All cells arise from other cells 3.2.2 ( 3.2 Cells)

Question 1

EQ : Describe how crossing over occurs during meiosis I (2)

Answer 1

• Chromosomes in each pair twist around each other
• Chromatids break and rejoin to chromatid on sister chromosome

Question 2

summarise the stage of the cell cycle in eukaryotic cells

Answer 2

stage 1 interphase :
• DNA replicates semi conservatively
- leading to 2 chromatids joined at centromere
• Number of organelles and volume of cytoplasm increases
Stage 2 mitosis :
• nucleus divided
• to produce 2 nuclei with identical copies of DNA produced by parent cell
Stage 3 cytokinesis:
• Cytoplasm and cell membrane divide
• To form 2 new genetic identical daughter cells

Question 3

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

Answer 3

stage 1 prophase :
• chromosomes condense , becoming shorter / thicker so visible - appear as 2 sister chromatids joined by 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 to opposite poles of a cell
Stage 4 Telophase :
• Chromosomes uncoil , becoming longer and thinner
• Nuclear envelopes reform = 2 nuclei
• Spindle fibres / centrioles break down

Question 4

Why do some eukaryotic cells not undergo the cell cycle

Answer 4

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

Question 5

Explain the importance of mitosis in the life of an organism

Answer 5

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

Question 6

Describe how tumours and cancers form

Answer 6

Mitosis is a controlled process so :
• Mutations in DNA controlling mitosis can lead to uncontrolled cell divisions
• Tumour formed if this results in mass of abnormal cells
- malignant tumour = cancerous , can spread
- benign tumour = non cancerous

Question 7

Suggest how cancer treatments control rate of cell divisions

Answer 7

• Some disrupt spindle fibre activity/ formation
- so chromosomes can’t attach to spindle by their centromere
- So chromatids can’t be repeated to opposite poles ( no anaphase )
- So prevents / slows mitosis

Question 8

Describe how prokaryotic cells replicate

Answer 8

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 9

Describe how viruses replicate

Answer 9

1) attachment proteins attach to complementary receptors on host cell
2) inject viral nucleic acid into host cell
3) Infected host cell replicates virus particles :
- nucleic acid replicated
- cell produces viral protein / enzymes
- virus assembled then released

Question 10

EQ : suggest and explain how two environment variables could be changed to increase the growth rate of these cells

Answer 10

1 ) increase glucose conc :
increases respiration
2) increase conc of oxygen :
increases respiration