Microsporogenesis is the process of formation of microspores (pollen grains) from a microspore mother cell (pollen mother cell) through meiosis; each diploid mother cell gives four haploid microspores arranged in a tetrad. A mature pollen grain has a two-layered wall: an outer hard exine made of sporopollenin (very resistant) with germ pores, and an inner thin intine of cellulose and pectin. Inside, a mature pollen grain is two-celled, having a larger vegetative cell (with food and a tube nucleus) and a smaller generative cell that later forms two male gametes.

Double fertilization is a characteristic feature of flowering plants in which two fusion events take place inside the embryo sac using the two male gametes from one pollen tube. One male gamete fuses with the egg cell to form a diploid zygote; this is called syngamy (true fertilization). The other male gamete fuses with the two polar nuclei of the central cell to form a triploid (3n) primary endosperm nucleus; this is called triple fusion. Since two fusions (syngamy and triple fusion) occur, the process is called double fertilization.

Spermatogenesis is the process of formation of sperms (spermatozoa) from the germ cells in the testes, which begins at puberty. The diploid spermatogonia multiply by mitosis and some enlarge to form primary spermatocytes (2n). Each primary spermatocyte undergoes the first meiotic division to form two haploid secondary spermatocytes, which then undergo the second meiotic division to form four haploid spermatids. The spermatids are transformed into mature sperms by a process called spermiogenesis. It is stimulated by the hormones FSH and testosterone.

The menstrual cycle is the cyclic change in the reproductive system of a human female, of about 28 days. Its phases are: (1) Menstrual phase (days 1-5) - the uterine lining breaks down and is shed as menstrual flow. (2) Follicular (proliferative) phase (days 6-13) - under FSH the ovarian follicle matures and the uterine lining is rebuilt, and oestrogen rises. (3) Ovulation (about day 14) - a surge of LH causes the release of the ovum from the ovary. (4) Luteal (secretory) phase (days 15-28) - the corpus luteum secretes progesterone, which maintains the uterine lining; if fertilisation does not occur, it degenerates and the next cycle begins.

In a monohybrid cross, a pure tall plant (TT) is crossed with a pure dwarf plant (tt). All the F1 plants are Tt and are tall, because T (tall) is dominant over t (dwarf). When the F1 plants (Tt) are self-pollinated, the gametes T and t combine in all possible ways to give the F2 generation: TT, Tt, Tt and tt. This gives a phenotypic ratio of 3 tall : 1 dwarf and a genotypic ratio of 1 TT : 2 Tt : 1 tt.

In a dihybrid cross, two pairs of contrasting characters are studied together (for example, seed shape and seed colour in peas: round yellow RRYY crossed with wrinkled green rryy). All F1 plants are round and yellow (RrYy). When the F1 are self-pollinated, the F2 generation shows four phenotypes in the ratio 9 (round yellow) : 3 (round green) : 3 (wrinkled yellow) : 1 (wrinkled green). This 9 : 3 : 3 : 1 ratio illustrates Mendel's law of independent assortment.

F2 dihybrid ratio = 9 : 3 : 3 : 1

Sex-linked disorders are genetic disorders caused by genes located on the sex chromosomes (usually the X chromosome), so their inheritance is linked to the sex of the individual. Haemophilia is an X-linked recessive disorder in which the blood does not clot properly (a clotting factor is missing), so there is prolonged bleeding even from minor injuries. Colour blindness (red-green) is also an X-linked recessive disorder in which a person cannot distinguish between red and green colours. Because these genes are on the X chromosome, such disorders are more common in males (who have only one X chromosome).

Semiconservative replication is the method of DNA replication in which each of the two strands of the parent DNA acts as a template, and the newly formed DNA molecule contains one old (parental) strand and one newly synthesised strand. During replication, the enzyme helicase unwinds and separates the two strands; then the enzyme DNA polymerase adds new nucleotides to each template strand following complementary base pairing, forming two identical daughter DNA molecules. This mode was experimentally proved by Meselson and Stahl.

