the 5′ end picks this. End P, with the free phosphate group, is the 5′ end; nucleotides are added at the 3′ end, end Q.
2. BAt either end, as a strand can be lengthened in both directions
   A student who thinks a strand can grow at either end picks this. The two ends differ (a 5′ phosphate at P and a 3′ hydroxyl at Q), and synthesis adds nucleotides only at the 3′ end.
3. CBeside any base along the strand, held to it by hydrogen bonding
   A student who thinks the nucleotides in a strand are held together by hydrogen bonds picks this. Nucleotides in a strand are joined by covalent bonds between the phosphate of one and the sugar of the next, and a new nucleotide is added at the 3′ end.
4. DAt end Q, joined to the free –OH group on the sugar of the last nucleotide Correct
   End Q is the end at which a sugar carries a free hydroxyl (–OH) group: the 3′ end. During nucleic acid synthesis, nucleotides are added to the 3′ end of the growing strand, where the phosphate of the incoming nucleotide forms a covalent bond with the 3′ hydroxyl of the last sugar.

CED 1.6.A.2 · Read this in Fix

1. ASynthesis would continue normally, as nucleotides are added at the strand's 5′ end.
   A student who thinks nucleotides are added at the 5′ end picks this. Synthesis adds nucleotides at the 3′ end, which now ends in a sugar with no hydroxyl group, so no more nucleotides can be joined.
2. BNo further nucleotides could be added to the strand after the modified one. Correct
   Nucleotides are added to the 3′ end of a growing strand, where the phosphate of the incoming nucleotide forms a covalent bond with the 3′ hydroxyl of the last sugar. Once the modified nucleotide, which has no 3′ hydroxyl, is at the 3′ end, there is nothing for the next nucleotide to bond to, so synthesis of that strand stops.
3. CSynthesis would continue at the strand's other end, as strands can grow in either direction.
   A student who thinks a strand can grow at either end picks this. Nucleotides are added only to the 3′ end; the 5′ end is not extended, so synthesis stops when the 3′ end is blocked.
4. DThe next nucleotide would be joined to the modified one by hydrogen bonds instead.
   A student who thinks nucleotides in a strand can be held together by hydrogen bonds picks this. The nucleotides of a strand are joined by covalent bonds; hydrogen bonds join paired bases and cannot link a new nucleotide into the strand.

CED 1.6.A.2 · Read this in Fix

1. ATACGGT, with its 5′ end at the left
   A student who thinks the two strands run in the same direction picks this. The bases are correct, but DNA is antiparallel: the left end of the bottom strand is its 3′ end.
2. BUACGGU, with its 3′ end at the left
   A student who applies RNA's A–U pairing to DNA picks this. In DNA, adenine pairs with thymine, so the bottom strand contains T, not U.
3. CTACGGT, with its 3′ end at the left Correct
   Each base of the bottom strand pairs with the base above it, A with T and C with G, giving T, A, C, G, G, T from left to right. The strands are antiparallel: the top strand runs 5′ to 3′ from left to right, so the bottom strand runs 3′ to 5′ from left to right.
4. DATGCCA, with its 5′ end at the left
   A student who thinks the two strands of DNA are identical picks this. The strands are complementary, not identical: each base pairs with its partner (A with T, C with G), and the strands run in opposite directions.

