6.1 DNA and RNA Structure

1. AX is a large plasmid, as prokaryotic cells such as Cell 1 have no chromosomes.
   A student who thinks prokaryotes lack chromosomes picks this. Prokaryotes typically have a circular chromosome, the large loop X; the small extra circles, Y, are the plasmids.
2. BX is a linear chromosome folded into a loop, as all chromosomes are linear.
   A student who thinks every chromosome is linear picks this. Linear chromosomes, like Z in Cell 2, are typical of eukaryotes; prokaryotic chromosomes are typically circular.
3. CX is a circular chromosome, as is usual for the main DNA of a prokaryote. Correct
   Cell 1 has no nucleus, so it is the prokaryotic cell. Its large closed loop, X, is its chromosome: prokaryotes typically have a single circular chromosome in the cytoplasm. The small circles, Y, are plasmids.
4. DX is the outline of Cell 1's nucleus, which holds the DNA as in Cell 2.
   A student who thinks all cells have a nucleus picks this. Cell 1 is the prokaryotic cell, which has no nucleus; X is its circular chromosome lying in the cytoplasm.

CED 6.1.A.1.i · Read this in Fix

1. AIt is not a plasmid, as plasmids are a feature of prokaryotic cells.
   A student who thinks plasmids occur only in prokaryotes picks this. Both prokaryotes and eukaryotes can contain plasmids, and yeast is a well-known eukaryotic example.
2. BIt shows that yeast is a prokaryote, as circular DNA is a prokaryotic feature.
   A student who thinks circular DNA belongs only to prokaryotes picks this. Yeast cells have a nucleus and linear chromosomes, so yeast is eukaryotic; eukaryotes also have circular DNA in mitochondria and in plasmids.
3. CIt is a plasmid, as plasmids occur in eukaryotes as well as in prokaryotes. Correct
   A plasmid is an extra-chromosomal circular DNA molecule. Plasmids are best known in prokaryotes, but eukaryotes can contain them too, and this yeast molecule is one.
4. DIt carries no genes, as genes are carried on the yeast's chromosomes.
   A student who thinks genes occur only on chromosomes picks this. Plasmids, like other DNA molecules, can carry genes; the yeast plasmid carries several.

CED 6.1.A.2 · Read this in Fix

1. AEach has a structure made of two fused rings Correct
   Purines, adenine and guanine, have a double-ring structure; the pyrimidines, cytosine, thymine and uracil, each have a single ring.
2. BEach has a structure built from one ring only
   A student who swaps the ring structures of purines and pyrimidines picks this. A single ring is the feature of the pyrimidines; the purines have a double ring.
3. CEach pairs with the other in double-stranded DNA
   A student who thinks similar bases pair with each other picks this. Purines pair with pyrimidines: adenine with thymine and guanine with cytosine.
4. DEach occurs in DNA but is absent from RNA molecules
   A student who thinks DNA and RNA have entirely different bases picks this. Adenine and guanine are found in both DNA and RNA; thymine is the base found in DNA but not RNA.

CED 6.1.B.1.i · Read this in Fix

1. AProteins in the extract carried the hereditary information for making a capsule.
   A student who thinks proteins carry hereditary information picks this. Capsule-making bacteria still appeared when protein was destroyed, so protein was not needed to pass on the trait.
2. BDNA in the extract carried the hereditary information for making a capsule. Correct
   The capsule trait was passed on in every sample except the one in which DNA was destroyed. Destroying protein or RNA had no effect, so DNA was the molecule carrying the information.
3. CRNA in the extract, not DNA, carried hereditary information for the capsule trait.
   A student who thinks RNA is the hereditary material of cells picks this. Destroying RNA left the result unchanged; only destroying DNA stopped the trait being passed on.
4. DAll three kinds of molecule were needed together to pass on the capsule trait.
   A student who thinks heredity needs all of a cell's molecules picks this. The trait was still passed on when protein or RNA was destroyed; only DNA was needed.

