CBSE Class 12 Biotechnology — NCERT Solutions

Recombinant DNA technology was developed when scientists understood that DNA is the genetic material and learned how to isolate, cut, and join DNA molecules. The important steps were:

• discovery of the double-helical structure of DNA;
• understanding of replication, transcription, and translation;
• development of methods to isolate and purify DNA;
• discovery of enzymes such as restriction enzymes that cut DNA and ligase that joins DNA fragments.

At the same time, scientists found that foreign DNA fragments can enter bacteria and get integrated into their genome. Stanley Cohen had the skill to introduce plasmid DNA into E. coli and clone it, while Herbert Boyer could cut DNA using restriction enzymes. By combining these discoveries, they created the first recombinant DNA and started rDNA technology.

In crop improvement, rDNA technology is used to produce genetically modified crops with useful traits such as:

• disease resistance,
• drought resistance,
• salinity tolerance,
• improved quality of production and value of products.

These help farmers by reducing crop loss and allowing cultivation in adverse conditions.

In therapeutics, rDNA technology is used to produce important human proteins and medicines in bacteria or other systems, such as:

• human insulin for diabetes,
• growth hormone for dwarfism,
• interferon for cancer treatment,
• plasminogen activator and urokinase to dissolve blood clots.

These products are cheaper, available in larger quantity, and avoid some problems of animal-derived products.

The host–vector system is essential in rDNA technology because gene cloning works only when the insert DNA is carried by a suitable vector into a compatible host. The vector supplies the DNA sequences needed for replication in the host, while the host supplies the enzymes and proteins required for copying the recombinant DNA. After introduction, the recombinant DNA is propagated and may also be expressed using the host’s genetic machinery. So, gene cloning is basically a two-component system of host + vector.

A good vector must have these major characteristics:

1. It should be small in size so it can be easily introduced into the host and should accommodate a fairly large insert.
2. It must have an origin of replication (ori) so it can replicate autonomously inside the host.
3. It should contain unique restriction sites so the foreign DNA can be inserted at a specific place.
4. It must have at least one selectable marker to screen transformants, such as amp$^R$ or tet$^R$.

These features make a DNA molecule useful for cloning.

Isolation of DNA involves four main steps:

1. Cell disruption — the cells or tissues are broken open to release DNA. This may be done mechanically or enzymatically.
2. Protection from nucleases — the extracted material is protected from degrading enzymes like DNases by using a suitable buffer containing EDTA and mild alkaline conditions.
3. Separation of DNA from other molecules — proteins and other cell components are removed by treating with detergents like SDS, then chloroform-isoamyl alcohol is used for deproteinisation.
4. Precipitation and concentration — DNA in the aqueous phase is precipitated by adding ethanol or isopropanol, then collected by centrifugation.

Finally, RNase A may be added to remove RNA, and the DNA is stored at low temperature.

A biological detergent helps in lysis of the cell membrane or cell wall so that nucleic acids are released into the extraction medium. For example, SDS disrupts the lipid bilayer of membranes, and in plants CTAB also helps in breaking the tough cell wall. In addition, detergents like SDS help denature proteins and prevent nuclease activity, protecting the nucleic acid.

DNA fingerprinting is a technique used to identify an individual by studying the unique patterns of DNA variations inherited in the genome. It is based mainly on VNTRs (variable number tandem repeats), which are highly polymorphic and inherited from parents.

It can be explained through RFLP (Restriction Fragment Length Polymorphism) as follows:

1. DNA is isolated from the sample.
2. The DNA is cut with restriction enzymes at specific sites, producing fragments of different lengths.
3. These fragments are separated by agarose gel electrophoresis.
4. The DNA is transferred to a nylon/nitrocellulose membrane by Southern blotting.
5. A labelled VNTR probe is added and hybridisation occurs with complementary VNTR sequences.
6. The hybridised fragments are detected as band patterns.
7. The band pattern is specific to each individual and is used for forensics and paternity testing.

Thus, in RFLP, the different fragment lengths produced after restriction digestion help reveal the individual-specific DNA profile.

