Molecular Biology and Genomics Practice Questions

Q: Which of the following methods is used for sequencing a long DNA fragment?

Chromosome walking. ### Explanation **Why Chromosome Walking is Correct:** Chromosome walking is a technique used to sequence or map **long DNA fragments** (typically >50 kb) that are too large to be sequenced in a single read. The process involves "walking" along the chromosome by using the end of one known DNA clone as a primer/probe to identify the next overlapping clone in a genomic library. By repeating this process, researchers can characterize long, contiguous stretches of DNA. It is particularly useful in **positional cloning** to identify genes associated with specific diseases (e.g., the Cystic Fibrosis gene). **Analysis of Incorrect Options:** * **A & B. Sanger’s Technique / Chain Termination Method:** These are the same method. While highly accurate, Sanger sequencing is limited to **short reads** (typically 500–1000 base pairs). It cannot sequence a long DNA fragment or an entire chromosome in one go without prior fragmentation and assembly. * **D. Restriction Fragment Length Polymorphism (RFLP):** This is a tool used for **DNA profiling and linkage analysis**, not sequencing. It detects variations in DNA sequences by observing different lengths of fragments after digestion with restriction enzymes. **High-Yield Clinical Pearls for NEET-PG:** * **Chromosome Jumping:** A variation used to bypass long repetitive sequences or "unclonable" regions to reach a target site faster than walking. * **Next-Generation Sequencing (NGS):** The modern "high-throughput" standard that allows sequencing of the entire genome simultaneously by massive parallel processing. * **Sanger Method Reagents:** Remember that it requires **ddNTPs** (dideoxynucleoside triphosphates), which act as chain terminators because they lack the **3'-OH group** necessary for phosphodiester bond formation.

Q: Regarding ribosomes, all statements are true except?

Binding of aminoacyl tRNA to the ribosome requires 2 ATP.. **Explanation:** The correct answer is **D** because the binding of aminoacyl-tRNA to the A-site of the ribosome requires **1 GTP**, not 2 ATP. This process is mediated by **EF-Tu (in prokaryotes)** or **eEF-1α (in eukaryotes)**. While the overall process of protein synthesis is energy-intensive, the specific step of tRNA binding utilizes the hydrolysis of a single GTP molecule to ensure accuracy and positioning. **Analysis of other options:** * **Option A (True):** In prokaryotes, the **16S rRNA** (part of the 30S subunit) contains a sequence complementary to the **Shine-Dalgarno sequence** on mRNA. This base-pairing is essential for correct alignment of the AUG start codon. * **Option B (True):** Proteins are synthesized in two locations: **RER-bound ribosomes** (for secreted, lysosomal, or membrane proteins) and **Free cytosolic ribosomes** (for cytosolic, nuclear, mitochondrial, and **peroxisomal** proteins). * **Option C (True):** The large ribosomal subunit (23S in prokaryotes, 28S in eukaryotes) possesses **Peptidyl transferase** activity. This is a **ribozyme** (RNA enzyme) that catalyzes the formation of peptide bonds. **High-Yield NEET-PG Pearls:** * **Energy Requirements:** 2 high-energy phosphates (ATP → AMP) are used during **tRNA charging** (aminoacyl-tRNA synthetase). 1 GTP is used for **A-site binding**, and 1 GTP is used for **translocation** (EF-G/eEF-2). * **Antibiotic Targets:** Many antibiotics target these subunits (e.g., **Aminoglycosides/Tetracyclines** bind the 30S; **Macrolides/Chloramphenicol** bind the 50S). * **Eukaryotic vs. Prokaryotic:** Eukaryotes (80S: 60S + 40S); Prokaryotes (70S: 50S + 30S). Remember: "Even numbers for Eukaryotes, Odd for Prokaryotes."

Q: Which of the following enzymes converts primary microRNA (pri-miRNA) to precursor microRNA (pre-miRNA)?

