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 & 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

Explain the procedure for a root tip squash experiment.

Answer 7

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

Question 8

Explain how to prepare a temporary root tip mount.

Answer 8

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 9

Name 2 dyes that bind to chromosomes.

Answer 9

● toluidine blue (blue)

● acetic orcein (purple-red)

Question 10

Why is only the root tip used when calculating a mitotic index?

Answer 10

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

Question 11

What are tumour suppressor genes?

Answer 11

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

Question 12

What are proto-oncogenes?

Answer 12

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

Question 13

How can mutation to tumour suppressor genes & proto-oncogenes cause cancer?

Answer 13

● 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 14

Suggest how cancer treatments control the rate of cell division.

Answer 14

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

Question 15

How do prokaryotic cells replicate?

Answer 15

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 16

Estimate the exponential growth of bacteria within 8

hours. Assume binary fission occurs once every 20 mins & there is 1 bacterium at the start.

Answer 16

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

Question 17

Why are viruses classified as non-living?

Answer 17

They are acellular: no cytoplasm, no metabolism & can’t self-replicate.

Question 18

Outline how viruses replicate.

Answer 18

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 19

How do new viral particles leave the host cell?

Answer 19

a) Bud off & use cell membrane to form envelope.

b) Cause lysis of host cell.

Question 20

Why is it so difficult to develop effective treatments

against viruses?

Answer 20

Replicate inside living cells = difficult to kill them without killing host cells.

Question 21

What happens during prophase?

Answer 21

• DNA condenses and coils
• Nuclear envelope breaks down
• Centrioles move to opposite poles

Question 22

What happens during metaphase?

Answer 22

• Spindle fibres attach to centromeres

- Chromosomes line up at the equator

Question 23

What happens during anaphase?

Answer 23

• Centromeres divide

- Chromatids move to opposite poles of the cell

Question 24

What happens during telophase?

Answer 24

• Chromosomes uncoil

- Nuclear envelope reforms