3.2.2 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

what happens during prophase?

Answer 7

1. chromosomes condense, becoming visible
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

what happens in 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 & are pulled to opposite poles of the cell
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. 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 the field of view and number of cells in a stage of mitosis
3. calculate mitotic index (proportion of cells undergoing mitosis)

Question 12

outline how to prepare a temporary mount of root tissue

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 at 45 degrees to press down coverslip & obtain a single layer of cells. avoid trapping air bubbles

Question 13

name two 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 & proto-oncogenes?

Answer 15

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 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-oncogenes cause cancer?

Answer 17

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 18

suggest 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

why are viruses classed 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 synthesis 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

they replicate inside of living cells = difficult to kill them without killing host cells