Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics Indian Medical PG Practice Questions and MCQs

Question 381: A nucleic acid was analyzed and found to contain 32% adenine, 18% guanine, 17% cytosine, and 33% thymine. What type of nucleic acid must this be?

A. Single-stranded RNA
B. Single-stranded DNA
C. Double-stranded RNA
D. Double-stranded DNA (Correct Answer)

Explanation: ### Explanation **1. Why the Correct Answer is Right (Chargaff’s Rule)** The correct answer is **Double-stranded DNA (dsDNA)**. This determination is based on **Chargaff’s Rule**, which states that in a double-stranded DNA molecule, the molar ratio of purines to pyrimidines is 1:1. Specifically: * **Adenine (A) = Thymine (T)** * **Guanine (G) = Cytosine (C)** In the given data: * A (32%) is approximately equal to T (33%). * G (18%) is approximately equal to C (17%). The presence of **Thymine** confirms it is DNA (not RNA), and the near-equal proportions of complementary bases confirm it is **double-stranded**, where base-pairing occurs via hydrogen bonds. **2. Why the Incorrect Options are Wrong** * **A & C (RNA Options):** These are incorrect because the analysis shows the presence of **Thymine**. RNA contains **Uracil** instead of Thymine. * **B (Single-stranded DNA):** In ssDNA, there is no complementary strand to mandate base-pairing. Therefore, the ratios of A:T and G:C are usually unequal and random. Since the provided percentages show A≈T and G≈C, the molecule must be double-stranded. **3. NEET-PG High-Yield Pearls** * **Chargaff’s Rule Applicability:** It applies **only** to double-stranded DNA/RNA. It does **not** apply to single-stranded viruses (e.g., $\phi$X174) or mitochondrial DNA during certain replication phases. * **Base Pairing:** A-T pairs have **2 hydrogen bonds**, while G-C pairs have **3 hydrogen bonds**. * **Melting Temperature ($T_m$):** DNA with a higher G-C content has a higher $T_m$ because triple hydrogen bonds require more energy to denature. * **Formula:** Purines (A+G) = Pyrimidines (T+C). In this question: $32+18 = 50\%$ and $33+17 = 50\%$.

Question 382: The aromatase enzyme converts which of the following?

A. Estrogen to androgen
B. Androgen to estrogen (Correct Answer)
C. Progesterone to estrogen
D. Androgen to progesterone

Explanation: **Explanation:** The **aromatase enzyme** (also known as estrogen synthetase or CYP19A1) is a member of the Cytochrome P450 superfamily. Its primary function is the conversion of **androgens to estrogens**. Specifically, it catalyzes the aromatization of the "A-ring" of steroid precursors, converting **androstenedione to estrone** and **testosterone to estradiol**. This process occurs primarily in the granulosa cells of the ovaries, the placenta, adipose tissue, and the brain. **Analysis of Options:** * **Option A (Estrogen to androgen):** This is the reverse of the physiological pathway. There is no single enzyme that converts estrogens back into androgens in the human body. * **Option C (Progesterone to estrogen):** Progesterone is a precursor in the steroidogenesis pathway, but it must first be converted to androgens (via 17α-hydroxylase and 17,20-lyase) before it can be aromatized into estrogens. * **Option D (Androgen to progesterone):** This is biochemically incorrect; progesterone precedes androgens in the biosynthetic hierarchy. **High-Yield Clinical Pearls for NEET-PG:** * **Aromatase Inhibitors (e.g., Letrozole, Anastrozole):** These are first-line treatments for postmenopausal women with ER-positive breast cancer, as they block the peripheral production of estrogen in adipose tissue. * **Aromatase Deficiency:** A rare condition leading to virilization of the female fetus and mother during pregnancy, as androgens cannot be converted to estrogens and cross the placenta. * **PCOS Connection:** In Polycystic Ovary Syndrome, there is often an imbalance where high LH levels lead to excess androgen production that exceeds the aromatization capacity of the granulosa cells.

