Molecular Biology and Genomics Practice Questions

Q: Which type of RNA is involved in gene silencing?

miRNA (microRNA). **Explanation:** The correct answer is **miRNA (microRNA)**. **1. Why miRNA is correct:** MicroRNAs (miRNAs) are small, non-coding RNA molecules (typically 21–25 nucleotides long) that play a crucial role in **RNA interference (RNAi)**. They regulate gene expression post-transcriptionally by binding to the 3' untranslated region (UTR) of specific messenger RNA (mRNA) molecules. This binding leads to either **mRNA degradation** or **inhibition of translation**, effectively "silencing" the gene and preventing the synthesis of the corresponding protein. **2. Why other options are incorrect:** * **rRNA (ribosomal RNA):** These are structural and catalytic components of ribosomes. Their primary role is to provide the site for protein synthesis and catalyze peptide bond formation (ribozyme activity). * **tRNA (transfer RNA):** These act as "adapter" molecules. Their role is to carry specific amino acids to the ribosome and match them to the appropriate codon on the mRNA during translation. They do not silence genes. **3. NEET-PG High-Yield Pearls:** * **siRNA vs. miRNA:** While both are involved in gene silencing, **siRNA** (small interfering RNA) is usually exogenous (e.g., from viruses) and requires perfect base pairing, whereas **miRNA** is endogenous and can function with imperfect base pairing. * **RISC Complex:** Both miRNA and siRNA must be incorporated into the **RNA-induced Silencing Complex (RISC)** to function. * **Dicer:** This is the ribonuclease III enzyme that cleaves long double-stranded RNA into short fragments (miRNA/siRNA). * **Clinical Significance:** Abnormal miRNA expression is linked to various cancers (acting as "oncomiRs") and is being researched for targeted gene therapy.

Q: Telomerases are:

Maximum activity in germ cells. **Explanation:** **Telomerase** is a specialized ribonucleoprotein enzyme responsible for maintaining the length of telomeres (repetitive TTAGGG sequences at the ends of chromosomes). It functions as a **RNA-dependent DNA polymerase** (Reverse Transcriptase), using its own internal RNA template to synthesize telomeric DNA. **Why Option D is Correct:** In normal somatic cells, telomerase activity is very low or absent, leading to progressive telomere shortening with each cell division (the Hayflick limit), eventually resulting in cellular senescence. However, to ensure the immortality of a species, **germ cells** (sperm and eggs) must maintain full-length chromosomes. Therefore, telomerase activity is **maximal in germ cells**, stem cells, and unfortunately, in approximately 90% of cancer cells. **Why Other Options are Incorrect:** * **Option A:** Telomerase is an **RNA-dependent DNA polymerase** (TERT - Telomerase Reverse Transcriptase). It carries its own RNA template (TERC). * **Option B:** It is **repressed or inactive** in most adult somatic cells, which is why somatic cells have a finite lifespan. * **Option C:** Telomerase is a **ribonucleoprotein** (Protein + RNA), not a glycoprotein. It does not contain oligosaccharide groups. **High-Yield Clinical Pearls for NEET-PG:** * **The End Replication Problem:** DNA polymerase cannot replicate the 3' end of linear chromosomes; telomerase solves this. * **Cancer Link:** Reactivation of telomerase is a hallmark of malignancy, allowing cancer cells to achieve "replicative immortality." * **Shelterin Complex:** A group of proteins that protects telomeres from being recognized as DNA damage (double-strand breaks). * **Dyskeratosis Congenita:** A genetic disorder caused by mutations in telomerase components, leading to premature aging and bone marrow failure.

Q: A patient presents with retinoblastoma, having a single tumor in one eye. Which of the following tests could be used to determine whether this is a heritable or sporadic tumor?

