Molecular Biology and Genomics Practice Questions

Q: Mitosis is arrested in which phase by using colchicine in cytogenetic analysis?

Metaphase. **Explanation:** **Correct Answer: B. Metaphase** The fundamental mechanism of **Colchicine** involves its binding to **tubulin dimers**, which inhibits the polymerization of microtubules. This prevents the formation of the mitotic spindle apparatus. During mitosis, the spindle fibers are essential for aligning chromosomes at the equatorial plate and subsequently pulling sister chromatids apart. By disrupting these fibers, colchicine arrests the cell cycle specifically at **Metaphase**. In cytogenetic analysis (Karyotyping), metaphase is the ideal stage for observation because chromosomes are at their **maximum state of condensation**, making them clearly visible and distinguishable under a light microscope. **Analysis of Incorrect Options:** * **A. Prophase:** During prophase, chromatin condenses and the nuclear envelope begins to break down. While spindle formation starts here, the arrest occurs later when the cell attempts to organize chromosomes for division. * **C. Anaphase:** This phase involves the migration of sister chromatids to opposite poles. Since colchicine prevents spindle formation, the cell can never reach the stage of chromatid separation. * **D. Telophase:** This is the final stage where nuclear membranes reform. Arrest occurs much earlier in the cycle. **High-Yield Clinical Pearls for NEET-PG:** * **Karyotyping:** Colchicine is the standard "mitotic poison" used to obtain a "Metaphase Spread" for diagnosing numerical or structural chromosomal abnormalities (e.g., Down Syndrome). * **Gout:** Clinically, colchicine is used in acute gout to inhibit neutrophil motility and chemotaxis by disrupting their microtubules. * **Taxanes (e.g., Paclitaxel):** Unlike colchicine which prevents assembly, Taxanes **stabilize** microtubules (preventing disassembly), also leading to mitotic arrest.

Q: Why is *Aquaticus thermophilus* preferred over *E. coli* in PCR?

It is thermostable at temperatures at which DNA denatures.. **Explanation:** The Polymerase Chain Reaction (PCR) involves repeated cycles of high-temperature heating to denature double-stranded DNA into single strands. **1. Why Option A is Correct:** The DNA polymerase from *Escherichia coli* is thermolabile; it denatures and loses activity at the high temperatures (typically 94–95°C) required for DNA denaturation. In contrast, **Taq polymerase**, isolated from the thermophilic bacterium ***Thermus aquaticus*** (often referred to as *Aquaticus thermophilus*), is **thermostable**. It remains functional throughout multiple heating cycles, eliminating the need to manually add fresh enzyme after every denaturation step, thus making the process automated and efficient. **2. Why Incorrect Options are Wrong:** * **Option B:** Standard Taq polymerase actually **lacks 3' to 5' exonuclease (proofreading) activity**, leading to a higher error rate compared to some other polymerases like *Pfu*. * **Option C:** While PCR is precise, the "precision" is a result of the biochemical properties of the enzyme and primers, not the primary reason for choosing a thermophilic source. * **Option D:** All DNA polymerases, including Taq, **require a primer** with a free 3'-OH group to initiate DNA synthesis. **High-Yield Clinical Pearls for NEET-PG:** * **Taq Polymerase Optimum Temperature:** Its peak activity occurs at **72°C**. * **Steps of PCR:** Denaturation (95°C) → Annealing (50–65°C) → Extension (72°C). * **RT-PCR:** Uses Reverse Transcriptase to convert RNA to cDNA; used for diagnosing RNA viruses like **SARS-CoV-2** and **HIV** (viral load). * **Components of PCR:** Template DNA, Primers, dNTPs, Taq Polymerase, and **$Mg^{2+}$ ions** (essential cofactor).

Q: Which of the following vectors can carry the largest genome?

