4.5 Cell Cycle

1. ADNA is replicated in interphase, before the nucleus divides in mitosis. Correct
   DNA is replicated in S phase of interphase (G1, S, G2); mitosis then divides the nucleus and cytokinesis divides the cytoplasm.
2. BDNA is replicated in mitosis, at the same time as the chromosomes condense.
   A student who thinks DNA is copied during mitosis picks this. DNA is replicated in S phase, so each chromosome already consists of two sister chromatids when it condenses in prophase.
3. CThe nucleus and the cytoplasm divide together, in one single event.
   A student who thinks mitosis and cytokinesis are one process picks this. Mitosis divides the nucleus and cytokinesis then divides the cytoplasm; a cell can even complete mitosis without dividing its cytoplasm.
4. DEach stage of the cycle, from G1 to cytokinesis, lasts about equally long.
   A student who thinks every stage lasts the same length of time picks this. The stages differ greatly in length; in most dividing cells interphase takes up most of the cycle.

CED 4.5.A.1.i · Read this in Fix

1. AIts DNA is being replicated as the chromosomes condense and become visible.
   A student who thinks DNA is copied during mitosis picks this. The chromosomes already consist of two sister chromatids each, because the DNA was replicated in S phase.
2. BIts centromeres are moving toward opposite poles, where they will form the spindle.
   A student who confuses centromeres with centrosomes picks this. The structures moving apart are the labeled centrosomes; centromeres are the points where each chromosome's sister chromatids are joined.
3. CIts homologous chromosomes are pairing up so that they can line up together.
   A student who thinks homologous chromosomes pair in mitosis picks this. The two long and two short chromosomes are scattered separately; in mitosis each chromosome lines up on its own.
4. DIts chromatids are condensing, and its centrosomes are moving toward opposite poles. Correct
   The cell is in prophase: the sister chromatids are condensed into visible chromosomes, the nuclear envelope is breaking down, spindle fibers are forming, and the arrows show the centrosomes moving toward opposite poles.

CED 4.5.B.1.iii · Read this in Fix

1. AEach type of cell carries different genes, and only some cell types carry the genes for dividing.
   A student who thinks different cell types carry different genes picks this. Almost all body cells carry the same genome; liver cells, for example, divide when the liver is damaged, so they carry the genes needed.
2. BCell division is regulated, so whether a cell divides depends on its type and conditions. Correct
   The cell cycle is a highly regulated series of events. Intestinal cells are continually stimulated to divide, liver cells can be stimulated to re-enter the cycle after damage, and most mature nerve cells stay out of the cycle. The differences come from regulation, not from differences in the genes the cells carry.
3. CEach cell divides when it decides for itself that the body needs more cells of its own type.
   A student who explains cell behavior by needs or intentions picks this. Cells do not decide anything; division is controlled by responses to signals, as when damage to the liver stimulates liver cells to divide.
4. DLiver and nerve cells are resting in interphase, the stage in which cells are inactive.
   A student who thinks interphase is a resting stage picks this. Cells that are not dividing are alive and active; nerve and liver cells carry out demanding functions, and interphase is when dividing cells are busiest preparing to divide.

CED 4.5.A.1 · Read this in Fix

1. A0.8 hours
   A student who thinks DNA is copied during mitosis picks this, using the 10 cells in mitosis: 10/200 × 16 h = 0.8 hours. DNA is replicated in S phase, before mitosis begins.
2. B4.0 hours
   A student who thinks every stage lasts the same time picks this: 16 h ÷ 4 stages = 4.0 hours. The unequal counts show that the stages differ in length.
3. C6.9 hours
   A student who divides the S-phase count by the number of cells not in S picks this: 60/140 × 16 h = 6.9 hours. The fraction of the cycle is the count in S divided by all 200 cells.
4. D4.8 hours Correct
   DNA is replicated in S phase. 60 of 200 cells (0.30) are in S, so S takes about 0.30 of the 16-hour cycle: 0.30 × 16 h = 4.8 hours.

Working DNA is replicated in S phase. In a population of cells dividing independently, the fraction of cells in a stage is about the fraction of the cycle spent in it. Fraction in S = 60/200 = 0.30. Time for DNA replication ≈ 0.30 × 16 h = 4.8 hours.

