CBSE Class 12 Biology Chapter 11: Biotechnology - Principles and Processes NCERT Solutions

Here are 10 recombinant proteins used in medical practice, along with their therapeutic applications:

1. Human insulin – Used to treat diabetes mellitus by regulating blood sugar levels.
2. Human growth hormone – Used to treat dwarfism and other growth disorders.
3. Blood clotting factor VIII/IX – Used to treat haemophilia, a genetic disorder causing impaired blood clotting.
4. TPA (tissue plasminogen activator) – Used to dissolve blood clots in heart attack and stroke patients, restoring blood flow.
5. PDGF (platelet derived growth factor) – Stimulates wound healing and tissue repair.
6. Interferon – Used to treat viral infections and certain types of cancer by boosting the immune system.
7. Erythropoietin (EPO) – Stimulates red blood cell production, used to treat anaemia, especially in kidney disease patients.
8. Hepatitis B vaccine – A recombinant vaccine used for the prevention of Hepatitis B infection.
9. Herpes Vaccine – Used for the prevention of infections caused by the Herpes simplex virus.
10. DNase I – Used in the treatment of cystic fibrosis to break down DNA in mucus, making it easier to clear.

Here is a representation of a restriction enzyme, its substrate DNA, the cutting site, and the resulting products:

Name of the Restriction Enzyme: Bam HI

Substrate DNA: A DNA molecule containing the recognition sequence for Bam HI.

Recognition Site (The site at which it cuts DNA):

The recognition site for Bam HI is a palindromic sequence: 5'—GGATCC—3'

The enzyme cuts between the G and A on both strands:

5'—G | GATCC—3'

3'—CCTAG | G—5'

Product it produces:

Bam HI produces DNA fragments with 'sticky ends'. These are short, single-stranded overhangs that are complementary to each other and can easily anneal with other DNA fragments cut by the same enzyme.

The resulting fragments will have the following structure:

5'—G—3'

3'—CCTAGG—5' and 5'—GATCC—3'

3'—G—5'

(Diagrammatic representation would show the DNA double helix with the enzyme approaching the recognition site, the cut being made, and the two resulting fragments with their sticky ends.)

Enzymes are generally bigger than DNA in molecular size. This is because enzymes are proteins, and proteins are macromolecules composed of amino acids. DNA, while a large molecule, is a polymer of nucleotides. Typically, a single protein molecule (enzyme) is made up of many amino acids, and the molecular weight of amino acids is generally larger than that of nucleotides. Furthermore, a single DNA molecule in a cell can contain millions of nucleotides, but the enzymes that act on it, like restriction enzymes or polymerases, are individual protein molecules. The principle of gel electrophoresis also supports this: while small DNA fragments move easily through the gel matrix, larger molecules like proteins (if denatured) would move differently or not at all, indicating size differences.

The molar concentration of DNA in a human cell is a complex value that depends on various factors, including the cell type and its ploidy. However, a commonly cited approximate value for the DNA content in a diploid human cell nucleus is around 6 picograms (pg). To convert this mass to molar concentration, we need to consider the average molecular weight of a base pair. The average molecular weight of a DNA base pair is approximately 660 Daltons (Da). A human diploid cell contains approximately 6.6 x 10^9 base pairs. Therefore, the total mass of DNA is roughly 6.6 x 10^9 bp * 660 Da/bp ≈ 4.36 x 10^12 Da. Since 1 Da ≈ 1.66 x 10^-24 g, the mass is approximately 7.2 x 10^-12 g or 7.2 pg. If we consider the volume of a typical human cell nucleus (e.g., ~10 micrometers in diameter, volume ~524 µm³), the concentration can be calculated. However, the source text provides a simplified value: 2 mg/ml of cell extract. This value is a practical measure often used in laboratory contexts rather than a precise molar concentration within the nucleus itself. For a precise molar concentration, one would need to know the exact volume of the nucleus and the total moles of DNA, which is highly variable.

No, eukaryotic cells do not naturally possess restriction endonucleases. Restriction endonucleases are enzymes that cut DNA at specific recognition sequences. These enzymes are primarily found in bacteria, where they function as a defense mechanism against invading viral DNA (bacteriophages). Bacteria modify their own DNA by methylation at the recognition sites, preventing the restriction enzymes from cutting their own genome. Eukaryotic cells have a different strategy for DNA protection and modification. While they possess various nucleases, they lack the specific type of restriction endonucleases found in prokaryotes. All known restriction endonucleases have been isolated from various strains of bacteria.