YAC (Yeast Artificial Chromosome). **Explanation:** In recombinant DNA technology, the choice of a vector depends primarily on the size of the DNA insert it can accommodate. **Why YAC is the Correct Answer:** **Yeast Artificial Chromosomes (YACs)** are engineered chromosomes containing yeast telomeres, centromeres, and replication origins. They are designed to mimic natural chromosomes in *Saccharomyces cerevisiae*. Because they function like eukaryotic chromosomes, they possess the highest carrying capacity of all common vectors, typically ranging from **100 kb to 3,000 kb (3 Mb)**. This makes them indispensable for large-scale genomic mapping, such as the Human Genome Project. **Why the Other Options are Incorrect:** * **Plasmids (A):** These are small, circular extrachromosomal DNA molecules. They have the lowest capacity, generally carrying inserts of **<10 kb**. * **Cosmids (D):** These are hybrid vectors (plasmid + Lambda phage *cos* sites). They can package DNA into phage heads, allowing for inserts of **30–45 kb**. * **BAC (B):** Based on the F-plasmid of *E. coli*, BACs are more stable than YACs but have a lower capacity, typically carrying **100–300 kb**. **High-Yield Clinical Pearls for NEET-PG:** * **Order of Capacity (Ascending):** Plasmid < Bacteriophage < Cosmid < BAC < YAC. * **Stability:** While YACs carry the most DNA, **BACs are often preferred** in labs because they are more structurally stable and easier to manipulate. * **Expression Vectors:** If the goal is to produce a human protein (e.g., Insulin) in bacteria, an **Expression Vector** (containing a promoter and ribosome binding site) must be used.

Q: Cells cultured from patients with a certain autosomal recessive genetic disorder exhibit low activity for the nucleotide excision repair process. This disorder is characterized by marked sensitivity to sunlight (ultraviolet light), leading to the formation of multiple skin cancers and premature death. What is this disorder?

Xeroderma Pigmentosa. ### Explanation **Correct Option: C. Xeroderma Pigmentosa (XP)** Xeroderma Pigmentosa is an autosomal recessive disorder caused by a defect in the **Nucleotide Excision Repair (NER)** pathway. * **Mechanism:** UV light causes the formation of **pyrimidine dimers** (usually thymine dimers) in DNA. In healthy individuals, the NER pathway (involving UV-specific endonucleases) identifies and removes these dimers. In XP patients, this repair mechanism is deficient. * **Clinical Presentation:** This leads to extreme photosensitivity, severe sunburn with minimal exposure, and a 1000-fold increased risk of developing cutaneous malignancies (Basal Cell Carcinoma, Squamous Cell Carcinoma, and Melanoma) at a very young age. **Why Incorrect Options are Wrong:** * **A. Acute Intermittent Porphyria:** A metabolic disorder of heme biosynthesis due to **Porphobilinogen deaminase** deficiency. It presents with abdominal pain and neuropsychiatric symptoms but, unlike other porphyrias, it does **not** feature photosensitivity. * **B. Alkaptonuria:** A disorder of tyrosine metabolism due to **Homogentisate oxidase** deficiency. It is characterized by ochronosis (darkening of connective tissues) and urine that turns black upon standing, not DNA repair defects. * **D. Ataxia-Telangiectasia:** While this also involves DNA repair, the defect is in the **ATM gene**, which is responsible for repairing **double-stranded DNA breaks** (non-homologous end joining), not NER. It presents with cerebellar ataxia and telangiectasia. **High-Yield Clinical Pearls for NEET-PG:** * **Enzyme Deficient in XP:** UV-specific endonuclease (excisionase). * **Inheritance:** Autosomal Recessive. * **Key Association:** "Children of the Night" (must avoid all sunlight). * **Other DNA Repair Disorders:** * **Lynch Syndrome:** Mismatch repair (MSH2, MLH1). * **Bloom Syndrome/Werner Syndrome:** DNA Helicase. * **Fanconi Anemia:** DNA cross-link repair.

Q: Termination of RNA molecule synthesis is signaled by a specific sequence in the DNA template strand, which is recognized by which termination protein?