CED 4.5.A.1.i · Read this in Fix

1. ADNA per cell rises steadily through G1, S and G2, along with protein per cell.
   A student who thinks DNA increases gradually as a cell grows picks this. DNA is flat in G1 and G2 and rises only in S; only protein rises throughout interphase.
2. BDNA per cell doubles during mitosis, once the chromosomes have condensed.
   A student who thinks DNA is copied during mitosis picks this. The DNA doubled between 8 and 14 hours (S) and did not change during M (19 to 20 hours).
3. CProtein per cell rises in G1, but DNA per cell stays constant until S. Correct
   From 0 to 8 hours (G1) protein rises from 1.0 to about 1.4 while DNA stays at 1.0; DNA doubles only between 8 and 14 hours (S). The cell grows and makes new cell components in G1 before its DNA is replicated.
4. DProtein per cell does not change when a cell divides, as each new cell is full-sized.
   A student who thinks division produces two full-sized cells picks this. At 20 hours protein per cell falls from 2.0 to 1.0: the contents are shared between two daughter cells, each of which then grows again.

CED 4.5.A.1.ii · Read this in Fix

1. AEach would have about as many organelles as the parent, since division copies the whole cell.
   A student who thinks division produces two full copies of the parent cell picks this. Cytokinesis shares out what the parent cell contains; without duplication in G1 there is nothing extra to share.
2. BEach would get half of the parent's chromosomes, as well as half of its organelles.
   A student who thinks mitosis halves the chromosome number picks this. DNA replication and mitosis are unaffected, so each daughter cell receives a full set of chromosomes.
3. CEach would make up its missing organelles, as it would sense that it needed more.
   A student who explains cell behavior by needs picks this. Cells do not sense what they need and act to meet it; the daughter cells carry the same mutation, so they cannot duplicate their organelles either and each starts with about half as many.
4. DEach would start with about half as many organelles as its parent started with. Correct
   Organelles are normally duplicated in G1, so each daughter cell can receive about as many as its parent started with. If they are not duplicated, the parent's organelles are shared between the two daughter cells, so each starts with about half as many.

CED 4.5.A.1.ii · Read this in Fix

1. AThe DNA condenses into visible chromosomes so that it can then be copied.
   A student who thinks DNA must be condensed before it can be replicated picks this. Replication in S phase happens while the DNA is extended as chromatin; condensation follows in prophase.
2. BThe DNA, as extended chromatin, is copied to make sister chromatids. Correct
   X lies between G1 and G2, so it is S phase. In S phase the DNA is replicated while in the form of chromatin, so that each chromosome comes to consist of two sister chromatids joined at a centromere.
3. CThe number of chromosomes in the nucleus doubles as each one is copied.
   A student who counts each sister chromatid as a chromosome picks this. After S phase each chromosome consists of two chromatids joined at one centromere, so the chromosome number is unchanged.
4. DThe cell rests, between its growth in G1 and its later preparation for mitosis.
   A student who thinks interphase includes a resting stage picks this. S phase is a stage of intense activity in which the whole genome is replicated.

CED 4.5.A.1.iii · Read this in Fix

1. AIn G1: 20; end of S phase: 20 Correct
   Each chromatid is one DNA molecule, so the 40 DNA molecules are 40 sister chromatids, joined in pairs at 20 centromeres: 20 chromosomes at the end of S phase. Replication copies each chromosome without changing their number, so in G1 the nucleus had 20 chromosomes, each one DNA molecule.
2. BIn G1: 20; end of S phase: 40
   A student who counts each sister chromatid as a separate chromosome picks this: the 40 chromatids are counted as 40 chromosomes, twice the 20 present in G1. Sister chromatids joined at a centromere form one chromosome, so there are still 20 chromosomes at the end of S phase.
3. CIn G1: 40; end of S phase: 40
   A student who thinks DNA is copied only during mitosis picks this: with no replication in S phase, the 40 DNA molecules would be 40 unreplicated chromosomes, already present in G1. The DNA is replicated in S phase, so the 40 molecules are the sister chromatids of 20 chromosomes.
4. DIn G1: 10; end of S phase: 10
   A student who counts each strand of the double helix as a DNA molecule picks this: 40 strands make 20 double-stranded chromatids, which form 10 replicated chromosomes, and S phase does not change the number. Each chromatid is one double-stranded DNA molecule, so the 40 molecules are the 40 chromatids of 20 chromosomes.

