Biotechnology : Principles and Processes — NCERT Solutions

Definition: A palindromic sequence in DNA is a sequence of base pairs that reads the same on both strands in the 5′→3′ direction.

5 Examples of Palindromic DNA Sequences (Recognition sites of restriction enzymes):

Creating a palindromic sequence (following base-pair rules):

Let us construct a 6-base palindrome:

• Choose the first three bases: 5′–GAA–3′
• The complementary strand (3′→5′) reads: 3′–CTT–5′, i.e., 5′–TTC–3′
• For palindrome: the second half of the top strand must be the complement of the first half read in reverse.
• Complement of GAA (read 3′→5′) = CTT, so the top strand becomes: 5′–GAATTC–3′
• Bottom strand: 3′–CTTAAG–5′, i.e., 5′–GAATTC–3′ ✓

$$5'-\text{G-A-A-T-T-C}-3'$$
$$3'-\text{C-T-T-A-A-G}-5'$$

Both strands read GAATTC in the 5′→3′ direction — confirming it is a palindrome. (This is the recognition sequence of EcoRI.)

Concept: In the context of meiosis, recombinant DNA (genetic recombination) is produced during crossing over.

Stage: Recombinant DNA is made during Prophase I of Meiosis I, specifically at the Pachytene stage.

Explanation:

1. During Prophase I, homologous chromosomes pair up in a process called synapsis, forming bivalents (tetrads).

2. At the Pachytene stage, the paired homologous chromosomes (held together by the synaptonemal complex) undergo crossing over — a process in which non-sister chromatids of homologous chromosomes exchange segments of DNA.

3. The physical exchange of DNA segments between non-sister chromatids results in recombinant chromosomes containing new combinations of alleles — this is the natural formation of recombinant DNA.

4. The points of exchange are visible as chiasmata (singular: chiasma) during the Diplotene stage of Prophase I.

Summary:
$$\text{Meiosis I} \rightarrow \text{Prophase I} \rightarrow \text{Pachytene stage} \rightarrow \text{Crossing over} \rightarrow \text{Recombinant DNA formed}$$

Conclusion: In meiosis, recombinant DNA is naturally produced during the Pachytene stage of Prophase I through the process of crossing over between non-sister chromatids of homologous chromosomes.

Concept: Reporter enzymes (reporter genes) are used to visually or biochemically identify whether a host cell has been successfully transformed with foreign DNA.

How a Reporter Enzyme Monitors Transformation:

1. Reporter gene cloning: A reporter gene encoding an easily detectable enzyme (e.g., β-galactosidase encoded by the lacZ gene, or Green Fluorescent Protein – GFP, or luciferase) is cloned into the vector along with the gene of interest.

2. **Insertion inactivation principle (example with lacZ):**

• The lacZ gene is present in the vector (e.g., pUC19 plasmid).
• The foreign DNA (gene of interest) is inserted within the lacZ gene at the multiple cloning site (MCS).
• This insertion disrupts the lacZ gene → the transformed cells cannot produce functional β-galactosidase.

3. Detection using chromogenic substrate:

• When the growth medium contains the substrate X-gal (5-bromo-4-chloro-3-indolyl-β-D-galactopyranoside):
• Non-transformed cells or cells with intact lacZ (no insert): β-galactosidase cleaves X-gal → blue colonies.
• Transformed cells with recombinant DNA (insert disrupts lacZ): no functional β-galactosidase → white colonies.

4. Blue-White Screening: This technique allows easy visual identification of recombinant (transformed) colonies — white colonies contain the foreign DNA insert.

5. In addition to selectable marker: The selectable marker (e.g., antibiotic resistance gene) confirms that the cell has taken up the vector. The reporter enzyme (*lacZ*/β-galactosidase) further confirms that the foreign DNA has been inserted into the vector (recombinant), not just the empty vector.

Conclusion: A reporter enzyme like β-galactosidase, when its gene is disrupted by insertion of foreign DNA, allows visual screening of transformed cells through blue-white colony selection, providing an additional and confirmatory method beyond the selectable marker to identify successful transformation with recombinant DNA.

(a) Origin of Replication (ori):

• The origin of replication (ori) is a specific sequence of nucleotides in a DNA molecule from which DNA replication is initiated.
• It is a necessary component of any vector (plasmid or viral) used in recombinant DNA technology.
• When a foreign DNA fragment is linked to the ori-containing vector, it can replicate within the host cell using the host's replication machinery.
• The ori sequence also controls the copy number of the plasmid — i.e., how many copies of the recombinant DNA are produced per cell.
• Example: The plasmid pBR322 contains an ori sequence that allows it to replicate autonomously in E. coli.