CED 6.1.A.1 · Read this in Fix

1. AIts RNA molecule stores the genetic information that is passed to new viruses. Correct
   Genetic information is usually stored in DNA but in some cases in RNA. This virus has RNA and no DNA, so its RNA is the molecule that stores its genetic information and passes it to the new virus particles.
2. BIts protein coat stores the genetic information that is passed on to new viruses.
   A student who thinks proteins carry hereditary information picks this. The coat protects the RNA; the information that is passed on is stored in the RNA.
3. CIt uses the tobacco cell's DNA, as only DNA can store genetic information.
   A student who thinks only DNA can be genetic material picks this. New virus particles carry the virus's own genetic information, not the tobacco cell's; it is stored in the virus's RNA.
4. DIts genetic information is kept in a nucleus inside each virus particle.
   A student who thinks all genetic information is kept in a nucleus picks this. A virus is not a cell and has no nucleus; this one is only a protein coat around an RNA molecule.

CED 6.1.A.1 · Read this in Fix

1. AHistones are sections of the DNA molecule that carry some of the chromosome's genes.
   A student who thinks every part of a chromosome is DNA picks this. The model shows histones as separate protein structures that the DNA winds around.
2. BHistones hold the two strands of the DNA together by pairing with its bases.
   A student who merges histone binding with base pairing picks this. The two strands are joined by pairing between their own bases; the model shows histones as spools around which the double helix winds.
3. CHistones change the order of the bases so that the DNA fits inside the nucleus.
   A student who thinks packing DNA changes its sequence picks this. Winding DNA around histones changes how tightly it is packed, not the order of its bases.
4. DHistones are proteins that DNA winds around, packing the DNA into a shorter structure. Correct
   The model shows the DNA double helix wound around histone proteins, and this wound DNA is folded further into a condensed chromosome. Histones therefore package a long DNA molecule into a compact one.

CED 6.1.A.1.ii · Read this in Fix

1. AIts DNA will be less tightly packed, so its chromosomes will be less condensed. Correct
   Histones are the proteins that DNA winds around to become condensed. With far fewer histones, less DNA can be wound and packed, so the chromosomes are expected to be less condensed.
2. BIts DNA will have a changed base sequence, as histones set the order of its bases.
   A student who thinks packing DNA changes its sequence picks this. Histones package DNA; the base sequence is set by the DNA itself and is not altered by how it is packed.
3. CIts two DNA strands will come apart, as histones hold the base pairs together.
   A student who thinks histones join the two strands picks this. The strands are held together by base pairing between A and T and between G and C, which does not depend on histones.
4. DIts chromosomes will lose some genes, as histones are sections of the DNA.
   A student who thinks histones are part of the DNA picks this. Histones are proteins, separate from the DNA, so making fewer of them does not remove any genes.

CED 6.1.A.1.ii · Read this in Fix

1. AIn each species, the four bases are present in about equal percentages.
   A student who expects equal amounts of all four bases picks this. The percentages range from 14.8% to 35.2%; only base partners are about equal.
2. BIn each species, A and G are about equal, as are T and C, in percentage.
   A student who thinks similar bases pair with each other picks this. In species 1, A is 30.8% but G is 19.2%; the equal pairs are A with T and G with C.
3. CAll four species have about the same percentage of each of the four bases.
   A student who thinks shared pairing rules mean identical DNA picks this. The species differ widely; for example, G is 14.8% in species 3 and 27.6% in species 2.
4. DIn each species, the percentage of A is about equal to T, and G to C. Correct
   In every species, %A ≈ %T (for example 30.8 and 31.0) and %G ≈ %C (19.2 and 19.0), as expected when A pairs with T and G with C. The proportions differ between species: A is 22.4% in species 2 but 35.2% in species 3.

