(a) Describe the steps of constructing a recombinant DNA and its cloning. (20 marks) (b) Discuss the major morphological…

Recombinant DNA Construction and Cloning

Recombinant DNA (rDNA) technology involves combining genetic material from different sources to create novel DNA sequences and multiplying them within host organisms.

The process begins with the isolation of pure target DNA containing the gene of interest and vector DNA, using cell lysis, enzymatic digestion of contaminants (proteases, RNases), and ethanol precipitation. Both target and vector DNAs are treated with specific restriction endonucleases (molecular scissors) that generate compatible cohesive or blunt ends. DNA ligase then catalyzes phosphodiester bond formation between the complementary ends, yielding circular recombinant molecules.

Cloning vectors are tailored to insert size: plasmids (such as pBR322 or pUC19, carrying inserts up to 10 kb), cosmids (30–45 kb), bacterial artificial chromosomes (BACs, 100–300 kb), and yeast artificial chromosomes (YACs, up to 1 Mb). The construct is introduced into competent host cells—predominantly Escherichia coli via heat-shock or electroporation, or Saccharomyces cerevisiae for eukaryotic post-translational expression.

Selection and screening distinguish transformed recombinant cells from non-transformants and non-recombinants. Selectable markers utilize antibiotic resistance genes (e.g., ampicillin, tetracycline resistance). In blue-white screening, insertional inactivation of the lacZ gene prevents functional β-galactosidase synthesis; recombinant colonies remain white on media containing X-gal and IPTG, whereas non-recombinant vectors produce blue colonies.

Morphological Modifications in Horse Evolution

The evolutionary lineage of the horse (Eohippus to Equus) represents an orthogenetic progression driven by climatic shifts from dense, humid Eocene forests to dry, open Miocene grasslands. This transition necessitated adaptations for cursorial locomotion and a tough, fibrous diet.

The sequential morphological modifications include:

1. Locomotor System and Digits: Progressive reduction of digits from a four-toed forefoot and three-toed hindfoot in Eohippus (Hyracotherium), through three-toed tridactyl forms (Mesohippus, Merychippus), to the functional monodactyly of modern Equus, where the enlarged third digit bears all weight while the second and fourth digits are reduced to non-functional splint bones. Limbs elongated distally, with fusion of the radius-ulna and tibia-fibula.
2. Dentition: Shift from low-crowned, bunodont teeth (adapted for soft forest foliage) to high-crowned, prismatic hypsodont molars with complex enamel folding and cementum infilling, capable of resisting wear from silica-rich grasses. Premolars underwent molarization.
3. Cranial and Body Size: Progressive increase in body stature, along with elongation of the facial skull anterior to the eyes to accommodate hypsodont tooth roots and facilitate simultaneous grazing and predator scanning.

Extensive fossil beds in North America trace this phylogeny, while India’s Siwalik deposits yield crucial Neogene fossils like Hipparion theobaldi and Equus sivalensis, demonstrating regional radiation.

Salient Features of Zoological Nomenclature

Zoological nomenclature provides a standardized, universal system for naming animal taxa, governed by the International Commission on Zoological Nomenclature (ICZN) through the International Code of Zoological Nomenclature.

1. Binomial Nomenclature: Every species name consists of a Latinized binomen: a capitalized generic name followed by a lowercase specific epithet, both italicized in print (e.g., Panthera tigris).
2. Principle of Priority: The valid name of a taxon is the oldest available name published after 1 January 1758 (Linnaeus’ Systema Naturae, 10th edition). Junior synonyms and homonyms are suppressed.
3. Type Concept (Typification): Every nominal taxon must be anchored to a physical name-bearing type specimen (holotype, lectotype, or neotype) deposited in a recognized museum, ensuring stability if taxa are split or revised.
4. Authorship and Date Citation: The original describer’s surname and publication year follow the binomen without intervening punctuation (e.g., Homo sapiens Linnaeus, 1758); parentheses indicate subsequent generic reassignments.
5. Standardized Suffixes: Higher taxonomic ranks require mandated endings, such as -oidea for superfamily, -idae for family (e.g., Felidae), and -inae for subfamily.

These biotechnological, evolutionary, and taxonomic frameworks collectively form the empirical bedrock of modern zoological science, enabling precise identification, evolutionary reconstruction, and genetic manipulation of fauna.