CED 1.6.A.3 · Read this in Fix

1. AStrands rich in C–G pairs have stronger covalent bonds in their backbones, which break as the strands separate.
   A student who thinks heating separates the strands by breaking covalent bonds picks this. When the strands separate, only the hydrogen bonds between paired bases break; the covalent backbone of each strand stays intact.
2. BSpecies living in hot places make C–G-rich DNA so that their strands stay paired when heated.
   A student who explains a pattern by the organism's needs picks this. The data show how separation temperature depends on base composition; the trend is explained by the number of hydrogen bonds between paired bases, not by a purpose, and no data on habitats are given.
3. CC–G-rich strands have more hydrogen bonds linking the nucleotides along each strand to one another.
   A student who thinks hydrogen bonds join the nucleotides along a strand picks this. Nucleotides within a strand are joined by covalent bonds; hydrogen bonds join the paired bases of the two strands, and these are what break when the strands separate.
4. DC–G pairs have more hydrogen bonds than A–T pairs, so strands rich in them need more energy to separate. Correct
   The separation temperature rises steadily, from about 82 °C at 30% C–G pairs to about 98 °C at 70%. Strands separate when the hydrogen bonds between paired bases break; a C–G pair has three hydrogen bonds and an A–T pair two, so DNA with a higher proportion of C–G pairs has more hydrogen bonds to break and needs a higher temperature. (Interactions between neighboring stacked bases also contribute to the stability of C–G-rich DNA.)

CED 1.6.A.3 · Read this in Fix

1. AI and III
   A student who thinks DNA is always double-stranded picks this, including sample III because it contains thymine. In sample III, A (33%) does not equal T (22%) and G (18%) does not equal C (27%), so its bases cannot all be paired.
2. BI and IV Correct
   In a double-stranded molecule every base is paired, so the amount of A equals the amount of its partner (T in DNA, U in RNA) and the amount of G equals the amount of C. Sample I (DNA: A = T = 30%, G = C = 20%) and sample IV (RNA: A = U = 27%, G = C = 23%) fit this. Samples II and III do not, so they are most likely single-stranded.
3. CI only
   A student who thinks RNA is never double-stranded picks this. RNA is typically single-stranded, but some RNA, such as the genomes of some viruses, is double-stranded; sample IV, with A = U and G = C, fits a double-stranded RNA.
4. DAll of them
   A student who thinks all nucleic acids are double helices picks this. In samples II and III the paired bases are not present in equal amounts (for example, 31% G and 19% C in sample II), so these molecules cannot be fully double-stranded.

CED 1.6.A.4.iii · Read this in Fix

1. AIt is one strand, part of which folds back so that its bases pair with each other. Correct
   The diagram shows one continuous chain with one 5′ end and one 3′ end. The chain folds back on itself, and bases in one part pair with complementary bases in another part of the same strand (A with U, C with G). The molecule contains uracil, so it is RNA, which is typically single-stranded.
2. BIt is made of two strands, as nucleic acids typically form a two-stranded helix.
   A student who thinks every nucleic acid is a double helix picks this. The diagram shows a single chain with one 5′ end and one 3′ end; parts of one strand pair with each other.
3. CIts paired bases are joined by covalent bonds, like those along its backbone.
   A student who has the bonds in nucleic acids the wrong way round picks this. Paired bases are held together by hydrogen bonds; covalent bonds join the nucleotides along the backbone.
4. DIt is DNA rather than RNA, as some of its bases are paired with each other.
   A student who thinks RNA bases never pair, so paired bases mean DNA, picks this. The molecule contains uracil, so it is RNA, and an RNA strand can fold back so that its bases pair, adenine with uracil and cytosine with guanine.

CED 1.6.A.4.iii · Read this in Fix

1. AThey could still make new DNA, but they could not make new RNA.
   A student who has thymine and uracil the wrong way round picks this. Thymine is a base of DNA; RNA uses uracil, so RNA synthesis is not affected.
2. BThey could make neither new DNA nor new RNA molecules.
   A student who thinks RNA is made of the same components as DNA picks this. RNA contains uracil, not thymine, so the cells can still make RNA.
3. CThey could make new DNA and RNA normally, using uracil for thymine.
   A student who thinks thymine and uracil are interchangeable picks this. Uracil is a base of RNA, not DNA; without thymine-containing nucleotides the cells cannot make normal new DNA, although they can still make RNA.
4. DThey could still make new RNA but could not make new DNA. Correct
   Thymine is found in DNA but not in RNA, which contains uracil in its place. Without thymine-containing nucleotides the cells cannot build new DNA, but they can still make RNA from nucleotides containing A, U, G and C.

CED 1.6.A.4.ii · Read this in Fix