Rho (r) factor. **Explanation:** In prokaryotic transcription, the termination of RNA synthesis occurs via two primary mechanisms: **Rho-independent** (intrinsic) and **Rho-dependent** termination. 1. **Why the Correct Answer is Right:** The **Rho (ρ) factor** is an ATP-dependent hexameric helicase protein. It binds to a specific C-rich sequence on the nascent RNA chain known as the **rut (Rho utilization) site**. Once bound, it moves along the RNA toward the RNA polymerase. When the polymerase pauses at a termination sequence on the DNA template, the Rho factor catches up, uses its ATPase activity to unwind the RNA-DNA hybrid, and facilitates the release of the RNA transcript. 2. **Analysis of Incorrect Options:** * **s factor (Sigma factor):** This is involved in the **initiation** of transcription. It helps RNA polymerase recognize and bind to the promoter sequence. It dissociates shortly after transcription begins. * **d factor (Delta) and e factor (Epsilon):** These are not standard nomenclature for transcription termination factors in prokaryotes. In eukaryotes, various subunits exist for DNA polymerases (like Pol δ and ε), but they are involved in DNA replication, not RNA termination. 3. **High-Yield Clinical Pearls for NEET-PG:** * **Rifampicin:** Inhibits the **initiation** of transcription by binding to the β-subunit of bacterial RNA polymerase (used in TB treatment). * **Actinomycin D:** Inhibits transcription by intercalating into the DNA template, preventing elongation (used in oncology). * **Alpha-amanitin:** Found in *Amanita phalloides* (death cap mushroom); it specifically inhibits **RNA Polymerase II** in eukaryotes, leading to severe liver failure.

Q: What enzyme is required for the conversion of DNA to RNA?

RNA polymerase. **Explanation:** The process of converting DNA into RNA is known as **Transcription**. This is a fundamental step in the central dogma of molecular biology, where genetic information stored in DNA is transcribed into messenger RNA (mRNA), transfer RNA (tRNA), or ribosomal RNA (rRNA). **Why RNA Polymerase is Correct:** **RNA polymerase** is the primary enzyme responsible for transcription. It binds to a specific DNA sequence called the promoter, unwinds the DNA helix, and catalyzes the synthesis of an RNA strand by adding ribonucleotides complementary to the DNA template (in a 5' to 3' direction). In eukaryotes, there are three main types: RNA Pol I (rRNA), RNA Pol II (mRNA), and RNA Pol III (tRNA). **Why Other Options are Incorrect:** * **DNA Polymerase:** This enzyme is used in **DNA Replication**, not transcription. It synthesizes a new DNA strand from a DNA template. * **DNA Ligase:** Known as the "molecular glue," this enzyme joins DNA fragments together (e.g., Okazaki fragments) by forming phosphodiester bonds. * **Primase:** This is a specialized RNA polymerase that creates a short RNA **primer** to provide a 3'-OH group, which is essential for DNA polymerase to begin DNA replication. **Clinical Pearls for NEET-PG:** * **Rifampicin**, a key anti-tubercular drug, acts by inhibiting the **beta-subunit of bacterial DNA-dependent RNA polymerase**. * **Alpha-amanitin** (from the *Amanita phalloides* mushroom) specifically inhibits **RNA Polymerase II**, leading to severe liver failure. * In prokaryotes, a single RNA polymerase (with a **Sigma factor** for initiation) handles all transcription, whereas eukaryotes require three distinct enzymes.

Q: The Shine-Dalgarno sequence in prokaryotes is primarily associated with which molecular process?

