Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics Indian Medical PG Practice Questions and MCQs

Question 181: Which has more gene content: X or Y?

A. X contains 2.8% more than Y (Correct Answer)
B. Y contains 2.8% more than X
C. X contains 0.28% more than Y
D. Y contains 0.28% more than X

Explanation: **Explanation:** The human genome is organized into 23 pairs of chromosomes. Among the sex chromosomes, there is a significant disparity in size and gene density between the **X and Y chromosomes**. 1. **Why Option A is correct:** The X chromosome is significantly larger and more gene-rich than the Y chromosome. In terms of total genomic DNA content, the **X chromosome accounts for approximately 5%** of the haploid genome, while the **Y chromosome accounts for only about 2%**. The specific difference in DNA content between a cell with an X chromosome and one with a Y chromosome (e.g., in sperm) is approximately **2.8%**. Therefore, X contains 2.8% more genomic material than Y. 2. **Why other options are incorrect:** * **Option B:** This is factually incorrect as the Y chromosome is the smallest human chromosome (along with chromosome 21) and contains the fewest genes (approx. 50–200), whereas the X chromosome contains over 800–900 genes. * **Options C & D:** These options use the incorrect decimal value (0.28%). The actual physical difference in DNA mass is roughly 10-fold higher than this value. **High-Yield Clinical Pearls for NEET-PG:** * **Flow Cytometry:** This 2.8% difference in DNA content is the physiological basis for **"Sperm Sorting"** (pre-conceptual sex selection), where flow cytometry distinguishes X-bearing sperm from Y-bearing sperm based on fluorescence intensity. * **Gene Dosage:** To compensate for the double dose of genes in females (XX), one X chromosome is randomly inactivated (**Lyonization**) to form a **Barr Body**. * **Pseudoautosomal Regions (PAR):** These are homologous sequences on X and Y chromosomes that allow them to pair during meiosis. * **Holandric Inheritance:** Traits determined by genes on the Y chromosome (e.g., SRY gene for testis determination) are passed strictly from father to son.

Question 182: Mitochondrial codons are an exception to the universality of codons. For example, the initiation codon in mammals is AUG, which codes for methionine. However, in mitochondria, methionine is coded by some other codon. Which among the following is that initiation codon?

A. AGA
B. AAG
C. AUA (Correct Answer)
D. AUG

Explanation: ### Explanation **1. Why AUA is the Correct Answer** The genetic code is "nearly" universal, but human mitochondrial DNA (mtDNA) exhibits specific deviations. In the standard nuclear genetic code, **AUG** is the universal initiation codon (coding for Methionine). However, in mammalian **mitochondria**, both **AUG and AUA** function as codons for **Methionine** and can act as initiation codons. Additionally, **AUU** can sometimes serve as an initiator in mitochondria. This flexibility is a classic example of the non-universality of the mitochondrial genome. **2. Analysis of Incorrect Options** * **Option A (AGA):** In the standard code, AGA codes for Arginine. In human mitochondria, however, AGA (along with AGG) functions as a **Stop Codon**, rather than coding for an amino acid. * **Option B (AAG):** This codes for **Lysine** in both the standard genetic code and the mitochondrial code. It does not function as an initiation codon. * **Option D (AUG):** While AUG is the primary initiation codon in the nuclear genome (and also works in mitochondria), the question specifically asks for the *other* codon that codes for methionine/initiation in the context of mitochondrial exceptions. **3. NEET-PG High-Yield Clinical Pearls** * **Mitochondrial Exceptions to the Genetic Code:** * **UGA:** Standard = Stop; Mitochondrial = **Tryptophan** (Trp). * **AUA:** Standard = Isoleucine; Mitochondrial = **Methionine** (Met). * **AGA/AGG:** Standard = Arginine; Mitochondrial = **Stop Codons**. * **Maternal Inheritance:** mtDNA is inherited exclusively from the mother. * **Heteroplasmy:** The presence of a mixture of more than one type of organellar genome (normal and mutated) within a cell, explaining the variable severity of mitochondrial diseases (e.g., MELAS, LHON).

