Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics Indian Medical PG Practice Questions and MCQs

Question 521: Classic human disorders related to genomic imprinting include all EXCEPT?

A. Prader-Willi syndrome
B. Angelman syndrome
C. Beckwith-Wiedemann syndrome
D. Duchenne muscular dystrophy (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Duchenne muscular dystrophy** #### 1. Why Duchenne Muscular Dystrophy (DMD) is the Correct Answer Duchenne Muscular Dystrophy is an **X-linked recessive disorder** caused by a mutation in the *DMD* gene (encoding the protein dystrophin) on the X chromosome. It is characterized by progressive muscle degeneration. It is **not** related to genomic imprinting. Genomic imprinting involves the differential expression of genes depending on whether they are inherited from the mother or the father, usually regulated by DNA methylation. DMD follows classic Mendelian inheritance patterns. #### 2. Why the Other Options are Incorrect * **A & B. Prader-Willi (PWS) and Angelman (AS) Syndromes:** These are the classic "textbook" examples of imprinting disorders involving chromosome **15q11-q13**. * **PWS:** Loss of the *paternally* expressed gene (maternal imprinting). * **AS:** Loss of the *maternally* expressed gene (paternal imprinting). * **C. Beckwith-Wiedemann Syndrome (BWS):** This is a pediatric overgrowth disorder (macroglossia, omphalocele, hemihyperplasia) caused by alterations in imprinting at the **11p15.5** locus (involving genes like *IGF2* and *H19*). #### 3. High-Yield Clinical Pearls for NEET-PG * **Mechanism:** Imprinting occurs during gametogenesis via **DNA methylation** (specifically at CpG islands) by DNA methyltransferases. * **Uniparental Disomy (UPD):** A common cause of imprinting disorders where an individual receives two copies of a chromosome from one parent and none from the other. * **Other Imprinting Disorders:** Silver-Russell Syndrome (Chr 11/7) and Albright Hereditary Osteodystrophy (Pseudohypoparathyroidism Type 1A). * **DMD Mnemonic:** Remember "DMD is X-linked" (Dystrophin gene is the largest known human gene).

Question 522: Regarding the wobble hypothesis, what is the true statement about the variation?

A. 3' end of anticodon
B. 5' end of anticodon (Correct Answer)
C. mRNA
D. tRNA

Explanation: ### Explanation The **Wobble Hypothesis**, proposed by Francis Crick, explains why there are fewer tRNA molecules (approx. 40-50) than there are codons (61 sense codons). It states that while the first two bases of a codon pair strictly with the anticodon via standard Watson-Crick base pairing, the **third base of the codon** and the **first base of the anticodon** can undergo non-standard pairing. #### Why Option B is Correct The "wobble" occurs at the **5' end of the anticodon** (the first position of the tRNA anticodon). This flexibility allows a single tRNA to recognize and bind to multiple codons that differ only in their third base (the 3' end of the mRNA codon). For example, if the 5' position of the anticodon contains **Inosine (I)**, it can pair with U, C, or A at the 3' position of the mRNA. #### Why Other Options are Incorrect * **Option A (3' end of anticodon):** The 3' end of the anticodon corresponds to the first base of the mRNA codon. Pairing here is absolute and must follow strict Watson-Crick rules (A-U, G-C) to ensure translational fidelity. * **Option C & D (mRNA/tRNA):** These options are too broad. While the wobble effect involves both mRNA and tRNA, the specific "variation" or "wobble position" refers to a specific nucleotide location (the 5' end of the anticodon or the 3' end of the codon), not the entire molecule. #### High-Yield Clinical Pearls for NEET-PG * **Wobble Rules:** * If the 5' anticodon base is **G**, it can pair with **U or C**. * If the 5' anticodon base is **U**, it can pair with **A or G**. * If the 5' anticodon base is **I (Inosine)**, it can pair with **A, U, or C**. * **Significance:** The wobble hypothesis explains the **degeneracy of the genetic code** (multiple codons coding for one amino acid) and protects against silent mutations at the third nucleotide position. * **Directionality:** Always remember: **5' of Anticodon pairs with 3' of Codon.**

Question 523: In humans, the genetic code is represented by a code of three nucleotides. If one amino acid is coded by more than one triplet, then this is known as?