Working Each chromatid is one DNA molecule, so 40 DNA molecules at the end of S phase = 40 chromatids. Each replicated chromosome consists of two sister chromatids joined at one centromere: chromosomes at the end of S phase = 40 / 2 = 20. Replication in S phase does not change the number of chromosomes, so in G1 there were also 20 chromosomes, each one DNA molecule (20 DNA molecules, doubled to 40 in S phase).

CED 4.5.A.1.iii · Read this in Fix

1. AIt is resting after its DNA was replicated and makes few new proteins until mitosis begins.
   A student who thinks interphase is a resting stage picks this. G2 is a stage of active protein synthesis and ATP production.
2. BIt has twice as many chromosomes as in G1, since each one was copied in S phase.
   A student who counts each sister chromatid as a chromosome picks this. Each chromosome now consists of two chromatids joined at one centromere, so the chromosome number is the same as in G1.
3. CIt has replicated its DNA, and it is making proteins and ATP and copying its centrosome. Correct
   G2 follows S phase, so the DNA has already been replicated. In G2 the cell synthesizes proteins, produces ATP in large quantities and replicates its centrosome in preparation for mitosis.
4. DIt is copying the centromere of each chromosome so that the copies can organize the spindle.
   A student who confuses centromeres with centrosomes picks this. In G2 the centrosome, a structure in the cytoplasm, is replicated; centromeres are regions of the chromosomes.

CED 4.5.A.1.iv · Read this in Fix

1. AATP made in G2 is stockpiled, like glycogen, so that the cell can draw on it slowly through the next cell cycle.
   A student who thinks ATP is a long-term energy store picks this. A cell holds only a small amount of ATP and regenerates it continuously; long-term stores are molecules such as glycogen and fats.
2. BMaking proteins in G2 and moving chromosomes in mitosis both require energy, which is supplied by ATP. Correct
   G2 prepares the cell for mitosis: it synthesizes proteins and replicates its centrosomes, and mitosis itself involves forming a spindle and moving chromosomes. These processes use energy supplied by ATP, which is produced in large quantities in G2.
3. CATP made in G2 is used mainly to replicate the cell's DNA, which is copied once mitosis has begun.
   A student who thinks DNA is copied during mitosis picks this. DNA replication has already happened in S phase, before G2.
4. DThe cell makes more ATP in G2 because it knows that it is about to divide and will need energy.
   A student who explains cell activity by intentions picks this. A cell does not know or plan anything; the reasoning must connect the ATP to the processes that use it.

CED 4.5.A.1.iv · Read this in Fix

1. AThe fibroblasts remain in G0, as cells in G0 have permanently lost the ability to divide.
   A student who thinks cells in G0 can never divide again picks this. Many cells in G0, including fibroblasts, can re-enter the cell cycle in response to appropriate cues.
2. BThe fibroblasts enlarge to fill the wound without dividing, as tissues grow by cell enlargement.
   A student who thinks tissues grow mainly by cells enlarging picks this. Cells can enlarge only so far; replacing lost tissue requires new cells made by mitosis.
3. CThe fibroblasts near the wound re-enter the cell cycle and divide, supplying new cells. Correct
   Cells in G0 do not divide but can re-enter the cell cycle in response to appropriate cues. The chemical signals released at the wound are such cues, so some fibroblasts divide and supply cells for tissue repair.
4. DThe fibroblasts divide only once they decide, on their own, that the wound needs new cells.
   A student who explains cell behavior by intentions picks this. Cells do not decide; re-entry into the cell cycle is a response to signals such as those released at the wound.

CED 4.5.A.1.v · Read this in Fix

1. AOnly the few liver cells that were already dividing before surgery went on to replicate DNA.
   A student who thinks nondividing cells cannot divide again picks this. Only about 1% of cells were replicating DNA before surgery, yet 30% were at 24 hours, so many nondividing cells re-entered the cycle.
2. BLiver cells divided at a high rate both before surgery and throughout the week after it.
   A student who thinks all cells divide continuously picks this. Only about 1% of liver cells were replicating DNA before surgery and in the sham group, and the rise after tissue removal lasted only a few days.
3. CThe remaining liver cells divided because they recognized the liver needed more cells.
   A student who explains cell behavior by needs or intentions picks this. The data show that removing tissue was followed by many cells re-entering the cycle; cells respond to signals, they do not recognize needs, and the graph gives no evidence about intentions.
4. DRemoving liver tissue caused many nondividing liver cells to re-enter the cell cycle. Correct
   Before surgery only about 1% of liver cells were replicating DNA, but 24 hours after tissue was removed 30% were. Far more cells than were dividing before must have re-entered the cycle. The sham group stayed near 1%, so the operation alone did not cause this; the loss of liver tissue did.

