Chapter 7

Question 1

Binary fission

Answer 1

• Prokaryotic cells divide through binary fission.
• It involves coordinated growth, DNA replication, and cell division.
• This results in two daughter cells from one parent cell.
• DNA replication takes up most of the cell cycle in rapidly dividing prokaryotes.

Question 2

Bacterial cell cycle

Answer 2

• Replicates chromosome.
• Cell divides by the plasma membrane and cell wall growing inward, forming two daughter cells.

Question 2

Mitosis and Binary Fission

Answer 2

• Mitosis evovled from Binary Fission, Differences in modern eukaryotes’ mitosis show how this transition might have happened.
  
  • Chloroplasts and mitochondria divide independently of host cell, using binary fission

Question 3

Mitotic Cell Division

Answer 3

Eukaryotes:
- Multiple, large, linear chromosomes.
- DNA in the nucleus.
Prokaryotes:
- One small, circular chromosome.
- DNA in the cytoplasm.

Question 4

Mitosis and the Eukaryotic Cell Cycle

Answer 4

Cell cycle: A period of growth followed by nuclear
division and cytokinesis

Mitosis is the basis for:
- Growth and maintenance of body mass in multicellular eukaryotes
- The reproduction of many single-celled eukaryotes

• Mitosis divides replicated DNA equally and precisely
• Ensures the two cells resulting from a cell division have the same genetic information as the parent cell entering division

Question 5

Chromosomes and Ploidy

Answer 5

• Eukaryotic DNA is divided into individual, linear chromosomes located in the nucleus.
  
  • Chromosomes carry genetic information and are divided during mitosis.
  • Ploidy refers to the number of chromosome sets in a cell or species.
    
    • Haploid (n): One complete set of chromosomes.
    • Diploid (2n): Two complete sets of chromosomes.
  • Humans are diploid: n = 23, 2n = 46.

Question 6

Sister Chromatids

Answer 6

DNA replication creates two identical molecules called sister chromatids for each chromosome. (Chromosome segretion: distributes it equally)

Question 7

The Eukaryotic Cell Cycle: Interphase

Answer 7

• After cell division, interphase begins and lasts until the next mitosis.
  Interphase has sub-phases:
• G1 phase: Growth of the cell.
• DNA replication happens to prepare for mitosis

Question 8

3 phases of interphase

Answer 8

1. G1 phase: The cell performs its functions and may grow.
2. S phase: DNA replication and chromosome duplication occur.
3. G2 phase: The cell continues to grow and prepares for mitosis and cytokinesis.

Question 9

Mitotic cell cycle (5 stages)

Answer 9

After interphase, mitosis consists of five stages
1. Prophase
2. Prometaphase
3. Metaphase
4. Anaphase
5. Telophase

Question 10

Stage 1: Prophase

Answer 10

• Chromosomes become condensed into compact, rod-like shapes.
  
  • The spindle begins to form in the cytoplasm.

Question 11

Stage 2: Prometaphase

Answer 11

• The nuclear envelope breaks down.
• The spindle extends into the area where the nucleus was.
• Microtubules from opposite spindle poles attach to the kinetochores of each chromosome.

Question 12

Stage 3: Metaphase

Answer 12

• The spindle is fully formed.
  
  • Chromosomes align at the spindle’s midpoint.
  • Spindle microtubules position the chromosomes.

Question 13

Stage 4: Anaphase

Answer 13

• The spindle pulls sister chromatids apart, moving them to opposite poles.
  
  • Chromosome segregation is completed.

Question 14

Stage 5: Telophase

Answer 14

• Chromosomes relax back to their interphase state.
  
  • New nuclear envelopes form around each set of chromosomes.

Question 15

Cytokinesis

Answer 15

• Completes cell division by splitting the cytoplasm between daughter cells.
  
  • In animals:
    
    • A band of microfilaments forms inside the plasma membrane, creating a belt.
    • Microfilaments tighten, forming a furrow in the membrane.
    • The furrow deepens, dividing the cytoplasm into two separate parts.

Question 16

Plant Cytokinesis

Answer 16

• Cell wall material is deposited at the former spindle midpoint.
  
  • The deposition forms a continuous cell plate, dividing the daughter cells.

Question 17

Spindle Formation

Answer 17

• In animal cells:
  
  • Centrosomes, with a pair of centrioles, divide and move apart.
  • Microtubules radiate from centrosomes to form the spindle.
  • In plant cells:
    
    • No centrosomes are present.
    • Spindles form through the self-assembly of microtubules.

Question 18

Cell cycle control

Answer 18

• Proteins and enzymes regulate cell progression (G1 → S, G2 → M, and through Mitosis).
• Internal controls create checkpoints to:
  –>Ensure each stage is complete before moving to the next.
  –>Guarantee accurate cell division.

Question 19

Cyclins - CDKs(Cyclin dependant kinases)

Answer 19

• Protein kinases that phosphorylate target proteins to regulate the cell cycle.
• Active only when bound to the correct cyclin.

Question 20

Internal Controls

Answer 20

• Cyclin-CDK complexes control cell cycle progression.
• Checkpoints ensure stages are complete or conditions are correct before continuing.
• Additional controls regulate cyclin-CDK activity.

Question 21

External Controls

Answer 21

Surface receptors detect external signals:
- Hormones, growth factors, or neighboring cells.

Adjust cell division to match organism needs:
- Speed up, slow down, or stop division.
- Signals activate pathways that regulate cyclin-CDK activity

Question 22

Cyclins

Answer 22

• Proteins that activate CDKs by binding to them.
• Levels fluctuate during the cell cycle.

Question 23

Maturation-Promoting Factor (MPF)

Answer 23

A cyclin-CDK complex that:
- Activates enzymes.
- Triggers nuclear envelope breakdown.
- Promotes chromosome condensation.
- Cyclins are degraded after their role is complete.

Question 24

Contact Inhibition

Answer 24

- Prevents cell division when cells are too crowded. - Communicated via cell surface receptors detecting cell density.

Question 25

Gap Filling

Answer 25

- If cells are removed, remaining cells divide to fill the space. - Division stops once the gap is filled.

Question 26

Cancer Cells

Answer 26

- Loss of contact inhibition. - May produce excess growth factors or have cell cycle regulation failures.

Question 27

Asymmetric Cell Division

Answer 27

-Not all divisions produce identical daughter cells. - Common in stem cells found in plant meristems and animal tissues. Outcome: - One daughter cell remains a stem cell. - The other becomes a differentiating progenitor cell.

Question 28

Cells Cannot Divide Indefinitely

Answer 28

- Most multicellular cells have limited division capacity. - **Cellular Senescence**: Loss of ability to proliferate over time. - **Causes of Senescence**: - **DNA Damage**. - **Telomere Shortening**.

Question 29

Cancer

Answer 29

Loss of Cell Division Control: - Cells divide uncontrollably, forming a tumour. - Tumour interferes with normal body functions. Cancer Cell Mutations: - Multiple mutations in cancer cells. - Often affect the cyclin-cdk regulation system, surface receptors, or signal transduction pathways. Oncogenes: - Mutated genes that drive cancer development.

Question 30

Apoptosis (Programmed Cell Death)

Answer 30

- An ancient mechanism, present in all multicellular eukaryotes. - Triggered by internal or external signals.