Stirred tank bioreactors offer several significant advantages over shake flasks for large-scale biotechnological production:

• Controlled Environment: Bioreactors are equipped with sophisticated systems to precisely control parameters like temperature, pH, oxygen levels, and foam. This ensures optimal growth conditions for the microorganisms or cells being cultured.
• Sampling Ports: They have ports that allow for the periodic withdrawal of small samples of the culture for monitoring growth, product formation, and other critical parameters without contaminating the entire culture.
• Foam Control: Bioreactors often include systems to control foam, which can be a problem in vigorously aerated cultures and can interfere with gas exchange and lead to contamination.
• Sterility Maintenance: The closed system of a bioreactor helps maintain sterility, reducing the risk of contamination by unwanted microorganisms.
• Scalability: Bioreactors are designed for large-scale production, allowing for the efficient cultivation of large volumes of cells or microorganisms needed for industrial applications. Shake flasks are typically limited to small-scale laboratory use.
• Efficient Mixing and Oxygen Availability: While aeration and mixing are mentioned, the stirred mechanism ensures that oxygen and nutrients are distributed evenly throughout the entire volume of the culture, maximizing cell growth and productivity.

A palindromic DNA sequence is a sequence that reads the same forwards and backwards on the complementary strands. This means the sequence on the 5' to 3' direction of one strand is identical to the sequence on the 5' to 3' direction of the complementary strand when read in the opposite direction.

Here are five examples of palindromic DNA sequences:

1. Sequence: 5'—GGATCC—3' Complementary Strand: 3'—CCTAGG—5' (Reads as GGATCC forwards and backwards)
2. Sequence: 5'—AAGCTT—3' Complementary Strand: 3'—TTCGAA—5' (Reads as AAGCTT forwards and backwards)
3. Sequence: 5'—ACGCGT—3' Complementary Strand: 3'—TGCGC A—5' (Reads as ACGCGT forwards and backwards)
4. Sequence: 5'—ACTAGT—3' Complementary Strand: 3'—TGATCA—5' (Reads as ACTAGT forwards and backwards)
5. Sequence: 5'—AGGCCT—3' Complementary Strand: 3'—TCCGGA—5' (Reads as AGGCCT forwards and backwards)

To create a palindromic sequence, you can write a sequence (e.g., 5'—XYZ—3') and then write its complementary sequence (3'—X'Y'Z'—5') and reverse the complementary sequence to read it from 5' to 3'. If the original sequence matches the reversed complementary sequence, it's a palindrome.

Recombinant DNA, in the context of genetic recombination during meiosis, is primarily made during the **Pachytene** stage of Meiosis I. During this stage, homologous chromosomes synapse to form bivalents, and the synaptonemal complex is fully formed. It is at this point that crossing over occurs between non-sister chromatids. Crossing over involves the breakage and rejoining of DNA segments between homologous chromosomes, leading to the exchange of genetic material and the formation of new combinations of alleles on the chromosomes. These exchanged segments result in the creation of recombinant DNA molecules.

A reporter enzyme provides a visual or easily detectable signal that indicates whether a host cell has successfully incorporated foreign DNA. While a selectable marker (like an antibiotic resistance gene) allows for the selection of transformed cells from a mixed population, a reporter enzyme can further differentiate between transformed cells and even indicate the activity of the foreign gene or the process of transformation itself.

For example, the $\beta$-galactosidase (LacZ) gene is often used as a reporter. If a foreign DNA fragment is inserted into the LacZ gene within a plasmid vector, it disrupts the gene's function. When the transformed cells are grown on a medium containing a substrate for $\beta$-galactosidase (like X-gal), cells that have the intact LacZ gene (non-transformed or cells where insertion did not occur in LacZ) will produce the enzyme and turn blue. Cells that have been transformed and have the foreign DNA inserted into the LacZ gene will not produce functional $\beta$-galactosidase and will remain white. This allows for easy identification of successfully transformed colonies.

