Molecular Biology and Genomics Practice Questions

Q: Which of the following is NOT a characteristic of the genetic code?

Contains punctuation signals. The genetic code is the set of rules by which information encoded in genetic material is translated into proteins. Understanding its properties is fundamental for NEET-PG biochemistry. ### **Explanation of the Correct Answer** **C. Contains punctuation signals:** This is the correct answer because the genetic code is **commaless**. Once translation begins at the start codon (AUG), the mRNA is read continuously, three nucleotides at a time, without skipping any bases or using "punctuation" marks between codons. There are no spacers between the triplets. While there are "Stop" codons (UAA, UAG, UGA) that signal the end of translation, they do not act as internal punctuation between individual amino acids. ### **Analysis of Incorrect Options** * **A. Degenerate:** This is a true characteristic. Most amino acids are coded by more than one codon (e.g., Leucine has six). This provides a buffer against mutations (often at the 3rd "wobble" position). * **B. Universal:** The code is consistent across almost all organisms, from bacteria to humans. *Exception:* Mitochondrial DNA (e.g., UGA codes for Tryptophan instead of Stop). * **D. Non-overlapping:** Each nucleotide is part of only one codon. In a sequence ABCDEF, the codons are ABC and DEF, never BCD. ### **High-Yield Clinical Pearls for NEET-PG** * **Wobble Hypothesis:** Proposed by Francis Crick; explains why the 3rd base of a codon can have non-traditional pairing, allowing one tRNA to recognize multiple codons. * **Initiation Codon:** **AUG** (codes for Methionine in eukaryotes and N-formylmethionine in prokaryotes). * **Nonsense Mutations:** Occur when a point mutation changes an amino acid codon into a **Stop codon** (UAA, UAG, UGA), leading to premature protein termination. * **Frameshift Mutations:** Result from the insertion or deletion of nucleotides (not in multiples of three), which alters the entire reading frame because the code is commaless and non-overlapping.

Q: Which of the following mutations is potentially lethal?

Insertion of one base. ### Explanation **1. Why "Insertion of one base" is the correct answer:** The insertion of a single nucleotide (or any number not divisible by three) results in a **Frameshift Mutation**. Since the genetic code is read in non-overlapping triplets (codons), adding one base shifts the entire reading frame downstream from the mutation site. This typically leads to: * **Garbled translation:** Every amino acid after the mutation is altered. * **Premature Stop Codons:** Most frameshifts result in a truncated, non-functional protein (Nonsense-mediated decay). Because this fundamentally destroys the protein's primary structure and function, it is far more likely to be **lethal** than a single amino acid substitution. **2. Why the other options are incorrect:** * **Options A and C (Substitution):** These are **Point Mutations** (specifically Transversion and Transition). These usually result in a **Missense mutation** (one amino acid change) or a **Silent mutation** (no change). While some (like Sickle Cell Anemia) cause disease, many are benign or non-lethal. * **Option B (Methylcytosine for Cytosine):** This is a form of **Epigenetic modification** rather than a traditional mutation. DNA methylation is a physiological process used for gene silencing and regulation. While abnormal methylation is linked to cancer, it is not a structural "lethal mutation" in the context of sequence disruption. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Frameshift Examples:** Duchenne Muscular Dystrophy (DMD) is often caused by frameshift deletions/insertions, whereas the milder Becker’s MD usually involves in-frame mutations. * **Tay-Sachs Disease:** Often caused by a 4-base pair insertion in the HEXA gene. * **Transition vs. Transversion:** Transitions (Purine to Purine) are more common than Transversions (Purine to Pyrimidine). * **Nonsense Mutation:** A point mutation that creates a premature stop codon (UAG, UAA, UGA).

Q: A 43-year-old woman with a BMI of 32 presents with abnormal uterine bleeding. An endometrial biopsy reveals endometrial adenocarcinoma of endometrioid histology. Her family history is significant for colon cancer (mother diagnosed at 66) and endometrial cancer (paternal aunt diagnosed at 67). Suspecting a hereditary condition, which of the following DNA repair mechanisms is likely defective?