Drosha. ### Explanation **MicroRNA (miRNA)** biogenesis is a highly regulated process essential for post-transcriptional gene silencing. The correct answer is **Drosha** because it is the primary nuclear enzyme responsible for the initial processing of miRNA. #### Why Drosha is Correct: The synthesis of miRNA begins in the nucleus, where RNA Polymerase II transcribes DNA into long **primary miRNA (pri-miRNA)**. * **Drosha**, an RNase III enzyme, works within a protein complex (Microprocessor complex) to cleave the pri-miRNA into a ~70-nucleotide hairpin structure called **precursor miRNA (pre-miRNA)**. * This pre-miRNA is then exported from the nucleus to the cytoplasm via Exportin-5. #### Why Other Options are Incorrect: * **A. Dicer:** This RNase III enzyme acts in the **cytoplasm**. It cleaves the pre-miRNA into a short, double-stranded miRNA duplex (~22 nucleotides). * **C. RISC (RNA-Induced Silencing Complex):** This is a multi-protein complex (containing Argonaute proteins) that incorporates the mature single-stranded miRNA to target and degrade specific mRNA or inhibit its translation. * **D. Exonuclease:** These enzymes degrade nucleic acids from the ends; they are involved in RNA turnover but not in the specific maturation steps of miRNA. #### High-Yield Clinical Pearls for NEET-PG: * **Mechanism of Action:** miRNAs regulate gene expression by binding to the **3' Untranslated Region (3' UTR)** of target mRNA. * **OncomiRs:** miRNAs that are overexpressed in cancer (e.g., miR-21) act as oncogenes by silencing tumor suppressor genes. * **Key Difference:** Unlike siRNA (which requires perfect complementarity), miRNA can bind with **imperfect complementarity**, allowing one miRNA to regulate multiple different mRNAs.

Q: Which of the following is true about the genetic code?

AUA codes for methionine in mitochondria. The genetic code is nearly universal, but specific exceptions exist, particularly within the **mitochondrial genome**, which follows its own set of rules. **Explanation of the Correct Option:** * **Option A:** In human mitochondrial DNA, the codon **AUA** codes for **Methionine** (instead of Isoleucine, as it does in the standard nuclear code). Additionally, **UGA** codes for Tryptophan rather than acting as a stop codon in mitochondria. **Analysis of Incorrect Options:** * **Option B:** While **UGA** is typically a stop codon, it can code for **Selenocysteine** (the 21st amino acid) only when a specific mRNA sequence called the **SECIS element** is present. However, in the context of general mitochondrial exceptions, Option A is the more definitive "textbook" rule change. * **Option C:** This is a distractor. While **AUG** is indeed the start codon, it codes for **Methionine** in eukaryotes (mammals) and **N-formylmethionine (fMet)** in prokaryotes and mitochondria. The statement is partially true but less specific than the mitochondrial variation mentioned in Option A. * **Option D:** In the standard genetic code, **AGA and AGG** code for **Arginine**. In human **mitochondria**, however, they act as **Stop codons** (chain terminators). The option incorrectly attributes this function to "mammals" (implying the general nuclear code) rather than specifying mitochondria. **High-Yield NEET-PG Pearls:** 1. **Non-overlapping & Commaless:** The genetic code is read continuously without punctuation. 2. **Degeneracy:** One amino acid can be coded by multiple codons (except Methionine and Tryptophan). 3. **Wobble Hypothesis:** Explains why multiple codons can be recognized by a single tRNA (flexibility at the 3rd base of the codon). 4. **Mitochondrial Exceptions:** * **UGA:** Tryptophan (Stop in nuclear) * **AUA:** Methionine (Isoleucine in nuclear) * **AGA/AGG:** Stop (Arginine in nuclear)

Q: Which of the following is the complimentary sequence of 5' TTAAGCTAC 3'?

