Answer The best part of the plant for making virus-free plants is the meristem tissue, specifically the apical meristem. The apical meristem is located at the tips of roots and shoots and is responsible for the growth of the plant.

There are several reasons why the apical meristem is ideal for producing virus-free plants:

1. Low Virus Concentration: The apical meristem often has a lower concentration of viruses compared to other parts of the plant. This is because viruses typically spread more slowly to these rapidly dividing cells.

2. Rapid Cell Division: The cells in the meristem are rapidly dividing, which can outpace the replication of the virus, making it easier to obtain virus-free tissue.

3. Totipotency: Meristematic cells are totipotent, meaning they have the potential to develop into any type of plant cell. This allows for the regeneration of a whole plant from a small piece of meristem tissue.

4. Regeneration Capability: Techniques such as meristem culture and micropropagation can be used to regenerate whole plants from meristem tissue. These regenerated plants are often free from the viruses that infected the parent plant.

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Advantages of producing plants by micropropagation:

• The number of plants can be obtained in very less time and in less space.
• Plants can be obtained throughout the year.
• Sterile plants can be multiplied by this method.
• An economical and easy method of plant propagation.

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In vitro propagation of an explant typically involves a culture medium that contains several key components:

1. Macronutrients: Essential elements like nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur.
2. Micronutrients: Trace elements such as iron, manganese, zinc, copper, molybdenum, boron, and cobalt.
3. Vitamins: Essential vitamins like thiamine (B1), nicotinic acid (B3), pyridoxine (B6), and others.
4. Carbon Source: Usually sucrose or glucose to provide energy.
5. Plant Growth Regulators: Hormones such as auxins, cytokinins, and gibberellins to regulate growth and development.
6. Agar or Gelrite: A gelling agent to solidify the medium.
7. Water: Sterile, distilled water to dissolve all components and support plant tissue hydration.

These components create an optimal environment for the growth and development of plant tissues in vitro.

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toxin is inactive:

In bacteria, the toxin is present in an inactive form, called prototoxin, which gets converted into active form when it enters the body of an insect.

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Transgenic bacteria contain foreign gene that is intentionally introduced into its genome. They are manipulated to express the desirable gene for the production of various commercially important products.

An example of transgenic bacteria is E.coli. In the plasmid of E.coli, the two DNA sequences corresponding to A and $B$ chain of human insulin are inserted, so as to produce the respective human insulin chains. Hence, after the insertion of insulin gene into the bacterium, it becomes transgenic and starts producing chains of human insulin. Later on, these chains are extracted from E.coli and combined to form human insulin.

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The production of genetically modified (GM) or transgenic plants has several advantages.

(i) Most of the GM crops have been developed for pest resistance, which increases the crop productivity and therefore, reduces the reliance on chemical pesticides.

(ii) Many varieties of GM food crops have been developed, which have enhanced nutritional quality. For example, golden rice is a transgenic variety in rice, which is rich in vitamin $A$.

(iii) These plants prevent the loss of fertility of soil by increasing the efficiency of mineral usage.

(iv) They are highly tolerant to unfavourable abiotic conditions.

(v) The use of GM crops decreases the post harvesting loss of crops.

However, there are certain controversies regarding the use of genetically modified crops around the world. The use of these crops can affect the native biodiversity in an area. For example, the use of $\mathrm{Bt}$ toxin to decrease the amount of pesticide is posing a threat for beneficial insect pollinators such as honey bee. If the gene expressed for Bt toxin gets expressed in the pollen, then the honey bee might be affected. As a result, the process of pollination by honey bees would be affected. Also, genetically modified crops are affecting human health. They supply allergens and certain antibiotic resistance markers in the body. Also, they can cause genetic pollution in the wild relatives of the crop plants. Hence, it is affecting our natural environment.

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Cry proteins are encoded by cry genes. These proteins are toxins, which are produced by Bacillus thuringiensis bacteria. This bacterium contains these proteins in their inactive from. When the inactive toxin protein is ingested by the insect, it gets activated by the alkaline $\mathrm{pH}$ of the gut. This results in the lysis of epithelial cell and eventually the death of the insect. Therefore, man has exploited this protein to develop certain transgenic crops with insect resistance such as Bt cotton, Bt corn, etc.

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Gene therapy is a technique for correcting a defective gene through gene manipulation. It involves the delivery of a normal gene into the individual to replace the defective gene, for example, the introduction of gene for adenosine deaminase (ADA) in ADA deficient individual. The adenosine deaminase enzyme is important for the normal functioning of the immune system. The individual suffering from this disorder can be cured by transplantation of bone marrow cells. The first step involves the extraction of lymphocyte from the patient’s bone marrow. Then, a functional gene for ADA is introduced into lymphocytes with the help of retrovirus. These treated lymphocytes containing ADA gene are then introduced into the patient’s bone marrow. Thus, the gene gets activated producing functional Tlymphocytes and activating the patient’s immune system.

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DNA cloning is a method of producing multiple identical copies of specific template DNA. It involves the use of a vector to carry the specific foreign DNA fragment into the host cell. The mechanism of cloning and transfer of gene for growth hormone into E.coli is represented below.

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Recombinant DNA technology (rDNA) is a technique used for manipulating the genetic material of an organism to obtain the desired result. For example, this technology is used for removing oil from seeds. The constituents of oil are glycerol and fatty acids. Using rDNA, one can obtain oilless seeds by preventing the synthesis of either glycerol or fatty acids. This is done by removing the specific gene responsible for the synthesis.

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Golden rice is a genetically modified variety of rice, Oryza sativa, which has been developedas a fortified food for areas where there is a shortage of dietary vitamin A. It contains a precursor of pro-vitamin A, called beta-carotene, which has been introduced into the rice through genetic engineering. The rice plant naturally produces beta-carotene pigment in its leaves. However, it is absent in the endosperm of the seed. This is because beta-carotene pigment helps in the process of photosynthesis while photosynthesis does not occur in endosperm. Since beta-carotene is a precursor of pro-vitamin $\mathrm{A}$, it is introduced into the rice variety to fulfill the shortage of dietary vitamin $\mathrm{A}$. It is simple and a less expensive alternative to vitamin supplements. However, this variety of rice has faced a significant opposition from environment activists. Therefore, they are still not available in market for human consumption.

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No, human blood does not include the enzymes, nucleases and proteases. In human beings, blood serum contains different types of protease inhibitors, which protect the blood proteins from being broken down by the action of proteases. The enzyme, nucleases, catalyses the hydrolysis of nucleic acids that is absent in blood.