CED 4.5.A.1.v · Read this in Fix

1. AThey have stopped dividing but are alive, carrying out the functions of their cell type. Correct
   Nondividing cells may exit the cell cycle or be held at a particular stage. The starch-storing cells no longer divide, but they are alive and metabolically active, carrying out their specialized function.
2. BThey are dead, since a cell that has stopped dividing can no longer carry out life processes.
   A student who thinks nondividing cells are dead picks this. Storing starch is an active process; cells that stop dividing remain alive.
3. CThey have fewer chromosomes than the tip cells, since a specialized cell needs fewer.
   A student who thinks different cell types have different numbers of chromosomes picks this. These cells were produced by mitosis from the dividing tip cells, and mitosis does not remove chromosomes; specializing to store starch does not reduce the chromosome number.
4. DThey are resting in interphase, doing little until they are able to divide again.
   A student who thinks interphase is a resting stage picks this. The specialized cells are not waiting to divide; they are actively storing starch, and interphase in dividing cells is itself a busy stage.

CED 4.5.A.1.vi · Read this in Fix

1. AMost cells grown without the nutrient died, leaving few cells in S, G2 or mitosis.
   A student who thinks cells that stop dividing are dead picks this. The data show where living cells were in the cycle: almost all were in G1, held before DNA replication. Nothing in the data shows that cells died.
2. BMost cells without the nutrient were held in G1, prior to DNA replication. Correct
   Without the nutrient, 92% of cells were in G1 compared with 45% in the culture with it, and very few were in S, G2 or mitosis. The cells were held at a particular stage, G1, before DNA replication.
3. CWith the nutrient, G1 is the stage that cells pass through most quickly.
   A student who thinks a stage with many cells is passed through quickly picks this. G1 holds the largest share of cells (45%), so it is the longest stage, not the shortest.
4. DWith the nutrient, cells spend about the same length of time in each stage.
   A student who thinks all stages last the same time picks this. The percentages (45, 30, 15 and 10) are very unequal, so the stages differ in length.

CED 4.5.A.1.vi · Read this in Fix

1. AIt has half as many chromosomes, since the parent's chromosomes are shared out.
   A student who thinks mitosis halves the chromosome number picks this. The DNA was replicated before mitosis, so each daughter cell receives a full set.
2. BIt has the same number of chromosomes but a new mix of the parent's genes.
   A student who thinks mitosis produces genetic variation picks this. Sister chromatids are identical copies, so each daughter cell receives the same genetic information as the parent.
3. CIt carries just the genes needed by the type of skin cell it will become.
   A student who thinks each cell type carries only its own genes picks this. Mitosis passes the complete genome to both daughter cells, whatever they later become.
4. DIt has the same number of chromosomes, carrying the same genetic information as the parent. Correct
   Mitosis transfers a complete genome to each daughter cell: every chromosome was replicated in S phase and one sister chromatid of each went to each daughter cell. Each daughter cell is genetically identical to the parent, apart from rare new mutations.

CED 4.5.B.1 · Read this in Fix

1. ADiagram 1
   A student who thinks homologous chromosomes separate in mitosis picks this: A and a, and B and b, go to different daughter cells. In mitosis the sister chromatids of each chromosome separate, so each daughter cell receives A, a, B and b.
2. BDiagram 2
   A student who thinks whole chromosomes are shared out between daughter cells picks the diagram in which one daughter cell receives A and a and the other B and b. Every chromosome is split, so each daughter cell gets a copy of each.
3. CDiagram 3 Correct
   Each chromosome is replicated in S phase and its two sister chromatids go to different daughter cells, so both daughter cells receive A, a, B and b, the same set as the parent cell.
4. DDiagram 4
   A student who thinks mitosis mixes genetic information picks the diagram in which the daughter cells have four chromosomes but different combinations (A, A, B, b and a, a, B, b). Both daughter cells should be identical to the parent.

