Cell cycle and mitosis

• Describe binary fission in bacteria.

  1. Replication of circular DNA;

  2. Replication of plasmids;

  3. Division of cytoplasm to produce daughter cells;
• The cell growth rate of the bacterium Bacillus subtilis is proportional to its mass immediately after binary fission. Suggest and explain how two environmental variables could be changed to increase the growth rate of these cells.

  1. Increased concentration of glucose; Increased respiration;

  3. Increased concentration of oxygen; Increased respiration;

  5. Increased temperature; Increased enzyme activity;

  7. Increased concentration of phosphate; Increased

  ATP/DNA/RNA;

  9. Increased concentration of nucleotides;

  10. Increased DNA synthesis;
• This question is about mitosis in cells. The image below shows the arrangement of the genetic material in a cell during prophase. Describe and explain the arrangement of the genetic material shown in the above image.

  1. Chromosomes are becoming distinct;

  2. Because still condensing;

  OR

  3. Chromosomes arranged at random;

  4. Because no spindle activity;
• Name the fixed position occupied by a gene on a DNA molecule

  Locus/loci;
• Give reasons why trout eggs produced by meiosis are genetically different.

  1. Independent segregation

  2. Crossing over;
• A student prepared a stained squash of cells from the tip of an onion root and observed it using an optical microscope. During the preparation of the slide, he:

  • cut the first 5 mm from the tip of an onion root and placed it on a glass slide • covered this tip with a drop of stain solution and a cover slip

  • warmed the glass slide

  • pressed down firmly on the cover slip.

  He identified and counted nuclei in different stages of the cell cycle.

  Explain why the student:

  1. used only the first 5 mm from the tip of an onion root.

  1. Where mitosis occurs;

  2. No dividing cells in tissue further away

  3. To get soft tissue that will squash;

  4. Length that will fit under cover slip;
• A student prepared a stained squash of cells from the tip of an onion root and observed it using an optical microscope. During the preparation of the slide, he:

  • cut the first 5 mm from the tip of an onion root and placed it on a glass slide • covered this tip with a drop of stain solution and a cover slip

  • warmed the glass slide

  • pressed down firmly on the cover slip.

  He identified and counted nuclei in different stages of the cell cycle.

  Explain why the student:

  2. pressed down firmly on the cover slip.

  2. Thin layer of cells so light passes through making cells visible;
• What is the name given to the division of cytoplasm during the cell cycle?

  Cytokinesis;
• Describe and explain what the student should have done when counting cells to make sure that the mitotic index he obtained for this root tip was accurate.

  1. Examine large number of fields of view to ensure representative sample;

  2. Repeat count to ensure figures are correct;

  3. Count only whole cells to standardise counting;
• A scientist treated growing tips of onion roots with a chemical that stops roots growing. After 24 hours, he prepared a stained squash of these root tips. Figure 2 is a drawing showing the chromosomes in a single cell observed in the squash of one of these root tips in anaphase. This cell was typical of other cells in anaphase in these root tips. Use all of this information to suggest how the chemical stops the growth of roots.

  1. Stops anaphase

  2. By stopping spindle fibres forming;

  3. Preventing separation of sister chromatids;

  4. So no new cells added to root tip;
• When preparing the cells for observation the scientist placed them in a solution that had a slightly higher (less negative) water potential than the cytoplasm. This did not cause the cells to burst but moved the chromosomes further apart in order to reduce the overlapping of the chromosomes when observed with an optical microscope. Suggest how this procedure moved the chromosomes apart.

  1. Water moves into the cell's cytoplasm by osmosis;

  2. Cell's cytoplasm gets bigger;
• The dark stain used on the chromosomes binds more to some areas of the chromosomes than others, giving the chromosomes a striped appearance. Suggest ways the structure of the chromosome could differ along its length to result in the stain binding more in some areas.

  1. Differences in base sequences

  2. Differences in interaction with histones

  3. Differences in condensation;
• What is a homologous pair of chromosomes?

  Two chromosomes that carry the same genes
• Name the process by which bacterial cells divide.

  Binary fission
• A microbiologist investigated the ability of different plant oils to kill the bacterium Listeria monocytogenes. She cultured the bacteria on agar plates. She obtained the bacteria from a broth culture.

  Describe aseptic techniques she would have used when transferring a sample of broth culture on to an agar plate.

  Explain why each was important.

  1. Keep lid on Petri dish to prevent unwanted bacteria contaminating the dish.

  2. Wear gloves OR Wear mask OR Wash hands to prevent contamination from bacteria on hands / mouth

  3. Use sterile pipette OR Flame the loop of the container of the culture to maintain a pure culture of bacteria
• 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.

  1. Push hard - spread tissue;

  2. Not push sideways - avoid rolling cells together and breaking chromosomes.
• Describe the appearance and behaviour of chromosomes during mitosis.

  During prophase 

  1. Chromosomes condense, shorten, thicken andbecome visible; 

  2. Chromosomes appear as two sister chromatids joined at the centromere; 

  During metaphase

  3. Chromosomes line up on the equator of the cell;

  4. Chromosomes attached to spindle fibres; 

  5. By their centromere; 

  During anaphase 

  6. The centromere divides; 

  7. Sister chromatids are pulled to opposite poles 

  During telophase 

  8. Chromosomes unwind, become longer and thinner.