Molecular Biology and Genomics Practice Questions

Q: All of the following are true about a genomic library, EXCEPT?

Only exons are present.. **Explanation:** The core concept distinguishing a **Genomic Library** from a **cDNA Library** is the source material. A genomic library is created by taking the entire nuclear DNA of an organism, digesting it with restriction enzymes, and cloning the resulting fragments. **Why Option C is the correct answer (The Exception):** In eukaryotes, genomic DNA contains both **coding regions (exons)** and **non-coding regions (introns, promoters, and enhancers)**. Therefore, a genomic library contains the entire genome, including introns. In contrast, a **cDNA library** is synthesized from mature mRNA (where introns have already been spliced out), meaning it contains **only exons**. **Analysis of Incorrect Options:** * **Option A:** True. By definition, a library is a collection of DNA fragments that have been inserted into host cells for storage and propagation. * **Option B:** True. To find a specific gene within the thousands of clones in a library, researchers use labeled **oligonucleotide probes** that are complementary to the sequence of interest (Colony Hybridization). * **Option D:** True. Vectors (such as plasmids, bacteriophages, BACs, or YACs) are essential vehicles used to carry the DNA fragments into host bacteria (like *E. coli*) for replication. **High-Yield Clinical Pearls for NEET-PG:** * **Genomic Library:** Represents the entire genome; same for every cell in an organism. Useful for studying gene structure, introns, and regulatory elements. * **cDNA Library:** Represents only the "expressed" genes; varies from tissue to tissue (e.g., a liver cDNA library differs from a brain cDNA library). * **Vector Capacity:** For large genomic fragments, **YACs (Yeast Artificial Chromosomes)** have the largest capacity (up to 1000 kb), followed by **BACs (Bacterial Artificial Chromosomes)**.

Q: During protein biosynthesis, high-energy bonds are NOT utilized in which one of the following steps?

Formation of the peptide bond (peptidyl transferase step). ### Explanation The process of translation is energy-intensive, requiring the hydrolysis of high-energy phosphate bonds (ATP and GTP) at various stages. However, the actual formation of the peptide bond is a unique exception. **1. Why Option C is Correct:** The formation of the peptide bond is catalyzed by **peptidyl transferase** (a ribozyme component of the large ribosomal subunit). This step does **not** require the hydrolysis of a new high-energy phosphate bond (ATP or GTP). Instead, it utilizes the **inherent energy** stored in the high-energy ester bond between the amino acid and its tRNA, which was previously created during the "charging" phase. The reaction is essentially an energetically neutral transesterification. **2. Why the Other Options are Incorrect:** * **A. Formation of aminoacyl-tRNA:** This is the "charging" step catalyzed by aminoacyl-tRNA synthetase. It requires the hydrolysis of **ATP to AMP + PPi** (equivalent to 2 high-energy bonds). * **B. Binding of aminoacyl-tRNA to the A site:** This step requires **EF-Tu** (in prokaryotes) or **eEF-1** (in eukaryotes) and the hydrolysis of **one GTP**. * **D. Translocation:** The movement of the ribosome along the mRNA requires **EF-G** (prokaryotes) or **eEF-2** (eukaryotes) and the hydrolysis of **one GTP**. **Clinical Pearls & High-Yield Facts:** * **Energy Tally:** For every single amino acid added to a polypeptide chain, **4 high-energy bonds** are consumed (2 from ATP during charging, 1 for A-site binding, and 1 for translocation). * **Ribozyme Activity:** Peptidyl transferase is not a protein but an RNA enzyme (23S rRNA in prokaryotes, 28S rRNA in eukaryotes). * **Antibiotic Link:** **Chloramphenicol** specifically inhibits the peptidyl transferase enzyme in 50S bacterial ribosomes, making it a classic target for pharmacological inhibition.

Q: Which of the following statements are true regarding Polymerase Chain Reaction (PCR)?