A. Degeneracy (Correct Answer)
B. Frame-shift mutation
C. Ambiguity
D. Mutation

Explanation: ### Explanation **1. Why "Degeneracy" is the Correct Answer:** The genetic code consists of 64 possible codons (triplets) that encode for only 20 standard amino acids. Because there are more codons than amino acids, most amino acids are specified by multiple codons. This phenomenon is termed **Degeneracy** (or Redundancy). * **Mechanism:** Degeneracy primarily occurs at the **third position** of the codon (the 3' end), often explained by the **Wobble Hypothesis**. For example, Leucine is coded by six different codons (UUA, UUG, CUU, CUC, CUA, CUG). This provides a protective mechanism against mutations, as a change in the third nucleotide may still result in the same amino acid (Silent Mutation). **2. Why Other Options are Incorrect:** * **Ambiguity:** This would mean one codon codes for multiple different amino acids. The genetic code is **non-ambiguous**; each specific codon always codes for only one specific amino acid. * **Frame-shift Mutation:** This occurs when the addition or deletion of nucleotides (not in multiples of three) shifts the reading frame, altering all subsequent amino acids. It is a type of mutation, not a property of the code itself. * **Mutation:** This is a general term for any permanent change in the DNA sequence. While degeneracy helps mitigate the effects of mutations, it is not a synonym for the process. **3. High-Yield Clinical Pearls for NEET-PG:** * **Universal Code:** The genetic code is nearly universal across all species. **Exception:** Human mitochondrial DNA (e.g., UGA codes for Tryptophan instead of a Stop codon). * **Non-overlapping & Comma-less:** The code is read sequentially from a fixed starting point without skipping any nucleotides. * **Initiation Codon:** **AUG** (Methionine). In prokaryotes, it codes for N-formylmethionine. * **Stop Codons (Nonsense Codons):** UAA (Ochre), UAG (Amber), UGA (Opal). These do not code for any amino acid.

Question 524: What enzyme is used by RNA to synthesize DNA?

A. Reverse Transcriptase (Correct Answer)
B. DNA Polymerase
C. RNA Polymerase
D. DNA Topoisomerase

Explanation: ### Explanation **Correct Answer: A. Reverse Transcriptase** **Understanding the Concept:** In the "Central Dogma" of molecular biology, genetic information typically flows from DNA to RNA (Transcription). However, certain viruses and retrotransposons utilize an enzyme called **Reverse Transcriptase** (also known as RNA-dependent DNA polymerase) to reverse this flow. This enzyme uses a single-stranded RNA template to synthesize a complementary DNA (cDNA) strand. This is a critical step for retroviruses to integrate their genetic material into the host cell's genome. **Analysis of Incorrect Options:** * **B. DNA Polymerase:** These enzymes synthesize DNA using a **DNA template** (DNA-dependent DNA polymerase). They are primarily involved in DNA replication and repair. * **C. RNA Polymerase:** These enzymes synthesize RNA using a **DNA template** (DNA-dependent RNA polymerase) during the process of transcription. * **D. DNA Topoisomerase:** These enzymes do not synthesize nucleic acids; instead, they manage DNA topology by relieving torsional strain (supercoiling) during replication and transcription by creating transient breaks in the DNA backbone. **High-Yield Clinical Pearls for NEET-PG:** * **Retroviruses:** Human Immunodeficiency Virus (HIV) is the most clinically significant virus utilizing reverse transcriptase. * **Pharmacology Link:** Nucleoside Reverse Transcriptase Inhibitors (NRTIs) like **Zidovudine** and Non-Nucleoside Reverse Transcriptase Inhibitors (NNRTIs) like **Efavirenz** are cornerstones of HAART (Highly Active Antiretroviral Therapy). * **Telomerase:** This is a specialized reverse transcriptase that carries its own internal RNA template to maintain the ends of eukaryotic chromosomes (telomeres). * **Laboratory Use:** Reverse transcriptase is essential in **RT-PCR**, used to detect RNA viruses (like SARS-CoV-2) by first converting viral RNA into cDNA.