This method complements selectable markers by providing a direct visual confirmation of successful gene insertion and expression, helping researchers distinguish between cells that merely survived selection and those that truly contain the desired recombinant DNA construct.

(a) Origin of Replication (ori):

The origin of replication is a specific DNA sequence on a plasmid or other vector where DNA replication begins. Any piece of DNA, when linked to this sequence, can be replicated within a host cell. The 'ori' sequence is crucial because it controls the number of copies of the linked DNA that are made within the host cell. Vectors designed for high copy numbers utilize origins that support rapid and numerous replications, allowing for the recovery of many copies of the target DNA. It ensures that the foreign DNA is faithfully copied along with the host cell's own DNA during cell division.

(b) Bioreactors:

Bioreactors are large vessels designed for the biological conversion of raw materials into specific products using microorganisms, plant cells, animal cells, or their enzymes. They provide a controlled environment that optimizes conditions for growth and product formation. The most common type is the stirred tank bioreactor, which is typically a cylindrical vessel with a curved base. It features systems for agitation (stirring), oxygen delivery (sparging), foam control, temperature control, and pH monitoring. These features ensure efficient mixing, uniform oxygen availability, and maintenance of optimal growth parameters, facilitating large-scale production of biotechnological products like vaccines, enzymes, and therapeutic proteins.

(c) Downstream Processing:

Downstream processing refers to the series of operations required to recover and purify a product (like a therapeutic protein or enzyme) from a biological culture after it has been produced in a bioreactor. This typically involves several steps, including separation (e.g., centrifugation, filtration to remove cells or debris) and purification (e.g., chromatography to isolate the target molecule from other cellular components). After purification, the product is often formulated with suitable preservatives and stabilizers. If the product is a drug, it must undergo rigorous clinical trials to ensure its safety and efficacy before it can be marketed.

(a) PCR (Polymerase Chain Reaction):

PCR is a powerful in vitro technique used to amplify a specific segment of DNA exponentially. It does not require a living organism for replication. The process involves three main steps repeated cyclically:

1. Denaturation: The double-stranded DNA template is heated to a high temperature (around 94-98°C) to separate the two strands into single strands.
2. Annealing: The temperature is lowered (around 50-65°C) to allow short synthetic DNA sequences called primers to bind (anneal) to complementary regions on the single-stranded DNA templates.
3. Extension: The temperature is raised to an optimal temperature for a thermostable DNA polymerase (like Taq polymerase, isolated from Thermus aquaticus) to synthesize new DNA strands, extending from the primers.

PCR is widely used for amplifying desired genes for cloning, genetic testing, forensic analysis, and diagnostics. Its advantages include high output, greater efficiency, reduced error rates, minimal human interference, and its automated and cyclic nature.

(b) Restriction enzymes and DNA:

Restriction enzymes are a class of enzymes, primarily found in bacteria, that act as 'molecular scissors' to cut DNA strands. They recognize specific short DNA sequences, known as recognition sites, which are often palindromic. Upon binding to the recognition site, the enzyme cleaves the DNA backbone. Restriction enzymes function as endonucleases, meaning they cut within the DNA molecule, rather than at the ends (exonucleases). They play a crucial role in genetic engineering by allowing scientists to isolate specific genes or DNA fragments. Bacteria use them as a defense mechanism to degrade foreign DNA, such as that from viruses, by cutting it at various recognition sites. The cleavage often results in 'sticky ends' (short overhangs) or 'blunt ends', depending on the enzyme. It is believed that bacteria evolved these enzymes to resist viral attacks and help remove viral DNA sequences.

(c) Chitinase:

Chitinase is an enzyme that catalyzes the hydrolysis of chitin, a long-chain polymer of N-acetylglucosamine. While not directly involved in the core processes of recombinant DNA technology like restriction enzymes or PCR, chitinase can be used in specific biotechnological applications. For instance, in the process of isolating plasmid DNA from bacterial cells, the bacterial cell wall needs to be broken down. If the bacteria have chitin in their cell walls (though less common than peptidoglycan), chitinase could potentially be used. More relevantly, chitinase is sometimes used in the preparation of protoplasts from fungal cells or in the study of plant defense mechanisms, as chitin is a component of fungal cell walls and insect exoskeletons. In some contexts, it might be used in downstream processing or in specific genetic engineering applications involving organisms with chitinous structures.