(a) Food Web (10 marks) (b) Biological Rhythms (10 marks) (c) AIDS (10 marks) (d) Chromosome Painting (10 marks) (e) Red Data…

(a) Food Web

A food web is an interconnected network of multiple food chains operating within an ecological community, reflecting the complex, non-linear trophic interactions among organisms, unlike a single linear food chain. Energy flows through defined trophic levels—primary producers (autotrophs), primary consumers (herbivores), secondary/tertiary consumers (carnivores), and decomposers/detritivores, with only about 10% of energy passing from one level to the next (Lindeman's rule). Within ecosystems, grazing pathways (originating from living autotrophic biomass) and detritus pathways (originating from decaying organic matter) do not function in isolation; rather, they cross-link extensively as generalist predators consume prey from both pathways. This structural complexity provides ecological stability through functional redundancy: if one species population fluctuates or declines, alternative trophic channels buffer the system against sudden collapse and mitigate trophic cascades. For example, in the Sundarbans mangrove ecosystem, mangrove leaf litter feeds detritivorous crabs and molluscs, while phytoplankton supports zooplankton and juvenile fish; both grazing and detrital pathways converge on higher predators like estuarine crocodiles and the Bengal tiger.

(b) Biological Rhythms

Biological rhythms are self-sustaining, periodic physiological and behavioural fluctuations governed by endogenous pacemakers and entrained by environmental cues (zeitgebers) such as light-dark and temperature cycles. These rhythms manifest across various periodicities, including circadian (approximately 24-hour cycles, e.g., sleep-wake cycles), circatidal (approximately 12.4 hours, e.g., fiddler crab foraging), and circannual rhythms (seasonal reproduction, hibernation, and bird migration). In mammals the master pacemaker is the suprachiasmatic nucleus (SCN) of the anterior hypothalamus, which drives nocturnal melatonin secretion by the pineal gland; in birds and lower vertebrates the pineal itself is photoreceptive and acts as a clock. At the cellular level, the molecular clock relies on an autoregulatory transcription-translation feedback loop (TTFL), primarily driven by heterodimeric transcription factors CLOCK and BMAL1, which activate the transcription of Period (PER) and Cryptochrome (CRY) genes, whose protein complexes then repress their own transcription; the discovery of this loop in Drosophila earned Hall, Rosbash and Young the 2017 Nobel Prize in Physiology or Medicine. These rhythms confer immense adaptive fitness by anticipating daily and seasonal ecological changes, enabling organisms to optimize foraging efficiency, maximize reproductive success, and synchronize predator-avoidance strategies.

(c) AIDS (Acquired Immunodeficiency Syndrome)

Acquired Immunodeficiency Syndrome (AIDS) is a chronic, life-threatening condition caused by the Human Immunodeficiency Virus (HIV), an enveloped lentivirus possessing a single-stranded RNA genome and key enzymes including reverse transcriptase, integrase, and protease. The viral envelope glycoprotein gp120 binds specifically to the CD4 receptor along with chemokine coreceptors (CCR5 or CXCR4) on helper T-lymphocytes, macrophages, and dendritic cells. Following membrane fusion via gp41, viral RNA is reverse-transcribed into proviral DNA and integrated into the host genome. Transmission occurs sexually, through infected blood and needle sharing, and vertically from mother to child. Progressive viral replication results in severe CD4+ T-cell depletion and immune collapse, rendering the host vulnerable to opportunistic pathogens like Mycobacterium tuberculosis and malignancies such as Kaposi's sarcoma. In India, the National AIDS Control Organisation (NACO) spearheads management via targeted interventions among high-risk groups, including female sex workers and intravenous drug users. Free antiretroviral therapy (ART) under the 'Treat All' policy (2017), the HIV and AIDS (Prevention and Control) Act, 2017, and the National AIDS and STD Control Programme Phase V have markedly reduced transmission and AIDS-related deaths; the India HIV Estimates 2023 put people living with HIV at about 25 lakh, with adult prevalence of about 0.2%.

(d) Chromosome Painting

Chromosome painting is a molecular cytogenetic technique based on Fluorescence In Situ Hybridization (FISH) that uses complex, fluorochrome-labeled probe sets to visualize and differentiate whole individual chromosomes or distinct chromosomal regions under fluorescence microscopy. These whole-chromosome probes (WCPs) are generated by isolating individual metaphase chromosomes using dual-laser flow sorting or microdissection, followed by degenerate oligonucleotide-primed PCR (DOP-PCR) amplification and fluorescent labeling. During hybridization, the probes bind specifically to complementary sequences along the target chromosome, creating a distinctive colour signal. Chromosome painting is invaluable in clinical cytogenetics and oncology for identifying cryptic structural aberrations, complex translocations, marker chromosomes, and aneuploidies in leukemias and solid tumors. Furthermore, comparative cross-species chromosome painting (Zoo-FISH) plays a crucial role in evolutionary biology by elucidating conserved syntenic blocks, reconstructing ancestral mammalian karyotypes, and mapping chromosomal rearrangements that have driven evolutionary divergence.

(e) Red Data Book

The Red Data Book is a foundational conservation documentation initiated in 1966 by the International Union for Conservation of Nature (IUCN) to catalogue the global risk of extinction across plant, animal, and fungal taxa. Species are evaluated against standardized, quantitative criteria—including population size reduction, geographic range fragmentation, and extinction probability models—and categorized into Extinct (EX), Extinct in the Wild (EW), Critically Endangered (CR), Endangered (EN), Vulnerable (VU), Near Threatened (NT), Least Concern (LC), and Data Deficient (DD). In India, state-level assessments and national Red Data Books are compiled by the Zoological Survey of India (ZSI) and Botanical Survey of India (BSI). Prominent Indian species documented include the Critically Endangered Great Indian Bustard (Ardeotis nigriceps) and Gharial (Gavialis gangeticus), as well as the Endangered Lion-tailed Macaque (Macaca silenus). The original Red Data Books used colour-coded pages: black for extinct, red for endangered, amber for vulnerable, white for rare, green for recovered and grey for insufficiently known species. The Red Data Book serves as an essential empirical baseline for guiding statutory protections under the Wildlife (Protection) Act, 1972, identifying Key Biodiversity Areas, and formulating targeted recovery programs.