Molecular Biology and Genomics Practice Questions

Q: Mutations that completely disrupt the function of a gene are used in which of the following techniques?

Targeted gene disruption. **Explanation:** **Targeted gene disruption**, commonly known as **Gene Knockout**, is the correct answer. This technique involves the deliberate "silencing" or inactivation of a specific gene to study its function. By using homologous recombination to replace a functional gene with an inactive or mutated DNA sequence, researchers can observe the resulting phenotypic changes. This allows for the determination of the gene's physiological role and the creation of animal models for human genetic diseases. **Analysis of Incorrect Options:** * **Non-sense mutation (A):** This is a type of point mutation where a codon is changed to a premature stop codon (UAG, UAA, UGA). While it disrupts gene function, it is a *type* of mutation, not a laboratory *technique* used to study gene function. * **Restriction Fragment Length Polymorphism (RFLP) (B):** This is a technique used to detect variations in DNA sequences (polymorphisms) based on different patterns of DNA fragments after digestion with restriction enzymes. It is used for mapping and diagnosis, not for disrupting gene function. * **Knock-in (D):** This technique involves inserting a functional gene (often a mutated version or a reporter gene) into a specific locus. Unlike a knockout, which aims to eliminate function, a knock-in aims to replace or add a specific function. **High-Yield Clinical Pearls for NEET-PG:** * **Knockout Mice:** The most common model for studying Mendelian disorders. * **CRISPR-Cas9:** The modern, most efficient tool for targeted gene disruption and editing. * **RNA Interference (RNAi):** Often confused with knockout; RNAi causes gene "knockdown" (temporary/partial suppression) rather than permanent disruption at the DNA level.

Q: What type of mutation involves the substitution of a purine for a pyrimidine or vice versa?

Transversion. ### Explanation **1. Why Transversion is Correct:** Point mutations (single base substitutions) are classified into two categories: transitions and transversions. A **transversion** occurs when a **purine (A, G)** is replaced by a **pyrimidine (C, T)** or vice versa. Structurally, this involves swapping a two-ringed structure for a single-ringed structure (or vice versa), which causes a more significant distortion of the DNA helix than a transition. **2. Why the Other Options are Incorrect:** * **B. Transition:** This is a substitution where a purine is replaced by another purine (A ↔ G) or a pyrimidine is replaced by another pyrimidine (C ↔ T). Transitions are more common in the genome than transversions. * **C. Insertion:** This involves the addition of one or more extra nucleotides into the DNA sequence. If not in multiples of three, it leads to a **frameshift mutation**. * **D. Deletion:** This involves the removal of one or more nucleotides. Like insertions, these are categorized as "indels" and often result in truncated or non-functional proteins. **3. High-Yield Clinical Pearls for NEET-PG:** * **Frequency:** Although there are twice as many possible transversion pathways as transitions, **transitions** occur more frequently in nature due to the spontaneous deamination of 5-methylcytosine to thymine. * **Sickle Cell Anemia:** A classic example of a transversion mutation (GAG → GTG), where Adenine (purine) is replaced by Thymine (pyrimidine) in the β-globin gene, leading to Glutamate being replaced by Valine. * **Silent vs. Missense:** Substitutions can be **silent** (no amino acid change), **missense** (different amino acid), or **nonsense** (creates a premature stop codon: UAA, UAG, UGA).

Q: The gene determining testicular development is located on which chromosome?

Y chromosome. **Explanation:** The correct answer is the **Y chromosome**. The primary determinant of male sexual differentiation is the **SRY gene** (Sex-determining Region of the Y chromosome), located on the short arm (p arm) of the Y chromosome. This gene encodes the **Testis-Determining Factor (TDF)**, a transcription factor that triggers the undifferentiated gonads to develop into testes. Once testes are formed, they secrete testosterone (from Leydig cells) and Anti-Müllerian Hormone (from Sertoli cells) to complete male phenotypic development. **Analysis of Incorrect Options:** * **Option A (X chromosome):** While the X chromosome contains genes essential for ovarian development and general survival, it lacks the SRY gene. In the absence of a Y chromosome (e.g., Turner Syndrome, 45,XO), testes do not develop. * **Option C & D (Chromosomes 21 & 18):** These are autosomes. While they carry genes for general development, they do not determine primary sex. Trisomy 21 (Down Syndrome) and Trisomy 18 (Edwards Syndrome) result in multisystem anomalies but do not switch the sex-determination pathway. **High-Yield Clinical Pearls for NEET-PG:** * **SRY Gene Location:** Short arm of the Y chromosome (Yp11.3). * **Swyer Syndrome (46,XY Pure Gonadal Dysgenesis):** Occurs due to a mutation or deletion of the SRY gene; individuals have a female phenotype despite a male genotype. * **XX Male Syndrome (de la Chapelle Syndrome):** Occurs when the SRY gene translocates onto an X chromosome during meiosis, resulting in a male phenotype with a 46,XX karyotype. * **SOX9 Gene:** A downstream target of SRY; it is also critical for testis differentiation.