CED 6.1.B.1 · Read this in Fix

1. AEach group of organisms evolved the same pairing rules on its own because it needed them.
   A student who thinks traits arise because organisms need them picks this. Independent origins of identical rules in every lineage are far less likely than inheritance of the rules from a shared ancestor.
2. BThe pairing rules arose in a common ancestor and were kept in all of its descendants. Correct
   A feature shared by every known organism is best explained by inheritance from a common ancestor, with the feature conserved because changes to it would disrupt the storage and passing on of genetic information.
3. CPlants and animals evolved from present-day bacteria and inherited their pairing rules.
   A student who thinks living species are ancestors of other living species picks this. Present-day bacteria, plants and animals are all descendants of a common ancestor that lived long ago.
4. DAll organisms have the same DNA sequence, so their base-pairing rules are also the same.
   A student who thinks shared pairing rules mean identical DNA picks this. Species differ in their DNA sequences; only the rules for pairing bases are shared.

CED 6.1.B.1 · Read this in Fix

1. AThe two strands would be farther apart at that site, as pyrimidines have two rings each.
   A student who swaps the ring structures of purines and pyrimidines picks this. The model shows pyrimidines (C and T) as single rings, so two of them make a shorter pair, not a longer one.
2. BThe two strands would be closer together at that site, as each pyrimidine has one ring. Correct
   Each normal pair has one double-ring purine and one single-ring pyrimidine, and all such pairs span the same distance. Two single-ring pyrimidines together are shorter than a purine–pyrimidine pair, so the backbones would have to come closer at that site.
3. CThe width would not change at that site, as every base takes up the same space.
   A student who thinks all bases are the same size picks this. The model shows purines with two rings and pyrimidines with one, so a pair's length depends on which bases are paired.
4. DOne pyrimidine would change into a purine so that the pair fits the usual width again.
   A student who thinks molecules change to fit where they are needed picks this. A base does not change its structure to suit its position; a mismatched pair stays mismatched.

CED 6.1.B.1.ii · Read this in Fix

1. AT 28%, G 22%, C 28%
   A student who thinks similar bases pair (A with G, T with C) picks this, setting G equal to A at 22% and sharing the rest between T and C. In DNA, A pairs with T, so T must equal A.
2. BU 22%, G 28%, C 28%
   A student who thinks DNA contains uracil picks this. Uracil is found in RNA; in DNA, adenine pairs with thymine, so the sample is 22% T.
3. CT 28%, G 25%, C 25%
   A student who thinks A + T must equal G + C picks this, making A + T = 50% and so T = 28%. In fact %A = %T and %G = %C; it is A + G that equals T + C.
4. DT 22%, G 28%, C 28% Correct
   A pairs with T, so %T = %A = 22%. Then G + C = 100 − 22 − 22 = 56%, and because G pairs with C, %G = %C = 56/2 = 28%.

Working In double-stranded DNA, %A = %T and %G = %C. %T = 22%. %G + %C = 100 − 22 − 22 = 56%, so %G = %C = 56/2 = 28%. Check: 22 + 22 + 28 + 28 = 100%.

CED 6.1.B.1.iii · Read this in Fix

1. AThymine is present, and uracil absent
   A student who thinks RNA has the same bases as the DNA it is made from picks this. Thymine and no uracil is the result expected for DNA, the opposite of the hypothesis.
2. BIts four bases are present in equal amounts
   A student who thinks nucleic acids always contain equal amounts of their four bases picks this. Base composition varies, and equal amounts would not distinguish RNA from DNA; the hypothesis predicts uracil in place of thymine.
3. CUracil is present, and thymine is absent Correct
   RNA contains uracil in place of thymine, and the other three bases (adenine, guanine and cytosine) are shared with DNA. Finding uracil and no thymine is the result predicted for RNA.
4. DNone of adenine, guanine or cytosine is present
   A student who thinks DNA and RNA have entirely different bases picks this. RNA contains adenine, guanine and cytosine, as DNA does; the two differ only in thymine and uracil.

CED 6.1.B.1.iii · Read this in Fix