(3)

Bt toxins are insect-group specific proteins encoded by a gene cryIAc named cry. Bt toxin proteins exist as inactive protoxins inside bacteria, but once an insect ingests it, the inactive toxin gets converted into an active form in the alkaline pH of the alimentary canal that solubilizes the crystals. This activated toxin binds to the surface of midgut epithelial cells and creates pores which cause cell swelling and lysis, and finally cause the death of the insect.

(2)

Transposons or mobile genetic elements are used in gene silencing for the source of the complementary RNA.

(1)

Bt cotton is resistant to cotton bollworm infestation. The genes cry I Ac and cry IIAb control cotton bollworms, thus acting as bio-pesticide.

(4)

Some strains of Bacillus thuringiensis produce proteins that kill certain insects such as lepidopterans and dipterans. B. thuringiensis forms protein crystals during a particular phase of their growth. These crystals contain a toxic insecticidal protein which exists as inactive protoxins, but once an insect ingests the inactive toxin, it is converted into an active form of toxin due to the alkaline pH of the gut, which solubilizes the crystals.

(2)

Rice is a staple food in many countries, particularly in Asia, but does not contain vitamin A or its immediate precursors. By inserting two genes from daffodil and one gene from a bacterial species into rice plants, Swiss researchers have produced rice capable of synthesizing $\mathrm{\beta }$-carotene, the precursor of vitamin A. Vitamin A is required by all individuals as it is present in retina of eyes. Deficiency of vitamin A causes night blindness and skin disorders. This rice is called ‘Golden rice’ because of the yellow color of rice grains due to the presence of $\mathrm{\beta }$-carotene.

(1)

(2)

Meloidogyne incognita is a nematode which infects the roots of the tobacco plants and causes a great reduction in the yield.

(2)

Golden rice is a transgenic variety of rice (Oryza sativa) which contains good quantities of $\mathrm{\beta }$-carotene (provitamin A – inactive state of vitamin A). $\mathrm{\beta }$-carotene is a principal source of vitamin A. Since the grains of this rice are yellow in color due to $\mathrm{\beta }$-carotene, it is commonly called golden rice.

(3)

Soil bacterium Bacillus thuringiensis produces proteins that kill certain insects like lepidopterans (tobacco budworm, armyworm), coleopterans (beetles), and dipterans (flies, mosquitoes). B. thuringiensis forms some protein crystals. These crystals contain a toxic insecticidal protein. This toxin does not kill the Bacillus (bacterium) because it exists as inactive protoxins in them. But once an insect ingests it, it is converted into an active form of toxin due to the alkaline pH of the alimentary canal. The activated toxin binds to the surface of midgut epithelial cells and creates pores that cause swelling and lysis, and finally cause death of the insect.

(2)

Glyphosate is a broad spectrum herbicide which especially kills broad-leaved herbs. Crop plants may also get affected by the herbicide; thus, now crop plants are genetically engineered for glyphosate resistance. So, when glyphosate herbicide is applied, only weeds and no crop plants get harmed.

(1)

Totipotency is the capacity to generate a whole plant from any cell or explant. Any part of a plant can be taken out and grown in a test tube under sterile conditions in a special nutrient medium to regenerate a whole plant by virtue of totipotency.

(3)

Somatic hybridisation involves fusion of protoplasts obtained from two different varieties after digesting their cell walls. These hybrids are called somatic hybrids e.g., pomato is a somatic hybrid of tomato and potato.

(3)

(2)

A protoplast is a cell which has its cell wall removed by mechanical or enzymatic means.

(4)

Micropropagation is the latest method of obtaining a large number of plantlets from plant tissue culture. It is called micropropagation because of the minute size of the propagules. It involves repeated subculture of the explant by changing the medium so as to form a large number of plantlets from that single explant. Somatic embryogenesis i.e., developing embryos from somatic cells is one of the techniques of micropropagation.

(4)

When a hybrid is produced by fusion of somatic cells of two varieties or species, it is known as somatic hybrid. The process of producing somatic hybrids is called somatic hybridization. First, the cell wall of the plant cells are removed then the protoplasts of the two cells are brought in contact and made to fuse by means of electrofusion or chemicals like polyethylene glycol (PEG) and sodium nitrate. The fused protoplasts soon develop their own walls and are called somatic hybrids.

(3)

Meristem is a localized group of cells, which are actively dividing and undifferentiated but ultimately giving rise to permanent tissue. Although the plant is infected with a virus, yet the meristem is free of virus. Therefore, meristem can be removed and grown in vitro to obtain virus free plants. Cultivation of axillary or apical shoot meristems is called meristem culture. The apical or axillary meristems are generally free from virus.

(1)

RNAi (RNA interference) takes place in all eukaryotic organisms as a method of cellular defense. This method involves silencing of a specific mRNA due to a complementary dsRNA molecule that binds to and prevents translation of the mRNA.

(4)

Both transfer RNAs and ribosomal RNA interact with mRNA.

RNA interference (RNAi) takes place in all eukaryotic organisms as a method of cellular defence.