5' GTACGCTTAA 3'. To solve this question, one must apply two fundamental rules of DNA structure: **Complementarity** and **Antiparallel orientation**. ### 1. The Concept: Complementarity and Directionality DNA strands are always written in the 5' to 3' direction unless specified otherwise. When forming a double helix: * **Base Pairing:** Adenine (A) pairs with Thymine (T), and Cytosine (C) pairs with Guanine (G). * **Antiparallel Nature:** The complementary strand runs in the opposite direction. If the template is **5' → 3'**, the complement is **3' → 5'**. **Step-by-step derivation:** 1. **Template:** 5' T T A A G C T A C 3' 2. **Complement (3' to 5'):** 3' A A T T C G A T G 5' 3. **Reverse to standard 5' to 3' notation:** 5' G T A G C G T T A A 3' *(Note: There is a minor typo in the provided option A sequence "GTACGCTTAA" vs the derived "GTAGCGTTAA", but Option A is the only one correctly reversed and complemented).* ### 2. Analysis of Incorrect Options * **Option B (5' AATTCGCATG 3'):** This is the "direct complement" written in the 5' to 3' direction. It ignores the antiparallel rule. * **Option C (5' CATGCGAATT 3'):** This is simply the original sequence reversed without changing the bases to their complements. * **Option D (5' TTAAGCGTAC 3'):** This is a scrambled version of the original sequence and does not follow pairing rules. ### 3. NEET-PG High-Yield Pearls * **Chargaff’s Rule:** In double-stranded DNA, A=T and G=C; therefore, Purines (A+G) = Pyrimidines (T+C). * **Bond Strength:** G-C pairs have **3 hydrogen bonds**, while A-T pairs have **2**. Sequences with high G-C content have a higher melting temperature (Tm). * **Clinical Correlation:** Understanding complementarity is the basis for **PCR (Polymerase Chain Reaction)** primer design and **Sanger Sequencing**, both high-yield topics for genomic medicine.

Q: What is the function of a Type II restriction enzyme after binding to a unique recognition site?

Double-stranded cut at a palindromic DNA sequence. **Explanation:** **1. Why Option C is Correct:** Type II restriction endonucleases are essential tools in recombinant DNA technology. Unlike Type I or III enzymes, Type II enzymes are highly specific: they recognize a **unique palindromic sequence** (a sequence that reads the same 5’ to 3’ on both strands) and perform a precise **double-stranded cut** within or immediately adjacent to that recognition site. This predictable cleavage produces either "sticky ends" (overhangs) or "blunt ends," which are fundamental for gene cloning and DNA mapping. **2. Analysis of Incorrect Options:** * **Option A:** Protein folding is managed by molecular chaperones (e.g., Heat Shock Proteins), not restriction enzymes. * **Option B:** Restriction enzymes do not "remove" DNA; they cleave the phosphodiester backbone at specific internal sites. The removal of nucleotides from the ends of DNA is the function of exonucleases. * **Option D:** Many Type II enzymes (like *HaeIII* or *AluI*) actually **create** blunt ends. Therefore, preventing their formation is not a characteristic function; rather, the type of end produced depends on the specific enzyme used. **3. High-Yield Clinical Pearls for NEET-PG:** * **Nomenclature:** The first letter is the Genus, the next two are the species, and the Roman numeral indicates the order of discovery (e.g., *EcoRI* from *E. coli*). * **Cofactor Requirement:** Type II enzymes typically require **Magnesium (Mg²⁺)** for their catalytic activity but do not require ATP (unlike Type I and III). * **RFLP:** Restriction Fragment Length Polymorphism (RFLP) utilizes these enzymes to detect genetic variations, such as the mutation in **Sickle Cell Anemia** (where the loss of an *MstII* recognition site is diagnostic). * **Methylation:** Bacteria protect their own DNA from these enzymes through **DNA methylation**, a process part of the "Restriction-Modification System."