CED 4.5.B.1 · Read this in Fix

1. AThey carry the same genetic information as the parent, except for any rare new mutations. Correct
   Mitosis plays a role in asexual reproduction: every cell of a fragment, and every cell it produces by mitosis, carries a complete copy of the parent's genome. The offspring are genetically identical to the parent except for rare new mutations.
2. BThey can differ genetically from one another, as each mitotic division mixes up genes.
   A student who thinks mitosis produces genetic variation picks this. Mitosis gives genetically identical daughter cells, so the offspring of fragmentation do not differ except through rare mutations.
3. CThey may inherit features that the parent developed during its life, such as insect damage.
   A student who thinks acquired characteristics are inherited picks this. Insect damage to the parent's leaves does not change its DNA, so the new plants grow normal leaves.
4. DThey each carry only part of the parent's genes, as each fragment receives only some.
   A student who thinks genetic information is shared out between cells picks this. Every cell in the fragment carries the complete genome because mitosis copies all of it to each cell.

CED 4.5.B.1.i · Read this in Fix

1. AGrowth was mainly by cells getting larger, as liver mass rose about 16-fold between newborn and adult.
   A student who thinks organs grow by their cells enlarging picks this. The 16-fold rise in mass is matched by a 15-fold rise in cell number; mean cell volume rose only about 4%.
2. BGrowth was mainly by mitosis, as the number of cells rose about 15-fold while cell volume barely changed. Correct
   From newborn to adult, liver mass rose 16-fold (0.4 g to 6.4 g) and the number of cells about 15-fold (100 to 1,540 million), while mean cell volume rose only from 4,000 to 4,150 µm³. Nearly all the growth came from more cells, produced by mitosis.
3. CCells did not need to grow between divisions, as the mean cell volume stayed about the same at all ages.
   A student who thinks division produces full-sized daughter cells picks this. Each division halves a cell's volume, so a steady mean volume over many divisions shows that each new cell grew before it divided again.
4. DCell division in the liver stopped by the adult stage, as the adult liver had finished growing.
   A student who thinks cell division stops once growth is complete picks this. The table gives no data on division in adults, so it cannot support this claim, and adult livers keep cells able to divide, for example to repair damage.

CED 4.5.B.1.i · Read this in Fix

1. A33%
   A student who divides the change by the new value picks this: (30 − 20)/30 × 100 = 33%. Percent change is calculated relative to the original value, 20%.
2. B10%
   A student who subtracts the two percentages picks this: 30% − 20% = 10 percentage points. The percent change compares that difference with the original 20%.
3. C65%
   A student who compares the raw counts of mitotic cells picks this: (66 − 40)/40 × 100 = 65%. Different numbers of cells (200 and 220) were counted, so the counts must first be converted to percentages.
4. D50% Correct
   The mitotic index is 40/200 = 20% in water and 66/220 = 30% with the chemical. Percent change = (30 − 20)/20 × 100 = 50%.

Working Mitotic index (water) = 40/200 × 100 = 20%. Mitotic index (chemical) = 66/220 × 100 = 30%. Percent change = (30 − 20)/20 × 100 = 50%.

CED 4.5.B.1.ii · Read this in Fix

1. AThe chemical increases the percentage of root-tip cells that are in mitosis.
   A student who thinks the null hypothesis is the expected effect picks this. An increase is the alternative hypothesis; the null hypothesis is that there is no effect.
2. BNo root-tip cells grown in the chemical solution will be found to be in mitosis.
   A student who thinks 'null' means zero picks this. The null hypothesis is about no difference between the groups, not about no cells dividing.
3. CThe chemical has no effect on the percentage of root-tip cells in mitosis. Correct
   A null hypothesis states that the variable tested has no effect: here, that the percentage of cells in mitosis is the same in treated and untreated root tips. The data are then tested against this.
4. DEqual numbers of root-tip cells will be found in interphase and in mitosis.
   A student who thinks a null hypothesis always predicts equal numbers in each category picks this. Even untreated root tips have far more cells in interphase than in mitosis; the null hypothesis concerns the effect of the chemical.

