Grade 12 Life Sciences: Core Concepts and Terminology

Comprehensive vocabulary flashcards covering the key concepts, anatomical structures, physiological processes, and evolutionary milestones from the Grade 12 Life Sciences curriculum.

Nucleotide

The monomer or building block of nucleic acids (DNA and RNA), composed of a phosphate group, a sugar molecule, and a nitrogenous base.

Double Helix

The natural twisted, ladder-like shape of a DNA molecule consisting of two complementary antiparallel strands joined together by weak hydrogen bonds.

DNA Replication

The process occurring during interphase of the cell cycle in which a DNA molecule makes an exact, identical copy of itself.

DNA Profiling

A technique that produces a distinct pattern of bands on X-ray film representing an individual's unique DNA sequence, used in forensic investigations and paternity testing.

Transcription

The first stage of protein synthesis occurring in the nucleus, in which a segment of DNA serves as a template to form a complementary strand of messenger RNA (mRNA).

Codon

A sequence of three consecutive nitrogenous bases on an mRNA molecule that codes for a specific amino acid.

Anti-codon

A sequence of three consecutive nitrogenous bases on a tRNA molecule that is complementary to a specific codon on mRNA.

Translation

The second stage of protein synthesis occurring in the cytoplasm at the ribosome, where tRNA molecules deliver specific amino acids that link via peptide bonds to form a protein.

Homologous Chromosomes

A matching pair of chromosomes of the same shape and size, one inherited from each parent, possessing alleles for the same characteristics at the same loci.

Bivalent

A pair of homologous chromosomes that lie physically adjacent to and in contact with each other during Prophase I of meiosis.

Crossing Over

The exchange of genetic material between non-sister chromatids of homologous chromosomes during Prophase I of meiosis, resulting in new genetic combinations.

Chiasma

The microscopic point of contact and overlap where non-sister chromatids cross over during Prophase I of meiosis.

Karyotype

A photographic or diagrammatic representation showing the full set of chromosomes of a somatic cell arranged according to number, shape, and size.

Non-disjunction

The failure of homologous chromosome pairs to separate during Anaphase I, or sister chromatids to separate during Anaphase II of meiosis, leading to gametes with abnormal chromosome numbers.

External Fertilisation

A type of fertilisation occurring outside the female's body where sperm and egg cells are released into an aquatic environment to fuse.

Internal Fertilisation

A type of fertilisation in which sperm are deposited directly inside the female reproductive tract, removing the dependency on an external water source.

Ovipary

A reproductive strategy in which eggs are laid outside the maternal body and embryos develop externally, deriving all nutrition from yolk.

Ovovivipary

A reproductive strategy where eggs are fertilised internally and retained inside the mother's body until hatching, with the embryo nourished solely by the egg's yolk.

Vivipary

A reproductive strategy where the embryo develops internally within the maternal uterus and receives continuous nutrition from the mother via a placenta.

Amniotic Egg

A shelled egg possessing extra-embryonic membranes (amnion, allantois, chorion, and yolk sac) that protect the developing embryo from desiccation and mechanical injury on land.

Precocial Development

A mode of development where hatchlings emerge in a well-developed state with open eyes, down feathers or fur, and the ability to move and forage independently.

Altricial Development

A developmental pattern where offspring hatch or are born underdeveloped, naked, blind, and immobile, requiring extensive parental care.

Sertoli Cells

Specialised supportive cells located in the seminiferous tubules of the testes that provide nourishment and support to developing spermatids.

Acrosome

A cap-like vesicle located at the anterior tip of a sperm head containing digestive enzymes required to penetrate the outer layer of the ovum.

Graafian Follicle

A fluid-filled ovarian follicle containing a mature ovum that secretes oestrogen and ruptures at ovulation.

Corpus Luteum

A temporary endocrine structure formed from the ruptured Graafian follicle after ovulation that secretes progesterone to maintain the uterine lining.

