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

(a) Lysosomes: structure, kinds, chemical nature, origin and functions

Structure: Lysosomes are spherical, single-membrane-bound vesicles, about 0.1–1.2 µm across, discovered by de Duve (1955). The membrane is heavily glycosylated on its inner face, which protects it from its own enzymes, and has an ATP-driven proton pump maintaining an internal pH of about 4.5–5.

Kinds:

1. Primary lysosomes: newly formed, containing inactive enzymes and no substrate.
2. Secondary lysosomes (digestive vacuoles): primary lysosomes fused with a phagosome (heterophagosome) or autophagosome.
3. Residual bodies: contain undigested material, which is expelled or retained (as lipofuscin in ageing cells).
4. Autophagic vacuoles (autolysosomes): formed by fusion with vesicles enclosing the cell's own organelles.

Chemical nature: about 40–50 acid hydrolases (proteases, lipases, nucleases, phosphatases, glycosidases, sulphatases), with acid phosphatase as the marker enzyme.

Origin: The enzymes are synthesised on the rough ER, glycosylated, and in the cis-Golgi tagged with mannose-6-phosphate, which binds M6P receptors in the trans-Golgi network. Clathrin-coated vesicles bud off and fuse with endosomes, giving lysosomes; the receptors are recycled.

Functions: intracellular digestion (heterophagy) of bacteria and food particles, autophagy (removal of worn organelles and recycling in starvation), extracellular digestion through enzyme release (e.g. in fertilisation by the acrosome, and in bone remodelling), autolysis in metamorphosis and apoptosis. In plant cells, the lytic vacuole has comparable functions. Their defects cause storage diseases, such as Tay–Sachs disease.

(b)(i) Multiple alleles and their characteristics

Multiple alleles are three or more alternative forms of one gene occupying the same locus on homologous chromosomes, arising by mutation of the same gene in different ways.

Characteristics:

• A diploid individual carries only two of the alleles, though the population carries many.
• Alleles occupy the same locus and show no crossing over between them.
• They may show a dominance series or codominance.
• They give many genotypes in the population; for n alleles there are n(n + 1)/2 genotypes.
• They act on the same character, with a range of expression.

Examples: human ABO blood groups (Iᴬ, Iᴮ, i), with Iᴬ and Iᴮ codominant and both dominant over i; eye colour at the white locus in Drosophila (white, eosin, apricot, wild type); and self-incompatibility alleles (S alleles) in plants.

(b)(ii) Difference from pseudoalleles

Pseudoalleles are closely linked genes at separate loci of the same functional unit, which act like alleles in the heterozygote, e.g. lozenge (lz) in Drosophila.

• Locus: multiple alleles are at one locus; pseudoalleles are at adjacent but distinct loci.
• Recombination: no crossing over between multiple alleles; pseudoalleles recombine at a very low frequency, giving wild type.
• Cis–trans effect: pseudoalleles in the trans arrangement (a b⁺/a⁺ b) give a mutant phenotype and in the cis arrangement (a b/a⁺ b⁺) give wild type; multiple alleles show no such position effect.

(c) Polymerase chain reaction (PCR)

PCR (Kary Mullis, 1983) amplifies a specific DNA segment exponentially in vitro.

Requirements: template DNA, two oligonucleotide primers (about 18–25 bases) flanking the target, thermostable Taq polymerase (from Thermus aquaticus), dNTPs, Mg²⁺, buffer, and a thermocycler.

Procedure: each cycle has three steps:

1. Denaturation at about 94–95 °C separates the strands.
2. Annealing at about 50–65 °C lets the primers bind.
3. Extension at about 72 °C, where Taq polymerase adds nucleotides from the primers' 3′ ends.

An initial denaturation and a final extension are added.

Efficiency: The product doubles each cycle, so n cycles give up to 2ⁿ copies, about a billion copies after 30 cycles. In practice efficiency is 90–100% per cycle at first, and then a plateau is reached as reagents are exhausted. Taq has no proofreading activity, with an error rate of about 10⁻⁴–10⁻⁵ per base, so high-fidelity polymerases are used when accuracy matters.