Same as western blot test. **Explanation:** Polymerase Chain Reaction (PCR) is a revolutionary molecular biology technique used to amplify specific sequences of **DNA**. By using thermal cycling, primers, and DNA polymerase (Taq polymerase), a single copy of a DNA fragment can be multiplied into millions of copies. **Analysis of Options:** * **Correct Answer (B/C):** *Note: There appears to be a discrepancy in the provided key.* **DNA amplification (Option B)** is the fundamental definition of PCR. However, if the question implies a comparison of diagnostic utility, PCR and **Western Blot** are both confirmatory tests (e.g., in HIV diagnosis, PCR detects the proviral DNA/RNA, while Western Blot detects specific proteins/antibodies). * **Option A (Protein amplification):** This is incorrect. PCR only amplifies nucleic acids (DNA/RNA). Proteins are detected via methods like ELISA or Western Blot, but they cannot be "amplified" in the same enzymatic fashion as DNA. * **Option D (Detection of infecting organisms):** While PCR is used for this purpose, it is a *clinical application* rather than the definition of the technique itself. **Clinical Pearls for NEET-PG:** * **Components:** Needs template DNA, Primers, dNTPs, and **Taq Polymerase** (thermostable, derived from *Thermus aquaticus*). * **Steps:** 1. Denaturation (94°C), 2. Annealing (55°C), 3. Extension (72°C). * **RT-PCR:** Reverse Transcriptase PCR is used for **RNA viruses** (like HIV, SARS-CoV-2) to convert RNA into cDNA before amplification. * **Real-Time PCR (qPCR):** Used for **quantification** of viral load (e.g., HBV, HCV). * **Nested PCR:** Increases sensitivity and specificity by using two sets of primers in two successive runs.

Q: Which of the following is not required for Polymerase Chain Reaction (PCR)?

Radio-labeled DNA probe. **Explanation:** Polymerase Chain Reaction (PCR) is an *in vitro* enzymatic method used to amplify specific DNA sequences. The process mimics natural DNA replication but occurs in a thermal cycler through three repeating steps: Denaturation, Annealing, and Extension. **Why Option D is the Correct Answer:** A **Radio-labeled DNA probe** is used in **Southern Blotting** or **In Situ Hybridization** to detect a specific DNA sequence after amplification or separation. It is **not** a component of the PCR reaction mixture itself. PCR produces millions of copies of DNA; visualization is typically done via gel electrophoresis with ethidium bromide or real-time fluorescence, rather than radioactive probes. **Why the other options are incorrect:** * **A. Primer:** Essential. These are short, synthetic oligonucleotides (usually 18–25 base pairs) that provide a 3'-OH group for DNA polymerase to initiate synthesis. Two primers (forward and reverse) are required. * **B. DNA fragments:** Essential. This is the **template DNA** containing the target sequence that needs to be amplified. * **C. DNA polymerase:** Essential. A heat-stable enzyme, typically **Taq Polymerase** (derived from *Thermus aquaticus*), is required to extend the primers and synthesize new DNA strands at high temperatures. **High-Yield Clinical Pearls for NEET-PG:** * **Taq Polymerase:** Optimum temperature is **72°C**. It lacks 3' to 5' exonuclease activity (no proofreading). * **RT-PCR:** Used for RNA viruses (like SARS-CoV-2); involves converting RNA to cDNA using **Reverse Transcriptase** before PCR. * **Components of PCR Mix:** Template DNA, Primers, Taq Polymerase, **dNTPs** (Deoxynucleotide triphosphates), and **Magnesium ions ($Mg^{2+}$)** which act as a cofactor for the polymerase.

Q: Which of the following techniques is used for detection of variations in DNA sequence and gene expression?

Hypothalamus. **Explanation** The question asks for the technique used to detect variations in DNA sequence and gene expression. However, there appears to be a **discrepancy between the question and the provided options**, as the options list neuroanatomical structures rather than molecular biology techniques. In the context of **Molecular Biology**, the correct answer to the question "Which technique detects variations in DNA sequence and gene expression?" should be **Microarray (DNA Microarray)**. Microarrays allow for the simultaneous analysis of thousands of genes to detect Single Nucleotide Polymorphisms (SNPs) and quantify mRNA expression levels. Regarding the provided options (Neuroanatomy context): * **Hypothalamus (Correct per key):** While not a molecular technique, the hypothalamus is the master regulator of homeostasis, controlling the endocrine system, autonomic nervous system, thirst, hunger, and temperature. * **Thalamus:** Acts as the primary sensory relay station for all senses except olfaction. * **Putamen:** A part of the basal ganglia involved in regulating movements and learning. * **Limbic Cortex:** Involved in emotion, memory, and behavior. **High-Yield NEET-PG Pearls:** 1. **DNA Microarray:** Uses nucleic acid hybridization to study the "transcriptome." 2. **Southern Blot:** Detects specific DNA sequences. 3. **Northern Blot:** Detects RNA (gene expression). 4. **Western Blot:** Detects proteins using antibodies. 5. **RT-PCR:** The gold standard for measuring specific gene expression (mRNA levels). 6. **Sanger Sequencing:** The "gold standard" for determining the exact DNA sequence.

