(a) Describe the structure, kinds, chemical nature, origin and functions of lysosomes. 20 (b) Explain multiple alleles and their…

(a) Lysosomes

Lysosomes are membrane-bound vesicles, approximately 0.1–1.0 µm in diameter, containing hydrolytic enzymes that function optimally at an acidic pH of 4.5–5.0. They are not storage compartments for inactive zymogens; rather, the enzymes are active at the internal acidic pH but are kept separate from cellular substrates until the lysosome fuses with a target compartment. This compartmentalization prevents the digestion of the cell’s own components.

Structure and Kinds Lysosomes are enclosed by a single lipid bilayer membrane rich in glycoproteins, which protects the cytoplasm from the hydrolytic enzymes within. They are classified into three functional types:

1. Primary Lysosomes: Newly formed vesicles budding from the trans-Golgi network, containing enzymes but no substrate.
2. Secondary Lysosomes (Digestive Vacuoles): Formed by the fusion of primary lysosomes with phagocytic or autophagic vacuoles. Digestion occurs here.
3. Residual Bodies (Lysosomes): Vacuoles containing undigested material that persists after the digestive process, eventually expelled via exocytosis.

Chemical Nature and Origin The enzymes are acid hydrolases, including proteases, lipases, nucleases, and glycosidases. They are synthesized on the rough endoplasmic reticulum and transported to the Golgi apparatus. In the Golgi, mannose-6-phosphate (M6P) tags are added to the enzymes, allowing them to be sorted into vesicles that bud off to form lysosomes. This pathway is part of the Golgi-Endoplasmic Reticulum-Lysosome (GERL) system.

Functions Lysosomes perform two major functions:

1. Heterophagy: Digestion of extracellular material taken up by phagocytosis (e.g., bacteria in macrophages) or pinocytosis.
2. Autophagy: Digestion of the cell’s own organelles or proteins. This is crucial for recycling nutrients during starvation and removing damaged organelles (mitophagy). Additionally, lysosomes play a role in programmed cell death (apoptosis) and the breakdown of waste products in senescent cells.

(b) Multiple Alleles and Pseudoalleles

Multiple Alleles Multiple alleles refer to the existence of three or more alternative forms of a gene in a population, all occupying the same locus on homologous chromosomes. An individual diploid organism can possess only two of these alleles at a time.

• Characteristics:
• Dominance Hierarchy: Often, a clear dominance series exists (e.g., in Drosophila eye color: w⁺ > w^e > w^c).
• Codominance: Some alleles may be codominant, expressing both phenotypes simultaneously.
• Population Frequency: The frequency of alleles is determined by Hardy-Weinberg equilibrium principles, though selection can alter these frequencies.
• Examples: The human ABO blood group system is a classic example, with alleles I^A, I^B, and i. I^A and I^B are codominant, while both are dominant over i. In Drosophila, the white eye gene has multiple alleles such as w⁺ (wild type), w (white), w^e (eosin), and w^c (coral).

Difference from Pseudoalleles Pseudoalleles are genes located at different loci but so closely linked that they behave like a single unit during inheritance, mimicking multiple alleles.

• Key Distinction: Multiple alleles are alternative forms of the same gene at the same locus. Pseudoalleles are distinct genes at different loci.
• Recombination: Multiple alleles do not recombine with each other (as they are at the same locus). Pseudoalleles can recombine, although the recombination frequency is very low (often <0.5%) due to tight linkage. For example, the star and asteroid mutations in Drosophila are pseudoalleles; they produce similar phenotypes but are located at different positions on the chromosome.

(c) Polymerase Chain Reaction (PCR)

PCR is an in vitro technique used to amplify specific DNA sequences exponentially. It was developed by Kary Mullis in 1983.

Procedure The process involves repeated cycles of temperature changes:

1. Denaturation (94–95°C): The double-stranded DNA template is heated to separate it into single strands.
2. Annealing (50–65°C): The temperature is lowered to allow short, synthetic primers to bind (anneal) to the complementary sequences flanking the target region.
3. Extension (72°C): The temperature is raised to the optimal activity temperature of Taq polymerase, which synthesizes new DNA strands by adding dNTPs to the 3' end of the primers.

Requirements

• Template DNA: The source DNA containing the target sequence.
• Primers: Two short oligonucleotides (18–25 bases) complementary to the 3' ends of the target sequence.
• Taq Polymerase: A thermostable DNA polymerase isolated from Thermus aquaticus, capable of withstanding high temperatures.
• dNTPs: Deoxynucleotide triphosphates (dATP, dTTP, dCTP, dGTP) as building blocks.
• Mg²⁺ Ions: Essential cofactor for polymerase activity.
• Buffer Solution: To maintain optimal pH and ionic strength.

Efficiency and Limitations PCR achieves exponential amplification, theoretically producing 2^ncopies aftern cycles. In practice, 20–30 cycles yield millions to billions of copies.

• Error Rate: Taq polymerase lacks 3' to 5' exonuclease proofreading activity, resulting in an error rate of approximately 10⁻⁴ per base per cycle. This can lead to mutations in the amplified product.
• Limitations: PCR is less efficient with highly degraded DNA or complex mixtures. Non-specific amplification can occur if primer design is poor.
• Applications: PCR is fundamental in forensic science (DNA fingerprinting in criminal cases), medical diagnosis (e.g., RT-PCR for SARS-CoV-2), and phylogenetic studies. Its speed and sensitivity make it indispensable in modern molecular biology.