3.2.2 All cells arise from other cells

Question 1

Viruses - Describe viral replication.(4)

Answer 1

1. Attachment proteins attach to receptors;
   For ‘attachment protein’ accept gp41/gp120/ glycoprotein but ignore ‘receptor protein’.
2. Virus injects nucleic acid (into host cell);
   For this mp accept ‘genetic material’ for ‘nucleic acid’?
3. Host cell replicates viral nucleic acid;
   Accept ‘RNA/DNA’ for ‘nucleic acid’.
4. Host cell produces (viral) protein/capsid/enzymes;
5. Virus (particles) assembled and released (from cell);

Question 2

Describe binary fission in
bacteria. (3)

Answer 2

1. Replication of (circular) DNA;
   Accept nucleoid
   Reject chromosome
   Reject mitosis
2. Replication of plasmids;
3. Division of cytoplasm (to produce daughter cells);

Question 3

Describe how bacteria
divide.(2)

Answer 3

1. Binary fission;
2. Replication of (circular) DNA;
3. Division of cytoplasm to produce 2 daughter cells;
4. Each with single copy of (circular) DNA;

Question 4

What is a tumour?
(2)

Answer 4

1. Mass of cells;
   Accept abnormal growth for ‘mass’
2. Many cells in mitosis/dividing cells
   OR
   Uncontrolled cell division;

Question 5

Describe and
explain the arrangement of the genetic
material in prophase (2)

Answer 5

1. Chromosomes (are) becoming visible/distinct;
2. Because (still) condensing;
   OR
   Accept ‘chromosomes are condensed’ for 2 marks.
   Accept shorten or thicken for ‘condensed’
3. Chromosomes (arranged) at random/not lined up;
4. Because no spindle (activity);
   OR
   Because not attached to spindle fibres;

Question 6

Describe and explain Chromosome Behaviour in all Stages (8)

Answer 6

(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 7

Describe the role of
the spindle fibres and the behaviour of
the chromosomes during mitosis (5)

Answer 7

1. (In) prophase, chromosomes condense;
   Accept chromatin for ‘chromosomes’ and for ‘condense’, shorten and thicken
2. (In) prophase OR metaphase, centromeres attach to spindle fibres;
3. (In) metaphase, chromosomes/pairs of chromatids at equator/centre of spindle/cell;
4. (In) anaphase, centromeres divide;
5. (In) anaphase, chromatids (from each pair) pulled to (opposite) poles/ends (of cell);
   Accept for ‘chromatids’, chromosomes but reject homologous chromosomes
6. (In) prophase/metaphase/anaphase, spindle fibres shorten;

Question 8

state name given to
the division of cytoplasm during the cell
cycle. (1)

Answer 8

cytokinesis

Question 9

Give two pieces of evidence when a cell is undergoing mitosis (2)

Answer 9

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 10

Describe Evidence presence when a cell in anaphase (2)

Answer 10

1. Chromosomes / chromatids are (in two groups) at poles of spindle / at ends of spindle;
   Do not accept ‘ends of cell’
2. V-shape shows that (sister) chromatids have been pulled apart at their centromeres / that centromeres of
   (sister) chromatids have been pulled apart.

Question 11

During the cell cycle,
the amount of DNA in a cell changes.
Explain how the behaviour of
chromosomes causes these changes in
the amount of DNA per cell (2)

Answer 11

(Increase)
1. Chromosomes / DNA replicates;
(First decrease)
2. Homologous chromosomes separate;
(Second decrease)
3. Sister chromatids separate.

Question 12

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

Answer 12

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);
   Accept description of ‘cannot separate’ e.g cannot move to poles
   Ignore ‘split’
4. (So, no) anaphase;

Question 13

Describe the
appearance of chromosomes in anaphase
(1)

Answer 13

Chromatids are being pulled to opposite poles/ends (of the cell) by spindles/spindle fibres;

Question 14

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

Answer 14

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 15

Req Prac 2 Suggest why the student soaked the root tips in hydrochloric acid

Answer 15

1. To break down links between/separate cell walls;
2. Allowing the stain to pass/diffuse into the cells
   OR
   Allowing the cells to be (more easily) squashed;

Question 16

Req Prac 2 Pressing the coverslip downwards enabled the student to observe the stages of mitosis clearly.
Explain why.

Answer 16

1. To break down links between/separate cell walls;
2. Allowing the stain to pass/diffuse into the cells
   OR
   Allowing the cells to be (more easily) squashed;

Question 17

Req Prac 2 describe why a scientist uses only the first 5 mm from the tip of an onion root.(1)

Answer 17

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;

Question 18

Req Prac 2 -
Describe how you would determine
a reliable mitotic index (MI) from tissue
observed with an optical microscope.

Answer 18

1. Count cells in mitosis in field of view;
2. Divide this by total number of cells in field of view;
3. Repeat many/> 10 times
   OR
   Select (fields of view) at random;

Question 19

Req Prac 2 -
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 19

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 20

Req Prac 2 -
suggest why different student may get a
different mitotic index using the same
methos (assume no errors) (2)

Answer 20

1. (Garlic) grown for different lengths of time
   OR
   (Garlic) grown in different conditions;
   Accept suitable descriptions of conditions, eg in different temperatures
2. The root tips from different (garlic) plants/roots/bulbs/species;
3. Single field of view is not representative of a root tip
   OR
   Different fields of view are different samples;

Question 21

The scientists
measured the percentage change in
tumour volume.
Suggest why they recorded both
percentage change and tumour
volume.(2)

Answer 21

Percentage change
1. To allow comparison as tumours may differ in volume/size (at the start of the investigation);
Tumour volume
2. (As) tumours may differ in length/width/shape
OR
(As) volume is (best) indication of the number of cells in tumour;