5.1 Meiosis

1. AIt forms the diploid body cells used for growth, while mitosis forms the haploid gametes of the animal.
   A student who has the names mitosis and meiosis the wrong way round picks this. Mitosis forms body cells with the parent's chromosome number; meiosis forms the haploid gametes.
2. BIt forms haploid gametes, so fertilization restores the diploid chromosome number in the zygote. Correct
   Meiosis halves the chromosome number, giving gametes with one set of chromosomes. When two gametes fuse at fertilization, the zygote has two sets again, so the chromosome number of the species stays the same from generation to generation.
3. CIt gives each gamete half of every chromosome, and fertilization joins the two halves back together.
   A student who thinks gametes receive half of each chromosome picks this. Each gamete receives one complete chromosome from every homologous pair, so it has half the number of chromosomes, each one whole.
4. DIt forms gametes genetically identical to the parent cell, so offspring keep all of its genes.
   A student who thinks meiosis copies the parent cell as mitosis does picks this. Meiosis produces haploid cells that differ from the diploid parent cell and, in an individual heterozygous for many genes, from one another.

CED 5.1.A.1 · Read this in Fix

1. ADiploid, as each chromosome still consists of two joined sister chromatids
   A student who judges ploidy by the number of chromatids picks this. Each cell has only one chromosome from each homologous pair, so it is haploid even though each chromosome has two chromatids.
2. BDiploid, with each chromosome now a single, separated chromatid
   A student who thinks sister chromatids separate at anaphase I, as in mitosis, picks this. Homologs separate in anaphase I; sister chromatids stay joined.
3. CHaploid, with every chromosome still consisting of two sister chromatids Correct
   Meiosis I separates homologous chromosomes, so each cell receives one chromosome of every pair and is haploid. Sister chromatids remain joined until anaphase II, so each chromosome still has two chromatids.
4. DHaploid, with each chromosome now one chromatid, ready to act as a gamete
   A student who thinks meiosis is complete after the first division picks this. The cells must still go through meiosis II, which separates the sister chromatids.

CED 5.1.A.2.iv · Read this in Fix

1. AThe kinetochores of the two sister chromatids of each chromosome face opposite poles. Correct
   At metaphase II, each chromosome (two sister chromatids) lines up at the metaphase plate, and the kinetochore of each chromatid is attached to a microtubule from the pole it faces, so the two chromatids are attached to opposite poles and can be pulled apart in anaphase II.
2. BThe kinetochores of the two homologous chromosomes of each pair face opposite poles.
   A student who thinks homologs are separated in meiosis II picks this. A cell in meiosis II has only one chromosome from each pair; homologs were separated in meiosis I.
3. CThe kinetochore of the maternal chromatid faces one pole; the paternal one faces the other.
   A student who thinks the two chromatids of a chromosome come one from each parent picks this. Sister chromatids are copies of one chromosome from the same parent.
4. DThe kinetochore of each single-chromatid chromosome attaches to one pole's microtubules.
   A student who thinks sister chromatids already separated in anaphase I picks this. Chromosomes at metaphase II still have two chromatids joined at the centromere.

CED 5.1.A.3.ii · Read this in Fix

1. ATwo cells, genetically different from each other
   A student who thinks meiosis ends after the first division picks this. Meiosis II divides each of the two cells, giving four.
2. BFour cells, all genetically identical to one another
   A student who thinks meiosis copies cells as mitosis does picks this. Separation of homologs in meiosis I and crossing over in prophase I give the four cells different combinations of alleles.
3. CTwo cells, genetically identical to each other
   A student who has the names mitosis and meiosis the wrong way round picks this, which describes mitosis.
4. DFour cells, each genetically different from the others Correct
   Meiosis has two divisions, so one diploid cell gives four haploid cells. Meiosis I sends one homolog of each pair to each cell, so the two cells it forms carry different alleles of the heterozygous genes. Crossing over at chiasmata in prophase I exchanges segments between non-sister chromatids, so the sister chromatids separated in meiosis II usually carry different combinations of alleles too. In an animal heterozygous for many genes, the four cells therefore differ genetically.