CED 4.5.B.1.ii · Read this in Fix

1. AReject the null hypothesis; the difference is too large to be explained by chance alone. Correct
   Expected counts are 176 interphase and 44 mitotic cells. χ² = (154 − 176)²/176 + (66 − 44)²/44 = 2.75 + 11.00 = 13.75. With 2 − 1 = 1 degree of freedom the critical value is 3.84; 13.75 > 3.84, so the difference is unlikely to be due to chance alone.
2. BFail to reject the null hypothesis; a χ² value this large shows the data fit well.
   A student who reads a large χ² as a good fit picks this. χ² = 13.75 measures how far the observed counts are from the expected ones; it is well above the critical value of 3.84, so the null hypothesis is rejected.
3. CReject the null hypothesis; the result proves that the chemical stimulates mitosis.
   A student who thinks a significant result proves the hypothesis picks this. Rejecting the null hypothesis shows that the difference is unlikely to be due to chance; it supports, but does not prove, an effect of the chemical.
4. DFail to reject the null hypothesis; with 219 degrees of freedom, χ² would need to be far larger.
   A student who takes degrees of freedom as the number of cells minus one picks this. For chi-square, degrees of freedom are the number of categories minus one: 2 − 1 = 1, giving a critical value of 3.84.

Working Water: 160/200 = 0.80 in interphase and 40/200 = 0.20 in mitosis. Expected for 220 treated cells: interphase 0.80 × 220 = 176; mitosis 0.20 × 220 = 44. χ² = (154 − 176)²/176 + (66 − 44)²/44 = 484/176 + 484/44 = 2.75 + 11.00 = 13.75. Degrees of freedom = 2 categories − 1 = 1; critical value at p = 0.05 = 3.84. Because 13.75 > 3.84, reject the null hypothesis.

CED 4.5.B.1.ii · Read this in Fix

1. ANo other set, since a high percentage in the treated tips alone would show the effect
   A student who thinks the treated group alone shows a treatment's effect picks this. Without untreated root tips there is no way to know what percentage of cells would be in mitosis without the chemical.
2. BRoot tips grown in water alone, under the same conditions as the treated root tips Correct
   A control must differ from the treated group only in the independent variable. Root tips grown in water under the same conditions show the percentage of cells in mitosis without the chemical, so any difference can be attributed to the chemical.
3. CRoot tips grown in water alone but kept warmer than the treated tips to speed their growth
   A student who thinks a control may differ in other conditions picks this. Temperature affects how fast cells divide, so a difference could be caused by temperature rather than the chemical.
4. DMore root tips grown in the chemical, since a large sample replaces a control
   A student who thinks a large sample can replace a control picks this. More treated root tips make the estimate more reliable, but they cannot show what happens without the chemical.

CED 4.5.B.1.ii · Read this in Fix

1. AHomologous chromosomes lie side by side in pairs at the equator, each pair beside the next.
   A student who thinks homologous chromosomes pair in mitosis picks this. In metaphase of mitosis each chromosome lines up independently.
2. BThe chromosomes drift to the equator by themselves, before any spindle fibers have formed.
   A student who thinks chromosomes move on their own picks this. The spindle begins to form in prophase, and spindle fibers align the chromosomes at the equator.
3. CEach replicated chromosome lies at the equator, its sister chromatids attached to fibers from opposite poles. Correct
   In metaphase, spindle fibers align the chromosomes along the equator of the cell. Each replicated chromosome lines up on its own, with its two sister chromatids attached by spindle fibers to opposite poles.
4. DThe centrosomes of the chromosomes line up at the equator and are held there by spindle fibers.
   A student who confuses centrosomes with centromeres picks this. Centrosomes are at the poles; it is the centromeres of the chromosomes that lie at the equator.

CED 4.5.B.1.iv · Read this in Fix

1. AIts two sister chromatids will separate, and one will move to each pole. Correct
   R is attached at its centromere to spindle fibers from both poles. In anaphase its sister chromatids separate and are pulled toward opposite poles, so each daughter cell receives a copy of R, and likewise of S.
2. BIt will move whole to one pole, while its homologue, S, moves to the other.
   A student who thinks homologous chromosomes separate in mitosis picks this. R and S are not paired; each splits into its two chromatids, so each pole receives a copy of both.
3. CIt will move to a pole on its own, after its spindle fibers have detached.
   A student who thinks chromosomes move on their own picks this. The chromatids are pulled toward the poles by the spindle fibers attached at the centromere.
4. DIts genes will be divided, so that each pole receives half of them.
   A student who thinks each daughter cell receives half of the genetic information picks this. The two sister chromatids are identical copies, so each pole receives every gene on R.