Question 383: During meiosis, the process of pairing of homologous chromosomes which permits large segments of DNA to be exchanged is known as:

A. Crossing over
B. Non-disjunction
C. Alignment
D. Synapsis (Correct Answer)

Explanation: **Explanation:** **1. Why Synapsis is Correct:** Synapsis is the highly specific process during the **Zygotene** stage of Prophase I where homologous chromosomes (one maternal and one paternal) pair up side-by-side. This pairing is mediated by a protein structure called the **synaptonemal complex**. Synapsis is the essential physical prerequisite that "permits" the exchange of genetic material by ensuring that corresponding gene sequences are perfectly aligned before the actual exchange occurs. **2. Why Other Options are Incorrect:** * **Crossing over:** While this is the actual *exchange* of DNA segments, the question asks for the "process of pairing" that *permits* this exchange. Crossing over occurs during the **Pachytene** stage, *after* synapsis has been established. * **Non-disjunction:** This is a pathological failure of homologous chromosomes or sister chromatids to separate properly during anaphase. It leads to aneuploidy (e.g., Trisomy 21) rather than normal genetic exchange. * **Alignment:** This is a general descriptive term often used for chromosomes lining up at the metaphase plate, but it is not the specific biological term for homologous pairing in meiosis. **3. NEET-PG High-Yield Pearls:** * **Stages of Prophase I (Mnemonic: LZPDD):** 1. **Leptotene:** Chromatin condensation. 2. **Zygotene:** **Synapsis** begins; Synaptonemal complex forms. 3. **Pachytene:** **Crossing over** occurs (mediated by recombinase). 4. **Diplotene:** Synaptonemal complex dissolves; **Chiasmata** (X-shaped structures) become visible. 5. **Diakinesis:** Terminalization of chiasmata. * **Clinical Correlation:** Errors in the synaptonemal complex or recombination can lead to infertility or spontaneous abortions due to chromosomal instability.

Question 384: What is the genetic component of a Superfemale?

A. XX
B. XXY
C. XO
D. XXX (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. XXX** **Concept Overview:** The term **"Superfemale"** (also known as **Triple X Syndrome** or Trisomy X) refers to a chromosomal abnormality characterized by the presence of an extra X chromosome in each cell of a female, resulting in a **47,XXX** karyotype. This occurs due to **nondisjunction** during meiosis, most commonly in the maternal germ cells. Despite the extra chromosome, most individuals are phenotypically female because the additional X chromosomes are inactivated as **Barr bodies** (a 47,XXX female will have two Barr bodies). **Analysis of Incorrect Options:** * **A. XX:** This is the normal female genotype (46,XX). * **B. XXY:** This represents **Klinefelter Syndrome**. These individuals are phenotypically male but often present with gynecomastia, small testes, and infertility. * **C. XO:** This represents **Turner Syndrome** (45,X). These individuals are phenotypically female but present with short stature, webbed neck, and primary amenorrhea. **Clinical Pearls for NEET-PG:** 1. **Barr Body Calculation:** The number of Barr bodies is always $N-1$ (where $N$ is the number of X chromosomes). Therefore, a Superfemale (XXX) has **2 Barr bodies**. 2. **Clinical Presentation:** Most Triple X females are asymptomatic. However, they may be taller than average and have an increased risk of learning disabilities or delayed speech development. Fertility is usually normal. 3. **Maternal Age:** Similar to Down syndrome, the risk of Triple X syndrome increases with advanced maternal age. 4. **Incidence:** It occurs in approximately 1 in 1,000 female births.

Question 385: Which of the following is NOT included in the IF4F (Iron-Folic acid fortification) strategy?