Transcription is the process of copying the genetic information from one strand of DNA into a molecule of messenger RNA (mRNA). Only one strand of the DNA (the template strand) is copied. The enzyme RNA polymerase binds to the promoter region and moves along the template, adding ribonucleotides according to complementary base pairing (A of DNA pairs with U of RNA, and so on), forming the RNA. The three steps are initiation, elongation and termination. In eukaryotes the RNA is then processed (splicing) before it leaves the nucleus.

Translation is the process of synthesis of a protein (polypeptide) from the information present in the mRNA. It takes place on the ribosomes. The mRNA attaches to a ribosome, and transfer RNA (tRNA) molecules bring specific amino acids according to the codons on the mRNA (each tRNA has an anticodon complementary to a codon). The amino acids are joined one by one by peptide bonds as the ribosome moves along the mRNA (initiation, elongation and termination), forming a polypeptide chain. Translation stops when a stop codon is reached.

The Hardy-Weinberg principle states that in a large, randomly mating population, the allele frequencies and genotype frequencies remain constant from generation to generation, provided there are no disturbing factors; such a population is said to be in genetic equilibrium. If p is the frequency of one allele and q the frequency of the other (with p + q = 1), then the genotype frequencies are given by the equation p^2 + 2pq + q^2 = 1, where p^2 and q^2 are the frequencies of the two homozygotes and 2pq is the frequency of the heterozygote.

p^2 + 2pq + q^2 = 1

Immunity is the ability of the body to resist and fight against disease-causing organisms (pathogens) and their harmful products. Innate (natural or non-specific) immunity is present from birth and provides a general, non-specific defence through barriers such as the skin, mucous membranes, phagocytic cells and inflammation. Acquired (adaptive or specific) immunity develops during a person's lifetime after exposure to a specific pathogen; it is specific to that pathogen, involves antibodies and memory cells, and includes active immunity (produced by one's own body after infection or vaccination) and passive immunity (ready-made antibodies received from outside).

Sewage (the waste water from towns and cities) is treated in sewage treatment plants (STPs) mainly with the help of heterotrophic microbes. Primary treatment is a physical process in which floating and suspended solids are removed by sequential filtration and sedimentation, forming primary sludge; the liquid part is the primary effluent. Secondary (biological) treatment is where aerobic microbes are grown as flocs and consume the organic matter in the effluent, greatly reducing its BOD (biochemical oxygen demand); the microbial mass then settles as sludge, part of which is digested by anaerobic bacteria to produce biogas.

The main steps are: (1) isolation of the desired DNA (gene) and of the vector DNA; (2) cutting both the desired DNA and the vector with the same restriction enzyme to get compatible (sticky) ends; (3) joining (ligation) of the desired DNA into the vector using DNA ligase to form the recombinant DNA; (4) introduction (transformation) of the recombinant DNA into a suitable host cell; and (5) selection and multiplication of the transformed host cells so that the gene is expressed and the desired product is obtained (using a bioreactor and downstream processing).

The main population interactions between two species are: (1) Mutualism - both species benefit (for example, lichens, and pollination of flowers by insects). (2) Competition - both species are harmed as they compete for the same resources. (3) Predation - one species (predator) kills and eats the other (prey), benefiting the predator. (4) Parasitism - one species (parasite) benefits at the expense of the other (host), which is harmed. (5) Commensalism - one species benefits while the other is neither harmed nor benefited (for example, an orchid growing on a tree).

The flow of energy in an ecosystem is unidirectional: it starts from the sun, is trapped by producers (green plants) during photosynthesis, and then passes from the producers to herbivores (primary consumers) and then to carnivores (higher consumers) through the food chain. At each transfer, most of the energy is lost as heat in respiration and other activities. According to Lindeman's ten per cent law, only about 10 per cent of the energy present at one trophic level is transferred to and stored at the next higher trophic level; the remaining 90 per cent is lost. This is why food chains usually have only 3 to 5 trophic levels.

only about 10 percent energy transferred per level

In-situ (on-site) conservation is the conservation of species in their natural habitat, where the whole ecosystem is protected so that the species continue to live and evolve in their natural surroundings; examples include national parks, wildlife sanctuaries, biosphere reserves and sacred groves. Ex-situ (off-site) conservation is the conservation of species outside their natural habitat, in specially protected places, and is used especially for threatened species; examples include zoological parks (zoos), botanical gardens, seed banks, gene banks and cryopreservation.