CED 4.5.B.1.v · Read this in Fix

1. A0.8 µm/min
   A student who divides by the whole time shown on the graph picks this: 12 µm ÷ 16 min = 0.75, about 0.8 µm/min. The chromatid moved only from 4 to 10 minutes.
2. B2.0 µm/min Correct
   The chromatid moves only while the distance is falling, from 4 to 10 minutes. Rate = (14 − 2) µm ÷ (10 − 4) min = 12 µm ÷ 6 min = 2.0 µm/min.
3. C0.5 µm/min
   A student who divides time by distance picks this: 6 min ÷ 12 µm = 0.5, which is minutes per micrometer, not micrometers per minute.
4. D0.2 µm/min
   A student who divides the coordinates of one point picks this: 2 µm ÷ 10 min = 0.2 µm/min at the end of anaphase. The rate is the change in distance divided by the change in time.

Working During anaphase (4 to 10 min) the distance falls from 14 µm to 2 µm. Rate = (14 − 2) µm ÷ (10 − 4) min = 12 µm ÷ 6 min = 2.0 µm/min.

CED 4.5.B.1.v · Read this in Fix

1. AEach new nucleus is receiving half as many chromosomes as the parent cell had in G1.
   A student who thinks mitosis halves the chromosome number picks this. Each set contains one copy of every chromosome, the same number as the parent cell had in G1.
2. BThe chromosomes are breaking down and disappearing, as they exist only during mitosis.
   A student who thinks chromosomes exist only during mitosis picks this. The chromosomes are decondensing into chromatin; they remain in each nucleus throughout interphase.
3. CHomologous chromosomes have been pulled apart into the two new nuclei that are now forming.
   A student who thinks homologous chromosomes separate in mitosis picks this. Each set contains two long and two short chromosomes: both members of each homologous pair are in each new nucleus.
4. DA nuclear envelope is forming around each set of chromosomes as the spindle breaks down. Correct
   The cell is in telophase: the chromatids have reached the poles, the spindle (a few remaining fibers) is breaking down, a new nuclear envelope is developing around each set of chromosomes, and the cell is beginning to pinch in two.

CED 4.5.B.1.vi · Read this in Fix

1. AOne cell with one nucleus, as the nucleus divides only as the cell splits
   A student who thinks mitosis and cytokinesis are one process picks this. The nucleus divides by mitosis, which the drug does not affect, so two nuclei form even though the cytoplasm does not divide.
2. BOne cell with two nuclei that each hold just half of the 8 chromosomes
   A student who thinks mitosis halves the chromosome number picks this. DNA replication and mitosis are unaffected, so each nucleus receives all 8 chromosomes.
3. COne cell with two nuclei, each nucleus holding 8 chromosomes Correct
   Mitosis (nuclear division) and cytokinesis (cytoplasmic division) are separate processes. With the spindle working, the chromatids are separated and two nuclei form, each with 8 chromosomes, but without cytokinesis they remain in one cell.
4. DOne cell with two nuclei, each holding a different mix of the parent's genes
   A student who thinks mitosis mixes genetic information picks this. The two nuclei receive identical sister chromatids, so they carry the same genetic information.

CED 4.5.B.1.vii · Read this in Fix

1. ACell A divides by a cleavage furrow, and cell B divides by forming a cell plate. Correct
   Cell A, with no wall, is pinching in at the middle: a cleavage furrow, as in animal cells. Cell B has a wall and is building a partition from the center outward between its nuclei: a cell plate, as in plant cells. Both are carrying out cytokinesis.
2. BCell B has not yet started cytokinesis, which will begin when a furrow forms in it.
   A student who thinks all cells divide their cytoplasm by a cleavage furrow picks this. Cell B is already dividing its cytoplasm by forming a cell plate, the method used by cells with walls.
3. CEach of the four nuclei contains half the chromosomes that its parent cell had.
   A student who thinks mitosis halves the chromosome number picks this. Each nucleus produced by mitosis has the same number of chromosomes as the parent cell had in G1.
4. DNeither cell now contains any chromosomes, as chromosomes exist only during mitosis.
   A student who thinks chromosomes disappear after mitosis picks this. The chromosomes are present in each nucleus as decondensed chromatin, which is why they are not visible.