A. 4A
B. 4G
C. 4E
D. 4S (Correct Answer)

Explanation: This question pertains to the **eukaryotic translation initiation complex**, specifically the **eIF4F complex**, which is a critical assembly required for the binding of mRNA to the 40S ribosomal subunit. ### **Explanation of the eIF4F Complex** The eIF4F complex is a heterotrimeric protein complex composed of three essential subunits: 1. **eIF4E:** The **Cap-binding protein**. It recognizes and binds to the 7-methylguanosine cap at the 5' end of mRNA. This is often the rate-limiting step of translation. 2. **eIF4A:** An **ATP-dependent RNA helicase**. It unwinds the secondary structures (hairpins) in the 5' untranslated region (UTR) of the mRNA to allow the ribosome to scan for the start codon (AUG). 3. **eIF4G:** A large **scaffolding protein**. It serves as a bridge, coordinating the assembly by binding to eIF4E, eIF4A, and the poly(A) binding protein (PABP), circularizing the mRNA. **Why Option D is Correct:** **eIF4S** is not a recognized component of the eukaryotic translation initiation machinery. It is a distractor and does not exist within the eIF4F complex. ### **Why Other Options are Incorrect:** * **Option A (4A):** Incorrect because eIF4A is the helicase component of the eIF4F complex. * **Option B (4G):** Incorrect because eIF4G is the scaffold component of the eIF4F complex. * **Option C (4E):** Incorrect because eIF4E is the cap-binding component of the eIF4F complex. ### **High-Yield Clinical Pearls for NEET-PG:** * **eIF2:** Responsible for bringing the initiator methionyl-tRNA (Met-tRNAi) to the 40S subunit. It is regulated by phosphorylation (inhibited during cellular stress). * **eIF4E Regulation:** Overexpression of eIF4E is linked to many cancers as it promotes the translation of oncogenic mRNAs. * **Kozak Sequence:** The sequence (ACCAUGG) surrounding the AUG start codon that helps the ribosome identify the correct initiation site. * **Shine-Dalgarno Sequence:** The prokaryotic equivalent of the ribosome binding site (not found in eukaryotes).

Question 386: Which sequence is present immediately downstream of the Shine-Dalgarno sequence in prokaryotes?

A. AUG (Correct Answer)
B. UAG
C. UGA
D. UAA

Explanation: **Explanation:** In prokaryotic translation initiation, the **Shine-Dalgarno (SD) sequence** plays a critical role in mRNA recognition. It is a purine-rich leader sequence (typically AGGAGG) located approximately 8 base pairs upstream of the start codon. The SD sequence is complementary to the 3' end of the **16S rRNA** (part of the 30S ribosomal subunit). This base-pairing ensures that the ribosome is correctly positioned so that the P-site is aligned directly over the **AUG start codon**, which is located immediately downstream. This alignment is essential for the initiation of protein synthesis and ensures the correct reading frame is established. **Analysis of Options:** * **Option A (AUG):** Correct. This is the universal start codon in prokaryotes (coding for N-formylmethionine) and is positioned downstream of the SD sequence to initiate translation. * **Options B, C, and D (UAG, UGA, UAA):** Incorrect. These are **Stop Codons** (Amber, Opal, and Ochre, respectively). Their presence immediately downstream of an SD sequence would result in immediate termination of translation rather than initiation. **High-Yield Facts for NEET-PG:** * **Kozak Sequence:** The eukaryotic functional equivalent of the Shine-Dalgarno sequence, which helps the 40S subunit identify the start codon. * **16S rRNA:** A component of the small ribosomal subunit (30S) in prokaryotes; its interaction with the SD sequence is a classic example of RNA-RNA interaction. * **Streptomycin:** An aminoglycoside that acts by binding to the 30S subunit, interfering with the initiation complex and the reading of the mRNA.

Question 387: Which of the following statements about DNA is true?

A. The nucleotides of one strand form bonds with nucleotides of the opposite strand. (Correct Answer)
B. Cytosine and Uracil differ by one ribose sugar.
C. The information from DNA is copied in the form of tRNA.
D. Each nucleotide pair includes two purines.

