ANWSER
Question 1
Explain the process of DNA replication during the cell cycle. At what phase does it occur, and why is it crucial for cell division?
Answer:
DNA replication occurs during the S phase (Synthesis phase) of the cell cycle, which is part of interphase. The process involves the following steps:
1. Initiation: Replication begins at specific sites called origins of replication, where the DNA double helix is unwound by helicase, forming a replication fork.
2. Elongation: DNA polymerase synthesizes new strands by adding complementary nucleotides to each template strand. The leading strand is synthesized continuously, while the lagging strand is synthesized discontinuously as Okazaki fragments.
3. Termination: Replication ends when the entire DNA molecule is copied, and ligase joins the Okazaki fragments on the lagging strand.
Significance:
DNA replication is crucial for cell division because it ensures that each daughter cell receives an identical copy of the genetic material. This fidelity is essential for maintaining genetic continuity and proper cellular function.
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Question 2
Explain the phases of the cell cycle and discuss the significance of each phase in ensuring proper cell division.
Answer:
The cell cycle consists of four main phases:
1. G1 Phase (Gap 1): The cell grows and prepares for DNA replication. It checks for favorable conditions and ensures the cell is ready for the next phase.
2. S Phase (Synthesis): DNA replication occurs, ensuring each daughter cell will have a complete set of chromosomes.
3. G2 Phase (Gap 2): The cell continues to grow and prepares for mitosis. It checks for DNA replication errors and repairs them.
4. M Phase (Mitosis): The cell divides into two genetically identical daughter cells through mitosis (nuclear division) and cytokinesis (cytoplasmic division).
Significance:
– G1 and G2 phases allow the cell to grow and ensure conditions are suitable for division.
– S phase guarantees accurate genetic material duplication.
– M phase ensures equal distribution of chromosomes to daughter cells.
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Question 3a
Explain the process of digestion, detailing the roles of mechanical and chemical digestion in breaking down macronutrients into absorbable units.
Answer:
Digestion Process:
1. Mechanical Digestion: Begins in the mouth with chewing (mastication) and continues in the stomach through churning. This breaks food into smaller pieces, increasing surface area for enzymatic action.
2. Chemical Digestion: Involves enzymes that hydrolyze macronutrients:
– Carbohydrates: Salivary amylase (mouth) and pancreatic amylase (small intestine) break starches into disaccharides. Brush border enzymes (e.g., lactase) further break disaccharides into monosaccharides.
– Proteins: Pepsin (stomach) and pancreatic proteases (small intestine) break proteins into peptides and amino acids.
– Lipids: Bile emulsifies fats, and lipases (small intestine) break them into fatty acids and glycerol.
Absorbable Units: Monosaccharides (glucose), amino acids, and fatty acids/glycerol are absorbed in the small intestine.
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Question 3b
State the enzymes involved and the corresponding products of the following reactions:
Answer:
– Maltose โ Glucose + Glucose
Enzyme: Maltase
– Isomaltose โ Glucose + Glucose
Enzyme: Isomaltase
– Sucrose โ Glucose + Fructose
Enzyme: Sucrase
– Lactose โ Glucose + Galactose
Enzyme: Lactase
– ฮฑ-limit dextrin โ Glucose + Glucose
Enzyme: ฮฑ-dextrinase
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Question 4
Compare and contrast active transport and facilitative diffusion in the absorption of monosaccharides. Include the role of sodium ions in active transport and the importance of specific glucose transporters such as SGLT-1 in the process.
Answer:
| Feature | Active Transport | Facilitative Diffusion |
|———————-|———————————————-|——————————————–|
| Energy Requirement | Requires ATP (energy-dependent) | Passive (no energy required) |
| Direction | Against concentration gradient | Along concentration gradient |
| Sodium Role | Sodium-glucose symport (SGLT-1 uses Na+ gradient) | Not involved |
| Transporters | SGLT-1 (intestine), SGLT-2 (kidney) | GLUT transporters (e.g., GLUT2, GLUT4) |
SGLT-1 Importance:
– Located in the intestinal epithelium, SGLT-1 couples glucose transport with sodium, enabling absorption even when glucose concentration is low in the gut.
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Question 5a
Describe the three main organelles that are unique to eukaryotic cells with emphasis on their structure and role in cell operations.
Answer:
1. Nucleus:
– Structure: Surrounded by a double membrane (nuclear envelope) with pores; contains chromatin (DNA + proteins).
– Role: Stores genetic material and directs protein synthesis (transcription).
2. Mitochondria:
– Structure: Double-membrane-bound; inner membrane folded into cristae; contains matrix with mitochondrial DNA.
– Role: Produces ATP via aerobic respiration (powerhouse of the cell).
3. Endoplasmic Reticulum (ER):
– Structure: Network of membranous tubules (rough ER has ribosomes; smooth ER lacks ribosomes).
– Role: Rough ER synthesizes proteins; smooth ER detoxifies and synthesizes lipids.
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Question 5b
Compare and contrast prokaryotic and eukaryotic cells, paying particular attention to their organelles and structural characteristics. Give an example each.
Answer:
| Feature | Prokaryotic Cells | Eukaryotic Cells |
|———————|———————————————|——————————————–|
| Nucleus | Absent (DNA in nucleoid) | Present (membrane-bound) |
| Organelles | No membrane-bound organelles | Membrane-bound organelles (e.g., mitochondria) |
| Size | Smaller (1โ10 ยตm) | Larger (10โ100 ยตm) |
| Cell Wall | Peptidoglycan (bacteria) | Cellulose (plants) or absent (animals) |
| Example | *Escherichia coli* (bacterium) | *Homo sapiens* (human) |