2.2 Mitosis and the cell cycle

Question 1

State what the cell cycle is

Answer 1

A cycle of division with intermediate growth periods

Question 2

Outline the stages of the cell cycle

Answer 2

1. Interphase
2. Mitosis or meiosis
3. Cytokinesis

Question 3

Explain why the cell cycle does not occur in some cells

Answer 3

After differentiation, some types of cell in multicellular organisms no longer have the ability to divide

Question 4

What is the difference between the cell cycle and mitosis?

Answer 4

Cell cycle includes growth period between divisions ; mitosis is only 10% of the cycle and refers only to nuclear division

Question 5

Outline what happens during interphase

Answer 5

G1: cell synthesises proteins for replication
S: DNA replicates
G2: organelles divide

Question 6

State the purpose of mitosis

Answer 6

produces 2 genetically identical daughter cells for:
- Growth
- Cell replacement / tissue repair
- Asexual reproduction

Question 7

Name the stages of mitosis

Answer 7

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

Question 8

Outline what happens during prophase

Answer 8

1. Chromosomes condense, becoming visible
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 9

Outline what happens during metaphase

Answer 9

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

Question 10

Outline what happens during anaphase

Answer 10

requires energy from ATP hydrolysis
1. Spindle fibres contract = centromeres divide
2. Sister chromatids separate into 2 distinct chromosomes and are pulled to opposite poles of cell
3. Spindle fibres break down

Question 11

Outline what happens during telophase

Answer 11

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

Question 12

Explain the procedure for a root tip squash experiment

Answer 12

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

Question 13

Outline how to prepare a temporary root tip mount

Answer 13

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 to press down coverslip and obtain a single layer of cells. Avoid trapping air bubbles

Question 14

Name 2 dyes that bind to chromosomes

Answer 14

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

Question 15

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

Answer 15

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

Question 16

What are tumour suppressor genes?

Answer 16

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

Question 17

What are proto-oncogenes?

Answer 17

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

Question 18

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

Answer 18

• 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 19

Suggest how cancer treatments control the rate of cell division

Answer 19

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

Question 20

How do prokaryotic cells replicate?

Answer 20

Binary fission:
1. DNA loop replicates. Both copies attached to cell membrane. 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 loop but a variable number of plasmids

Question 21

Estimate 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 21

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

Question 22

Why are viruses classified as non-living?

Answer 22

They are acellular: no cytoplasm, no metabolism and cannot self-replicate

Question 23

Outline how viruses replicate

Answer 23

1. Attachment proteins attach to receptors on host cell membrane
2. Enveloped viruses fuse with cell membrane or move in via endocytosis and 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 24

How do new viral particles leave the host cell?

Answer 24

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

Question 25

Why is it so difficult to develop effective treatments against viruses?

Answer 25

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