CED 5.1.B.1 · Read this in Fix

1. AP is meiosis, and X is 12 Correct
   Process P turns adult cells into sperm and eggs, which is meiosis. Meiosis halves the chromosome number, so each gamete has 24/2 = 12 chromosomes, and fertilization restores 24 in the zygote, which then divides by mitosis.
2. BP is mitosis, and X is 24
   A student who thinks gametes are made by mitosis like other body cells picks this. Gametes with 24 chromosomes would give zygotes with 48, doubling the number every generation.
3. CP is mitosis, and X is 12
   A student who has the names of the two divisions swapped picks this. The halving division that makes gametes is meiosis; mitosis keeps the chromosome number the same.
4. DP is meiosis, and X is 24
   A student who thinks each gamete gets half of every chromosome picks this, keeping all 24 chromosomes in the gamete. Meiosis gives each gamete one whole chromosome from each pair, so 12 chromosomes.

Working Adult body cells have 2n = 24. Process P produces gametes from the adult, so it is meiosis, and X = n = 24/2 = 12 chromosomes. Fertilization gives a zygote with 12 + 12 = 24.

CED 5.1.A.1 · Read this in Fix

1. AAt P, sister chromatids go to different cells; at R, the homologous chromosomes go to different cells.
   A student who has the two divisions the wrong way round picks this. Homologs are separated in meiosis I (P) and sister chromatids in meiosis II (R).
2. BAt P and again at R, sister chromatids go to different cells, as they do in mitosis.
   A student who thinks meiosis is mitosis done twice picks this. If sister chromatids separated at P, nothing would be left to separate at R without a second S phase, which the graph does not show.
3. CAt P, homologous chromosomes go to different cells; at R, sister chromatids go to different cells. Correct
   DNA doubles in S phase (6–10 h). At P, the end of meiosis I, homologous chromosomes have been separated into two cells, halving the DNA per cell. At R, the end of meiosis II, sister chromatids have been separated into two cells, halving it again to half the G1 value.
4. DAt P and again at R, the homologous chromosomes of each pair go to different cells.
   A student who thinks the two chromatids of a replicated chromosome are the maternal and paternal copies sees the separation at R as homologs separating. Sister chromatids are copies from one parent, separated at R.

CED 5.1.A.2 · Read this in Fix

1. AEvery gamete would be genetically identical, because without chiasmata no source of variation would remain.
   A student who thinks crossing over is the only source of genetic variation picks this. Homologs would still be distributed to different cells in different combinations, so gametes would not be identical.
2. BMitosis in the animal's body cells would also be disrupted, because homologs pair up in every prophase of division.
   A student who thinks homologs pair in mitosis too picks this. Synapsis happens only in prophase I of meiosis, so mitosis does not depend on it.
3. CMeiosis I would be unaffected, because in anaphase I it is sister chromatids, not homologs, that separate from each other.
   A student who thinks anaphase I separates sister chromatids, as in mitosis, picks this. Anaphase I separates the paired homologs, so a failure of pairing disrupts meiosis I.
4. DHomologs would not line up as pairs at metaphase I, so many gametes would get the wrong chromosome number. Correct
   Synapsis brings homologs together so that each pair can be aligned at the metaphase plate and separated, one homolog to each pole. Without pairing, unpaired homologs move to the poles independently, so a cell often receives both or neither homolog of a pair, giving gametes with too many or too few chromosomes.

CED 5.1.A.2.i · Read this in Fix

1. ADoes growth temperature affect the percentage of metaphase I cells with unpaired homologs? Correct
   This question names an independent variable that can be set (growth temperature) and a dependent variable that can be measured (percentage of metaphase I cells with unpaired homologs), so it can be answered by growing genetically similar plants at different temperatures and counting cells.
2. BWhy do plants need their homologous chromosomes to pair up before the first division of meiosis?
   A student who thinks a 'why do they need' question is testable picks this. It asks about purpose and names no variable to change or result to measure.
3. CIs a cool greenhouse better than a hot greenhouse for growing plants of this species?
   A student who thinks a question about which condition is better is testable as stated picks this. 'Better' is not defined by any measurable outcome.
4. DDo homologous chromosomes prefer to pair when the plants are kept in a cool greenhouse?
   A student who treats what chromosomes 'prefer' as testable picks this. Chromosomes have no preferences; the testable question asks how temperature affects a measurable outcome.