Q: Excessive ultraviolet (UV) radiation is harmful to life. What is the primary mechanism by which ultraviolet radiation causes damage to biological systems?

Formation of thymidine dimers. ### Explanation **Correct Option: B. Formation of thymidine dimers** Ultraviolet (UV) radiation, specifically UV-B (280–320 nm), is non-ionizing radiation that is absorbed by the nitrogenous bases of DNA. The primary photochemical reaction involves the formation of **cyclobutane pyrimidine dimers**, most commonly between two adjacent **thymine** residues on the same DNA strand. This covalent cross-linking creates a "bulge" in the DNA helix, which distorts the structure and interferes with both transcription and DNA replication. **Analysis of Incorrect Options:** * **A. Inhibition of DNA synthesis:** While UV damage eventually leads to the arrest of the replication fork, this is a *consequence* of the damage, not the primary mechanism of the radiation itself. * **C. Ionization:** UV radiation is **non-ionizing**. Ionizing radiation (like X-rays or Gamma rays) works by ejecting electrons from atoms, creating free radicals and causing double-strand breaks. * **D. DNA fragmentation:** This is typically a result of ionizing radiation or late-stage apoptosis. UV radiation primarily causes localized point lesions (dimers). **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Repair Mechanism:** Pyrimidine dimers are repaired by **Nucleotide Excision Repair (NER)**. This involves the "cut and patch" mechanism using endonucleases (UV-specific), DNA polymerase, and ligase. * **Clinical Correlation:** A defect in the NER pathway (specifically the excinuclease enzyme) leads to **Xeroderma Pigmentosum**. Patients present with extreme photosensitivity and a high risk of skin cancers (Basal Cell Carcinoma, Squamous Cell Carcinoma, and Melanoma). * **Prokaryotic Repair:** Bacteria can also use **Photoreactivation** (via the enzyme DNA photolyase), a process not found in humans.

Q: Given the DNA template strand 5'-TTACGTAC-3', what will be the resulting mRNA sequence after transcription?

5'-GUACGUAA-3'. ### Explanation **1. Why Option D is Correct:** Transcription is the process where RNA polymerase synthesizes mRNA using a **DNA template strand**. This process follows two fundamental rules: * **Antiparallel Orientation:** The mRNA is synthesized in the 5' to 3' direction, meaning it must be complementary and antiparallel to the template strand. * **Base Pairing:** Adenine (A) pairs with Uracil (U) in RNA, and Cytosine (C) pairs with Guanine (G). Given Template: **5'- T T A C G T A C -3'** To find the mRNA, we read the template from 3' to 5' and pair accordingly: * Template 3' → 5' is: **C-A-T-G-C-A-T-T** * Complementary mRNA 5' → 3' is: **G-U-A-C-G-U-A-A** **2. Why Other Options are Incorrect:** * **Option A:** This is identical to the template DNA. mRNA cannot have Thymine (T) and must be complementary, not identical. * **Option B:** This is simply the template strand written in reverse. It ignores base-pairing rules and the presence of Uracil. * **Option C:** This is the "Coding Strand" sequence (substituting T for U). While the mRNA sequence matches the coding (non-template) strand, this option incorrectly retains Thymine (T) instead of Uracil (U). **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Coding vs. Template:** The **Coding Strand** (5'→3') has the same sequence as mRNA (except T is replaced by U). The **Template Strand** (3'→5') is what the RNA polymerase actually "reads." * **Directionality:** RNA synthesis always occurs in the **5' to 3' direction**. * **Inhibitors:** **Rifampicin** inhibits bacterial DNA-dependent RNA polymerase (used in TB), while **Actinomycin D** inhibits transcription in both prokaryotes and eukaryotes (used in chemotherapy). * **Alpha-amanitin:** Found in *Amanita phalloides* mushrooms; it specifically inhibits **RNA Polymerase II**, leading to severe liver failure.