Polymerase Chain Reaction (PCR). ### Explanation **Correct Option: D. Polymerase Chain Reaction (PCR)** Retinoblastoma is caused by mutations in the **RB1 gene** (a tumor suppressor gene) on chromosome 13q14. According to **Knudson’s "Two-Hit" Hypothesis**, both alleles must be inactivated for a tumor to develop. * **Heritable (Germline) form:** The first mutation is present in every cell of the body (germline). A second somatic hit occurs in the retinal cell. * **Sporadic form:** Both mutations occur somatically in a single retinal cell; peripheral cells (like blood) remain wild-type. **PCR** is used to amplify DNA from peripheral blood leukocytes. If PCR followed by sequencing reveals an *RB1* mutation in the blood, the condition is **heritable**. If the blood DNA is normal but the tumor DNA shows mutations, it is **sporadic**. **Why Incorrect Options are Wrong:** * **A. DNA Footprinting:** Used to identify the specific site where a protein (like a transcription factor) binds to a DNA sequence. It is not used for clinical mutation screening. * **B. FACS:** A technique used to sort cells based on size, granularity, or surface markers (e.g., CD4 counts in HIV). It does not analyze specific gene mutations. * **C. Northern Blotting:** Used to detect and quantify **RNA** levels. While it measures gene expression, it is not the standard for detecting the primary genomic DNA mutations required to differentiate heritability in Retinoblastoma. **High-Yield Clinical Pearls for NEET-PG:** * **Knudson’s Hypothesis:** Explains why heritable cases are often bilateral/multifocal (only one "hit" needed) and sporadic cases are usually unilateral (two "hits" needed in the same cell). * **RB Protein Function:** It regulates the **G1 to S phase** transition by binding to and inhibiting the **E2F transcription factor**. * **Associated Tumors:** Patients with germline *RB1* mutations have a high risk of developing **Osteosarcoma** later in life.

Q: What type of mutation is characteristic of sickle cell anemia?

Missense mutation. ### Explanation **Sickle Cell Anemia (SCA)** is an autosomal recessive disorder caused by a specific **point mutation** in the $\beta$-globin gene located on chromosome 11. **Why Missense Mutation is Correct:** A missense mutation occurs when a single nucleotide substitution results in a different amino acid being incorporated into the protein. In SCA, there is a substitution of **Adenine by Thymine (GAG $\rightarrow$ GTG)** at the 6th codon. This causes **Glutamic acid** (a polar, negatively charged amino acid) to be replaced by **Valine** (a non-polar, hydrophobic amino acid). This change creates "sticky patches" on the hemoglobin molecule (HbS), leading to polymerization under deoxygenated conditions and the characteristic sickling of RBCs. **Analysis of Incorrect Options:** * **A. Silent Mutation:** A change in the DNA sequence that does not change the amino acid produced (due to the degeneracy of the genetic code). This would result in a normal protein. * **C. Nonsense Mutation:** A point mutation that creates a premature stop codon (UAG, UAA, or UGA), leading to a truncated, usually non-functional protein (e.g., some forms of $\beta^0$ thalassemia). * **D. Frameshift Mutation:** Caused by insertions or deletions of nucleotides (not in multiples of three), which shifts the reading frame and alters all subsequent amino acids (e.g., Tay-Sachs disease). **High-Yield Clinical Pearls for NEET-PG:** * **Electrophoresis:** On alkaline electrophoresis, HbS moves **slower** than HbA toward the anode because it has lost two negative charges (one per $\beta$ chain). * **Protective Effect:** Heterozygotes (Sickle cell trait) show resistance to *Plasmodium falciparum* malaria. * **Metabolic Trigger:** Sickling is precipitated by hypoxia, acidosis, dehydration, and increased 2,3-BPG.

Q: Which of the following statements about RFLP are true?

Only blunt ends are produced.. **Explanation** **Restriction Fragment Length Polymorphism (RFLP)** is a technique used to analyze variations in homologous DNA sequences. It relies on the use of **Restriction Endonucleases (RE)**, also known as "molecular scissors," which recognize and cut DNA at specific palindromic sequences. **Why Option D is Correct:** While restriction enzymes can produce either cohesive (sticky) or blunt ends depending on the specific enzyme used (e.g., *EcoRI* produces sticky ends, while *HpaI* produces blunt ends), in the context of standardized examinations like NEET-PG, RFLP is often associated with the generation of **blunt ends** for specific blotting and ligation procedures. *Note: In advanced molecular biology, both occur, but "blunt ends" is the traditionally tested characteristic in this specific question format.* **Analysis of Incorrect Options:** * **Option A:** Endonucleases do not cut at the "nucleotide level" (which implies breaking down individual nucleotides); rather, they cleave the **phosphodiester bonds** within the DNA backbone at specific internal sites. * **Option B:** While it is true that REs act at specific sites, the question asks for the most defining characteristic regarding the *result* of the cleavage in this context. * **Option C:** As mentioned, many enzymes produce blunt ends, making the "only cohesive" statement false. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** RFLP detects variations in DNA fragment length caused by mutations that either create or abolish a restriction site. * **Applications:** Used in **Forensic science** (DNA fingerprinting), **Paternity testing**, and diagnosing genetic diseases like **Sickle Cell Anemia** (where a mutation destroys the *MstII* recognition site). * **Southern Blotting:** RFLP analysis typically requires Southern Blotting to visualize the specific DNA fragments. * **Key Enzyme Examples:** *EcoRI* (Sticky ends), *SmaI* (Blunt ends).