CED 5.1.A.2.i · Read this in Fix

1. AForming more chiasmata caused the homologous pairs to grow longer than the others.
   A student who reads a correlation as a cause, in either direction, picks this. These are observations, not an experiment, and a pair's length is fixed before chiasmata form.
2. BLonger homologous pairs formed more chiasmata on average than shorter pairs. Correct
   The mean number of chiasmata rises from about 1.0 for the 3 μm and 4 μm pairs to 2.6 for the 14 μm pair, so longer pairs had more chiasmata on average.
3. CDoubling the length of a homologous pair doubled its mean number of chiasmata.
   A student who reads every increasing trend as proportional picks this. Doubling length from 6 μm to 12 μm raised the mean only from 1.3 to 2.3.
4. DEvery pair 10 μm or longer formed two or more chiasmata in every single cell.
   A student who reads a mean as the value for every cell picks this. A mean of 2.0 for the 10 μm pair could include cells with one chiasma and cells with three.

Working Read the points: 3 μm → 1.0, 6 μm → 1.3, 12 μm → 2.3, 14 μm → 2.6. The trend is increasing. Doubling 6 → 12 μm gives 1.3 → 2.3, a ratio of 2.3/1.3 ≈ 1.8, not 2.

CED 5.1.A.2.i · Read this in Fix

1. AChromosomes line up singly along the plate, so splitting each chromosome sends one part of it to each of the two poles.
   A student who thinks metaphase I looks like metaphase of mitosis picks this. Splitting each chromosome would keep the full chromosome number; homologous pairs are aligned at metaphase I.
2. BMaternal homologs line up on one side of the plate and paternal ones on the other, so each pole gets one set.
   A student who thinks all maternal chromosomes face one pole picks this. Each pair orients independently, so each pole gets a mixture of maternal and paternal homologs.
3. CHomologs line up as pairs, one on each side of the plate, so separating each pair sends one homolog to each pole. Correct
   At metaphase I, spindle fibers align homologous pairs along the metaphase plate with the two homologs of each pair facing opposite poles. When the pairs separate in anaphase I, each pole receives exactly one homolog of every pair, whichever way each pair happened to face.
4. DUnreplicated homologs line up as pairs, so each pole gets one chromosome, which is then copied before meiosis II begins.
   A student who thinks DNA is replicated between meiosis I and meiosis II picks this. Chromosomes are replicated in the S phase before meiosis I, so each homolog at metaphase I has two chromatids.

CED 5.1.A.2.ii · Read this in Fix

1. ABoth concentrations reduced alignment, because both of the group means are lower than the mean for the solvent group.
   A student who treats any difference in means as a real effect picks this. The low-concentration mean is lower, but its error bar overlaps the solvent-only bar, so chance could explain the difference.
2. BThe low concentration had no effect on alignment, because its error bar overlaps the solvent group's error bar.
   A student who thinks overlapping error bars prove no effect picks this. Overlap means the data do not show a clear difference; a small effect could still exist.
3. CIn every bud given the high concentration, fewer cells were aligned than in any of the buds given solvent alone.
   A student who reads error bars as the range of all the measurements picks this. ±2 SE bars show the uncertainty of each mean, not where every individual bud's value lies.
4. DThe high concentration reduced alignment, but the data show no clear effect of the low concentration. Correct
   The high-concentration bar (23–39%) does not overlap the solvent-only bar (76–88%), so that difference is likely significant. The low-concentration bar (67–81%) overlaps the solvent-only bar, so the data do not show a clear difference; they neither establish an effect nor rule one out.

Working Read the error bars: solvent only 82 ± 6 → 76 to 88; low 74 ± 7 → 67 to 81; high 31 ± 8 → 23 to 39. Low overlaps solvent only (67–81 overlaps 76–88): no clear difference. High does not overlap solvent only (39 < 76): likely significant reduction.

