Molecular Biology and Genomics Practice Questions

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: 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.

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).

Question 1

The gene determining testicular development is located on which chromosome?

Accepted Answer

Y chromosome

Answer

X chromosome

Answer

Chromosome 21

Answer

Chromosome 18

Question 2

Restriction fragment length polymorphism uses which of the following techniques?

Accepted Answer

Southern blotting

Answer

Northern blotting

Answer

Western blotting

Answer

Eastern blotting

Question 3

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 4

Peptidyltransferase is an example of which of the following?

Accepted Answer

Ribozyme

Answer

Enzyme

Answer

Catalyst

Answer

Elongation Factor

Question 5

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 6

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 7

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 8

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 9

What type of mutation is characteristic of sickle cell anemia?

Accepted Answer

Missense mutation

Answer

Silent mutation

Answer

Nonsense mutation

Answer

Frameshift mutation

Question 10

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.

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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