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 cells 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

The cell cycle includes the 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

Prophase
Metaphase
Anaphase
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 a 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 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

Explain how to prepare a temporary root tip mount.

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 degrees 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?

Answer 15

Genes that code for proteins to trigger apoptosis (programmed death of damaged cells) / slow the cell cycle.

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 and proto-oncogenes cause cancer?

Answer 17

• Tumour suppressor: no production of a protein needed to slow the cell cycle.
• Proto-oncogenes: forms 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

Assume the exponential growth of bacteria within 8 hours. Assume binary fission occurs once every 20 minutes and 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 and 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.