PSEB SS Biology flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Double fertilisation

The angiosperm-specific process in which one male gamete fuses with the egg to form the zygote and the second fuses with two polar nuclei to form the triploid primary endosperm nucleus.

Embryo sac

The mature female gametophyte inside the ovule, typically containing seven cells and eight nuclei: one egg, two synergids, three antipodals and a binucleate central cell.

Spermatogenesis

The process in seminiferous tubules in which a primary spermatocyte undergoes meiosis I and II to yield four haploid spermatids that mature into sperm cells.

Corpus luteum

A temporary endocrine structure formed from the ruptured follicle after ovulation; it secretes progesterone, which maintains the endometrium and supports early pregnancy.

LH surge

A sharp rise in luteinising hormone around mid-cycle that triggers ovulation and converts the ruptured follicle into the corpus luteum.

ZIFT

Zygote intrafallopian transfer: fertilisation occurs outside the body, and the zygote or early embryo is placed into the fallopian tube.

Test cross

A cross between an individual of unknown dominant genotype and a homozygous recessive individual; offspring ratios reveal whether the unknown was homozygous or heterozygous.

Codominance

A condition in which both alleles in a heterozygote are fully expressed, as in AB blood group where IA and IB alleles each produce their own antigen.

Semi-conservative replication

DNA replication mode in which each daughter molecule keeps one parental strand and one newly synthesised strand, confirmed by the Meselson–Stahl experiment.

Stop codons

The three mRNA codons UAA, UAG and UGA that do not code for any amino acid and signal termination of translation at the ribosome.

Lac operon

An inducible operon in E. coli with regulator, promoter, operator and three structural genes; lactose binding to the repressor allows transcription of lactose-metabolising enzymes.

Hardy–Weinberg equilibrium

The principle that allele and genotype frequencies in a large, randomly mating population remain constant across generations without mutation, migration or selection.

Analogous organs

Structures with similar function but different evolutionary origin, such as wings of insects and birds; they indicate convergent evolution rather than common ancestry.

Natural selection

The differential survival and reproduction of individuals with heritable variations better suited to their environment, leading to changes in allele frequencies over generations.

Plasmodium

A protozoan parasite transmitted by female Anopheles mosquitoes that causes malaria; its life cycle includes sporozoites, merozoites and gametocytes in human hosts.

Cell-mediated immunity

Immune defence mediated by T lymphocytes that directly attack infected or abnormal cells; responsible for graft rejection and defence against intracellular pathogens.

EcoRI

A restriction endonuclease that recognises the palindromic sequence GAATTC and cuts between G and A, producing sticky ends useful in recombinant DNA construction.

Ti plasmid

A tumour-inducing plasmid of Agrobacterium tumefaciens used as a vector to deliver foreign genes into plant cells, with the tumour genes removed.

Polymerase chain reaction

A technique that amplifies DNA in vitro through repeated cycles of denaturation, primer annealing and extension by heat-stable Taq polymerase, doubling copies each cycle.

Gel electrophoresis

A method separating charged molecules such as DNA fragments through a gel under an electric field; negatively charged DNA migrates toward the positive electrode.

Biopiracy

The unauthorised commercial exploitation of biological resources or traditional knowledge without fair compensation to the country or community of origin.

Logistic growth

Population growth described by dN/dt = rN(K−N)/K, slowing as population size N approaches carrying capacity K and producing a sigmoid curve.

Ten percent law

The ecological rule that only about ten percent of energy at one trophic level is transferred to the next, the rest lost as heat and respiration.

Biodiversity hotspot

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

Ecological pyramid

A graphical representation of numbers, biomass or energy at successive trophic levels; it can be inverted for numbers or biomass but never for energy.

Biofertiliser

A preparation of living microorganisms such as Rhizobium, Azotobacter or Glomus that enriches soil nutrients by nitrogen fixation or phosphate solubilisation.

Primary productivity

The rate at which producers convert solar energy into organic matter; gross primary productivity minus respiration equals net primary productivity available to consumers.

Transcription unit

A segment of DNA comprising a promoter, structural gene and terminator that is transcribed as a single RNA molecule by RNA polymerase.

Genetic drift

Random change in allele frequencies in small populations due to chance events; it can fix or eliminate alleles independently of natural selection.

Auxin

A plant hormone, chiefly indole-3-acetic acid, that promotes cell elongation, apical dominance and root formation and is used in vegetative propagation.

Apical dominance

The suppression of lateral bud growth by auxin produced in the shoot apex; removing the apex releases buds and produces bushier growth.

Photoperiodism

The response of plants to relative lengths of light and dark periods, controlling flowering in short-day and long-day plants and dormancy in some species.