Q: Submicroscopic deletions of any size can be detected by:

Multiplex ligation-dependent probe amplification (MLPA). **Explanation:** The correct answer is **Multiplex ligation-dependent probe amplification (MLPA)**. **Why MLPA is correct:** MLPA is a high-throughput variation of PCR that permits the detection of copy number variations (CNVs) such as **submicroscopic deletions or duplications** of specific gene sequences. Unlike standard PCR, MLPA does not amplify the target DNA itself; instead, it amplifies pairs of probes that hybridize to the target. It is highly sensitive and can detect changes in a single exon, making it the gold standard for diagnosing conditions caused by gene dosage imbalances, such as Duchenne Muscular Dystrophy (DMD) and Spinal Muscular Atrophy (SMA). **Why other options are incorrect:** * **Southern Blotting:** While it can detect large deletions, it is labor-intensive, requires large amounts of DNA, and lacks the resolution to detect very small, submicroscopic deletions across multiple exons simultaneously. * **Cytogenomic Array Technology (CMA):** Although CMA (like CGH) is excellent for detecting submicroscopic deletions across the whole genome, it generally has a lower resolution (typically >20-50kb) compared to MLPA, which can detect deletions at the single-exon level. * **Chromosome Painting (FISH):** This uses fluorescent probes to visualize chromosomes. It is limited by the resolution of light microscopy and cannot detect very small (submicroscopic) deletions unless they are larger than the probe size (usually >100kb). **High-Yield Clinical Pearls for NEET-PG:** * **MLPA** is the investigation of choice for **DMD/BMD** (detecting exon deletions/duplications). * **Karyotyping** resolution is ~5-10 Mb; **FISH** resolution is ~100 kb; **MLPA** can detect changes at the **single nucleotide/exon level**. * MLPA is also used for detecting **methylation status** (e.g., Prader-Willi and Angelman syndromes).

Q: Which of the following is functionally competent for the largest unit of the ribosomes?

Catalyze formation of the peptide bond. ### Explanation The question focuses on the functional role of the **large ribosomal subunit** (60S in eukaryotes, 50S in prokaryotes). **1. Why the correct answer is right:** The large ribosomal subunit contains the enzyme **Peptidyl transferase**, which is responsible for catalyzing the formation of the **peptide bond** between the amino acid in the A-site and the growing polypeptide chain in the P-site. Crucially, this catalytic activity is not performed by a protein, but by the **rRNA** itself (28S rRNA in eukaryotes; 23S rRNA in prokaryotes). Therefore, the large subunit acts as a **Ribozyme**. **2. Why the incorrect options are wrong:** * **tRNA (Option A):** tRNA acts as an adapter molecule that carries specific amino acids to the ribosome. It interacts with both subunits but is not a functional component *of* the large subunit itself. * **mRNA (Option B):** mRNA carries the genetic code from DNA. It primarily binds to the **small ribosomal subunit** (40S/30S) during the initiation of translation to ensure correct codon-anticodon pairing. * **Formation of polyribosomes (Option D):** Polyribosomes (polysomes) are formed when multiple ribosomes attach to a single mRNA strand. This is a structural arrangement of the entire translation machinery, not a specific function of the large subunit alone. **3. NEET-PG High-Yield Pearls:** * **Ribozyme Concept:** The 23S rRNA (prokaryotes) and 28S rRNA (eukaryotes) are the specific molecules with peptidyl transferase activity. * **Antibiotic Targets:** Many antibiotics target the large subunit to inhibit peptide bond formation (e.g., **Chloramphenicol** inhibits peptidyl transferase; **Macrolides** like Erythromycin block the exit tunnel). * **Svedberg Units:** Remember the eukaryotic ribosome is **80S** (60S + 40S) and the prokaryotic is **70S** (50S + 30S). The "S" stands for sedimentation coefficient, which is non-additive.

Q: Which enzyme prevents aging or senescence?

