NEB Class 12 Biology flashcards

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

Binary fission

Asexual reproduction in which a parent cell divides into two identical daughter cells, typical of bacteria and amoeba; no meiosis or gamete fusion is involved.

Seminiferous tubules

Coiled tubules inside the testis where spermatogenesis occurs; their walls contain germ cells and Sertoli cells that nourish developing sperm.

Leydig cells

Interstitial cells of the testis located between seminiferous tubules; they secrete testosterone under stimulation by luteinizing hormone from the pituitary.

Sertoli cells

Tall supporting cells within seminiferous tubules that nourish and protect developing sperm cells and form the blood-testis barrier.

LH surge

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

Corpus luteum

The temporary endocrine gland formed from the ruptured follicle after ovulation; it secretes progesterone, which maintains the uterine endometrium for possible implantation.

Menstruation

Shedding of the uterine endometrium that occurs when progesterone and estrogen levels fall and no embryo has implanted, marking the start of a new cycle.

Oogenesis

Formation of ova in the ovary by meiosis; one functional ovum and polar bodies result, and meiosis II is completed only after fertilization.

Fertilization site

In humans, fusion of sperm and ovum normally occurs at the ampullary-isthmic junction of the fallopian tube, not in the uterus.

Blastocyst

The hollow ball of cells formed after cleavage; its trophoblast implants in the endometrium and its inner cell mass gives rise to the embryo.

Placenta

Temporary organ formed from chorionic villi and maternal uterine tissue that exchanges nutrients, gases and wastes between mother and fetus and secretes hormones.

hCG

Human chorionic gonadotropin, secreted by the developing placenta; it maintains the corpus luteum so progesterone continues to support the pregnancy.

Law of segregation

Mendel's principle that the two alleles of a gene separate during meiosis so each gamete carries only one allele for that gene.

Independent assortment

Mendel's principle that alleles of genes on different chromosomes segregate independently during gamete formation, producing new allele combinations.

Test cross

Crossing an individual of unknown genotype with a homozygous recessive individual; appearance of recessive offspring reveals the unknown parent was heterozygous.

Incomplete dominance

A condition in which the heterozygote shows an intermediate phenotype, as when red and white snapdragons produce pink flowers.

Codominance

A condition in which both alleles are fully expressed in the heterozygote, as in AB blood group where both A and B antigens appear.

Pleiotropy

A single gene affecting several apparently unrelated phenotypic traits, for example a gene causing both skeletal and eye defects.

Polygenic inheritance

Inheritance of a trait controlled by many genes with additive effects, producing continuous variation such as human skin color and height.

Sex-linked inheritance

Transmission of genes located on sex chromosomes; X-linked recessive traits like color blindness appear more often in males than females.

Semiconservative replication

Each daughter DNA molecule retains one parental strand and one newly synthesized strand, as shown by the Meselson-Stahl experiment.

Genetic code

The triplet base sequence of mRNA specifying amino acids; it is degenerate, non-overlapping, and includes start and stop codons.

Transcription

Synthesis of an RNA molecule complementary to a DNA template strand, catalyzed by RNA polymerase in the nucleus of eukaryotic cells.

Translation

Ribosome-mediated synthesis of a polypeptide in which tRNA anticodons pair with mRNA codons to add amino acids in sequence.

Natural selection

Differential survival and reproduction of individuals with heritable traits better suited to their environment, leading to adaptation over generations.

Hardy-Weinberg equilibrium

Allele and genotype frequencies in a population remain constant without mutation, migration, selection, or genetic drift, expressed as p² + 2pq + q² = 1.

Homologous organs

Structures with the same basic origin and anatomy but different functions, such as the forelimbs of humans, whales and bats, indicating common ancestry.

Analogous organs

Structures with different origins but similar function, such as wings of insects and birds, produced by convergent evolution rather than common descent.

Speciation

Formation of new species when populations become reproductively isolated and diverge genetically, often through geographic barriers.

Apomixis

Asexual seed formation in which an embryo develops without fertilization, producing offspring genetically identical to the parent plant.

Parthenocarpy

Development of fruit without fertilization of the ovule, resulting in seedless fruits such as banana; can be induced by growth hormones.

Linkage

Tendency of genes located close together on the same chromosome to be inherited together, reducing recombination frequency between them.