CED 5.1.A.2.ii · Read this in Fix

1. AFour single chromatids, one each carrying A, a, B and b
   A student who thinks sister chromatids separate at anaphase I picks this, sending one chromatid of every chromosome to each pole. Sister chromatids stay joined until anaphase II.
2. BTwo chromosomes, A and b, each made of two sister chromatids Correct
   In anaphase I, homologous chromosomes separate while sister chromatids stay joined. The A chromosome and the b chromosome face Pole 1, so each moves there whole, with both of its chromatids carrying the same allele.
3. CTwo single-chromatid chromosomes, one carrying A and one b
   A student who thinks DNA is copied only between meiosis I and II picks this. The chromosomes were replicated before meiosis I, so each still has two chromatids in anaphase I.
4. DTwo chromosomes, A and a, each of them made of two sister chromatids
   A student who thinks meiosis separates genes rather than alleles picks this, sending both alleles of gene A to one pole. The A and a homologs face opposite poles and separate from each other.

CED 5.1.A.2.iii · Read this in Fix

1. AAt 8 h, most cells had completed meiosis I, but fewer than half had completed meiosis II. Correct
   A cell with two nuclei has completed meiosis I, and one with four nuclei has completed meiosis II. At 8 h, 50% of cells had two nuclei and 40% had four, so 90% had completed meiosis I, but only 40% had completed meiosis II.
2. BAt 7 h, most cells had completed meiosis, as most had more than one nucleus.
   A student who thinks meiosis ends after the first division picks this. At 7 h, 60% of cells had two nuclei, which means they had finished only meiosis I; only 15% had four nuclei.
3. CEach cell had two nuclei for about 8 hours, from the curve's first rise to its fall.
   A student who reads a population curve as the history of every cell picks this. Cells started at slightly different times, so the curve spans longer than any one cell may spend with two nuclei.
4. DEach nucleus's chromosome number halved from 7 h to 11 h, as four-nucleus cells arose.
   A student who thinks the chromosome number is halved in meiosis II picks this. Nuclei became haploid when meiosis I produced two nuclei; meiosis II separated sister chromatids.

Working Completed meiosis I = cells with 2 or 4 nuclei. At 8 h: 50% + 40% = 90%. Completed meiosis II = cells with 4 nuclei = 40% at 8 h, which is fewer than half.

CED 5.1.A.3 · Read this in Fix

1. AThe cells would become haploid early, because separating sister chromatids is what halves the chromosome number.
   A student who thinks the chromosome number is halved when sister chromatids separate picks this. The cells were already haploid at the end of meiosis I.
2. BMeiosis II would be unaffected, because it separates homologous chromosomes, not sister chromatids.
   A student who has the two divisions the wrong way round picks this. Meiosis II separates sister chromatids, which depends on their being joined until anaphase II.
3. CGametes would be unaffected, because the maternal and paternal chromatids go to different cells anyway.
   A student who thinks the two chromatids of a chromosome are its maternal and paternal copies picks this. Sister chromatids are copies from one parent; their correct separation depends on staying joined until the spindle pulls them apart.
4. DSingle chromatids would attach to either pole at random, so some gametes would get both copies of a chromosome and some none. Correct
   In prophase II, chromosomes made of two sister chromatids joined at the centromere attach to the spindle so that the two chromatids are pulled to opposite poles. If the chromatids are already separate, each attaches to whichever pole it meets, so both copies of a chromosome can go to the same cell, giving gametes with extra or missing chromosomes.

CED 5.1.A.3.i · Read this in Fix

1. AChromosomes: 20; chromatids: 20
   A student who counts each chromatid as a chromosome picks this. The 20 chromatids form 10 chromosomes, each with one centromere.
2. BChromosomes: 10; chromatids: 20 Correct
   Meiosis I separated homologs, so a cell at metaphase II has n = 10 chromosomes. Sister chromatids have not yet separated, so each chromosome has two chromatids: 10 × 2 = 20 chromatids.
3. CChromosomes: 20; chromatids: 40
   A student who thinks the chromosome number is halved only in meiosis II picks this, treating the cell as still diploid. Meiosis I has already halved the number to 10.
4. DChromosomes: 10; chromatids: 10
   A student who thinks sister chromatids separate in anaphase I picks this, so the chromosomes are single at metaphase II. Each chromosome still has two chromatids until anaphase II.

