2.2 All cells arise from other cells

Question 1

State what the cell cycle is and outline its

stages.

Answer 1

cycle of division with intermediate growth
periods
1. interphase
2. mitosis or meiosis (nuclear division)
3. cytokinesis (cytoplasmic division)

Question 2

Explain why the cell cycle does not occur

in some cells.

Answer 2

After differentiation, some types of cell in
multicellular organisms (e.g. neurons) no
longer have the ability to divide.

Question 3

What is the difference between the cell

cycle and mitosis?

Answer 3

Cell cycle includes growth period
between divisions; mitosis is only 10% of
the cycle & refers only to nuclear
division.

Question 4

Outline what happens during interphase.

Answer 4

G1: cell synthesises proteins for replication e.g.
tubulin for spindle fibres & cell size doubles
S: DNA replicates = chromosomes consist of 2
sister chromatids joined at a centromere
G2: organelles divide

Question 5

State the purpose of mitosis.

Answer 5

produces 2 genetically identical daughter cells
for:
● Growth
● Cell replacement/ tissue repair
● Asexual reproduction

Question 6

Name the stages of mitosis.

Answer 6

1. Prophase
2. Metaphase
3. Anaphase
4. Telophase

Question 7

Outline what happens during prophase.

Answer 7

1. Chromosomes condense, becoming visible.
   (X-shaped: 2 sister chromatids joined at
   centromere)
2. Centrioles move to opposite poles of cell (animal
   cells) & mitotic spindle fibres form.
3. Nuclear envelope & nucleolus break down =
   chromosomes free in cytoplasm.

Question 8

Outline what happens during metaphase.

Answer 8

Sister chromatids line up at cell equator,
attached to the mitotic spindle by their
centromeres.

Question 9

Outline what happens during anaphase.

Answer 9

requires energy from ATP hydrolysis
1. Spindle fibres contract = centromeres divide.
2. Sister chromatids separate into 2 distinct
chromosomes & are pulled to opposite poles of
cell (looks like ‘V’ shapes facing each other).
3. Spindle fibres break down.

Question 10

Outline what happens during telophase.

Answer 10

1. Chromosomes decondense, becoming
invisible again.
2. New nuclear envelopes form around each
set of chromosomes = 2 new nuclei, each
with 1 copy of each chromosome.

Question 11

Explain the procedure for a root tip

squash experiment.

Answer 11

1. Prepare a temporary mount of root tissue.
2. Focus an optical microscope on the slide. Count
   total number of cells in the field of view and number
   of cells in a stage of mitosis.
3. Calculate mitotic index (proportion of cells
   undergoing mitosis).

Question 12

Outline how to prepare a temporary

mount of root tissue.

Answer 12

1. Place root in hydrochloric acid to halt cell division
   & hydrolyse middle lamella.
2. Stain root tip with a dye that binds to chromosomes.
3. Macerate tissue in water using mounted needle.
4. Use mounted needle at 45° to press down coverslip
   & obtain a single layer of cells. Avoid trapping air
   bubbles.

Question 13

Name 2 dyes that bind to chromosomes.

Answer 13

● toluidine blue (blue)

● acetic orcein (purple-red)

Question 14

Why is only the root tip used when

calculating a mitotic index?

Answer 14

● Meristematic cells at root tip are
actively undergoing mitosis.
● Cells further from root tip are
elongating rather than dividing.

Question 15

What are tumour suppressor genes &

proto-oncogenes?

Answer 15

Genes that code for proteins to trigger
apoptosis (programmed death of
damaged cells)/ slow cell cycle (e.g. p53
acts between G1 & S in interphase so
damaged DNA cannot replicate).

Question 16

What are proto-oncogenes?

Answer 16

Genes that code for proteins to stimulate
cell cycle to progress from one stage to
the next.

Question 17

How can mutation to tumour suppressor

genes & proto-oncogenes cause cancer?

Answer 17

● Tumour suppressor: no production of a protein
needed to slow the cell cycle.
● Proto-oncogenes: form permanently-activated
oncogenes.
● Disruption to cell cycle → uncontrolled cell division →
tumour

Question 18

Suggest how cancer treatments control

the rate of cell division.

Answer 18

Disrupt the cell cycle:
● prevent DNA replication
● disrupt spindle formation = inhibit metaphase
/ anaphase
NB: can also damage healthy cells

Question 19

How do prokaryotic cells replicate?

Answer 19

Binary fission:
1. DNA loop replicates. Both copies stay attached to cell
membrane. Plasmids replicate in cytoplasm.
2. Cell elongates, separating the 2 DNA loops.
3. Cell membrane contracts & septum forms.
4. Cell splits into 2 identical progeny cells, each with 1 copy
of the DNA loop but a variable number of plasmids.

Question 20

Estimate the exponential growth of
bacteria within 8 hours. Assume binary
fission occurs once every 20 minutes &
there is 1 bacterium at the start.

Answer 20

8 x 60 = 480 mins
480 / 20 = 24 divisions
2^24

Question 21

Why are viruses classified as non-living?

Answer 21

They are acellular: no cytoplasm, no

metabolism & cannot self-replicate.

Question 22

Outline how viruses replicate.

Answer 22

1. Attachment proteins attach to receptors on host cell
   membrane.
2. Enveloped viruses fuse with cell membrane or move in via
   endocytosis & release DNA/ RNA into cytoplasm OR
   viruses inject DNA/ RNA.
3. Host cell uses viral genetic information to synthesise new
   viral proteins/ nucleic acid.
4. Components of new viral particle assemble.

Question 23

How do new viral particles leave the host

cell?

Answer 23

a) Bud off & use cell membrane to form
envelope.
b) Cause lysis of host cell.

Question 24

Why is it so difficult to develop effective

treatments against viruses?

Answer 24

Replicate inside living cells = difficult to

kill them without killing host cells.