Q: On which of the following does tRNA act specifically?

Specific amino acid. **Explanation:** The core function of **tRNA (transfer RNA)** is to act as an "adapter molecule" during protein synthesis (translation). Its primary specificity lies in its ability to link a specific genetic code (codon) to a **specific amino acid**. **Why the correct answer is right:** Each tRNA molecule is "charged" with a specific amino acid at its 3' end (the CCA tail). This process is catalyzed by the enzyme **aminoacyl-tRNA synthetase**, which ensures high fidelity by matching the tRNA's anticodon with the correct amino acid. This specificity is crucial because it ensures that the ribosome incorporates the exact amino acid dictated by the mRNA template. **Analysis of incorrect options:** * **ATP (A):** While ATP is required for the activation of amino acids (forming aminoacyl-adenylate), tRNA does not act "specifically" on ATP; ATP is a universal energy donor used by many enzymes. * **Golgi body (B):** The Golgi apparatus is involved in post-translational modification and sorting of proteins, not the translation process where tRNA functions. * **Ribosome (D):** tRNA interacts with the ribosome (at the A, P, and E sites), but the ribosome serves as the physical workbench. The tRNA's *specificity* is defined by the amino acid it carries, not the ribosome itself, which remains the same for all tRNAs. **NEET-PG High-Yield Pearls:** * **Wobble Hypothesis:** Explains why there are fewer tRNAs (approx. 30-40) than codons (61), allowing the 3rd base of the anticodon to have non-traditional pairing. * **Charging:** The attachment of an amino acid to tRNA is called "charging," and the enzyme aminoacyl-tRNA synthetase is often referred to as the "true genetic decoder." * **Structure:** tRNA has a secondary **cloverleaf structure** and a tertiary **L-shaped structure**.

Question 1

All of the following are true about a genomic library, EXCEPT?

Accepted Answer

Only exons are present.

Answer

It is a collection of cloned DNA fragments.

Answer

Screening is done by oligonucleotide probes.

Answer

Vectors are used to carry and replicate the fragments.

Question 2

During protein biosynthesis, high-energy bonds are NOT utilized in which one of the following steps?

Accepted Answer

Formation of the peptide bond (peptidyl transferase step)

Answer

Formation of aminoacyl-tRNA

Answer

Binding of aminoacyl-tRNA to the A site of the ribosome

Answer

Translocation step

Question 3

Which of the following statements are true regarding Polymerase Chain Reaction (PCR)?

Accepted Answer

Same as western blot test

Answer

Protein amplification

Answer

DNA amplification

Answer

Detection of infecting organisms

Question 4

What is the function of restriction endonucleases?

Accepted Answer

To cut double-stranded DNA at specific recognition sequences.

Answer

To cut RNA at specific sequences.

Answer

To cut single-stranded DNA at specific sequences.

Answer

To break peptide bonds in proteins.

Question 5

Which of the following is not required for Polymerase Chain Reaction (PCR)?

Accepted Answer

Radio-labeled DNA probe

Answer

Primer

Answer

DNA fragments

Answer

DNA polymerase

Question 6

A mutation that completely disrupts the function of a gene is utilized in which of the following techniques?

Accepted Answer

Gene knockout

Answer

Nonsense mutation

Answer

Restriction fragment length polymorphism

Answer

Targeted gene disruption

Question 7

Which of the following techniques is used for detection of variations in DNA sequence and gene expression?

Accepted Answer

Hypothalamus

Answer

Thalamus

Answer

Putamen

Answer

Limbic cortex

Question 8

Excessive ultraviolet (UV) radiation is harmful to life. What is the primary mechanism by which ultraviolet radiation causes damage to biological systems?

Accepted Answer

Formation of thymidine dimers

Answer

Inhibition of DNA synthesis

Answer

Ionization

Answer

DNA fragmentation

Question 9

Given the DNA template strand 5'-TTACGTAC-3', what will be the resulting mRNA sequence after transcription?

Accepted Answer

5'-GUACGUAA-3'

Answer

5'-TTACGTAC-3'

Answer

3'-TTACGTAC-5'

Answer

5'-CATGCATT-3'

Question 10

On which of the following does tRNA act specifically?

Accepted Answer

Specific amino acid

Answer

ATP

Answer

Golgi body

Answer

Ribosome

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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