A genome is the complete genetic information present in a cell of an organism.

In prokaryotes, the genome includes the DNA present in the nucleoid region and plasmids. In eukaryotes, the genome includes the DNA present in the chromosomes of the nucleus as well as in organelles such as mitochondria and plastids/chloroplasts.

So, the main difference is that prokaryotic genome is usually simpler and includes nucleoid DNA plus plasmids, while eukaryotic genome is distributed in the nucleus and organelles.

Microorganisms need nutrients for energy production, growth, and multiplication. The main requirements are:

• Macronutrients in large amounts: carbon, oxygen, hydrogen, nitrogen, sulfur, phosphorus, potassium, calcium, magnesium, and iron.
• Carbon, oxygen, hydrogen, nitrogen, sulfur and phosphorus are needed for the synthesis of carbohydrates, lipids, proteins, and nucleic acids.
• K, Ca, Mg, and Fe act in various roles, especially as enzyme cofactors.
• Micronutrients/trace elements in small amounts: Mn, Zn, Co, Mo, Ni, and Cu.
• Growth factors such as certain amino acids, purines, pyrimidines, and vitamins are needed when the organism cannot synthesise them.
• Water is essential as the base of the culture medium.
• Some media also include anti-foaming agents like olive oil, sunflower oil, and silicones, though these are not nutrients.

The exact nutritional requirement depends on the species and its natural habitat.

Plant tissue culture (PTC) means growing plant cells, tissues or organs outside the plant body on an artificial nutrient medium under sterile and controlled conditions. It uses the property of totipotency, by which a plant cell can divide, differentiate, and regenerate into a whole plant.

Animal cell culture is the in vitro maintenance and proliferation of animal cells outside the living organism, provided they are supplied with suitable nutrients and growth conditions. It involves isolating cells from animal tissue and growing them in a suitable culture medium under laboratory conditions.

Stem cells are non-specialised, immature cells with two key properties: self-renewal and potency. They can divide repeatedly by mitosis to produce more stem cells, and they can also differentiate into specialised cell types. In humans, they are found in the umbilical cord, placenta, inner cell mass of the early embryo, foetal tissues, and some adult organs. They are useful in developing new cells, studying diseases, and testing drugs.

Primary metabolites are compounds produced directly from primary metabolic pathways and are essential for growth and development. Examples include amino acids, organic acids, and ethanol.

Secondary metabolites are produced by secondary metabolic pathways and are not directly required for basic growth, but they have important functions such as defence against pathogens and herbivores, tolerance to abiotic stress, and attraction of insects/animals for pollination or seed dispersal. Examples include antibiotics like penicillin, and plant products like taxol and shikonin.

Sewage water treatment uses a variety of microorganisms present in sewage, such as bacteria, fungi, protozoa, algae, nematodes, amoebae and viruses. The important bacteria mentioned are coliforms, streptococci, clostridia, micrococci, Proteus, Pseudomonas and lactobacilli.

Their role is to break down and stabilise organic matter in sewage during biological treatment. In the activated sludge process, aerobic microbes oxidise organic matter to CO₂ and H₂O and mineralise complex substances into inorganic forms. In trickling filters and oxidation ponds, microbes digest the organic load and reduce BOD. Thus, microorganisms help in degradation of sewage pollutants and treatment of wastewater before discharge.

Advantages of biodiesel:

• It contains more hydrogen and oxygen and less carbon than fossil fuels.
• This improves its combustion.
• It reduces particulate emissions from unburnt carbon.
• It can be used directly in regular diesel engines without any modification.

Entrepreneurship is the process in which an individual undertakes an activity by foreseeing a business opportunity, organises the required resources, and bears the risk involved in starting and running the enterprise.

Importance of entrepreneurship:

• It helps in capital creation by bringing together resources.
• It creates employment, which raises the standard of living and purchasing power.
• It helps in the development of the community by diversifying employment among small units.
• It is regarded as a panacea for unemployment.
• It promotes self-employment, empowerment, and social change.