RBSE SS Biology flashcards

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

Double fertilisation

In angiosperms, one male gamete fuses with the egg to form a diploid zygote, while the second fuses with two polar nuclei to form the triploid primary endosperm nucleus.

Embryo sac cell count

A mature angiosperm embryo sac has eight nuclei in seven cells: one egg, two synergids, three antipodals, and one central cell containing two polar nuclei.

Spermatogenesis outcome

One primary spermatocyte completes meiosis I and II to yield four haploid spermatids, each of which matures into a sperm cell after spermiogenesis.

Ovulation trigger

A mid-cycle surge of luteinising hormone from the anterior pituitary causes the mature Graafian follicle to rupture and release the secondary oocyte around day fourteen.

Law of segregation

The two alleles of a gene separate during gamete formation so that each gamete receives only one allele, and they reunite randomly at fertilisation.

Test cross ratio

Crossing an unknown genotype with a homozygous recessive parent gives a 1:1 dominant to recessive ratio if the unknown was heterozygous, or all dominant if homozygous.

Incomplete dominance

Neither allele is fully dominant, so the heterozygote shows an intermediate phenotype and the F2 generation gives a 1:2:1 phenotypic ratio.

Hardy-Weinberg conditions

Allele frequencies remain constant only in a large population with random mating and no mutation, migration, genetic drift or natural selection acting on the gene.

Malaria transmission

A female Anopheles mosquito injects Plasmodium sporozoites while feeding; the parasite multiplies in liver cells and red blood cells, causing fever and chills.

Active versus passive immunity

Active immunity develops after exposure to an antigen and generates memory cells; passive immunity is acquired by receiving ready-made antibodies, such as through colostrum.

Antibiotic limitation

Antibiotics such as penicillin target bacterial structures like the cell wall, so they do not harm viruses, which lack cell walls and independent metabolic machinery.

Biofortification

Plant breeding that raises the content of essential vitamins or minerals in edible crop parts, for example iron-rich bajra or vitamin A-rich carrot.

Restriction endonuclease

An enzyme that recognises a specific palindromic DNA sequence and cuts both strands there, often leaving single-stranded sticky ends used for joining fragments.

PCR three steps

Each PCR cycle denatures template DNA near 94 degrees Celsius, anneals primers near 50 to 60 degrees, then extends them with thermostable Taq polymerase near 72 degrees.

Bt toxin activation

The cry protein from Bacillus thuringiensis is an inactive protoxin in the bacterium and becomes an active pore-forming toxin only in the alkaline gut of the insect.

RNA interference

A double-stranded RNA expressed in the host is processed to silence a specific nematode mRNA, preventing the parasite from establishing infection in the plant.

Logistic growth equation

Logistic population growth follows dN/dt = rN(K minus N)/K, where r is the intrinsic rate and K is the carrying capacity of the environment.

Ten percent law

Only about ten percent of the energy at one trophic level is transferred to the next, so energy pyramids are always upright and food chains are short.

Ozone destruction

Chlorofluorocarbons release chlorine radicals in the stratosphere; each chlorine atom catalytically converts ozone into oxygen, thinning the protective ozone layer.

Biological oxygen demand

BOD measures the oxygen consumed by microbes decomposing organic matter in water; a high BOD indicates heavy organic pollution and low dissolved oxygen.

Species-area relationship

The number of species rises with habitat area according to log S equals log C plus Z log A, where Z is the slope of the regression line.

Fluid mosaic model

The membrane is a phospholipid bilayer with proteins embedded or attached, giving a mosaic of components that move laterally within a fluid lipid matrix.

Semi-autonomous organelles

Mitochondria and chloroplasts contain circular DNA and 70S ribosomes and divide by fission, evidence that they evolved from engulfed prokaryotes.

S phase of interphase

During S phase the DNA content of the cell doubles through replication, while the chromosome number stays the same because sister chromatids remain joined.

Meiosis versus mitosis

Meiosis has two divisions producing four haploid cells with crossing over, whereas mitosis is one division producing two genetically identical diploid cells.

Photolysis of water

At photosystem II, light energy splits water into electrons, protons and oxygen; the electrons replace those lost by chlorophyll and oxygen is released.

C4 versus C3 plants

C4 plants fix carbon dioxide with PEP carboxylase in mesophyll and have Kranz anatomy, avoiding photorespiration; C3 plants use RuBisCO directly.

Cohesion-tension theory

Water evaporating from leaves creates tension in the continuous xylem water column, and cohesion between water molecules pulls water upward from the roots.

Stomatal opening mechanism

Potassium ions enter guard cells, water follows osmotically, turgor pressure rises and the guard cells bow apart, opening the stomatal pore.

Haemoglobin dissociation curve

The sigmoid oxygen-haemoglobin curve shifts right when carbon dioxide rises, pH falls or temperature rises, releasing more oxygen to active tissues.

ADH action

Antidiuretic hormone increases water reabsorption from the collecting duct, producing concentrated urine and reducing water loss when body water is low.

Resting membrane potential

The resting potential of about minus seventy millivolts is maintained by the sodium-potassium pump and selective potassium leakage across the neuron membrane.