Question 525: Anticipation is seen in which of the following genetic phenomena?

A. Translocation
B. Chromosome breaking
C. Trinucleotide repeat expansion (Correct Answer)
D. Mitochondrial mutation

Explanation: ### Explanation **Correct Option: C. Trinucleotide repeat expansion** **Concept of Anticipation:** Anticipation is a genetic phenomenon where a disease becomes more severe or has an earlier age of onset in successive generations. This is the hallmark of **Trinucleotide Repeat Expansion** disorders. In these conditions, a specific sequence of three nucleotides (e.g., CAG, CGG) repeats multiple times within a gene. During gametogenesis (meiosis), these repeats are unstable and tend to expand. A larger number of repeats often correlates with increased disease severity and earlier clinical presentation, explaining the "anticipation" seen in pedigrees. **Analysis of Incorrect Options:** * **A. Translocation:** This involves the exchange of genetic material between non-homologous chromosomes (e.g., t(9;22) in CML). While it causes disease, it does not typically show progressive worsening across generations. * **B. Chromosome breaking:** This refers to structural instability (e.g., Fanconi anemia). While it leads to malignancies or birth defects, it is not characterized by the specific mechanism of anticipation. * **D. Mitochondrial mutation:** These exhibit **maternal inheritance** and **heteroplasmy**. While severity can vary among offspring due to the distribution of mutant mitochondria, it does not follow the predictable generational worsening defined as anticipation. **High-Yield Clinical Pearls for NEET-PG:** * **Fragile X Syndrome (CGG):** Most common cause of inherited intellectual disability; shows anticipation primarily during **maternal** transmission. * **Huntington Disease (CAG):** Shows anticipation primarily during **paternal** transmission (spermatogenesis). * **Myotonic Dystrophy (CTG):** Shows the most dramatic anticipation; a mildly affected mother can have a child with severe congenital myotonic dystrophy. * **Friedreich Ataxia (GAA):** The only common trinucleotide repeat disorder that is **Autosomal Recessive** (others are mostly Dominant or X-linked).

Question 526: Which of the following is true about eukaryotic initiation?

A. The initiator sequence spans the transcription start site (+1).
B. Distal regulatory elements (DREs) are exclusively found in eukaryotes.
C. Different alpha subunits are responsible for recognition specificity. (Correct Answer)
D. Trans-acting factors bind to the CAAT/GC box, which is a promoter-proximal element.

Explanation: In eukaryotic transcription initiation, the assembly of the pre-initiation complex (PIC) is a highly regulated process. **Explanation of the Correct Answer:** **Option C** is correct because the specificity of RNA polymerase recognition is mediated by various subunits and associated transcription factors. In eukaryotes, while the core RNA polymerase is complex, the **alpha-like subunits** (and specifically the TATA-binding protein/TBP within TFIID) are crucial for recognizing specific promoter sequences. In a broader context of "recognition specificity," different subunits or isoforms of transcription factors allow the cell to initiate transcription of specific gene sets in response to physiological needs. **Analysis of Incorrect Options:** * **Option A:** The **Initiator (Inr)** sequence is a core promoter element that *encompasses* the transcription start site (TSS), typically spanning from -2 to +4. However, the statement that it "spans the +1 site" is a general characteristic, but Option C is the more definitive functional truth regarding eukaryotic machinery complexity. * **Option B:** Distal regulatory elements (like enhancers and silencers) are **not exclusive** to eukaryotes; similar distal regulatory mechanisms have been identified in prokaryotes (e.g., NtrC binding sites in *E. coli*). * **Option D:** While trans-acting factors (transcription factors) do bind to CAAT and GC boxes, these are categorized as **upstream promoter elements**, usually located between -40 and -200. The distinction here is often technical; however, the functional specificity mentioned in Option C is a hallmark of eukaryotic complexity. **High-Yield Clinical Pearls for NEET-PG:** * **TATA Box:** Located at -25 to -30 (Hogness box); it is the site where TFIID (via TBP) binds to initiate PIC assembly. * **Alpha-Amanitin:** A toxin from the *Amanita phalloides* mushroom that specifically inhibits **RNA Polymerase II**, leading to severe liver failure. * **Enhancers:** Can be located thousands of base pairs away, upstream or downstream, and function via "DNA looping" to interact with the promoter.