Mismatch repair. **Explanation:** The patient presents with endometrial adenocarcinoma and a family history suggestive of **Lynch Syndrome** (Hereditary Non-Polyposis Colorectal Cancer - HNPCC). Lynch syndrome is the most common hereditary cause of endometrial cancer, often preceding the development of colon cancer in female carriers. **1. Why Mismatch Repair (MMR) is correct:** Lynch syndrome is caused by germline mutations in **DNA Mismatch Repair genes** (primarily *MLH1, MSH2, MSH6,* and *PMS2*). When the MMR system is defective, errors (mismatches) that occur during DNA replication are not corrected, leading to **Microsatellite Instability (MSI)** and subsequent oncogenesis. In women with Lynch syndrome, the lifetime risk of endometrial cancer (40–60%) equals or exceeds the risk of colorectal cancer. **2. Why other options are incorrect:** * **Nucleotide Excision Repair (A):** Defective in **Xeroderma Pigmentosum**. It repairs bulky DNA lesions caused by UV radiation. * **Homologous Recombination (B):** Defective in **BRCA1/2 mutations**. This pathway repairs double-strand breaks; defects are associated with hereditary breast and ovarian cancer (HBOC) syndromes. * **Base Excision Repair (D):** Defective in **MUTYH-associated polyposis (MAP)**. It repairs small, non-bulky DNA damage (e.g., oxidative damage). **Clinical Pearls for NEET-PG:** * **Amsterdam II Criteria:** Used to identify Lynch syndrome families (3-2-1 rule: 3 relatives with Lynch-associated cancers, 2 generations, 1 diagnosed before age 50). * **Screening:** For Lynch syndrome carriers, annual endometrial sampling is recommended starting at age 30–35. * **Sentinel Cancers:** In 50% of women with Lynch syndrome, endometrial cancer is the "sentinel" (first) malignancy diagnosed. * **Histology:** Lynch-associated endometrial cancers are often found in the lower uterine segment and may show prominent tumor-infiltrating lymphocytes.

Q: Which of the following cis-acting elements typically resides adjacent to or overlaps with many prokaryotic promoters?

Operator. ### Explanation **1. Why the Correct Answer is Right:** In prokaryotic gene regulation (the **Operon model**), the **Operator** is a specific DNA sequence (a **cis-acting element**) that serves as the binding site for regulatory proteins. It is strategically positioned between the promoter and the structural genes, often overlapping with the promoter itself. When a repressor protein binds to the operator, it physically obstructs RNA polymerase from binding to the promoter or moving forward, thereby inhibiting transcription. This spatial proximity is essential for the "on/off" switch mechanism of operons like *lac* and *trp*. **2. Why the Incorrect Options are Wrong:** * **A. Regulatory gene:** This is a DNA segment that codes for a regulatory protein (like a repressor). It is often located at a distance from the operon it regulates and is not a cis-acting element that overlaps the promoter. * **B. Structural gene(s):** These are the genes that code for the actual enzymes or proteins (e.g., *lacZ, lacY*). They are located downstream of the promoter and operator, not overlapping them. * **C. Repressor:** This is a **trans-acting factor** (a protein), not a cis-acting element (DNA sequence). It binds to the operator but is not part of the DNA architecture itself. **3. High-Yield Clinical Pearls for NEET-PG:** * **Cis-acting elements:** DNA sequences on the same molecule (e.g., Promoter, Operator, Enhancer). * **Trans-acting factors:** Diffusible molecules, usually proteins, that bind to DNA (e.g., Repressors, Activators). * **Polycistronic mRNA:** A hallmark of prokaryotes where one promoter/operator complex regulates multiple structural genes. * **Inducible vs. Repressible:** The *lac* operon is the classic example of an **inducible** system (turned on by the presence of lactose/allolactose), whereas the *trp* operon is **repressible**.

Q: What is the macromolecular complex that associates with introns during mRNA splicing?

Spliceosome. **Explanation:** The correct answer is **Spliceosome**. mRNA splicing is a critical post-transcriptional modification in eukaryotes where non-coding sequences (**introns**) are removed and coding sequences (**exons**) are joined together to form mature mRNA. 1. **Why Spliceosome is correct:** The spliceosome is a large macromolecular complex composed of **snRNPs** (small nuclear ribonucleoproteins, pronounced "snurps")—specifically U1, U2, U4, U5, and U6—and additional protein factors. These snRNPs recognize specific consensus sequences at the 5' splice site (GU) and 3' splice site (AG) to facilitate the transesterification reactions required for splicing. 2. **Why other options are incorrect:** * **Dicer:** An enzyme involved in the **RNA interference (RNAi)** pathway. It cleaves double-stranded RNA or pre-miRNA into short fragments (siRNA/miRNA) to silence gene expression. * **Nuclear body:** These are non-membrane-bound structures within the nucleus (e.g., Cajal bodies, Nucleoli) that serve various organizational functions but are not the specific machinery for intron removal. * **Splicer:** This is a distractor term and not a recognized biological complex in molecular biology. **Clinical Pearls for NEET-PG:** * **Systemic Lupus Erythematosus (SLE):** Patients often develop **anti-Smith (anti-Sm) antibodies**, which are directed against the proteins in snRNPs. This is a highly specific diagnostic marker for SLE. * **Spinal Muscular Atrophy (SMA):** Caused by a defect in the SMN1 gene, leading to impaired assembly of snRNPs. * **Beta-Thalassemia:** Some forms are caused by mutations at splice sites, leading to improper intron removal and non-functional hemoglobin chains.

