All Cells Arise From Other Cells

Question 1

State what the cell cycle is and outline its stages.

Answer 1

Cycle of cell division and intermediate growth periods.
1. Interphase
2. Mitosis or meiosis
3. Cytokinesis

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

Outline what happens during interphase.

Answer 3

G1: cell synthesise proteins for replication
S: DNA replicates = chromosomes consist of 2 sister chromatids joined at a centromere
G2: organelles divide

Question 4

Name the stages of mitosis

Answer 4

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

Question 5

Outline what happens during prophase.

Answer 5

1. Chromosomes condense becoming visible
2. Centrioles move to opposite poles of cell & mitotic spindle fibres form
3. Nuclear envelope and nucleolus break down = chromosomes free in cytoplasm

Question 6

Outline what happens during metaphase.

Answer 6

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

Question 7

Outline what happens during anaphase.

Answer 7

1. Spindle fibres contract = centromeres divide
2. Sister chromatids separate into 2 distinct chromosomes & are pulled to opposite poles of cell
3. Spindle fibres break down

Question 8

Outline what happens during telophase.

Answer 8

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

Question 9

Explain the procedure for a root tip squash experiment.

Answer 9

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

Question 10

Outline how to prepare a temporary mount of root tissue.

Answer 10

1. Place root in HCl to halt cell division and 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.

Question 11

Name two dyes that bind to chromosomes.

Answer 11

• toluidine blue
• acetic orcein

Question 12

What are tumour suppressor genes & proto-oncogenes?

Answer 12

Genes that code for proteins to trigger apoptosis/ slow cell cycle

Question 13

What are proto-oncogenes?

Answer 13

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

Question 14

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

Answer 14

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 15

Suggest how cancer treatments control the rate of cell division.

Answer 15

Disrupt the cell cycle:
- prevent DNA replication
- disrupt spindle formation = inhibit metaphase / anaphase

Question 16

How do prokaryotic cells replicate?

Answer 16

Binary fission:
1. DNA loop replicates. Plasmids replicate in cytoplasm.
2. Cell elongates, separating the 2 DNA loops.
3. Cell membrane contracts and septum forms.
4. Cell splits into 2 identical progeny cells, each with 1 copy of the DNA loops but varied plasmids

Question 17

Why are viruses classified as non-living?

Answer 17

They are acellular: no cytoplasm, no metabolism & cannot 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
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.