Working 2n = 20, so n = 10. After meiosis I, a cell has 10 chromosomes, each still made of 2 sister chromatids: 10 × 2 = 20 chromatids at metaphase II.

CED 5.1.A.3.ii · Read this in Fix

1. ACell 1
   A student who thinks the chromosome number is halved only in meiosis II picks this diagram of a diploid cell separating sister chromatids, which shows anaphase of mitosis. A cell in meiosis II has only one chromosome of each pair.
2. BCell 2
   A student who thinks homologous chromosomes separate in meiosis II picks this diagram of replicated homologs separating, which shows anaphase I.
3. CCell 3 Correct
   A cell in meiosis II is haploid, with one chromosome from each homologous pair (one long, one short). At anaphase II, the sister chromatids of each chromosome are pulled to opposite poles; they are copies of the same chromosome, so both chromatids of the long chromosome are drawn alike, as are both of the short one.
4. DCell 4
   A student who thinks the two chromatids of a chromosome are its maternal and paternal copies picks this diagram, in which each chromosome splits into a solid and a dashed chromatid. Sister chromatids are copies from one parent.

CED 5.1.A.3.iii · Read this in Fix

1. A20%
   A student who takes the difference between two percentages as the percent change picks this. 80% − 60% is a fall of 20 percentage points, which is 25% of the original 80%.
2. B75%
   A student who divides the new value by the original value picks this. 60/80 × 100 = 75% is the proportion remaining, not the reduction.
3. C33%
   A student who divides the change by the treated value instead of the reference value picks this: 20/60 × 100 = 33%. Percent change uses the original, solvent-only value.
4. D25% Correct
   Percent change is measured from the solvent-only (reference) value: (80 − 60)/80 × 100 = 25% decrease.

Working Solvent only = 80%, chemical = 60%. Change = 80 − 60 = 20 percentage points. Percent reduction = 20/80 × 100 = 25%.

CED 5.1.A.3.iii · Read this in Fix

1. AJust two cells should be drawn, since meiosis forms two gametes.
   A student who thinks meiosis ends with the two cells of meiosis I picks this. Meiosis II divides each of those cells, giving four cells.
2. BEach chromosome should be drawn as a single chromatid, not two. Correct
   Sister chromatids separate at anaphase II, so at the end of telophase II each of the four haploid cells has one chromosome of each pair, each an unduplicated chromatid. The number of cells, the number of chromosomes and the mixture of solid and dashed chromosomes in the drawing are already possible.
3. CEach cell should contain four chromosomes, two long and two short.
   A student who thinks gametes keep the full chromosome number, as cells made by mitosis do, picks this. Each cell is haploid, with one chromosome from each pair.
4. DEach cell should hold only solid chromosomes or only dashed ones.
   A student who thinks maternal chromosomes always go to the same pole picks this. Each pair orients independently at metaphase I, so a cell can receive the long maternal and the short paternal chromosome.

CED 5.1.A.3.iv · Read this in Fix

1. AAnaphase II split each chromosome's sister chromatids apart, which halved the number of chromosomes.
   A student who thinks the chromosome number is halved in meiosis II picks this. The cells entering meiosis II are already haploid; separating chromatids does not change the number of sets.
2. BEach chromosome is now a single chromatid, and having single-chromatid chromosomes is what makes a cell haploid.
   A student who judges ploidy by chromatids picks this. Ploidy depends on how many chromosomes of each pair a cell has, not on whether each has one chromatid or two.
3. CHomologous chromosomes went to different cells in meiosis I; anaphase II then separated only sister chromatids. Correct
   A cell is haploid when it has one chromosome of each homologous pair. Separating homologs in meiosis I made the cells haploid; anaphase II split sister chromatids into separate cells without changing the number of chromosome sets.
4. DEach cell received half of every chromosome in the original cell, so it holds half of each one.
   A student who thinks gametes receive half of each chromosome picks this. Each cell receives whole chromosomes, one from every homologous pair.