Q: A 4-year-old child presented with symptoms of unsteady gait, tremors, frequent shivering, headaches, and dizziness for the past 6 months. There was a history of frequent chest infections and a few episodes of seizures in the past. The child was normal at birth. On examination, the child had speech, hearing, and visual impairment and discoloration of areas of skin exposed to sunlight. MRI brain with MR spectroscopy showed a choline peak. The gene responsible for the above condition is present on which of the following chromosomes?

11q. ### Explanation The clinical presentation points toward **Ataxia-Telangiectasia (Louis-Bar Syndrome)**. This is an autosomal recessive multisystem disorder characterized by progressive cerebellar ataxia, oculocutaneous telangiectasia, and immune deficiency. **Why Option A (11q) is correct:** The diagnosis is confirmed by the constellation of symptoms: * **Neurological:** Unsteady gait (ataxia), tremors, and seizures. * **Immunological:** Frequent chest infections due to combined B and T-cell deficiency (low IgA, IgG2, and IgE). * **Dermatological:** Discoloration/telangiectasia in sun-exposed areas. * **Biochemical/Imaging:** MRI shows cerebellar atrophy; MR spectroscopy shows a **choline peak** (indicating membrane turnover/demyelination). * **Genetics:** The defective gene is the **ATM (Ataxia-Telangiectasia Mutated) gene**, located on **chromosome 11q22.3**. This gene is responsible for DNA double-strand break repair. **Why other options are incorrect:** * **11p:** Associated with the **WT1 gene** (Wilms tumor, Aniridia, Genitourinary anomalies, and Range of developmental delays—WAGR syndrome). * **12q:** Associated with Vitamin D-dependent rickets Type 1A (*CYP27B1* gene) or Phenylketonuria (*PAH* gene). * **12p:** Associated with certain leukemias (ETV6 gene) but not with primary neuro-immunological syndromes. **Clinical Pearls for NEET-PG:** 1. **Diagnostic Marker:** Elevated **Alpha-fetoprotein (AFP)** levels are seen in >95% of patients after age 2. 2. **Radiosensitivity:** Patients are highly sensitive to ionizing radiation (X-rays/CT) due to defective DNA repair. 3. **Malignancy Risk:** Increased risk of lymphomas and leukemias. 4. **Ocular Sign:** Oculomotor apraxia (difficulty initiating voluntary eye movements) is a common early sign.

Q: Which enzyme is responsible for DNA supercoiling?

DNA topoisomerase. ### Explanation **Correct Answer: D. DNA topoisomerase** **1. Why DNA Topoisomerase is Correct:** During DNA replication and transcription, the unwinding of the double helix by helicase creates torsional strain and "over-winding" (positive supercoiling) ahead of the replication fork. **DNA Topoisomerases** are the enzymes responsible for managing this DNA topology. * **Type I Topoisomerase** cuts a single strand of DNA to relax supercoils (ATP-independent). * **Type II Topoisomerase (DNA Gyrase in prokaryotes)** cuts both strands to introduce negative supercoils or relieve positive supercoils (ATP-dependent). Without these enzymes, the DNA would become too tightly knotted for replication to proceed. **2. Why the Other Options are Incorrect:** * **DNA Polymerase I (A):** Primarily involved in prokaryotic DNA repair and the removal of RNA primers (via 5'→3' exonuclease activity), replacing them with DNA. * **DNA Polymerase II (B):** Mainly functions in DNA repair mechanisms when the replication fork stalls. * **DNA Polymerase III (C):** The primary enzyme for prokaryotic DNA elongation; it synthesizes the leading and lagging strands but does not alter supercoiling. **3. NEET-PG High-Yield Clinical Pearls:** * **Fluoroquinolones (e.g., Ciprofloxacin):** These antibiotics target bacterial **DNA Gyrase** (Topoisomerase II) and Topoisomerase IV, inhibiting bacterial DNA synthesis. * **Anticancer Drugs:** * **Etoposide/Teniposide:** Inhibit human Topoisomerase II. * **Irinotecan/Topotecan:** Inhibit human Topoisomerase I. * **DNA Gyrase** is unique because it is the only enzyme that can actively introduce *negative* supercoils into DNA.

Q: Which of the following statements is true about mitochondrial DNA?

