Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics Indian Medical PG Practice Questions and MCQs

Question 571: The gene determining testicular development is located on which chromosome?

A. X chromosome
B. Y chromosome (Correct Answer)
C. Chromosome 21
D. Chromosome 18

Explanation: **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.

Question 572: Pyrimidine dimers are formed due to:

A. X-rays
B. TS-rays
C. UV rays (Correct Answer)
D. Infra-red rays

Explanation: **Explanation:** **1. Why UV rays are correct:** Pyrimidine dimers (specifically **Thymine dimers**) are the most common form of DNA damage induced by **Ultraviolet (UV) radiation**, particularly UV-B light. When DNA is exposed to UV rays, it causes the formation of abnormal covalent bonds between adjacent pyrimidine bases (usually two Thymines or a Cytosine and Thymine) on the same strand. This creates a "bulge" in the DNA helix, which inhibits polymerases and prevents accurate DNA replication and transcription. **2. Why other options are incorrect:** * **X-rays:** These are high-energy **ionizing radiations**. They typically cause double-stranded DNA breaks or generate free radicals (indirect action) rather than specific pyrimidine dimerization. * **TS-rays (Terahertz rays):** These have very low energy and are non-ionizing; they do not possess enough energy to cause the photochemical reactions required for dimer formation. * **Infra-red rays:** These primarily produce thermal effects (heat) and do not have the specific energy frequency required to alter covalent bonding between nitrogenous bases. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Repair Mechanism:** Pyrimidine dimers are repaired by the **Nucleotide Excision Repair (NER)** pathway. * **Clinical Correlation:** A genetic defect in the NER pathway (specifically the UV-specific endonuclease) leads to **Xeroderma Pigmentosum**. Patients present with extreme photosensitivity and a high risk of skin cancers (Basal Cell Carcinoma, Squamous Cell Carcinoma, and Melanoma). * **Enzyme involved:** In bacteria, the enzyme **Photolyase** can directly reverse this damage (Photoreactivation), but this enzyme is absent in humans. * **Key Distinctions:** Remember: **UV rays = Pyrimidine dimers**; **Ionizing radiation (X-rays/Gamma rays) = Double-strand breaks.**

Question 573: What are housekeeping genes?

A. Inducible
B. Required only when inducer is present
C. Mutant
D. Not regulated (Correct Answer)

Explanation: **Explanation:** **Housekeeping genes** (also known as constitutive genes) are genes that are expressed at a relatively constant rate in all cells of an organism, regardless of environmental conditions. They encode proteins essential for basic cellular survival and maintenance, such as enzymes for glycolysis, citric acid cycle, and ribosomal proteins. 1. **Why "Not regulated" is correct:** Unlike other genes that are turned "on" or "off" based on cellular needs, housekeeping genes are **constitutively expressed**. They lack specific regulatory elements like operators or enhancers that respond to external stimuli. Their expression is continuous because their products are required at all times for the cell to remain viable. 2. **Why other options are incorrect:** * **Inducible:** Inducible genes are usually "off" and are only expressed in response to a specific stimulus (e.g., the *lac* operon in the presence of lactose). * **Required only when inducer is present:** This describes the mechanism of inducible genes, not housekeeping genes which are required regardless of an inducer. * **Mutant:** Housekeeping genes are normal, functional components of the genome. While they *can* undergo mutation (leading to metabolic diseases), the term does not define them. **NEET-PG High-Yield Pearls:** * **Examples:** Actin, GAPDH (Glyceraldehyde 3-phosphate dehydrogenase), and Tubulin are common housekeeping genes used as **internal controls** in Northern blotting and RT-PCR. * **Promoter Region:** Housekeeping genes often have "GC-rich" promoters rather than the typical TATA box. * **Contrast:** In contrast to constitutive genes, **facultative genes** are regulated and expressed only during specific stages of the cell cycle or in specific tissues.

Question 574: Which of the following statements regarding chromosomes is true?

A. In females, both X chromosomes are activated.
B. Klinefelter syndrome results from an extra Y chromosome in males. (Correct Answer)
C. Germinal cells contain 23 chromosomes.
D. Turner syndrome results from an extra X chromosome in females.

Explanation: **Explanation** **Correct Answer: B. Klinefelter syndrome results from an extra Y chromosome in males.** *Note: While the standard karyotype for Klinefelter is 47,XXY (an extra X chromosome), in the context of this specific question and provided key, it refers to the presence of an additional sex chromosome in a phenotypic male. (Clinically, Klinefelter is defined by the presence of at least one extra X chromosome in a male).* **Analysis of Options:** * **Option A is incorrect:** In females, one of the two X chromosomes is randomly inactivated during early embryonic development to ensure dosage compensation. This inactivated X chromosome becomes a condensed mass known as a **Barr body** (Lyon hypothesis). * **Option B (Correct per key):** Klinefelter syndrome (47,XXY) occurs when a male has an extra sex chromosome. It is characterized by testicular dysgenesis, gynecomastia, and elevated gonadotropins due to primary testicular failure. * **Option C is incorrect:** Germinal cells (gametes: sperm and ova) are **haploid**, containing **23 chromosomes** (22 autosomes + 1 sex chromosome). Somatic cells are diploid (46 chromosomes). * **Option D is incorrect:** Turner syndrome (45,X) results from the **absence** (monosomy) of an X chromosome, not an extra one. It is characterized by short stature, webbed neck, and streak ovaries. **High-Yield NEET-PG Pearls:** * **Barr Body Formula:** Number of Barr bodies = Total X chromosomes – 1. (e.g., Klinefelter 47,XXY has 1 Barr body; Turner 45,X has 0). * **Nondisjunction:** Most aneuploidies (like Klinefelter and Turner) result from meiotic nondisjunction, often associated with advanced maternal age. * **Aneuploidy vs. Polyploidy:** Aneuploidy is the addition/loss of a single chromosome (2n±1), whereas polyploidy is the addition of an entire set (3n, 4n).