Question 183: Which one of the following binds to specific nucleotide sequences that are upstream of the start site of transcription?

A. Helicase
B. Histone protein
C. Primase
D. RNA polymerase (Correct Answer)

Explanation: ### Explanation **1. Why RNA Polymerase is Correct:** Transcription initiation requires **RNA polymerase** to recognize and bind to specific DNA sequences known as **promoters**. These promoter sequences (such as the TATA box in eukaryotes or the Pribnow box in prokaryotes) are located **upstream** (5' direction) of the transcription start site (+1). In eukaryotes, RNA polymerase II, along with general transcription factors, forms the pre-initiation complex at these sites to ensure that the gene is transcribed at the correct location and frequency. **2. Why the Other Options are Incorrect:** * **Helicase (A):** This enzyme is primarily involved in **DNA replication**. It breaks hydrogen bonds to "unzip" the double helix at the replication fork. It does not specifically bind upstream promoters to initiate transcription. * **Histone Protein (B):** Histones are structural proteins around which DNA wraps to form nucleosomes. While they regulate DNA accessibility, they bind to DNA non-specifically via electrostatic interactions (positive charge of histones to negative charge of DNA) rather than specific upstream nucleotide sequences. * **Primase (C):** This is an RNA polymerase used in **DNA replication** to synthesize short RNA primers. It provides a 3'-OH group for DNA polymerase to begin synthesis; it does not initiate gene transcription at promoter sites. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **RNA Polymerase Types (Eukaryotes):** * **Pol I:** rRNA (except 5S) * **Pol II:** mRNA (and snRNA, miRNA) — *Target of alpha-amanitin poisoning.* * **Pol III:** tRNA and 5S rRNA. * **Promoter Sequences:** The **TATA box** (Hogness box) is usually located at -25 bp, while the **CAAT box** is further upstream at -75 bp. * **Rifampicin:** A key antitubercular drug that acts by inhibiting **bacterial DNA-dependent RNA polymerase**, preventing transcription initiation.

Question 184: An autosomal dominant gene is one which is expressed in which state?

A. Homozygous state
B. Heterozygous state (Correct Answer)
C. Both homozygous and heterozygous states
D. Neither homozygous nor heterozygous states

Explanation: ### Explanation **Concept Overview:** In genetics, the **phenotype** (physical expression) of a gene depends on its **dominance**. An **Autosomal Dominant (AD)** trait is one where the presence of a single mutant allele on an autosome (non-sex chromosome) is sufficient to cause the disease or express the trait. **Why Option B is Correct:** By definition, a dominant gene is expressed in the **heterozygous state** (Aa). In medical genetics, most patients with autosomal dominant disorders (e.g., Marfan syndrome, Achondroplasia) are heterozygotes, having inherited one mutant allele from an affected parent and one normal allele from the other. **Analysis of Incorrect Options:** * **Option A (Homozygous state):** While a dominant gene *can* be expressed in a homozygous state (AA), this is not the defining characteristic of dominance. In many AD conditions, the homozygous state is lethal or results in a much more severe phenotype (e.g., homozygous Achondroplasia is often fatal in the neonatal period). * **Option C (Both states):** While technically true that the gene is expressed in both, the question asks for the state in which a dominant gene is *distinguished* from a recessive one. A recessive gene requires both alleles (homozygous) for expression, whereas a dominant gene is uniquely expressed even when only one allele is present (heterozygous). * **Option D:** This is biologically impossible for a functional gene. **High-Yield Clinical Pearls for NEET-PG:** 1. **Vertical Transmission:** AD disorders typically show a vertical pattern in pedigrees (seen in every generation). 2. **50% Risk:** An affected heterozygous parent has a 50% chance of passing the trait to each offspring. 3. **Variable Expressivity:** Individuals with the same AD genotype may show different degrees of clinical severity (e.g., Neurofibromatosis Type 1). 4. **Reduced Penetrance:** Some individuals inherit the AD gene but do not manifest the disease clinically. 5. **Pleiotropy:** A single AD gene mutation can affect multiple organ systems (e.g., Marfan syndrome affecting eyes, heart, and skeleton).

