all cells arise from other cells

Question 1

Describe the stages of the cell cycle in eukaryotic cells

Answer 1

1. Interphase
   ● (S phase) DNA replicates semi-conservatively
   ○ Leading to 2 chromatids (identical copies) joined at a centromere
   ● (G1/G2) number of organelles & volume of cytoplasm increases, protein synthesis
2. Mitosis
   ● Nucleus divides
   ● To produce 2 nuclei with identical copies of DNA produced by parent cell
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

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

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.
● 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