Tamil Nadu HSE Second Year Zoology key terms

The process in the seminiferous tubules of the testis by which diploid spermatogonia divide by mitosis and then meiosis to form haploid spermatozoa, supported by Sertoli cells and stimulated by FSH and testosterone.

The formation of haploid ova in the ovary, beginning before birth and completed only after fertilisation, producing one large ovum and polar bodies from each primary oocyte.

The fusion of a haploid sperm nucleus with a haploid ovum nucleus to form a diploid zygote, normally occurring in the ampullary region of the fallopian tube in humans.

The single diploid cell formed immediately after fertilisation, containing the combined genetic material of both parents and capable of repeated mitotic cleavage to form an embryo.

The attachment and embedding of the blastocyst into the endometrium of the uterus, occurring about six to seven days after fertilisation and essential for establishing pregnancy.

A temporary organ formed from maternal and fetal tissues that allows exchange of nutrients, gases and wastes between mother and fetus and secretes hormones such as hCG and progesterone.

Human chorionic gonadotropin, a hormone secreted by the developing chorion that maintains the corpus luteum and progesterone production in early pregnancy and is detected by pregnancy tests.

A reduction division producing four haploid cells from one diploid cell, with crossing over in prophase I and independent assortment in metaphase I, generating genetic variation in gametes.

Exchange of corresponding segments between non-sister chromatids of homologous chromosomes during pachytene of prophase I, producing new allele combinations on recombinant chromosomes.

A cross between an individual showing the dominant phenotype and a homozygous recessive individual, used to reveal whether the dominant parent is homozygous or heterozygous.

A condition in which the heterozygote shows an intermediate phenotype rather than the dominant one, as in pink flowers of snapdragon crosses between red and white parents.

A situation where both alleles are fully expressed in the heterozygote, illustrated by human AB blood group in which A and B antigens appear together on red cells.

The tendency of genes located close together on the same chromosome to be inherited together, reducing the frequency of recombinant offspring below the independent assortment expectation.

The synthesis of an RNA molecule from a DNA template catalysed by RNA polymerase, occurring in the nucleus of eukaryotes and producing mRNA, tRNA or rRNA.

The process on ribosomes in which the base sequence of mRNA is read in codons to assemble amino acids into a polypeptide chain, using tRNA as the adapter molecule.

The set of rules relating mRNA codons of three bases to specific amino acids, being degenerate, non-overlapping and nearly universal among organisms.

The differential survival and reproduction of individuals whose heritable variations make them better adapted to their environment, causing allele frequencies to change over generations.

The rapid diversification of a single ancestral lineage into many species occupying different ecological niches, as seen in Darwin's finches of the Galapagos Islands.

Structures in different species that share a common evolutionary origin and basic plan but may differ in function, such as the forelimbs of humans, whales and bats.

The principle that allele and genotype frequencies in a large, randomly mating population remain constant across generations in the absence of mutation, migration, selection and genetic drift.

Random change in allele frequencies in small populations due to chance sampling of gametes, which can fix or eliminate alleles independently of natural selection.

Non-specific, present-from-birth defence including physical barriers, phagocytes, inflammation and complement, acting rapidly against a broad range of pathogens without memory.

Pathogen-specific defence mediated by B and T lymphocytes, characterised by memory and developed after exposure to an antigen either naturally or through vaccination.

An immunoglobulin protein secreted by plasma cells that binds specifically to an antigen, neutralising it or marking it for destruction by other immune mechanisms.

Any foreign molecule, often a protein or polysaccharide on a pathogen, that is recognised by lymphocytes and triggers a specific immune response.

A preparation of weakened, killed or subunit antigen that stimulates protective immune memory without causing disease, giving active artificial immunity against the target pathogen.

A bacterial enzyme that cuts DNA at a specific recognition sequence, used in genetic engineering to generate fragments and to insert genes into vectors.

A small circular extrachromosomal DNA molecule in bacteria, widely used as a cloning vector because it replicates independently and can carry an inserted foreign gene.

A DNA molecule formed by joining genetic material from two different sources, typically a foreign gene inserted into a vector, and central to biotechnology applications.

An in vitro technique that repeatedly denatures, anneals primers and extends them with Taq polymerase to amplify a specific DNA segment into millions of copies.

A biological agent such as Bacillus thuringiensis or Trichoderma used to control pests without synthetic chemical pesticides, reducing environmental pollution.

A preparation containing living microorganisms such as Rhizobium, Azospirillum or mycorrhizal fungi that enrich soil nutrients and improve plant growth without chemical fertilisers.

Conversion of atmospheric nitrogen gas into ammonia by microorganisms such as Rhizobium in root nodules or free-living Azotobacter, making nitrogen available to plants.

A graphical representation of numbers, biomass or energy at successive trophic levels, usually upright for energy because only about ten percent passes to the next level.

The feeding position an organism occupies in a food chain, such as producer, primary consumer or secondary consumer, determining its energy source and ecological role.

The circulation of nutrient elements such as carbon, nitrogen and phosphorus between living organisms and the abiotic environment through reservoirs and transformation processes.

A self-regulating unit formed by a community of interacting organisms together with the non-living environment, through which energy flows and matter cycles.

A region with exceptionally high species richness and endemism that has already lost a large proportion of its original habitat and is a priority for conservation.

Protection of species within their natural habitats, as in national parks, wildlife sanctuaries and biosphere reserves, allowing natural ecological processes to continue.

Conservation of threatened species outside their natural habitat in facilities such as zoos, botanical gardens, seed banks and cryopreservation units for later reintroduction.

The structural and functional unit of the kidney, comprising a glomerulus, Bowman's capsule and tubules that filter blood and reabsorb useful substances to form urine.

The pressure-driven movement of water and small solutes from blood in the glomerulus into Bowman's capsule, forming filtrate while retaining cells and large proteins.

The arrangement of the loop of Henle and vasa recta in opposite flow directions that creates an osmotic gradient in the medulla, enabling concentration of urine.

The junction between two neurons or between a neuron and a target cell, across which a neurotransmitter is released to transmit an impulse in one direction.

A rapid reversal of membrane potential caused by sodium influx and potassium efflux along an axon, forming the electrical signal that propagates nerve impulses.

A chemical such as acetylcholine released from an axon terminal into a synapse, binding receptors on the next cell to excite or inhibit it and transmit the signal.

A chemical messenger secreted by an endocrine gland into the blood, acting on specific target cells with receptors to regulate metabolism, growth, reproduction or homeostasis.

Control of hormone secretion in which the level of a product or hormone inhibits or stimulates its own production, maintaining stable internal conditions around a set point.

The maintenance of a stable internal environment within narrow limits despite external changes, achieved through negative feedback by nervous and endocrine systems.