Question 527: Which method is used to map genes located on a particular locus in a chromosome?

A. Genomic imprinting
B. Segregation analysis
C. Genome-wide analysis
D. Linkage analysis (Correct Answer)

Explanation: ### Explanation **Linkage Analysis (Correct Answer)** Linkage analysis is the primary method used to map genes to specific chromosomal loci. It is based on the principle that genes located physically close to each other on the same chromosome are likely to be inherited together during meiosis. This occurs because the probability of **recombination (crossing over)** between two loci is proportional to the distance between them. By tracking the co-segregation of a disease trait with genetic markers (like SNPs or microsatellites) within families, scientists can determine the specific "locus" or position of a gene on a chromosome. **Why other options are incorrect:** * **Genomic Imprinting:** This is an epigenetic phenomenon where the expression of a gene depends on whether it was inherited from the mother or the father (e.g., Prader-Willi and Angelman syndromes). It describes gene *expression*, not gene *mapping*. * **Segregation Analysis:** This is a statistical technique used to determine the **mode of inheritance** (e.g., autosomal dominant vs. recessive) of a trait within families. It does not identify the physical location of the gene. * **Genome-wide Analysis (GWAS):** While used to find genetic associations with diseases, GWAS looks for common genetic variants (SNPs) across the entire genome in large populations. It is used for complex polygenic traits rather than mapping a specific gene to a particular locus via family inheritance patterns. **High-Yield Clinical Pearls for NEET-PG:** * **LOD Score (Logarithm of Odds):** Used in linkage analysis to estimate the likelihood of linkage. A LOD score **≥ 3.0** is considered definitive evidence that two loci are linked. * **Recombination Frequency:** 1% recombination frequency = **1 Centimorgan (cM)**, which is the unit of genetic distance. * **Syntenic Genes:** Genes located on the same chromosome are called syntenic, but they are only "linked" if they are close enough to be inherited together.

Question 528: What is epigenetics?

A. Alteration in nucleotide sequence
B. Alteration in chromosome number
C. Alteration in gene expression (Correct Answer)
D. Alteration in chromosome size

Explanation: **Explanation:** **Epigenetics** refers to the study of heritable changes in gene expression that occur **without** any alteration in the underlying DNA sequence. The correct answer is **C** because epigenetics focuses on how genes are "turned on or off" through biochemical modifications. The primary mechanisms of epigenetic regulation include: 1. **DNA Methylation:** Usually occurs at CpG islands; typically silences gene expression. 2. **Histone Modification:** Acetylation (increases transcription) or methylation (can increase or decrease transcription). 3. **Non-coding RNAs:** Such as miRNA and siRNA that regulate translation. **Why other options are incorrect:** * **Option A:** Alteration in nucleotide sequence defines a **mutation** (e.g., point mutations, insertions, or deletions). * **Option B:** Alteration in chromosome number refers to **aneuploidy** (e.g., Trisomy 21) or **polyploidy**. * **Option C:** Alteration in chromosome size usually results from large-scale structural aberrations like **translocations, deletions, or duplications**. **High-Yield Clinical Pearls for NEET-PG:** * **Genomic Imprinting:** A classic epigenetic phenomenon where only one allele (maternal or paternal) is expressed. Examples include **Prader-Willi Syndrome** (paternal deletion/maternal imprinting) and **Angelman Syndrome** (maternal deletion/paternal imprinting) on chromosome 15. * **Cancer:** Hypermethylation of tumor suppressor gene promoters (like *p16* or *BRCA1*) is a common epigenetic event in oncogenesis. * **Drug Link:** **5-Azacytidine** is a DNA methyltransferase inhibitor used in treating myelodysplastic syndromes by reversing epigenetic silencing.

Question 529: In DNA, Cytosine is paired with which of the following?