(b) Bioreactors:

• A bioreactor is a vessel or container in which raw materials are biologically converted into specific products using microorganisms, plant, animal, or human cells, or their enzymes, under optimally controlled conditions.
• Bioreactors provide the optimal conditions required for the desired product by controlling: temperature, pH, substrate concentration, dissolved oxygen, vitamins, and salts.
• The most commonly used type is the stirred tank bioreactor, which is cylindrical with a curved base to facilitate mixing. It has:
• A stirrer/agitator for mixing
• An oxygen delivery system
• A foam control system
• A temperature control system
• A pH control system
• Sampling ports
• Bioreactors can have volumes ranging from a few litres to $10^5$ litres, enabling large-scale production of biotechnological products such as enzymes, antibiotics, vaccines, and recombinant proteins.

(c) Downstream Processing:

• After the biosynthetic stage (fermentation/production) in a bioreactor, the product must be subjected to a series of processes before it is ready for marketing as a finished product. These processes are collectively called downstream processing.
• It includes:

1. Separation of the product from the culture medium or cells (e.g., centrifugation, filtration).
2. Purification of the product to remove contaminants (e.g., chromatography, precipitation).
3. Formulation with suitable preservatives and stabilisers.
4. Quality control testing — each batch must be tested for purity, potency, and safety.
5. Clinical trials (in the case of drugs/therapeutics).

• Downstream processing and quality control vary from product to product depending on its nature and intended use.

(a) PCR (Polymerase Chain Reaction):

• PCR is an in vitro technique used to amplify a specific segment of DNA (gene of interest) to produce millions of copies in a short time without using a living cell.
• Principle: It is based on the property of DNA polymerase to synthesise a new strand of DNA complementary to the template strand.
• Components required: Template DNA, two specific primers (short oligonucleotide sequences complementary to the flanking regions of the target DNA), thermostable DNA polymerase (e.g., Taq polymerase from Thermus aquaticus), dNTPs (deoxyribonucleoside triphosphates), and buffer.
• Steps (repeated in cycles, typically 30–40 cycles):

1. Denaturation (~94°C): The double-stranded DNA is heated to separate the two strands.
2. Annealing (~50–65°C): Primers bind (anneal) to their complementary sequences on each template strand.
3. Extension (~72°C): Taq polymerase extends the primers, synthesising new complementary DNA strands.

• Each cycle doubles the amount of DNA, so after $n$ cycles, $2^n$ copies are produced.
• Applications: Diagnosis of genetic diseases, forensic science, detection of pathogens, gene cloning.

(b) Restriction Enzymes and DNA:

• Restriction enzymes (restriction endonucleases) are molecular scissors that cut DNA at specific recognition sequences.
• They were first isolated from bacteria, where they serve as a defence mechanism against foreign (viral) DNA.
• They recognise short, specific palindromic sequences (4–8 bp) in double-stranded DNA and cleave both strands.
• Types of cuts:
• Sticky ends (cohesive ends): Staggered cuts producing single-stranded overhangs (e.g., EcoRI produces 5′-AATT overhangs).
• Blunt ends: Straight cuts at the same position on both strands (e.g., SmaI).
• Sticky ends are particularly useful in recombinant DNA technology because they can hydrogen-bond with complementary sticky ends of another DNA fragment, facilitating ligation by DNA ligase.
• Naming convention: First letter from genus, next two from species, strain designation, and order of discovery (e.g., *Eco*RI from Escherichia coli strain R, first enzyme).

(c) Chitinase:

• Chitinase is an enzyme that breaks down chitin, a polysaccharide that forms the cell wall of fungi.
• Role in biotechnology: To isolate DNA from fungal cells, the fungal cell wall (made of chitin) must first be broken down. Chitinase is used to digest the chitin in the fungal cell wall, allowing the release of the cellular contents including DNA.
• This is analogous to the use of lysozyme to break the peptidoglycan cell wall of bacteria, or cellulase to break the cellulose cell wall of plant cells, for DNA isolation.
• Source: Chitinase is produced by various bacteria, fungi, and plants.
• Other applications: Chitinase is also used in biocontrol of fungal pathogens in agriculture and in the production of chitooligosaccharides for pharmaceutical use.

(a) Plasmid DNA vs. Chromosomal DNA:

(b) RNA vs. DNA:

(c) Exonuclease vs. Endonuclease: