Molecular Biology and Genomics Practice Questions

Q: Barr bodies are not present in which of the following chromosomal complements?

XO. ### Explanation The presence of **Barr bodies** (sex chromatin) is a result of **Lyonization**, the process of random X-inactivation. To maintain dosage compensation between males and females, only one X chromosome remains active in a somatic cell; all additional X chromosomes are inactivated and condensed into heterochromatin (Barr bodies). The formula to determine the number of Barr bodies is: **Number of Barr Bodies = Total number of X chromosomes – 1** #### Why XO is the Correct Answer: * **XO (Turner Syndrome):** In this complement, there is only one X chromosome. Applying the formula (1 – 1 = 0), there are **no Barr bodies**. Since there is no "extra" X chromosome to inactivate, the cell remains negative for sex chromatin. #### Why the Other Options are Incorrect: * **XX (Normal Female):** Contains two X chromosomes. One undergoes inactivation (2 – 1 = 1), resulting in **1 Barr body**. * **XXY (Klinefelter Syndrome):** Despite being phenotypically male due to the Y chromosome, the presence of two X chromosomes leads to the inactivation of one (2 – 1 = 1), resulting in **1 Barr body**. * **XXX (Triple X Syndrome):** Contains three X chromosomes. Two are inactivated (3 – 1 = 2), resulting in **2 Barr bodies**. #### High-Yield Clinical Pearls for NEET-PG: * **Lyon Hypothesis:** X-inactivation occurs early in embryonic life (blastocyst stage, approx. 16th day) and is **irreversible and random**. * **XIST Gene:** Located on the X-inactivation center (Xic) of the X chromosome, it produces a non-coding RNA that "coats" the chromosome to be inactivated. * **Staining:** Barr bodies are typically visualized at the periphery of the nucleus using **Feulgen stain** or in neutrophils as **"Drumsticks."** * **Buccal Smear:** This is the classic clinical test used to detect Barr bodies.

Q: Which of the following statements about microRNAs (miRNAs) is false?

miRNAs can encode proteins.. **Explanation:** **1. Why Option B is the correct (false) statement:** MicroRNAs (miRNAs) are a class of **non-coding RNAs**. By definition, non-coding RNAs do not contain an open reading frame (ORF) and are not translated into proteins. Their primary function is regulatory, acting at the post-transcriptional level to modulate the expression of other protein-coding genes. **2. Analysis of incorrect options:** * **Option A & C:** These are true. miRNAs are potent **gene silencers**. They function by binding to the 3' untranslated region (3' UTR) of target messenger RNA (mRNA). This binding leads to either **mRNA degradation** (if the complementarity is perfect) or **translational repression** (if the complementarity is partial), thereby inhibiting gene expression. * **Option D:** This is true. Mature miRNAs are small molecules, typically ranging from **21 to 25 nucleotides** (though the broader category of small non-coding RNAs can extend up to 30 nucleotides). They are processed from longer primary transcripts (pri-miRNA) by enzymes **Drosha** (in the nucleus) and **Dicer** (in the cytoplasm). **Clinical Pearls & High-Yield Facts for NEET-PG:** * **RISC Complex:** miRNAs function as part of the **RNA-induced silencing complex (RISC)**. * **OncomiRs:** miRNAs that regulate oncogenes or tumor suppressor genes. Dysregulation of miRNAs is linked to various cancers (e.g., miR-21 is often overexpressed in tumors). * **Seed Sequence:** The 2nd to 7th nucleotides at the 5' end of the miRNA are crucial for target recognition. * **Therapeutic Potential:** miRNA mimics and "antagomirs" (antisense oligonucleotides) are being researched as potential targeted therapies for genetic disorders and malignancies.

Q: What is the codon for the initiation of protein synthesis?

AUG. ### Explanation **Correct Answer: A. AUG** In molecular biology, the **initiation of protein synthesis** (translation) is triggered by a specific start codon on the mRNA. **AUG** is the universal start codon. It serves two primary functions: 1. It signals the ribosome to begin translation. 2. It codes for the amino acid **Methionine** in eukaryotes and **N-formylmethionine (fMet)** in prokaryotes. --- ### Analysis of Incorrect Options: * **B. GLA:** This is not a standard genetic codon. It is likely a distractor. (Note: Glutamic acid is coded by GAA/GAG). * **C. UGA:** This is one of the three **Stop Codons** (Nonsense codons). It signals the termination of protein synthesis, not the initiation. * **D. UAG:** This is also a **Stop Codon** (specifically known as the "Amber" codon). --- ### NEET-PG High-Yield Pearls: * **Stop Codons (Nonsense Codons):** Remember them using the mnemonic "U Are Away, U Are Gone, U Go Away" (**UAA, UAG, UGA**). These do not code for any amino acid. * **Wobble Hypothesis:** Proposed by Francis Crick, it explains why multiple codons can code for a single amino acid (Degeneracy), usually differing at the 3rd base. * **Clinical Correlation:** Certain antibiotics target translation. For example, **Aminoglycosides** bind to the 30S ribosomal subunit and interfere with the initiation complex, causing misreading of the mRNA. * **Non-Standard Start:** In rare mitochondrial sequences, codons like GUG or UUG may occasionally act as start signals, but **AUG** remains the gold standard for the NEET-PG curriculum.

Q: Where is ribosomal RNA (rRNA) mainly produced?

Nucleolus. **Explanation:** The **nucleolus** is a non-membrane-bound sub-compartment within the nucleus and is the primary site for the **transcription and processing of ribosomal RNA (rRNA)**, as well as the assembly of ribosomal subunits. 1. **Why the Nucleolus is Correct:** The nucleolus contains "Nucleolar Organizer Regions" (NORs), which are clusters of genes (on chromosomes 13, 14, 15, 21, and 22) that code for 45S pre-rRNA. **RNA Polymerase I** transcribes these genes into 28S, 18S, and 5.8S rRNA. These rRNAs then combine with ribosomal proteins (imported from the cytoplasm) to form the 40S and 60S subunits before being exported back to the cytoplasm. 2. **Why Other Options are Incorrect:** * **Nucleus:** While the nucleolus is *inside* the nucleus, the nucleolus is the specific functional site for rRNA. The rest of the nucleus is primarily involved in DNA replication and mRNA/tRNA transcription. * **Ribosome:** These are the sites of **protein synthesis (translation)**, not rRNA production. They are composed *of* rRNA and proteins. * **Endoplasmic Reticulum (ER):** The Rough ER is studded with ribosomes and is involved in the synthesis of membrane-bound or secreted proteins. It has no role in rRNA synthesis. **High-Yield Clinical Pearls for NEET-PG:** * **RNA Polymerase I** transcribes all rRNA **except 5S rRNA** (which is transcribed by RNA Polymerase III in the nucleoplasm). * **Nucleolar hypertrophy** (enlarged nucleoli) is a classic histological marker of malignant cells due to high protein synthesis demands. * **Acrocentric Chromosomes:** The genes for rRNA are located on the short arms of acrocentric chromosomes (13, 14, 15, 21, 22).

Q: What is the most common form of DNA variation?

Single nucleotide polymorphism. ### Explanation **Correct Answer: A. Single nucleotide polymorphism (SNP)** **Why it is correct:** Single Nucleotide Polymorphisms (SNPs) are the most common type of genetic variation among people. A SNP represents a difference in a single nucleotide (adenine, thymine, cytosine, or guanine) at a specific position in the genome. For a variation to be classified as a SNP, it must occur in at least **1% of the population**. On average, SNPs occur once in every 300 to 1,000 nucleotides, meaning there are roughly 4 to 5 million SNPs in a person's genome. **Why the other options are incorrect:** * **B. Copy Number Variations (CNVs):** These involve structural variations where sections of the genome are repeated or deleted. While they account for significant genetic diversity by total base pair count, they occur much less frequently than SNPs. * **C. Transposons:** Also known as "jumping genes," these are DNA sequences that move locations within the genome. While they contribute to evolution, they are not the primary unit of common variation. * **D. Mutations:** In genetics, the term "mutation" usually refers to a permanent change in DNA that occurs in **less than 1%** of the population. SNPs are essentially common, non-pathogenic mutations that have persisted in the population. **High-Yield Facts for NEET-PG:** * **SNP Frequency:** Occurs approximately every 1 kb of DNA. * **Transition vs. Transversion:** Transitions (Purine to Purine or Pyrimidine to Pyrimidine) are more common than transversions. * **Clinical Significance:** SNPs are used as biological markers to locate genes associated with complex diseases (e.g., Diabetes, Hypertension) and are the basis for **Pharmacogenomics** (predicting an individual’s response to drugs). * **RFLP:** SNPs can create or destroy restriction enzyme sites, leading to Restriction Fragment Length Polymorphisms used in DNA fingerprinting.

Q: Peptidyl transferase is a/an

Ribozyme. **Explanation:** **1. Why Ribozyme is the correct answer:** Peptidyl transferase is the primary enzyme responsible for peptide bond formation during protein synthesis (translation). Unlike most enzymes which are proteins, peptidyl transferase is a **Ribozyme**—an RNA molecule with catalytic activity. Specifically, in the large ribosomal subunit (60S in eukaryotes, 50S in prokaryotes), it is the **23S rRNA** (prokaryotes) or **28S rRNA** (eukaryotes) that catalyzes the transfer of the amino acid from the tRNA in the P-site to the aminoacyl-tRNA in the A-site. **2. Why other options are incorrect:** * **Enzyme:** While peptidyl transferase functions as an enzyme, "Ribozyme" is the more specific and accurate biochemical classification required for NEET-PG. Most enzymes are proteins; ribozymes are the notable exception. * **Catalyst:** This is a broad term. While all ribozymes are biological catalysts, the question tests the specific structural nature of this molecule. * **Elongation factor:** Elongation factors (like EF-Tu or EF-G) are proteins that facilitate the translation process (e.g., bringing tRNA to the ribosome or translocation), but they do not possess the catalytic activity to form peptide bonds. **3. Clinical Pearls & High-Yield Facts:** * **Mechanism of Action:** Peptidyl transferase catalyzes the nucleophilic attack of the A-site amino group on the P-site ester linkage. * **Antibiotic Link:** **Chloramphenicol** is a high-yield antibiotic that acts by inhibiting the peptidyl transferase activity of the bacterial 50S ribosomal subunit. * **Other Ribozymes:** Apart from the ribosome, other examples include **SnRNAs** (involved in splicing) and **Ribonuclease P** (involved in tRNA processing).

Q: Which of the following is NOT a characteristic of restriction endonucleases?

They cut DNA randomly.. **Explanation:** Restriction Endonucleases (REs), often called "molecular scissors," are enzymes that recognize and cleave specific double-stranded DNA sequences. **Why Option D is the Correct Answer:** Restriction endonucleases do **not** cut DNA randomly. Their defining feature is **sequence specificity**. They recognize specific "recognition sites" (usually 4–8 base pairs long) and cleave the phosphodiester backbone only at or near those sites. Enzymes that cut DNA randomly are generally referred to as non-specific nucleases (e.g., DNase I). **Analysis of Incorrect Options:** * **Option A:** This is a core characteristic. REs are highly specific, ensuring that a particular DNA molecule will always be cut into the same set of fragments when treated with the same enzyme. * **Option B:** RE nomenclature follows a standard rule: the first letter is the genus, the next two are the species, and the Roman numeral indicates the order of discovery (e.g., **EcoRI** comes from ***E***scherichia ***co***li, strain **R**, **I**st discovered). * **Option C:** Most Type II REs (used in labs) recognize **palindromic sequences**, where the 5' to 3' sequence is identical on both strands (e.g., 5'-GAATTC-3' and 3'-CTTAAG-5'). **High-Yield Clinical Pearls for NEET-PG:** * **Biological Function:** In bacteria, REs serve as a defense mechanism against viral (bacteriophage) DNA. * **Protection:** The bacteria's own DNA is protected from these enzymes by **methylation** of the recognition sites. * **Blunt vs. Sticky Ends:** REs can produce "sticky ends" (staggered cuts like EcoRI) or "blunt ends" (straight cuts like SmaI). Sticky ends are preferred in recombinant DNA technology for easier ligation. * **RFLP:** Restriction Fragment Length Polymorphism uses REs to detect genetic variations/mutations.

Q: The gene coding for androgen receptors is located on which region of the sex chromosomes?

Long arm of the X-chromosome. **Explanation:** The **Androgen Receptor (AR) gene** is a critical mediator of male sexual development and function. It is located on the **long arm (q arm) of the X-chromosome**, specifically at the locus **Xq11-q12**. **1. Why the Correct Answer is Right:** The AR gene encodes a nuclear receptor that, upon binding to testosterone or dihydrotestosterone (DHT), acts as a transcription factor to regulate male gene expression. Because the gene is located on the X-chromosome, mutations in this gene follow an **X-linked recessive** inheritance pattern. This explains why conditions like Androgen Insensitivity Syndrome (AIS) primarily affect genotypic males (46, XY). **2. Why the Incorrect Options are Wrong:** * **Short arm of the X-chromosome (Xp):** This region contains genes like the *SHOX* gene (responsible for stature). Mutations here are associated with Turner Syndrome features but not androgen signaling. * **Short arm of the Y-chromosome (Yp):** This is the location of the **SRY gene** (Sex-determining Region Y), which initiates testis differentiation. While crucial for maleness, it does not code for the receptor itself. * **Long arm of the Y-chromosome (Yq):** This region contains the **AZF (Azoospermia Factor)** genes, which are essential for spermatogenesis, but not for androgen receptor synthesis. **3. Clinical Pearls for NEET-PG:** * **Androgen Insensitivity Syndrome (AIS):** Caused by mutations in the AR gene. Patients are 46, XY but have a female phenotype, "blind-ending" vagina, absent uterus, and undescended testes. * **Kennedy’s Disease (Spinal Bulbar Muscular Atrophy):** A trinucleotide repeat disorder (**CAG repeat**) occurring within the AR gene. * **Hormone Refractory Prostate Cancer:** Often involves amplification or hypersensitivity mutations of the AR gene located at Xq11-12.

Q: The test result of which investigation, for a child with developmental delay, is shown below?

DNA microarray. ***DNA microarray*** - **Array CGH** (comparative genomic hybridization) is the **first-line investigation** for unexplained developmental delay, detecting **copy number variations** (deletions/duplications) across the entire genome with high resolution. - The test result typically shows a **genomic plot** with **log2 ratios** displaying gains (duplications) and losses (deletions) as deviations from the baseline, making it easily identifiable. *Multiplex Ligation-dependent probe Amplification* - **MLPA** is a **targeted technique** that analyzes specific genomic regions or genes, not genome-wide screening like microarray. - Results are presented as **peak patterns** or **ratio plots** for specific probes, showing **dosage changes** in predetermined target sequences only. *Fluorescent in situ hybridisation* - **FISH** uses **fluorescent probes** to detect specific chromosomal regions and shows results as **colored signals** on metaphase chromosomes or interphase nuclei. - It's a **targeted approach** for known abnormalities rather than a genome-wide screening tool for developmental delay. *Chromosomal painting* - This technique uses **chromosome-specific probes** to "paint" entire chromosomes in different colors for detecting **large structural rearrangements**. - Results show **multicolored karyotypes** and are primarily used for complex rearrangements, not the subtle copy number changes typical in developmental delay.

Question 1

Barr bodies are not present in which of the following chromosomal complements?

Accepted Answer

XO

Answer

XXY

Answer

XX

Answer

XXX

Question 2

Which of the following statements about microRNAs (miRNAs) is false?

Accepted Answer

miRNAs can encode proteins.

Answer

miRNAs inhibit gene expression.

Answer

miRNAs are gene silencing RNAs.

Answer

miRNAs are typically 21-30 nucleotides in length.

Question 3

What is the codon for the initiation of protein synthesis?

Accepted Answer

AUG

Answer

GLA

Answer

UGA

Answer

UAG

Question 4

Where is ribosomal RNA (rRNA) mainly produced?

Accepted Answer

Nucleolus

Answer

Nucleus

Answer

Ribosome

Answer

Endoplasmic reticulum

Question 5

What is the most common form of DNA variation?

Accepted Answer

Single nucleotide polymorphism

Answer

Copy Number Variations (CNVs)

Answer

Transposons

Answer

Mutations

Question 6

Peptidyl transferase is a/an

Accepted Answer

Ribozyme

Answer

Enzyme

Answer

Catalyst

Answer

Elongation factor

Question 7

Which of the following is NOT a characteristic of restriction endonucleases?

Accepted Answer

They cut DNA randomly.

Answer

They cut DNA in a sequence-specific manner.

Answer

They are named according to the bacteria from which they are isolated.

Answer

Most of the DNA sequences they recognize are palindromic.

Question 8

The gene coding for androgen receptors is located on which region of the sex chromosomes?

Accepted Answer

Long arm of the X-chromosome

Answer

Short arm of the X-chromosome

Answer

Short arm of the Y-chromosome

Answer

Long arm of the Y-chromosome

Question 9

In E. coli, the structural genes of the lac operon are stimulated under which condition?

Accepted Answer

Presence of lactose and absence of glucose

Answer

Presence of glucose only

Answer

Presence of lactose only

Answer

Presence of glucose and absence of lactose

Question 10

The test result of which investigation, for a child with developmental delay, is shown below?

Accepted Answer

DNA microarray

Answer

Multiplex Ligation-dependent probe Amplification

Answer

Fluorescent in situ hybridisation

Answer

Chromosomal painting

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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