Topic 2 : 2.2 All cells arise from other cells

Question 1

Summarise the stages of the cell cycle in eukaryotic cells

Answer 1

Stage 1
Interphase

● DNA replicates semi-conservatively (S phase)
○ Leading to 2 chromatids (identical copies) joined at a centromere
● Number of organelles & volume of cytoplasm increases, protein synthesis (G1 / G2)

Stage 2
Mitosis

● Nucleus divides
● To produce 2 nuclei with identical copies of DNA produced by parent cell

Stage 3
Cytokinesis

● Cytoplasm and cell membrane (normally) divide
● To form 2 new genetically identical daughter cells

Question 2

Describe the behaviour of chromosomes & role of spindle fibres in mitosis

Answer 2

Stage 1
Prophase

● Chromosomes condense, becoming shorter / thicker so visible
○ Appear as 2 sister chromatids joined by a centromere
● Nuclear envelope breaks down
● Centrioles move to opposite poles forming spindle network
● Spindle fibres start to attach to chromosomes by their centromeres

Stage 2
Metaphase

● Spindle fibres attach to chromosomes by their centromeres
● Chromosomes align along equator

Stage 3
Anaphase

● Spindle fibres shorten / contract
● Centromere divides
● Pulling chromatids (from each pair) to opposite poles of cell

Stage 4
Telophase

● Chromosomes uncoil, becoming longer / thinner
● Nuclear envelopes reform = 2 nuclei
● Spindle fibres / centrioles break down

Question 3

Why do some eukaryotic cells not undergo the cell cycle?

Answer 3

● Within multicellular organisms, not all cells retain the ability to divide (eg. neurons)
● Only cells that do retain this ability go through a cell cycle

Question 4

Explain the importance of mitosis in the life of an organism

Answer 4

Parent cell divides to produce 2 genetically identical daughter cells for:
● Growth of multicellular organisms by increasing cell number
● Replacing cells to repair damaged tissues
● Asexual reproduction

Question 5

Describe how tumours and cancers form

Answer 5

Mitosis is a controlled process so:
● Mutations in DNA / genes controlling mitosis can lead to uncontrolled cell division
● Tumour formed if this results in mass of abnormal cells
○ Malignant tumour = cancerous, can spread (metastasis)
○ Benign tumour = non-cancerous

Question 6

Suggest how cancer treatments control rate of cell division

Answer 6

● Some disrupt spindle fibre activity / formation
○ So chromosomes can’t attach to spindle by their centromere
○ So chromatids can’t be separated to opposite poles (no anaphase)
○ So prevents / slows mitosis
● Some prevent DNA replication during interphase
○ So can’t make 2 copies of each chromosome (chromatids)
○ So prevents / slows mitosis

Question 7

Describe how prokaryotic cells replicate

Answer 7

Binary fission:
1. Replication of circular DNA
2. Replication of plasmids
3. Division of cytoplasm to produce 2 daughter cells
○ Single copy of circular DNA
○ Variable number of copies of plasmids

Question 8

Describe how viruses replicate

Answer 8

Being non-living, viruses do not undergo cell division
1. Attachment proteins attach to complementary receptors on host cell
2. Inject viral nucleic acid (DNA/RNA) into host cell
3. Infected host cell replicates virus particles:
a. Nucleic acid replicated
b. Cell produces viral protein / capsid / enzymes
c. Virus assembled then released