Question 185: The Watson-Crick model of DNA was proposed on the basis of which scientific technique?

A. X-ray crystallography (Correct Answer)
B. Chromatography
C. Gel electrophoresis
D. X-ray diffraction studies

Explanation: The Watson-Crick model of DNA (1953) is a cornerstone of molecular biology. While James Watson and Francis Crick built the physical model, their breakthrough relied entirely on interpreting data generated by **X-ray crystallography**. ### Why X-ray Crystallography is Correct X-ray crystallography involves directing X-rays at a crystallized molecule; the resulting diffraction pattern reveals the spatial arrangement of atoms. Watson and Crick utilized the famous **"Photo 51,"** an X-ray diffraction image of DNA produced by **Rosalind Franklin** and Maurice Wilkins. This image provided the critical evidence for the helical structure, the 20 Å width of the helix, and the 3.4 Å spacing between bases. ### Why Other Options are Incorrect * **Chromatography:** While Erwin Chargaff used paper chromatography to determine that the molar ratio of A=T and G=C (**Chargaff’s Rule**), this technique identifies chemical composition, not 3D structural geometry. * **Gel Electrophoresis:** This technique separates DNA fragments based on size and charge. It is essential for sequencing and blotting but was not used to determine the double-helix structure. * **X-ray diffraction studies:** While technically the *process* used, "X-ray crystallography" is the standard term for the scientific technique that encompasses the preparation of the sample and the mathematical analysis of the diffraction patterns to solve a structure. ### NEET-PG High-Yield Pearls * **B-DNA:** The Watson-Crick model describes the B-form of DNA, which is a **right-handed** helix. * **Dimensions:** One full turn is **3.4 nm** (10 base pairs); the distance between base pairs is **0.34 nm**. * **Bonds:** Phosphodiester bonds form the backbone (covalent), while **Hydrogen bonds** stabilize the base pairs (A=T has 2; G≡C has 3). * **Nobel Prize:** Awarded in 1962 to Watson, Crick, and Wilkins (Franklin had passed away).

Question 186: RNA dependent DNA polymerase is:

A. DNA polymerase
B. RNA polymerase
C. Reverse transcriptase (Correct Answer)
D. Phosphokinase

Explanation: **Explanation:** The central dogma of molecular biology typically flows from DNA to RNA to Protein. However, certain viruses and cellular processes utilize **Reverse Transcriptase**, an enzyme that catalyzes the synthesis of DNA using an RNA template. This process is known as **Reverse Transcription**. 1. **Why Reverse Transcriptase is correct:** The name of a polymerase is derived from the template it reads and the product it synthesizes. An **RNA-dependent DNA polymerase** reads an **RNA** template to synthesize a complementary **DNA** strand (cDNA). This enzyme is crucial for the replication of retroviruses like HIV and is also found in human telomerase (which uses an internal RNA template to extend telomeres). 2. **Why other options are incorrect:** * **DNA polymerase:** This is typically a **DNA-dependent DNA polymerase**, as it uses a DNA template to replicate DNA during the S-phase of the cell cycle. * **RNA polymerase:** This is a **DNA-dependent RNA polymerase**, which reads a DNA template to synthesize RNA (Transcription). * **Phosphokinase:** This is a general term for enzymes that catalyze the transfer of phosphate groups (phosphorylation) and is not involved in template-directed nucleic acid synthesis. **High-Yield Clinical Pearls for NEET-PG:** * **HIV/AIDS:** Reverse transcriptase is the primary target for **NRTIs** (e.g., Zidovudine, Tenofovir) and **NNRTIs** (e.g., Efavirenz). * **Telomerase:** A specialized reverse transcriptase (TERT) that maintains chromosomal stability; its activity is high in cancer cells and stem cells. * **PCR Technology:** In **RT-PCR**, reverse transcriptase is used to convert viral RNA (like SARS-CoV-2) into DNA before amplification. * **Hepatitis B Virus:** Unlike other DNA viruses, HBV uses a reverse transcriptase step during its replication cycle.

Question 187: Which one of the following causes a frame-shift mutation?