Q: Study the following pedigree carefully. What is the inheritance pattern of the disease in the family?

X-linked recessive type. ***X-linked recessive type*** - Shows **affected males** across multiple generations with **unaffected carrier females** transmitting the trait, which is characteristic of X-linked recessive inheritance. - Demonstrates **no male-to-male transmission** since affected fathers cannot pass X-linked traits to their sons (fathers give Y chromosome to sons). *Autosomal recessive type* - Would require **both parents to be carriers** for affected offspring, and would show equal distribution between **males and females**. - Typically shows **horizontal pattern** within a single generation rather than the vertical transmission seen across generations. *Autosomal dominant type* - Would show **affected individuals in every generation** with at least one affected parent for each affected child. - Would demonstrate **equal sex distribution** and **vertical transmission** from affected parent to child in consecutive generations. *X-Linked dominant type* - Would show **all daughters of affected males** being affected since fathers must pass their X chromosome to daughters. - Would typically show **more affected females than males** and affected males would have all affected daughters.

Q: Restriction fragment length polymorphism uses which of the following techniques?

Southern blotting. ### Explanation **1. Why Southern Blotting is Correct:** Restriction Fragment Length Polymorphism (RFLP) is a technique used to detect variations in homologous DNA sequences. The process involves: * **Digestion:** DNA is cut into fragments using **Restriction Endonucleases**. * **Separation:** Fragments are separated by size via gel electrophoresis. * **Southern Blotting:** The DNA fragments are transferred from the gel to a membrane and hybridized with a labeled **DNA probe**. Since RFLP specifically analyzes **DNA** variations, Southern blotting (the gold standard for DNA detection) is the integral technique used to visualize these fragments. **2. Why Other Options are Incorrect:** * **Northern Blotting:** This technique is used for the detection and quantification of **RNA**. It is used to study gene expression but cannot be used for RFLP as RFLP analyzes genomic DNA. * **Western Blotting:** This is used for the detection of specific **Proteins** using antibodies (e.g., confirmatory test for HIV). * **Eastern Blotting:** This is an extension of western blotting used to detect **post-translational modifications** of proteins (like carbohydrates or lipids). It is not a standard diagnostic tool for genetic polymorphisms. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Mnemonic (SNOW DROP):** * **S**outhern = **D**NA * **N**orthern = **R**NA * **O** = **O** (Nothing) * **W**estern = **P**rotein * **RFLP Applications:** Used in forensic "DNA fingerprinting," paternity testing, and identifying carriers of genetic diseases like Sickle Cell Anemia (where a mutation destroys a restriction site). * **Restriction Endonucleases:** Also known as "molecular scissors," they recognize specific palindromic sequences.

Q: The process used to prevent the recircularization of the stick ends of DNA is:

Homopolymer tailing. **Explanation:** **1. Why Homopolymer Tailing is Correct:** In recombinant DNA technology, when a vector is cut with restriction enzymes, it often results in "sticky ends" (complementary overhangs). These ends have a high tendency to re-anneal (recircularize) without incorporating the target DNA insert. **Homopolymer tailing** involves using the enzyme **Terminal Deoxynucleotidyl Transferase (TdT)** to add a string of identical nucleotides (e.g., poly-G) to the 3' ends of the vector and a complementary string (e.g., poly-C) to the 3' ends of the insert. Because the vector ends are now identical (G-G), they cannot base-pair with each other, effectively preventing recircularization. They can only pair with the complementary "tail" on the insert. **2. Why Other Options are Incorrect:** * **B. Ligation by restriction endonucleases:** This is a conceptual distractor. Restriction endonucleases *cut* DNA; they do not ligate it. DNA Ligase is the enzyme used for joining, and standard ligation often *promotes* recircularization rather than preventing it. * **C. Transfection:** This is the process of introducing foreign nucleic acids into eukaryotic cells (e.g., via liposomes or calcium phosphate). It is a downstream step in cloning and has no role in the enzymatic modification of DNA ends. **3. High-Yield Clinical Pearls for NEET-PG:** * **Terminal Deoxynucleotidyl Transferase (TdT):** Unlike DNA polymerase, TdT is **template-independent**. * **Clinical Correlation:** TdT is a crucial diagnostic marker in hematooncology. It is expressed in immature T and B cells; thus, it is a positive marker for **Acute Lymphoblastic Leukemia (ALL)** but negative in Mature B-cell lymphomas and AML (except M0). * **Alternative Method:** Another common way to prevent recircularization is using **Alkaline Phosphatase**, which removes the 5' phosphate group required by DNA ligase.

