2.2 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 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 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

1. Prophase
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 & 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 the mitotic spindle by their centromeres

Question 9

Outline what happens during anaphase

Answer 9

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

Question 12

Name 2 dyes that bind to chromosomes

Answer 12

● toluidine blue (blue)

● acetic orcein (purple-red)

Question 13

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

Answer 13

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

Question 14

What are tumour suppressor genes & proto-oncogenes?

Answer 14

Genes that code for proteins to trigger apoptosis (programmed death of damaged cells)/ slow cell cycle (e.g. p53 acts between G1 & S in interphase so damaged DNA cannot replicate).

Question 15

What are proto-oncogenes?

Answer 15

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

Question 16

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

Answer 16

● 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 17

Suggest how cancer treatments control the rate of cell division

Answer 17

Disrupt the cell cycle:
● prevent DNA replication
● disrupt spindle formation = inhibit metaphase
/ anaphase
NB: can also damage healthy cells

Question 18

How do prokaryotic cells replicate?

Answer 18

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 19

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 19

8 x 60 = 480 mins

480 / 20 = 24 divisions 224

Question 20

Why are viruses classified as non-living?

Answer 20

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

Question 21

Outline how viruses replicate

Answer 21

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 22

How do new viral particles leave the host cell?

Answer 22

a) Bud off & use cell membrane to form envelope.

b) Cause lysis of host cell.

Question 23

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

Answer 23

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