A. Deletion (Correct Answer)
B. Substitution of purine for pyrimidine
C. Substitution of pyrimidine for purine
D. Transition

Explanation: **Explanation:** **1. Why Deletion is Correct:** Genetic code is read in non-overlapping triplets called **codons**. A **Frame-shift mutation** occurs when the number of nucleotides inserted or deleted is **not a multiple of three**. This shifts the "reading frame" of the mRNA during translation. * **Deletion** of one or two nucleotides changes every subsequent codon downstream of the mutation. This usually results in a completely different amino acid sequence and often creates a premature **stop codon (UAA, UAG, UGA)**, leading to a truncated, non-functional protein. **2. Why the Other Options are Incorrect:** * **Options B & C (Transversion):** Substitution of a purine for a pyrimidine (or vice versa) is a **Point Mutation**. This only affects a single codon. It may result in a missense, nonsense, or silent mutation, but it does *not* shift the reading frame. * **Option D (Transition):** This is a point mutation where a purine is replaced by another purine (A↔G) or a pyrimidine by another pyrimidine (C↔T). Like transversions, transitions do not alter the reading frame. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Duchenne Muscular Dystrophy (DMD):** Caused by a **frame-shift mutation** (deletion) in the dystrophin gene, leading to a severe phenotype. * **Becker Muscular Dystrophy:** Caused by a **non-frame-shift** (in-frame) mutation, resulting in a milder phenotype. * **Tay-Sachs Disease:** Often caused by a 4-base pair insertion (frame-shift) in the HEXA gene. * **Cystic Fibrosis:** The most common mutation ($\Delta$F508) is an **in-frame deletion** of 3 nucleotides (one amino acid), meaning it is *not* a frame-shift mutation.

Question 188: What is the approximate number of base pairs in a human chromosome?

A. 3 million
B. 3 billion (Correct Answer)
C. 3.3 billion
D. 5 million

Explanation: **Explanation:** The human genome consists of the total genetic material stored within our cells. To answer this question correctly, it is vital to distinguish between the **haploid** and **diploid** genome sizes. 1. **Why 3 Billion is Correct:** The **haploid** human genome (the content of a single set of 23 chromosomes found in a sperm or egg cell) consists of approximately **3.2 to 3.3 billion base pairs**. In the context of standard medical examinations like NEET-PG, this is frequently rounded to **3 billion base pairs** (or 3 Gb). This represents the "blueprint" of a single human set of instructions. 2. **Analysis of Incorrect Options:** * **3.3 Billion (Option C):** While technically more precise for the haploid genome, standard textbooks and examiners often use "3 billion" as the benchmark figure. However, if the question asks for the **diploid** content (found in somatic cells), the answer would be approximately **6.6 billion base pairs**. * **3 Million / 5 Million (Options A & D):** These figures are far too small for eukaryotes. For comparison, the *Escherichia coli* (prokaryotic) genome is approximately 4.6 million base pairs. **High-Yield Clinical Pearls for NEET-PG:** * **Coding vs. Non-coding:** Only about **1–1.5%** of the human genome actually codes for proteins (exons). * **Mitochondrial DNA (mtDNA):** Unlike the nuclear genome, mtDNA is circular, double-stranded, and contains only **16,569 base pairs** encoding 37 genes. * **The "C-Value Paradox":** Genome size does not necessarily correlate with the complexity of the organism. * **Repeat Sequences:** Nearly 50% of the human genome consists of repetitive sequences (e.g., LINEs, SINEs, and satellite DNA), which are crucial for chromosomal structural integrity and regulation.

Question 189: The normal and mutated sequences of RNA coding for a protein are given below. Normal sequence: AGC-GCC-GAA-AGA-AAG-AUG-ACG. Mutated sequence: AGC-GCC-GAA-UGA-AAG-AUG-ACG. What type of mutation is shown above?