Q: What is the attachment site of miRNA to inhibit gene expression?

3' end of mRNA. **Explanation:** MicroRNAs (miRNAs) are small, non-coding RNA molecules (typically 21–25 nucleotides long) that play a crucial role in post-transcriptional gene regulation. **Why Option A is Correct:** The primary mechanism of miRNA action involves binding to the **3' Untranslated Region (3' UTR)** of the target mRNA. This binding occurs via base-pairing between the "seed sequence" (nucleotides 2–7) of the miRNA and the complementary sequence on the 3' end of the mRNA. Once attached, the miRNA-induced silencing complex (miRISC) inhibits translation or promotes the degradation of the mRNA, thereby "silencing" the gene. **Why Other Options are Incorrect:** * **Option B (5' end of mRNA):** While the 5' end is involved in translation initiation, it is not the standard physiological target site for miRNA-mediated inhibition. * **Option C (Sigma factor):** Sigma factors are prokaryotic transcription factors required for RNA polymerase binding to promoters. miRNAs are eukaryotic regulatory elements and do not interact with sigma factors. * **Option D (Cap binding protein):** Proteins like eIF4E bind to the 5' cap to initiate translation. While miRNAs may interfere with the function of these proteins indirectly, they do not physically attach to them to inhibit gene expression. **High-Yield NEET-PG Pearls:** * **Biogenesis:** miRNAs are processed by enzymes **Drosha** (in the nucleus) and **Dicer** (in the cytosol). * **Mechanism:** If the complementarity is perfect, mRNA is cleaved; if it is partial (common in humans), translation is repressed. * **Clinical Link:** Dysregulation of miRNAs is linked to various cancers (acting as "oncomiRs") and cardiovascular diseases. * **RNA Interference (RNAi):** Unlike siRNA (which is exogenous and double-stranded), miRNA is endogenous and single-stranded.

Question 1

Which of the following is NOT a characteristic of the genetic code?

Accepted Answer

Contains punctuation signals

Answer

Degenerate

Answer

Universal

Answer

Non-overlapping

Question 2

Which of the following mutations is potentially lethal?

Accepted Answer

Insertion of one base

Answer

Substitution of adenine for cytosine

Answer

Substitution of methylcytosine for cytosine

Answer

Substitution of guanine for cytosine

Question 3

Why do fetal cells continue to divide, while terminally differentiated adult cells do not?

Accepted Answer

Fetal cells possess numerous cyclin inhibitors that prevent adult cells from entering the S phase.

Answer

Phosphatase is absent in fetal cells.

Answer

Proteinase is absent in the fetus.

Answer

CDK (cyclin-dependent kinase) is absent in fetal cells.

Question 4

A 43-year-old woman with a BMI of 32 presents with abnormal uterine bleeding. An endometrial biopsy reveals endometrial adenocarcinoma of endometrioid histology. Her family history is significant for colon cancer (mother diagnosed at 66) and endometrial cancer (paternal aunt diagnosed at 67). Suspecting a hereditary condition, which of the following DNA repair mechanisms is likely defective?

Accepted Answer

Mismatch repair

Answer

Nucleotide excision repair

Answer

Homologous recombination

Answer

Base excision repair

Question 5

Which of the following is true about small silencing RNAs?

Accepted Answer

They are associated with gene knockdown technology.

Answer

They are mature oligomers in the human genome.

Answer

XIST is an example of a small interfering RNA (siRNA).

Answer

They are related to atherosclerosis.

Question 6

Post-translational modification is seen with which of the following drugs?

Accepted Answer

Streptomycin

Answer

Penicillin

Answer

Sulphonamides

Answer

Cephalosporins

Question 7

Choose the most appropriate statement about Epigenetics?

Accepted Answer

Heritable changes in gene expression

Answer

Irreversible modification of DNA

Answer

Change in genotype without change in phenotype

Answer

Change in nucleotide sequence

Question 8

Which of the following cis-acting elements typically resides adjacent to or overlaps with many prokaryotic promoters?

Accepted Answer

Operator

Answer

Regulatory gene

Answer

Structural gene(s)

Answer

Repressor

Question 9

What is the macromolecular complex that associates with introns during mRNA splicing?

Accepted Answer

Spliceosome

Answer

Splicer

Answer

Dicer

Answer

Nuclear body

Question 10

What is the attachment site of miRNA to inhibit gene expression?

Accepted Answer

3' end of mRNA

Answer

5' end of mRNA

Answer

Binding to sigma factor

Answer

Cap binding protein

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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