12th Standard Zoology March 2023: Online Practice Test

Explanation. Interspecific competition for the same limited resources can eliminate the weaker competitor, so it can lead to the extinction of a species.

Explanation. Statins, produced by the yeast Monascus purpureus, inhibit the enzyme that synthesises cholesterol and so lower blood cholesterol.

Explanation. Gastrulation converts the single-layered blastocyst (blastula) into a gastrula with three primary germ layers: ectoderm, mesoderm and endoderm. Attachment to the uterine wall is implantation and organ formation is organogenesis.

Explanation. AUG is the initiation codon and codes for methionine. UUU codes for phenylalanine, AUC for isoleucine and UAA is a stop codon.

Explanation. In the ZW-ZZ system the female is heterogametic (ZW) and the male homogametic (ZZ). It is found in birds and also in some reptiles and fishes (and butterflies and moths), so all of the above.

Explanation. Reptiles, including the dinosaurs, dominated the Earth during the Mesozoic era, which is therefore called the golden age of reptiles.

Explanation. The textbook states that the first human-like being was the hominid (Homo habilis). Ramapithecus was a man-like primate (ape), while Australopithecus and Homo erectus are separate stages of the line.

Explanation. Catadromous fishes (e.g. eels) migrate from fresh water to the sea to breed; anadromous fishes (e.g. salmon) move from sea to fresh water. Stenothermal and eurythermal refer to temperature tolerance.

Explanation. Spermarche is the first ejaculation of semen at puberty. Orchidectomy is removal of testes and azoospermia is absence of sperm in semen.

Explanation. Sporozoites injected by the mosquito first multiply asexually in the liver cells; this is exo-erythrocytic schizogony. Schizogony inside RBCs is the erythrocytic cycle.

Explanation. The interstitial Leydig cells of the testis secrete testosterone. Sertoli cells nourish developing sperms.

Explanation. Dodo, passenger pigeon and Steller's sea cow were hunted excessively by humans; over exploitation drove them to extinction.

Explanation. Allergens stimulate production of IgE antibodies, which bind to mast cells and cause release of histamine, producing allergic symptoms.

Explanation. DDT is not degraded or excreted, so its concentration increases at each trophic level (biomagnification). Fish-eating birds such as seagulls, at the top of the aquatic food chain, have the highest concentration.

Explanation. Bacteria do not form gametes; genetic material is transferred from donor to recipient cell through a conjugation tube, which is considered their mode of sexual reproduction.