All cells arise from other cells - 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

Name the stages of mitosis.

Answer 5

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

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

Outline what happens during prophase.

Answer 6

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 7

Outline what happens during metaphase.

Answer 7

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

Question 8

Outline what happens during anaphase.

Answer 8

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 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
   total number of cells in the field of view and number
   of cells in a stage of mitosis.
3. Calculate mitotic index (proportion of cells
   undergoing mitosis).

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

Outline how to prepare a temporary mount of root tissue.

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 11

Name 2 dyes that bind to chromosomes.

Answer 11

● toluidine blue (blue)
● acetic orcein (purple-red)

Question 12

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

Answer 12

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

Question 13

What are tumour suppressor genes & proto-oncogenes?

Answer 13

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 14

What are proto-oncogenes?

Answer 14

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

Question 15

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

Answer 15

● 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 16

Suggest how cancer treatments control the rate of cell division.

Answer 16

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

Question 17

How do prokaryotic cells replicate?

Answer 17

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 18

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 18

8 x 60 = 480 mins
480 / 20 = 24 divisions 224

Question 19

Why are viruses classified as non-living?

Answer 19

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

Question 20

Outline how viruses replicate.

Answer 20

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 21

How do new viral particles leave the host cell?

Answer 21

a) Bud off & use cell membrane to form envelope.
b) Cause lysis of host cell.

Question 22

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

Answer 22

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