2.2 All Cells Arise From Other Cells

Question 1

The cell cycle and its stages

Answer 1

• Cycle of division with intermediate growth periods
• Interphase, mitosis (nuclear division), 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

Difference between cell cycle and mitosis

Answer 3

• Cell cycle includes growth period between divisions
• Mitosis is only 10% of the cycle and refers to only nuclear divisions

Question 4

What happens during interphase?

Answer 4

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

Question 5

Purpose of mitosis

Answer 5

Produces 2 genetically identical daughter cells for:
- Growth
- Tissue repair
- Asexual reproduction

Question 6

Stages of mitosis

Answer 6

• Prophase
• Metaphase
• Anaphase
• Telophase

Question 7

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 and mitotic spindle fibres form
3) Nuclear envelope and nucleolus break down = chromosomes free in cytoplasm

Question 8

What happens during metaphase?

Answer 8

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

Question 9

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 and are pulled to opposite pole of cell
3) Spindle fibres break down

Question 10

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

Procedure for a root tip squash experiment

Answer 11

1) Prepare a temporary mount of root tissue
2) Focus on 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

Question 12

How to prepare a temporary root tip mount?

Answer 12

1) Place root in hydrochloric acid 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 cover slip and obtain single layer of cells. Avoid trapping air bubbles

Question 13

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

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 down
• 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
- Disruption spindle formation = inhibit metaphase/anaphase

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, separates the 2 DNA loops
3) Cell membrane contracts and septum forms
4) Cell splits into 2 identical progeny cells, each with 1 copy of DNA loop but a variable number of plasmids

Question 20

Estimate the exponential growth of bacterial 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 and 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, move in via endocytosis and release DNA/RNA into cytoplasm
3) Host cell uses viral genetic information to synthesis new viral proteins
4) Components of new viral particle assemble

Question 23

How do new viral particles leave the host cell?

Answer 23

• Bud off and use cell membrane to form envelope
• Cause lysis of host cell

Question 24

Why is it difficult to develop effective treatments against viruses?

Answer 24

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