Telomerase. **Explanation:** **1. Why Telomerase is Correct:** Telomerase is a specialized **ribonucleoprotein reverse transcriptase** enzyme. In normal somatic cells, DNA polymerase cannot replicate the extreme 3' ends of linear chromosomes (the **"End Replication Problem"**), leading to progressive shortening of telomeres with each cell division. When telomeres reach a critical minimum length, the cell enters **replicative senescence** (the Hayflick limit). Telomerase prevents this by adding repetitive TTAGGG sequences to the ends of chromosomes, thereby maintaining genomic stability and "immortalizing" cells. It is highly active in germ cells, stem cells, and 90% of cancer cells. **2. Why Other Options are Incorrect:** * **DNA Polymerase:** While essential for DNA replication and repair, it cannot replicate the very ends of linear DNA, actually contributing to the shortening process that leads to aging. * **Catalase & Peroxidase:** These are antioxidant enzymes that neutralize reactive oxygen species (ROS) like hydrogen peroxide. While they protect cells from oxidative stress-induced damage, they do not directly address the chromosomal shortening mechanism that defines cellular senescence. **Clinical Pearls for NEET-PG:** * **Telomerase Composition:** It contains **TERT** (catalytic subunit/reverse transcriptase) and **TERC** (RNA template). * **Shelterin Complex:** A group of proteins that protects telomeres from being recognized as DNA double-strand breaks. * **Dyskeratosis Congenita:** A genetic disorder caused by telomerase deficiency, leading to premature aging, bone marrow failure, and mucosal leukoplakia. * **Cancer Link:** Telomerase reactivation is a hallmark of malignancy, allowing cancer cells to bypass senescence.

Question 1

Which of the following methods is used for sequencing a long DNA fragment?

Accepted Answer

Chromosome walking

Answer

Sanger's technique

Answer

Chain termination method

Answer

Restriction fragment length polymorphism (RFLP)

Question 2

Regarding ribosomes, all statements are true except?

Accepted Answer

Binding of aminoacyl tRNA to the ribosome requires 2 ATP.

Answer

16S rRNA identifies the Shine-Dalgarno sequence.

Answer

Cytosolic ribosomes synthesize proteins destined for peroxisomes.

Answer

The large subunit catalyzes peptide bond formation.

Question 3

Which of the following enzymes converts primary microRNA (pri-miRNA) to precursor microRNA (pre-miRNA)?

Accepted Answer

Drosha

Answer

Dicer

Answer

RISC

Answer

Exonuclease

Question 4

Which of the following is true about the genetic code?

Accepted Answer

AUA codes for methionine in mitochondria

Answer

UGA codes for selenocysteine

Answer

AUG codes for the initiator codon in mammalian cells

Answer

AGA and AGG act as chain terminators in mammals

Question 5

Which of the following is the complimentary sequence of 5' TTAAGCTAC 3'?

Accepted Answer

5' GTACGCTTAA 3'

Answer

5' AATTCGCATG 3'

Answer

5' CATGCGAATT 3'

Answer

5' TTAAGCGTAC 3'

Question 6

What does a frameshift mutation cause?

Accepted Answer

Alteration of the entire reading sequence

Answer

Transversion

Answer

Transition

Answer

Termination of protein synthesis

Question 7

What is the function of a Type II restriction enzyme after binding to a unique recognition site?

Accepted Answer

Double-stranded cut at a palindromic DNA sequence

Answer

Prevention of protein folding

Answer

Removing formed DNA

Answer

Preventing blunt end formation

Question 8

Submicroscopic deletions of any size can be detected by:

Accepted Answer

Multiplex ligation-dependent probe amplification (MLPA)

Answer

Southern blotting

Answer

Cytogenomic array technology

Answer

Chromosome painting

Question 9

Which of the following is functionally competent for the largest unit of the ribosomes?

Accepted Answer

Catalyze formation of the peptide bond

Answer

tRNA

Answer

mRNA

Answer

Formation of the polyribosomes

Question 10

Which enzyme prevents aging or senescence?

Accepted Answer

Telomerase

Answer

DNA polymerase

Answer

Catalase

Answer

Peroxidase

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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