Cell Cycle - Mark Scheme Answers (Study Mind)

Question 1

Describe binary fission in bacteria.

Answer 1

1. Replication of (circular) DNA
2. Replication of plasmids;
3. Division of cytoplasm (to produce daughter cells);

Question 2

Describe and explain the arrangement of the genetic material shown in the
above image.

Answer 2

1. Chromosomes (are) becoming visible/distinct;
2. Because (still) condensing;

Question 3

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

Answer 3

Loci

Question 4

Name the three phases of mitosis shown by C, D and E on the figure
above.
Describe the role of the spindle fibres and the behaviour of the
chromosomes during each of these phases.

Answer 4

1. C = prophase and
   D = metaphase and
   E = anaphase;
2. (In) prophase, chromosomes condense;
3. (In) prophase OR metaphase, centromeres attach to spindle fibres;
4. (In) metaphase, chromosomes/pairs of chromatids at equator/centre of spindle/cell;
5. (In) anaphase, centromeres divide;
6. (In) anaphase, chromatids (from each pair) pulled to (opposite) poles/ends (of cell);
7. (In) prophase/metaphase/anaphase, spindle fibres shorten;

Question 5

Suggest why preventing the formation of spindle fibres (lines 2–3) stopped
the cell cycle.

Answer 5

1. Chromosomes/centromeres cannot attach (to spindle)
2. (So, no) metaphase;

Question 6

Suggest and explain why ABZ could be used as a treatment for cancer
even though it affects some healthy cells (lines 3–4).

Answer 6

1. Cancer cells divide more/uncontrollably/rapidly

Question 7

Explain why the student:
1. used only the first 5 mm from the tip of an onion root.

Answer 7

Where dividing cells are found / mitosis occurs;

Question 8

2. pressed down firmly on the cover slip.

Answer 8

Single / thin layer of cells / spread out cells so light passes through(making cells / nuclei visible);

Question 9

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 9

1. Examine large number of fields of view / many cells;
2. To ensure representative sample;
3. Repeat count;
4. To ensure figures are correct;

Question 10

Use all of this information to suggest how the chemical stops the growth of
roots.

Answer 10

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 11

(a) Give two pieces of evidence from Figure 1 that this cell was undergoing mitosis. Explain your answers.

Answer 11

1. The (individual) chromosomes are visible because they have condensed
2. (Each) chromosome is made up of two chromatids because DNA has replicated
3. The chromosomes are not arranged in homologous pairs, which they would be if it was meiosis;

Question 12

(c) 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.

Answer 12

1. Water moves into the cells/cytoplasm by osmosis
2. Cell/cytoplasm gets bigger

Question 13

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 13

Differences in base sequences

Question 14

In Figure 2 the chromosomes are arranged in homologous pairs.
What is a homologous pair of chromosomes?

Answer 14

(Two chromosomes that) carry the same genes;

Question 15

Name the process by which bacterial cells divide.

Answer 15

Binary fission

Question 16

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 16

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

Question 17

Describe the appearance and behaviour of chromosomes during mitosis.

Answer 17

(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.