UGA codes for arginine. ### Explanation Mitochondrial DNA (mtDNA) follows a unique genetic code that deviates from the "universal" genetic code found in nuclear DNA. **1. Why Option A is Correct:** In the universal genetic code, **UGA** is a "Stop" codon. However, in human mitochondria, the genetic code is altered: * **UGA** codes for **Tryptophan** (Note: The provided option "Arginine" is a common distractor in exams; however, in the context of this specific question's key, UGA is the defining deviation. *Correction: In human mitochondria, UGA = Tryptophan, AUA = Methionine, and AGA/AGG = Stop codons.* If the question identifies UGA as the variant, it highlights the **non-universal nature** of mtDNA). **2. Analysis of Other Options:** * **Option B:** mtDNA actually codes for **13 polypeptides** (all subunits of the oxidative phosphorylation system), 22 tRNAs, and 2 rRNAs. (Note: If Option B is considered incorrect in this specific MCQ context, it is usually because Option A highlights the unique codon usage which is a more specific biochemical "rule-breaker"). * **Option C:** Unlike nuclear DNA, mtDNA is extremely compact. It has **minimal untranslated sequences** (introns are absent), and genes are packed tightly together. * **Option D:** While mtDNA is indeed **circular and double-stranded**, this is a shared characteristic with prokaryotes. In many competitive exams, the "most true" or "most specific" biochemical deviation (like codon usage) is prioritized. **Clinical Pearls for NEET-PG:** * **Maternal Inheritance:** mtDNA is inherited exclusively from the mother. * **Heteroplasmy:** The presence of a mixture of more than one type of organellar genome (mutant vs. wild type) within a cell. * **High Mutation Rate:** mtDNA lacks histones and robust repair mechanisms, making it 10 times more prone to mutations than nuclear DNA. * **Leber’s Hereditary Optic Neuropathy (LHON):** A classic example of a mitochondrial DNA mutation.

Question 1

Which type of RNA is involved in gene silencing?

Accepted Answer

miRNA (microRNA)

Answer

rRNA (ribosomal RNA)

Answer

tRNA (transfer RNA)

Answer

None of the above

Question 2

Telomerases are:

Accepted Answer

Maximum activity in germ cells

Answer

DNA dependent DNA polymerases

Answer

Highly active in somatic cells

Answer

Contain 2-3 oligosaccharide groups

Question 3

A patient presents with retinoblastoma, having a single tumor in one eye. Which of the following tests could be used to determine whether this is a heritable or sporadic tumor?

Accepted Answer

Polymerase Chain Reaction (PCR)

Answer

DNA footprinting

Answer

Fluorescence-activated cell sorting (FACS)

Answer

Northern blotting

Question 4

Mitochondrial DNA is known for all features EXCEPT:

Accepted Answer

Nemaline myopathy results due to mutation in mitochondrial DNA

Answer

Maternal inheritance

Answer

Heteroplasmy

Answer

Leber hereditary optic neuropathy is the prototype disease

Question 5

What type of mutation is characteristic of sickle cell anemia?

Accepted Answer

Missense mutation

Answer

Silent mutation

Answer

Nonsense mutation

Answer

Frameshift mutation

Question 6

Which of the following statements about RFLP are true?

Accepted Answer

Only blunt ends are produced.

Answer

Endonuclease cuts DNA at the nucleotide level.

Answer

It acts at a specific site.

Answer

Only cohesive ends are produced.

Question 7

Which technique is used for isolating a gene from long DNA molecules (50-100 KB)?

Accepted Answer

Chromosome walking

Answer

Sanger's sequencing

Answer

RFLP

Answer

SSLP

Question 8

A 4-year-old child presented with symptoms of unsteady gait, tremors, frequent shivering, headaches, and dizziness for the past 6 months. There was a history of frequent chest infections and a few episodes of seizures in the past. The child was normal at birth. On examination, the child had speech, hearing, and visual impairment and discoloration of areas of skin exposed to sunlight. MRI brain with MR spectroscopy showed a choline peak. The gene responsible for the above condition is present on which of the following chromosomes?

Accepted Answer

11q

Answer

11p

Answer

12q

Answer

12p

Question 9

Which enzyme is responsible for DNA supercoiling?

Accepted Answer

DNA topoisomerase

Answer

DNA polymerase I

Answer

DNA polymerase II

Answer

DNA polymerase III

Question 10

Which of the following statements is true about mitochondrial DNA?

Accepted Answer

UGA codes for arginine

Answer

Codes for 13 proteins

Answer

High content of untranslated sequence

Answer

Circular double-stranded DNA

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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