Explanation: ### Explanation **Correct Option: A** The fundamental structure of the DNA double helix is maintained by **hydrogen bonds** between nitrogenous bases of opposing strands. According to Watson-Crick base pairing, Adenine (purine) pairs with Thymine (pyrimidine) via two hydrogen bonds, and Guanine (purine) pairs with Cytosine (pyrimidine) via three hydrogen bonds. This base-pairing mechanism is essential for DNA stability and the fidelity of replication. **Analysis of Incorrect Options:** * **Option B:** Cytosine and Uracil are both pyrimidines, but they differ by a **functional group**, not a sugar. Uracil is converted to Thymine by methylation (Thymine is 5-methyluracil). The difference in sugars (deoxyribose vs. ribose) distinguishes DNA from RNA, not specific bases. * **Option C:** The information from DNA is primarily copied into **mRNA (messenger RNA)** during transcription. While tRNA and rRNA are also transcribed from DNA, mRNA is the specific template that carries the genetic code for protein synthesis. * **Option D:** Each nucleotide pair consists of **one purine and one pyrimidine**. If two purines paired, the helix would bulge; if two pyrimidines paired, it would narrow. This 1:1 ratio ensures a constant helix diameter (20 Å). **NEET-PG High-Yield Pearls:** * **Chargaff’s Rule:** In double-stranded DNA, A+G (purines) = T+C (pyrimidines). This rule does not apply to single-stranded RNA. * **Bonding:** Phosphodiester bonds form the "backbone" (covalent), while hydrogen bonds connect the "rungs" (non-covalent). * **Denaturation:** The G-C bond is stronger than A-T due to the triple hydrogen bond; thus, DNA with high G-C content has a higher melting temperature ($T_m$). * **Z-DNA:** A rare left-handed helix, unlike the common right-handed B-DNA.

Question 388: The study of the structure and products of a gene is known as which of the following?

A. Genomics (Correct Answer)
B. Proteomics
C. Bioinformatics
D. Cytogenetics

Explanation: **Explanation:** **1. Why Genomics is Correct:** Genomics is the comprehensive study of the entire set of genes (the genome) of an organism. It involves the mapping, sequencing, and analysis of the **structure** (DNA sequence and organization) and the **products** (functional RNAs or proteins encoded) of genes. While genetics typically focuses on single genes or inheritance, genomics takes a large-scale approach to understand how the entire genetic blueprint functions and interacts. **2. Analysis of Incorrect Options:** * **Proteomics:** This is the large-scale study of **proteins**, particularly their structures and functions. While genes code for proteins, proteomics focuses on the "proteome" (the entire set of proteins expressed by a genome) rather than the gene itself. * **Bioinformatics:** This is an interdisciplinary field that uses **computational tools**, algorithms, and software to analyze and interpret complex biological data (like DNA sequences). It is the *method* used to study genomics, not the study of gene structure itself. * **Cytogenetics:** This is the branch of genetics that studies **chromosomes** and their relationship to cell behavior, particularly during mitosis and meiosis. It involves techniques like Karyotyping and FISH to detect numerical or structural chromosomal aberrations. **3. High-Yield Clinical Pearls for NEET-PG:** * **Pharmacogenomics:** A subfield of genomics that studies how an individual’s genetic makeup affects their response to drugs (e.g., *HLA-B*5701 testing before starting Abacavir). * **Structural vs. Functional Genomics:** Structural genomics deals with the 3D structure of every protein encoded by a genome, while functional genomics focuses on gene transcription, translation, and protein-protein interactions. * **The Human Genome Project:** Completed in 2003, it revealed that humans have approximately 20,000–25,000 genes.

Question 389: Which chromosome carries the gene for folic acid absorption?

