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 replicated 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 prophase

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

Question 3

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

Answer 3

• spindle fibres attach to chromosomes by their centromeres
• chromosomes align along the equator

Question 4

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

Answer 4

• spindle fibres shorten / contract
• centromere divides
• pulling chromatids from each pair to opposite poles of cell

Question 5

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

Answer 5

• chromosomes uncoil, becoming longer / thinner
• nuclear envelopes reform = 2 nuclei
• spindle fibres / centrioles break down

Question 6

Why do some eukaryotic cells not undergo the cell cycle?

Answer 6

• within multicellular organisms, not all cells retain the ability to divide
• only cells that do retain this ability go through a cell cycle

Question 7

Explain the importance of mitosis in the life of an organism

Answer 7

• growth of multicellular organisms by increasing cell number
• replacing cells to repair damaged tissues
• asexual reproduction

Question 8

Describe how tumours and cancers form

Answer 8

• 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 9

Describe how prokaryotic cells replicate

Answer 9

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 10

Describe how viruses replicate

Answer 10

1. Attachment proteins attach to complementary receptors on host cell
2. Inject viral nucleic acid into host cell
3. Infected host cell replicated virus particles:
   a. Nucleic acid replicated
   b. Cell produces viral protein / capsid / enzymes
   c. Virus assembles then released