A. Thymine
B. Guanine (Correct Answer)
C. Adenine
D. Uracil

Explanation: ### Explanation **Underlying Concept: Chargaff’s Rule and Base Pairing** In the DNA double helix, nitrogenous bases follow specific pairing rules known as **Chargaff’s Rules**. These rules state that a **Purine** (double-ring structure) always pairs with a **Pyrimidine** (single-ring structure) to maintain a constant distance between the two sugar-phosphate backbones. **Cytosine (C)**, a pyrimidine, specifically pairs with **Guanine (G)**, a purine. This pairing is stabilized by **three hydrogen bonds**, making the C-G bond stronger and more thermally stable than the A-T bond. **Analysis of Options:** * **B. Guanine (Correct):** Cytosine and Guanine are complementary. In DNA with high G-C content, the "melting temperature" ($T_m$) is higher due to the triple hydrogen bonding. * **A. Thymine:** This is a pyrimidine. Two pyrimidines do not pair together in DNA as it would narrow the helix diameter. Thymine pairs with Adenine via two hydrogen bonds. * **C. Adenine:** This is a purine, but it specifically pairs with Thymine (in DNA) or Uracil (in RNA) using two hydrogen bonds. * **D. Uracil:** This pyrimidine is found exclusively in **RNA**, replacing Thymine. It pairs with Adenine, not Cytosine. **High-Yield Clinical Pearls for NEET-PG:** 1. **Bond Strength:** G-C pairs have **3 hydrogen bonds**, while A-T pairs have **2**. Higher G-C content increases the DNA melting temperature ($T_m$). 2. **Deamination:** Spontaneous deamination of **Cytosine** converts it into **Uracil**. This is a common type of DNA damage repaired by Base Excision Repair (BER) using *Uracil DNA Glycosylase*. 3. **Methylation:** Cytosine methylation (at CpG islands) is a key mechanism of **epigenetic gene silencing**. 4. **Drug Link:** **5-Fluorouracil (5-FU)** is a pyrimidine analogue used in cancer chemotherapy that inhibits thymidylate synthase.

Question 530: Which of the following best describes microRNAs?

A. Involved in post-transcriptional gene regulation
B. Typically 20-22 nucleotides in length
C. A type of noncoding RNA
D. All of the above (Correct Answer)

Explanation: **Explanation:** MicroRNAs (miRNAs) are a class of small, endogenous, **noncoding RNA** molecules that play a pivotal role in the epigenetic regulation of gene expression. 1. **Noncoding Nature (Option C):** Unlike mRNA, miRNAs are not translated into proteins. Instead, they function as regulatory molecules that dictate the fate of other messenger RNAs. 2. **Size (Option B):** They are typically **20–22 nucleotides** long. They are processed from longer primary transcripts (pri-miRNA) by the enzymes **Drosha** (in the nucleus) and **Dicer** (in the cytoplasm). 3. **Mechanism of Action (Option A):** miRNAs facilitate **post-transcriptional gene silencing**. They incorporate into the **RISC** (RNA-induced silencing complex) and bind to the 3' untranslated region (UTR) of target mRNAs. This binding leads to either **translational repression** (if binding is partially complementary) or **mRNA degradation** (if binding is perfectly complementary). Since all three statements accurately describe the characteristics and functions of microRNAs, **Option D** is the correct answer. **High-Yield Clinical Pearls for NEET-PG:** * **OncomiRs:** miRNAs that act as oncogenes (e.g., miR-21) or tumor suppressors (e.g., let-7). Dysregulation is a hallmark of many cancers. * **siRNA vs. miRNA:** While both are small RNAs, siRNAs are usually exogenous (from viruses or labs) and require perfect base pairing, whereas miRNAs are endogenous and can function with imperfect pairing. * **Key Enzymes:** Remember **Dicer** and **Argonaute** (a protein within the RISC complex) as they are frequently tested in molecular biology sections.

Practice by Chapter

DNA Replication and Repair Mechanisms

Practice Questions

Transcription Factors and Gene Regulation

Practice Questions

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

Practice Questions

Single Nucleotide Polymorphisms

Practice Questions

Gene Therapy Approaches

Practice Questions

CRISPR-Cas9 and Genome Editing

Practice Questions

DNA Fingerprinting and Forensics

Practice Questions

Molecular Basis of Genetic Diseases

Practice Questions

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