A. Missense mutation
B. Nonsense mutation (Correct Answer)
C. Silent mutation
D. Frame shift mutation

Explanation: ### Explanation **1. Why Nonsense Mutation is Correct:** A **nonsense mutation** occurs when a single nucleotide substitution results in a **premature stop codon** (UAA, UAG, or UGA) within the mRNA sequence. * **Normal sequence:** ...GAA-**AGA**-AAG... (AGA codes for Arginine) * **Mutated sequence:** ...GAA-**UGA**-AAG... (UGA is a Stop Codon) The conversion of an amino acid codon (AGA) into a stop codon (UGA) leads to the premature termination of translation, resulting in a truncated, usually non-functional protein. **2. Analysis of Incorrect Options:** * **A. Missense mutation:** This involves a point mutation that changes one amino acid to a *different* amino acid (e.g., GAA to GUA). Here, the protein is not truncated but altered. * **C. Silent mutation:** This is a point mutation that changes the codon but, due to the **degeneracy of the genetic code**, it still codes for the *same* amino acid. There is no change in the protein primary structure. * **D. Frame shift mutation:** This occurs due to the **insertion or deletion** of nucleotides (not divisible by 3). This shifts the entire reading frame downstream. In this question, the number of nucleotides remains the same; only one base is substituted. **3. NEET-PG High-Yield Pearls:** * **Stop Codons:** Remember them as **U** **A**re **A**way (UAA), **U** **G**o **A**way (UGA), and **U** **A**re **G**one (UAG). * **Transition vs. Transversion:** In this case, Adenine (Purine) changed to Uracil (Pyrimidine), which is a **Transversion**. * **Clinical Correlation:** Nonsense mutations are responsible for severe phenotypes in diseases like **Duchenne Muscular Dystrophy** and certain types of **β-Thalassemia**. * **Aminoglycosides:** At low doses, drugs like Gentamicin can sometimes induce "translational read-through" of premature stop codons, a concept being researched for treating nonsense mutation-related genetic disorders.

Question 190: All of the following statements about human mitochondrial DNA are true EXCEPT:

A. It is circular.
B. AGA is a stop codon.
C. It has a low mutation rate. (Correct Answer)
D. It contains very few untranslated sequences.

Explanation: ### Explanation **Mitochondrial DNA (mtDNA)** is a unique, extranuclear genetic system that differs significantly from nuclear DNA (nDNA). **Why Option C is the Correct Answer (The False Statement):** Mitochondrial DNA actually has a **high mutation rate**—approximately 10 to 20 times higher than nuclear DNA. This is due to three primary factors: 1. **Lack of Histones:** mtDNA is "naked" and lacks the protective structural coating provided by histones. 2. **Reactive Oxygen Species (ROS):** mtDNA is located in the inner mitochondrial membrane, directly adjacent to the respiratory chain, exposing it to high levels of free radicals. 3. **Limited Repair Mechanisms:** While some repair exists, it lacks the robust, complex DNA repair machinery found in the nucleus. **Analysis of Other Options:** * **Option A (Circular):** True. mtDNA is a double-stranded, closed circular molecule (resembling bacterial DNA), which supports the endosymbiotic theory. * **Option B (AGA is a stop codon):** True. The mitochondrial genetic code is "non-universal." In mtDNA, **AGA and AGG** function as **Stop codons** (in the nucleus, they code for Arginine). Conversely, **UGA** codes for **Tryptophan** in mitochondria (it is a Stop codon in the nucleus). * **Option D (Few untranslated sequences):** True. mtDNA is highly efficient and "gene-dense." It contains almost no introns and very few non-coding (untranslated) sequences between genes. **High-Yield Clinical Pearls for NEET-PG:** * **Maternal Inheritance:** mtDNA is inherited exclusively from the mother (cytoplasmic inheritance). * **Heteroplasmy:** The presence of a mixture of more than one type of organellar genome (mutant and wild-type) within a cell. This explains the variable clinical severity in mitochondrial diseases. * **Key Diseases:** Leber’s Hereditary Optic Neuropathy (LHON), MELAS, and MERRF. * **Gene Count:** mtDNA encodes 13 polypeptides (subunits of the ETC), 22 tRNAs, and 2 rRNAs.

Practice by Chapter

DNA Replication and Repair Mechanisms

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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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