Cell Division

• Describe the appearance and behaviour of chromosomes during mitosis.

  (during prophase)

  • chromosomes condense;
  • chromosomes appear as two sister chromatids joined at the centromere;

  (during metaphase)

  • chromosomes line up on the equator;
  • chromosomes are attached to spindle fibres;
  • by their centromere;

  (during anaphase)

  • the centromere splits;
  • sister chromatids are pulled to opposite poles;

  (during telophase)

  • chromosomes uncoil and become longer/thinner;
• A student investigated mitosis in the tissue from an onion root tip. The student prepared a temporary mount of the onion tissue on a glass slide. She covered the tissue with a cover slip. She was then given the following instruction.

  “Push down hard on the cover slip, but do not push the cover slip sideways.”

  Explain why she was given this instruction.

  • push hard to spread/squash tissue;
  • don't push sideways to avoid rolling cells together or breaking chromosomes;
• The student concluded that the cell in the image below was in the anaphase stage of mitosis. Was she correct? Give two reasons for your answer.

  (yes, because)

  • chromatids are at the poles of the spindle;
  • the V-shape shows that the chromatids have been pulled apart at their centromeres;

  
• The figure below shows some cells from an onion root tip at different stages of the cell cycle.

  Place stages A to E in the correct order. Start with stage D.

  D C B E A

  
• The figure below shows how the amount of DNA per cell changed during interphase and meiosis in an animal.

  Explain how the behaviour of chromosomes causes these changes in the amount of DNA per cell between F and G.

  • (increase)
  • chromosomes/DNA replicates (first decrease);
  • homologous chromosomes separate (second decrease);
  • sister chromatids separate;

  
• What would happen to the amount of DNA per cell at fertilisation of cell G?

  • DNA would double;

  
• Describe binary fission in bacteria.

  • replication of circular DNA;
  • replication of plasmids;
  • division of cytoplasm to produce daughter cells;
• Suggest and explain which environmental variables could be changed to increase the growth rate of bacteria cells.

  • increased concentration of glucose = increased respiration;
  • increased concentration of oxygen = increased respiration;
  • increased temperature = increased enzyme activity;
  • increased concentration of phosphate = increased ATP/DNA/RNA;
  • increased concentration of nucleotides = increased DNA synthesis;
• Describe and explain the arrangement of the genetic material shown in the image below.

  (prophase, so)

  • chromosomes are becoming visible;
  • because they are still condensing;

  
• Name the fixed position occupied by a gene on a DNA molecule.

  Locus
• Describe the role of the spindle fibres and the behaviour of the chromosomes during prophase, metaphase and anaphase.

  (in prophase)

  • chromosomes condense;

  (in metaphase)

  • chromosomes line up at the equator

  (in anaphase)

  • centromeres divide;
  • chromatids are pulled to opposite poles

  (in prophase/metaphase/anaphase)

  • spindle fibres shorten;
• Give one reason why trout eggs produced by meiosis are genetically different.

  Independent assortment/crossing over