Cell cycle an dmitosis exam q

Question 1

1.c

Question 2

Suggest and explain how two environmental variables could be changed
to increase the growth rate of these cells. (4m)

Answer 2

1. Increased (concentration of) glucose;
2. Increased respiration;
3. Increased (concentration of) oxygen;
4. Increased respiration;
5. Increased temperature;
6. Increased enzyme activity;
7. Increased (concentration of) phosphate;
8. Increased ATP/DNA/RNA;
9. Increased (concentration of) nucleotides;
10. Increased DNA synthesis;

Question 3

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

Answer 3

Locus/loci;

Question 4

The offspring produced from farmed trout are sterile. Suggest and explain
why. (trouts have 80 chromosomes but they soemhow got the baby to have 120 chromosomes )

Answer 4

1. Too many/extra set/three copies of chromosomes;
2. (Homologous) chromosomes do not pair
3. (So) no meiosis;

Question 5

Suggest why preventing the formation of spindle fibres stopped the cell cycle.

Answer 5

1. Chromosomes/centromeres cannot attach (to spindle)
   OR
   Chromosomes cannot line up (on spindle);
2. (So, no) metaphase;
   OR
3. Chromatids cannot separate (on spindle);
4. (So, no) anaphase;

Question 6

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.
2. pressed down firmly on the cover slip.

Answer 6

1. Where dividing cells are found / mitosis occurs;
   OR
   No dividing cells / mitosis in tissue further away / more than 5 mm
   from tip;
   OR
   To get (soft) tissue that will squash;
   OR
   Length that will fit under cover slip;
2. Single / thin layer of cells / spread out cells so light passes through
   (making cells / nuclei visible);

Question 7

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.

Answer 7

1. Examine large number of fields of view / many cells;
   Mark as pairs only
   Accept large number / 20 or more for many
2. To ensure representative sample;
   Accept typical / reliable
   OR
3. Repeat count;
4. To ensure figures are correct;
   OR
5. Method to deal with part cells shown at edge /count only whole cells;
6. To standardise counting;

Question 8

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.

Answer 8

1. Stops anaphase / cell division / mitosis;
2. (By) stopping / disrupting / spindle fibres forming / attaching / pulling;
3. Preventing separation of (sister) chromatids;
4. (So) no new cells added (to root tip);

Question 9

The dark stain used on the chromosomes binds more to some areas of the
chromosomes than others, giving the chromosomes a striped appearance.
Suggest one way the structure of the chromosome could differ along its
length to result in the stain binding more in some areas.

Answer 9

Differences in base sequences
OR
Differences in histones/interaction with histones
OR
Differences in condensation/(super)coiling;

Question 10

What is a homologous pair

Answer 10

(Two chromosomes that) carry the same genes;

Question 11

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.
(b) Describe two aseptic techniques she would have used when transferring a
sample of broth culture on to an agar plate.
Explain why each was important.

Answer 11

(a) Binary fission;
Reject mitosis
1
(b) 1. Keep lid on Petri dish
OR
Open lid of Petri dish as little as possible.
2. To prevent unwanted bacteria contaminating the dish.
OR
L. monocytogenes may be dangerous / may get out.
OR
3. Wear gloves
OR
Wear mask
OR
Wash hands;
4. To prevent contamination from bacteria on hands / mouth
OR
Prevent spread of bacteria outside the lab;
OR
5. Use sterile pipette
OR
Flame the loop
OR
Flame the neck of the container of the culture;
6. To maintain a pure culture of bacteria

Question 12

Describe the appearance and behaviour of chromosomes during mitosis.

Answer 12

(a) (During prophase)
1. Chromosomes
coil / condense / shorten / thicken / become visible;
2. (Chromosomes) appear as (two sister) chromatids joined at the
centromere;
(During metaphase)
3. Chromosomes line up on the equator / centre of the cell;
4. (Chromosomes) attached to spindle fibres;
5. By their centromere;
(During anaphase)
6. The centromere splits / divides;
7. (Sister) chromatids / chromosomes are pulled to opposite poles /
ends of the cell / separate;
(During telophase)
8. Chromatids / chromosomes
uncoil / unwind / become longer / thinner

Question 13

A student investigated mitosis in the tissue from an onion root tip.
(a) 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

Answer 13

1. Push hard – spread / squash tissue;
2. Not push sideways – avoid rolling cells together / breaking
   chromosomes.