Blastocyst

A hollow, fluid-filled sphere of cells derived from the morula that embeds into the vascularised endometrium during implantation.

Complete Dominance

A genetic condition in which the presence of a dominant allele completely masks the phenotypic expression of a recessive allele in the heterozygous state.

Incomplete Dominance

A pattern of inheritance in which neither allele is dominant, producing an intermediate or blended phenotype in heterozygous individuals.

Co-dominance

A genetic inheritance pattern where two different alleles for a trait are equally expressed in the phenotype of a heterozygous individual.

Stem Cells

Undifferentiated biological cells that retain the capacity to divide continuously and differentiate into various specialised tissue types.

Reflex Arc

The specific neural pathway traversed by nerve impulses from a sensory receptor to an effector to produce an involuntary reflex action.

Synapse

The microscopic junction between the terminal axon of one neuron and the dendrite of an adjacent neuron across which impulses pass via neurotransmitters.

Accommodation

The physiological adjustment of the curvature of the eye lens via ciliary muscles and suspensory ligaments to obtain sharp visual focus on near or distant objects.

Pupillary Mechanism

The autonomic reflex action involving the antagonistic contraction and relaxation of circular and radial iris muscles to regulate the quantity of light entering the eye.

Cristae

Hair-cell sensory receptors located within the ampullae of the semicircular canals that detect dynamic balance, including changes in rotational speed and direction.

Maculae

Sensory receptor structures situated in the sacculus and utriculus of the inner ear vestibule that detect changes in the position of the head relative to gravity.

Organ of Corti

The receptor organ for hearing, situated within the middle chamber of the cochlea, which converts fluid pressure waves into nerve impulses.

Homeostasis

The maintenance of a chemically and physically constant internal environment (blood and tissue fluid) within narrow physiological limits.

Negative Feedback Mechanism

A self-regulating control loop in which a detected change or imbalance triggers a physiological response that counteracts and reverses the initial stimulus to restore equilibrium.

Osmoregulation

The homeostatic regulation of the water balance and solute concentration in the blood and tissue fluids of an organism, primarily regulated by antidiuretic hormone (ADH).

Vasodilation

The physiological widening of superficial arterioles and capillaries in the skin dermis, increasing cutaneous blood flow to facilitate heat loss by radiation.

Vasoconstriction

The narrowing of superficial arterioles and capillaries in the skin dermis, decreasing cutaneous blood flow to conserve internal body heat.

Apical Dominance

The physiological phenomenon in which auxins produced by the terminal bud inhibit the outgrowth of lateral (axillary) buds situated lower on the stem.

Phototropism

The directional growth response of a plant organ towards or away from a unilateral light stimulus, mediated by an unequal distribution of auxins.

Geotropism

The directional growth movement of a plant organ in direct response to the pull of gravity.

Natural Selection

The primary mechanism of biological evolution whereby organisms with inheritable traits best suited to their selective environment survive, reproduce, and pass on those favourable alleles.

Punctuated Equilibrium

An evolutionary theory proposing that species remain morphologically stable for long periods (stasis), interspersed with brief intervals of rapid speciation.

Geographic Speciation

The formation of new distinct biological species occurring when a parent population is divided by an extrinsic physical barrier that terminates gene flow.

Bipedalism

The habitual ability and mode of terrestrial locomotion involving standing erect and walking forward on two hind limbs.

Foramen Magnum

The large aperture located in the occipital bone of the skull through which the spinal cord connects to the brainstem; positioned forward in bipedal humans and backward in quadrupedal apes.

Prognathous

Possessing a prominent, forward-projecting lower and upper jaw facial profile relative to the cranial vault.

'Out of Africa' Hypothesis

The scientific hypothesis asserting that modern humans ($Homo\text{ }sapiens$) originated in Africa around 150,000 to 200,000 years ago and subsequently migrated outwards to replace archaic hominins across the globe.