Translation. **Explanation:** The **Shine-Dalgarno (SD) sequence** is a critical regulatory element in prokaryotic **Translation** (Option B). It is a purine-rich sequence (typically AGGAGG) located approximately 8 base pairs upstream of the AUG start codon on the mRNA. Its primary function is to serve as the **ribosomal binding site**. The SD sequence base-pairs with a complementary pyrimidine-rich sequence at the **3' end of the 16S rRNA** (part of the 30S small ribosomal subunit). This interaction ensures the correct alignment of the ribosome on the mRNA, allowing translation to begin at the proper initiation codon. **Analysis of Incorrect Options:** * **A. Transcription:** This is the synthesis of RNA from DNA. The key regulatory sequences here are **Promoters** (e.g., Pribnow box/TATA box), not the SD sequence. * **C. Replication:** This involves DNA synthesis. Key elements include the **Origin of Replication (OriC)** and primers, unrelated to mRNA-ribosome binding. * **D. Translocation:** This is a specific step *within* translation where the ribosome moves along the mRNA. While related to the overall process, the SD sequence is specifically involved in **Initiation**, not the translocation phase. **High-Yield Clinical Pearls for NEET-PG:** * **Kozak Sequence:** The eukaryotic functional equivalent of the Shine-Dalgarno sequence. * **16S rRNA:** The specific component of the prokaryotic 30S subunit that recognizes the SD sequence. * **Aminoglycosides:** This class of antibiotics (e.g., Streptomycin) acts by binding to the 30S subunit, interfering with the initiation and fidelity of translation. * **Polycistronic mRNA:** In bacteria, multiple SD sequences can exist on a single mRNA strand, allowing for the translation of multiple proteins from one transcript.

Question 1

Which of the following statements is true regarding DNA Polymerase III?

Accepted Answer

It synthesizes Okazaki fragments and requires an RNA primer.

Answer

It is essential for the process of translation.

Answer

Bacteria can survive and function without DNA Polymerase III.

Answer

It plays a role in DNA repair.

Question 2

Mitosis is arrested in which phase by using colchicine in cytogenetic analysis?

Accepted Answer

Metaphase

Answer

Prophase

Answer

Anaphase

Answer

Telophase

Question 3

Why is *Aquaticus thermophilus* preferred over *E. coli* in PCR?

Accepted Answer

It is thermostable at temperatures at which DNA denatures.

Answer

It possesses proofreading activity.

Answer

It is performed more precisely.

Answer

It does not require a primer.

Question 4

Which of the following vectors can carry the largest genome?

Accepted Answer

YAC (Yeast Artificial Chromosome)

Answer

Plasmids

Answer

BAC (Bacterial Artificial Chromosome)

Answer

Cosmids

Question 5

Which of the following is not required for protein synthesis (translation) in eukaryotes?

Accepted Answer

Amino acyl tRNA synthetase

Answer

RNA polymerase

Answer

Ribosomes

Answer

Peptidyl transferase

Question 6

Cells cultured from patients with a certain autosomal recessive genetic disorder exhibit low activity for the nucleotide excision repair process. This disorder is characterized by marked sensitivity to sunlight (ultraviolet light), leading to the formation of multiple skin cancers and premature death. What is this disorder?

Accepted Answer

Xeroderma Pigmentosa

Answer

Acute intermittent Porphyria

Answer

Alkaptonuria

Answer

Ataxia - Telangiectasia

Question 7

Termination of RNA molecule synthesis is signaled by a specific sequence in the DNA template strand, which is recognized by which termination protein?

Accepted Answer

Rho (r) factor

Answer

s factor

Answer

d factor

Answer

e factor

Question 8

Presence of which of the following in the expression vector ensures an increase in the yield of recombinant protein produced?

Accepted Answer

Inducible promoter

Answer

Gene coding for protease inhibitor

Answer

Translation initiation signals

Answer

Transcription and translation termination signals

Question 9

What enzyme is required for the conversion of DNA to RNA?

Accepted Answer

RNA polymerase

Answer

DNA Polymerase

Answer

DNA Ligase

Answer

Primase

Question 10

The Shine-Dalgarno sequence in prokaryotes is primarily associated with which molecular process?

Accepted Answer

Translation

Answer

Transcription

Answer

Replication

Answer

Translocation

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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