D2.1 Cell and nuclear division. Practice questions with markscheme.

• A: in every age group the abnormal embryos are split about evenly between an extra and a missing chromosome, yet among live births an extra chromosome is far more common, so most embryos missing a chromosome must die before birth; A is contradicted by the near 1 : 1 split, B by the rise from 26 % to only 31 % and 41 %, and D by the 26 % to 41 % in women under 38;

• B: meiosis halves the chromosome number, so each of the four cells produced from a diploid cell with 4 chromosomes has 2, one of each pair;

• A: crossing over recombines alleles and random orientation shuffles whole chromosomes; separating identical sister chromatids adds no variation (barring earlier crossover);

• A: supercoiling packages the very long DNA molecules for safe movement; DNA replication and transcription in fact require decondensed DNA;

• C: 120 min ÷ 20 min = 6 divisions; each division turns every mother cell into two daughter cells, so the number doubles each time: 2⁶ = 64;

• B: mitosis gives the bud a complete set of chromosomes identical to the mother cell's; only the division of the cytoplasm (cytokinesis) is unequal, so the bud starts smaller;

• D: nuclear division (mitosis/meiosis) distributes the replicated chromosomes; cytokinesis then splits the cytoplasm;

• B: anaphase: centromeres split and spindle microtubules pull the (now) chromosomes apart; metaphase aligns them at the equator;

• C: replication before meiosis I doubles the DNA from 6.0 to 12.0 pg (each chromosome becomes two chromatids), and meiosis I shares it between two nuclei, 6.0 pg each; the chromatids are not separated until anaphase II, so a cell at metaphase II holds 6.0 pg, as much as a G1 diploid nucleus; the student halved the G1 value instead of the replicated value, and 3.0 pg is the DNA in each nucleus only after meiosis II; C reaches 6.0 pg by a second error, as meiosis I does halve the DNA of a nucleus (12.0 to 6.0 pg), and B misses that same halving;

• D: before replication each of the 8 chromosomes is one DNA molecule; replication does not change the number of chromosomes, but each then consists of two chromatids (two DNA molecules) held together until anaphase, so 8 chromosomes contain 16 DNA molecules;

• the (rigid) cell wall prevents the membrane from being pinched inward;
• instead, vesicles (from the Golgi) gather at the equator and fuse, forming a cell plate;
• the plate develops into new membranes and a new (cellulose) wall separating the daughter cells, OWTTE;

• (a) [2]
  • cell 1, metaphase;
  • cell 2, telophase;
• (b) [2]
  • the centromeres divide and sister chromatids separate (anaphase);
  • spindle microtubules pull the (daughter) chromosomes to opposite poles;
• (c) [1]
  • two (daughter) nuclei/cells genetically identical to the parent (same diploid chromosome number);

• four chromosomes (two long and two short), each drawn as two sister chromatids joined at a centromere;
• chromosomes drawn condensed (short and thick) and scattered, not paired as homologues and not lined up at the equator;
• nuclear membrane drawn breaking down / fragmented (or annotated as breaking down);
• spindle microtubules drawn forming from the centrosomes/poles on opposite sides of the nucleus;
• annotation that the chromosomes are visible because they have condensed (by supercoiling), unlike the dispersed chromatin of interphase, OWTTE;

Reject chromosomes drawn as single chromatids, paired as bivalents, or already aligned at the equator.

• C: before replication each of the 8 chromosomes is one DNA molecule; replication does not change the number of chromosomes, but each then consists of two chromatids (two DNA molecules) held together until anaphase, so 8 chromosomes contain 16 DNA molecules;

• C: the child has three copies of chromosome 21 (trisomy 21), so one gamete carried two copies because the pair (or the chromatids) failed to separate in meiosis;

• (a) [2]
  • the number of cells falls as the concentration increases, from 1.96 × 10⁵ to 1.10 × 10⁵;
  • the number of nuclei stays the same / is 2.00 × 10⁵ at every concentration / doubles in every culture;
  • the percentage of cells with two nuclei rises, from 2 % to 82 %;
• (b) [2]
  • the number of nuclei doubled (from 1.00 to 2.00 × 10⁵) at every concentration, so nuclear division / mitosis was completed normally;
  • the number of cells rose less and less as the concentration increased, so fewer parent cells split into two daughter cells;
  • (so) the two nuclei produced by mitosis stayed in one cytoplasm, giving cells with two nuclei, OWTTE;

  One point about nuclei and one about cells are needed for [2].

• (c) [2]
  • in animal cells the cytoplasm is split by a ring of contractile actin and myosin proteins (at the equator, inside the membrane);
  • myosin pulls on the actin filaments so that the ring contracts / tightens;
  • the contracting ring pinches the cell membrane inwards (cleavage furrow) until the cytoplasm is split in two;
  • chromosomes are moved in mitosis by spindle microtubules (and microtubule motors), not by myosin, so mitosis is unaffected, OWTTE;

• (a) [2]
  • the chosen field is not a random / representative sample of the cells in the root tip;
  • so the mitotic index is overestimated / does not measure the proportion of dividing cells in the whole tip;
  • the choice depends on the student's judgement, which may differ between the two slides / may favour the result the student expects, OWTTE;
  • dividing cells are not spread evenly through a squash, so the index depends on which part of the squash is viewed;
• (b) [2]
  • only one root was sampled at each time / there are no repeats, and roots / seedlings differ, so the difference may be between the two roots rather than between the two times;
  • only 50 cells were classified in each root, so a difference of five dividing cells (9 against 4) could be due to chance / one cell changes the index by 0.02;
  • no statistical test was (or could be) done, so it is not known whether the difference is significant;
  • only two times on one day were sampled, so no pattern through the day can be shown;
  • other variables also changed with the time of day, such as light (on at 09:00, off at 21:00) or the temperature of the bench, OWTTE;

  Mark the first two answers only. Do not credit the choice of field of view, which is assessed in (a).

• (c) [3]
  • sample several roots (at least five), each from a different seedling, at each time;
  • classify more cells in each root tip (e.g. at least 200, from several fields of view);
  • choose the fields of view at random or by a fixed rule (such as a set distance from the tip), not by the number of dividing cells;
  • sample at more times through 24 hours (e.g. every 3 hours) and repeat on more than one day;
  • keep other variables constant: seedlings of the same age in a cabinet at constant temperature, the same length of tip cut, the same stain and staining time;
  • code the slides so that the person counting does not know the time each tip was cut;
  • calculate the mean (± standard deviation) mitotic index for each time and use a statistical test (e.g. a t-test) to judge whether differences are significant, OWTTE;

• (a) [2]
  • 60 hours: 3 (cells B1, B2 and C1);
  • 72 hours: 6;
• (b) [2]
  • times of 22, 25, 22, 24 and 23 h (for B, C, B1, B2 and C1);
  • mean = 116 ÷ 5 = 23.2 h;

  Award [2] for 23.2 h with no working. Cell A (formation not shown) and C2 (died) are not included; award [1] for a mean that includes either of them, correctly calculated.

• (c) [2]
  • daughter cells of the same parent divided at different times / the time between divisions varied (from 22 to 25 h);
  • so the cells fell out of step / were not dividing in synchrony;
  • one cell (C2) died, so its line of descendants was lost;
  • the first cell was already part of the way through its cycle when filming began, so it divided at 20 h, OWTTE;
• (d) [1]
  • every cell arose by division of a pre-existing (parent / mother) cell, each division giving two daughter cells;
  • no cell appeared except by the division of an existing cell, OWTTE;