A. Chromosome X
B. Chromosome XXI (Correct Answer)
C. Chromosome V
D. Chromosome XI

Explanation: **Explanation:** The correct answer is **Chromosome XXI (Chromosome 21)**. Folic acid absorption in the small intestine is primarily mediated by the **Proton-Coupled Folate Transporter (PCFT)**, which is encoded by the **SLC46A1 gene**. This gene is located on the long arm of Chromosome 21 (21q22.3). **Why Chromosome XXI is correct:** The SLC46A1 gene is responsible for the transmembrane protein that facilitates the uptake of dietary folates across the apical membrane of enterocytes. Mutations in this gene lead to **Hereditary Folate Malabsorption (HFM)**, a rare autosomal recessive disorder characterized by systemic folate deficiency, megaloblastic anemia, and neurological symptoms. **Analysis of Incorrect Options:** * **Chromosome X:** While many metabolic genes are X-linked (e.g., G6PD), genes related to primary intestinal folate transport are not located here. * **Chromosome V:** This chromosome carries genes like the *APC* gene (familial polyposis) but is not the primary locus for the SLC46A1 folate transporter. * **Chromosome XI:** This chromosome is home to the Beta-globin gene cluster and the insulin gene, but not the primary intestinal folate absorption gene. **Clinical Pearls for NEET-PG:** * **Hereditary Folate Malabsorption:** Patients present with low serum and CSF folate levels. It is treated with high-dose oral or parenteral folates (specifically **5-formyltetrahydrofolate/Leucovorin**). * **Down Syndrome Link:** Since Chromosome 21 is involved in folate metabolism, individuals with Trisomy 21 often exhibit altered folate pathways, which may contribute to the increased incidence of certain hematological conditions in these patients. * **Absorption Site:** Folic acid is primarily absorbed in the **proximal jejunum**.

Question 390: All are processing reactions in tRNA, except?

A. CCA tailing
B. Methylation of bases
C. Poly A tailing (Correct Answer)
D. Trimming of 5’ end

Explanation: **Explanation:** Post-transcriptional modification is essential for converting precursor tRNA (pre-tRNA) into functional mature tRNA. **Why Poly A tailing is the correct answer:** **Polyadenylation (Poly A tailing)** is a characteristic processing step for **mRNA**, not tRNA. In eukaryotes, a tail of 200–250 adenine residues is added to the 3’ end of mRNA to enhance stability and facilitate translation initiation. In contrast, adding a Poly A tail to tRNA or rRNA in some organisms actually signals for their degradation rather than maturation. **Analysis of incorrect options (Processing steps that DO occur in tRNA):** * **CCA tailing:** All mature tRNAs have a **CCA sequence** at their 3’ end, which is the attachment site for amino acids. While prokaryotic tRNA genes often encode this, eukaryotic tRNAs require the enzyme *nucleotidyltransferase* to add it post-transcriptionally. * **Methylation of bases:** tRNA undergoes extensive base modifications (e.g., methylation, pseudouridylation) to stabilize its tertiary structure and ensure precise codon-anticodon pairing. * **Trimming of 5’ end:** Pre-tRNA contains "leader" and "trailer" sequences. The 5’ leader sequence is removed by **RNase P** (a ribozyme), while the 3’ trailer is removed by RNase D. **High-Yield Clinical Pearls for NEET-PG:** * **RNase P** is a classic example of a **ribozyme** (RNA acting as a catalyst). * **Intron removal:** Some eukaryotic tRNAs contain introns in the anticodon loop; these are removed by a unique **endonuclease** rather than the standard spliceosome used for mRNA. * **Unusual Bases:** tRNA contains modified bases like **Dihydrouridine (D-loop)** and **Pseudouridine (TψC loop)**, which are diagnostic features of its "cloverleaf" secondary structure.

Practice by Chapter

DNA Replication and Repair Mechanisms

Practice Questions

Transcription Factors and Gene Regulation

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Epigenetics and DNA Methylation

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RNA Processing and Splicing

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miRNA and RNA Interference

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Protein Synthesis and Post-Translational Modifications

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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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