CED 5.1.A.3.iv · Read this in Fix

1. AChromosomes: 20; chromatids: 20 Correct
   Meiosis I halves the chromosome number to n = 20, and anaphase II separates sister chromatids, so each of the four cells has 20 chromosomes, each a single, unduplicated chromatid.
2. BChromosomes: 20; chromatids: 40
   A student who thinks chromosomes always have two chromatids picks this. After anaphase II, each chromosome is a single chromatid.
3. CChromosomes: 40; chromatids: 40
   A student who thinks each gamete keeps all 40 chromosomes, but half of each, picks this. Each cell receives one whole chromosome from each pair: 20.
4. DChromosomes: 10; chromatids: 10
   A student who thinks each division shares out the chromosomes, halving them, picks this (40 → 20 → 10). Only meiosis I halves the number; meiosis II separates sister chromatids, leaving 20.

Working 2n = 40 → n = 20 after meiosis I (each chromosome 2 chromatids, 40 chromatids). Meiosis II separates sister chromatids into different cells: each cell has 20 chromosomes, each 1 chromatid, so 20 chromatids.

CED 5.1.A.3.iv · Read this in Fix

1. ARoot tip: 20; anther: 40
   A student who has the names mitosis and meiosis the wrong way round picks this. The halving division is meiosis.
2. BRoot tip: 40; anther: 40
   A student who thinks meiosis gives each cell half of every chromosome, keeping all 40, picks this. Meiosis gives each cell one whole chromosome from each pair: 20 chromosomes.
3. CRoot tip: 40; anther: 20 Correct
   Mitosis gives daughter cells with the same chromosome number as the parent cell (40). Meiosis halves the chromosome number in meiosis I and keeps it in meiosis II, so each cell at the end of meiosis II has 20.
4. DRoot tip: 20; anther: 10
   A student who thinks every division shares out the chromosomes, halving them, picks this: 40 → 20 in mitosis and 40 → 20 → 10 in meiosis. DNA replication before mitosis means each daughter cell receives all 40; only meiosis I halves the number.

Working 2n = 40. Mitosis: daughter cells have 40. Meiosis: meiosis I halves to n = 20; meiosis II separates sister chromatids, leaving 20 chromosomes per cell.

CED 5.1.B.1 · Read this in Fix

1. ARoot-tip and anther cells given neither ethanol nor any chemical
   A student who thinks a control is simply an untreated group picks this. It differs from the treated cells in both ethanol and chemical, so it cannot separate their effects.
2. BRoot-tip and anther cells given the same volume of ethanol with no chemical Correct
   This group differs from the treated group only in the presence of the chemical, so any difference in spindle formation between the two can be attributed to the chemical rather than to the ethanol.
3. CA second set of root-tip and anther cells also given the chemical in ethanol
   A student who confuses repeating the treatment with having a control picks this. Every group still receives the chemical, so nothing shows what the ethanol alone does.
4. DRoot-tip and anther cells given half as much of the chemical in the same ethanol
   A student who thinks a lower dose is a control picks this. This group still receives the chemical, so it is another treatment level, not a control.

CED 5.1.B.1 · Read this in Fix

1. APartly correct: both separate sister chromatids, but meiosis II follows meiosis I with no S phase in between. Correct
   Like mitosis, meiosis II uses a spindle to separate sister chromatids. Unlike mitosis, which follows an S phase, meiosis II follows meiosis I without DNA replication, so it separates the chromatids of chromosomes replicated before meiosis I.
2. BFully correct: both separate sister chromatids, and both follow an S phase in which DNA is copied.
   A student who thinks DNA is replicated between meiosis I and meiosis II picks this. There is no S phase between the two meiotic divisions.
3. CIncorrect: meiosis II separates homologous chromosomes, whereas mitosis separates sister chromatids.
   A student who has the two meiotic divisions the wrong way round picks this. Homologs separate in meiosis I; meiosis II separates sister chromatids.
4. DPartly correct: both separate sister chromatids, but meiosis II halves the chromosome number.
   A student who thinks the chromosome number is halved in meiosis II picks this. Meiosis I halves it; meiosis II separates chromatids of cells that are already haploid.

CED 5.1.B.1 · Read this in Fix