2.2-All cells arise from other cells

Question 1

State what the cycle is and outline its stages.

Answer 1

Cycle of division with the intermediate growth periods.
1. Interphase
2. Mitosis or meiosis (nuclear division)
3. Cytokineses (cytoplasmic division)

Question 2

Explain what the cell cyle 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 the only 10% of the cycle and refers only to nuclear division.

Question 4

Outline what happens during interphase.

Answer 4

G1: cell synthesis proteins for replication e.g. tubulin for spundle fibres and 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:
1. Growth
2. Cell replacement/ tissue repair
3. Asexual reproduction

Question 6

Name the stages of mitosis.

Answer 6

1. Propase
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 and nucleolus break down = chromosomes free in cytoplasm.

Question 8

Outline what happens during metaphase.

Answer 8

Sister chromatids line up at the 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. Prepaer a temporary mount of root tissue.
2. Focus an optical microscope on the slide. Count total 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 to press down coverslip and obtain a single layer of cells. Avoid trapping air bubbles.

Question 13

Name 2 dyes that bind ti chromosomes.

Answer 13

• Toluidine blue (blue)
• Acetic orcein (purple-red)

Question 14

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

Answer 14

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

Question 15

What are tomour supressor genes?

Answer 15

Genes that code for proteins to trigger apoptosis (programmed death of damageed cells).

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-oncongenes cause cancer?

Answer 17

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

Question 18

Syggest how cancer treatments control the rate of cell division.

Answer 18

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

8x60 = 480 minutes
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

They replicate inside living cells so it is difficult to kill them without killing host cells.