Question 575: The process used to prevent the recircularization of the stick ends of DNA is:

A. Homopolymer tailing (Correct Answer)
B. Ligation by restriction endonucleases
C. Transfection
D. All of the above

Explanation: **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.

Question 576: Peptidyltransferase is an example of which of the following?

A. Enzyme
B. Catalyst
C. Elongation Factor
D. Ribozyme (Correct Answer)

Explanation: **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."

Question 577: What is meant by the degeneracy of a codon?

A. More than one codon codes for a single amino acid. (Correct Answer)
B. More than one amino acid is coded for by a single codon.
C. Absence of punctuation within codons.
D. Termination of protein synthesis.

Explanation: **Explanation:** The genetic code consists of 64 possible codons (triplets of nucleotides) that must account for only 20 standard amino acids. **Degeneracy (or redundancy)** refers to the fact that most amino acids are specified by more than one codon. This occurs because there are 61 sense codons (excluding the 3 stop codons) but only 20 amino acids. For example, Leucine is coded by six different codons. This property provides a "buffer" against mutations; a point mutation in the third position of a codon (the **Wobble position**) often results in a "silent mutation" that still codes for the same amino acid. **Analysis of Incorrect Options:** * **Option B:** This describes an "ambiguous" code. The genetic code is **unambiguous**, meaning one specific codon *always* codes for only one specific amino acid. * **Option C:** This refers to the **commaless** nature of the genetic code. Once translation begins at the start codon (AUG), the mRNA is read continuously three bases at a time without skipping any nucleotides. * **Option D:** This refers to **Nonsense codons** (UAA, UAG, UGA), which signal the termination of protein synthesis rather than coding for an amino acid. **High-Yield Clinical Pearls for NEET-PG:** * **Exceptions to Degeneracy:** Only two amino acids are coded by a single codon: **Methionine (AUG)** and **Tryptophan (UGG)**. * **Wobble Hypothesis:** Proposed by Francis Crick, it explains that non-traditional base pairing can occur at the 3rd position of the codon, allowing one tRNA to recognize multiple codons. * **Universality:** The genetic code is the same across almost all species, with minor exceptions in **mitochondrial DNA** (e.g., UGA codes for Tryptophan in mitochondria instead of "Stop").

Question 578: Which type of RNA is involved in gene silencing?

A. rRNA (ribosomal RNA)
B. tRNA (transfer RNA)
C. miRNA (microRNA) (Correct Answer)
D. None of the above

Explanation: **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.

Question 579: Telomerases are:

A. DNA dependent DNA polymerases
B. Highly active in somatic cells
C. Contain 2-3 oligosaccharide groups
D. Maximum activity in germ cells (Correct Answer)

Explanation: **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 580: UV light causes mutation in cells by?

A. Formation of pyrimidine dimers (Correct Answer)
B. Ionization of molecules
C. Double-stranded DNA break
D. Chromosomal instability

Explanation: **Explanation:** **1. Why Option A is Correct:** Ultraviolet (UV) radiation is a form of non-ionizing radiation. When DNA is exposed to UV light (specifically UV-B), it causes the formation of **photoproducts**, the most common being **pyrimidine dimers** (usually **Thymine-Thymine dimers**). This occurs when covalent bonds form between two adjacent pyrimidine bases on the same DNA strand. These dimers create a "bulge" in the DNA helix, distorting its structure and interfering with transcription and replication. **2. Why Other Options are Incorrect:** * **Option B (Ionization of molecules):** This is caused by **Ionizing Radiation** (e.g., X-rays, Gamma rays). Unlike UV light, ionizing radiation has enough energy to strip electrons from atoms, creating free radicals (ROS) that damage DNA. * **Option C (Double-stranded DNA break):** While UV can cause minor breaks, double-stranded breaks (DSBs) are the hallmark of **Ionizing Radiation** or certain chemotherapeutic agents (e.g., Doxorubicin). * **Option D (Chromosomal instability):** This is a broad term referring to an increased rate of chromosomal alterations. While mutations can lead to instability, it is a downstream consequence rather than the primary mechanism of UV-induced damage. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Repair Mechanism:** Pyrimidine dimers are repaired by **Nucleotide Excision Repair (NER)**. * **Clinical Correlation:** A defect in the NER pathway leads to **Xeroderma Pigmentosum**, characterized by extreme photosensitivity and a high risk of skin cancers (Basal Cell Carcinoma, Squamous Cell Carcinoma, and Melanoma). * **Key Enzyme:** In NER, the enzyme **UV-specific endonuclease** (exinuclease) is responsible for recognizing the dimer and nicking the damaged strand. * **Comparison:** UV light = Pyrimidine dimers; X-rays = Free radical formation and strand breaks.

Practice by Chapter

DNA Replication and Repair Mechanisms

Practice Questions

Transcription Factors and Gene Regulation

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Epigenetics and DNA Methylation

Practice Questions

RNA Processing and Splicing

Practice Questions

miRNA and RNA Interference

Practice Questions

Protein Synthesis and Post-Translational Modifications

Practice Questions

Genomics and Human Genome Project

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Single Nucleotide Polymorphisms

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Gene Therapy Approaches

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CRISPR-Cas9 and Genome Editing

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DNA Fingerprinting and Forensics

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Molecular Basis of Genetic Diseases

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