Q: Peptidyltransferase is an example of which of the following?

Ribozyme. **Explanation:** **Why Ribozyme is the correct answer:** Peptidyltransferase is the enzyme responsible for forming peptide bonds between adjacent amino acids during the elongation phase of translation. Unlike most enzymes, which are proteins, peptidyltransferase is a **Ribozyme**—an RNA molecule with catalytic activity. Specifically, in prokaryotes, this activity is mediated by the **23S rRNA** (of the 50S subunit), and in eukaryotes, it is mediated by the **28S rRNA** (of the 60S subunit). It catalyzes the nucleophilic attack of the amino group of the A-site aminoacyl-tRNA on the carboxyl group of the P-site peptidyl-tRNA. **Analysis of Incorrect Options:** * **A. Enzyme:** While peptidyltransferase acts as an enzyme, "Ribozyme" is the more specific and accurate biochemical classification required for NEET-PG. * **B. Catalyst:** This is a broad term. While all ribozymes are biological catalysts, the question tests the specific structural nature of the molecule. * **C. Elongation Factor:** Elongation factors (like EF-Tu or EF-G) are proteins that facilitate the movement of tRNA and mRNA through the ribosome, but they do not possess the catalytic activity to form peptide bonds. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of Action:** Peptidyltransferase is the target of the antibiotic **Chloramphenicol**, which inhibits the 50S subunit in bacteria. * **Other Ribozymes:** Examples include **SnRNAs** (involved in splicing/spliceosomes) and **RNase P** (involved in tRNA processing). * **Ribosome Structure:** Remember the mnemonic "23S (Prokaryotes) and 28S (Eukaryotes) are the catalysts."

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.

Question 1

Which of the following statements is true regarding the genetics of cancer?

Accepted Answer

Topoisomerase II is useful for breaking double-stranded DNA.

Answer

Increased telomerase activity enhances the anti-tumour effect.

Answer

Maximum synthesis of DNA occurs during the G2 phase.

Answer

Transition from G2 to M phase is important in the blockage of cancer spread.

Question 2

Mutations that completely disrupt the function of a gene are used in which of the following techniques?

Accepted Answer

Targeted gene disruption

Answer

Non-sense mutation

Answer

Restriction fragment length polymorphism

Answer

Knock-in

Question 3

What type of mutation involves the substitution of a purine for a pyrimidine or vice versa?

Accepted Answer

Transversion

Answer

Transition

Answer

Insertion

Answer

Deletion

Question 4

The gene determining testicular development is located on which chromosome?

Accepted Answer

Y chromosome

Answer

X chromosome

Answer

Chromosome 21

Answer

Chromosome 18

Question 5

Study the following pedigree carefully. What is the inheritance pattern of the disease in the family?

Accepted Answer

X-linked recessive type

Answer

Autosomal recessive type

Answer

Autosomal dominant type

Answer

X-Linked dominant type

Question 6

Restriction fragment length polymorphism uses which of the following techniques?

Accepted Answer

Southern blotting

Answer

Northern blotting

Answer

Western blotting

Answer

Eastern blotting

Question 7

The process used to prevent the recircularization of the stick ends of DNA is:

Accepted Answer

Homopolymer tailing

Answer

Ligation by restriction endonucleases

Answer

Transfection

Answer

All of the above

Question 8

Peptidyltransferase is an example of which of the following?

Accepted Answer

Ribozyme

Answer

Enzyme

Answer

Catalyst

Answer

Elongation Factor

Question 9

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 10

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

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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