CED 4.5.B.1.vii · Read this in Fix

1. ADo root-tip cells divide more at 25 °C because the root needs to grow faster?
   A student who explains cell division by the needs of the organism picks this. Whether a root 'needs' to grow faster cannot be measured by counting cells, so this question is not testable with this method.
2. BWill counting the cells prove that warmth makes root-tip cells divide more often?
   A student who thinks an experiment can prove a hypothesis picks this. Counting cells can support or fail to support an effect of temperature; it cannot prove one, so the question as worded cannot be answered by the method.
3. CIs 25 °C a better temperature than 15 °C for the cells of onion root tips?
   A student who thinks a question about which condition is 'better' is testable picks this. 'Better' does not name anything that can be measured, so the question cannot be answered from the cell counts.
4. DDoes the percentage of root-tip cells in mitosis differ between 15 °C and 25 °C? Correct
   This names the variable to be changed (temperature) and a variable that can be measured from the cell counts (the percentage of cells in mitosis), so it can be tested with the proposed method.

CED 4.5.A.1 · Read this in Fix

1. ASpindle fibers grow from both poles, so each chromosome attaches to fibers from each side. Correct
   The centrosomes organize the spindle. With one at each pole, spindle fibers from opposite poles attach to the two sister chromatids of each chromosome, so the chromatids can later be pulled in opposite directions.
2. BEach centrosome holds one chromatid of every chromosome and carries it along to one pole.
   A student who confuses centrosomes with centromeres picks this. Centrosomes are not part of the chromosomes; chromatids are moved by spindle fibers attached at their centromeres.
3. CEach pole collects half of the replicated chromosomes, which then form one of the new nuclei.
   A student who thinks whole chromosomes are shared between the daughter cells picks this. Each replicated chromosome is split, one sister chromatid going to each pole.
4. DThe cell membrane pinches in between the poles, which splits the nucleus into two parts.
   A student who thinks the nucleus divides as the cell is pinched in two picks this. The nucleus divides by mitosis, as chromatids are separated by the spindle; the cytoplasm divides afterward.

CED 4.5.B.1.iii · Read this in Fix

1. ARegrowing lost parts depends mainly on surviving cells enlarging to fill the gap.
   A student who thinks tissues grow by cells enlarging picks this. The cells that were not dividing survived the radiation but did not regrow the missing parts, so enlargement of existing cells was not enough.
2. BRegrowing lost parts depends on cells dividing by mitosis to produce new cells. Correct
   Killing only the dividing cells prevented regrowth, while the cells that survived could not regrow the missing parts on their own. Regrowth therefore depends on new cells produced by mitosis, which is how tissues are repaired.
3. CRegrowing lost parts depends on cells gaining the new genes for missing parts.
   A student who thinks each cell type carries only its own genes picks this. Cells produced by mitosis carry the complete genome, so no new genes are needed to make the missing tissues.
4. DRegrowing lost parts depends on reproductive cells, the only cells with all genes.
   A student who thinks only reproductive cells carry a complete set of genes picks this. Almost all body cells carry the complete genome, passed on to new cells by mitosis.

CED 4.5.B.1.i · Read this in Fix

1. AThe cell does not copy its DNA until the chromosomes condense in the next prophase.
   A student who thinks DNA is copied during mitosis, when the chromosomes become visible, picks this. The DNA is replicated in S phase of the interphase between divisions, so each chromosome already consists of two sister chromatids when it condenses in prophase.
2. BThe chromosomes break down and are rebuilt from new DNA before the next prophase.
   A student who thinks chromosomes exist only during mitosis picks this. The chromosomes persist as chromatin through interphase; each is replicated in S phase, not rebuilt.
3. CThe cell passes through interphase, in which it grows and replicates its DNA. Correct
   Mitosis alternates with interphase in the cell cycle: after one division the cell passes through G1, S and G2, growing and replicating its DNA, before the next prophase.
4. DThe cell rests in interphase, doing little until the next prophase begins.
   A student who thinks interphase is a resting stage picks this. In interphase the cell duplicates its organelles, replicates its DNA and prepares for mitosis.

CED 4.5.B.1.ii · Read this in Fix