Molecular Biology and Genomics Practice Questions

Q: DNA Gyrase is:

Prokaryotic DNA Topoisomerase II. ### Explanation **DNA Gyrase** is a specialized enzyme found in **prokaryotes** that belongs to the **Type II Topoisomerase** family. Its primary function is to introduce **negative supercoils** into DNA, which helps relieve the positive superhelical tension that builds up ahead of the replication fork. Unlike Type I topoisomerases, Type II enzymes (like DNA Gyrase) catalyze the breakage of both strands of the DNA double helix and require ATP for their activity. #### Analysis of Options: * **Option D (Correct):** DNA Gyrase is the bacterial version of Topoisomerase II. It is unique because it is the only enzyme capable of introducing negative supercoils, which is essential for compacting the bacterial chromosome and facilitating replication. * **Option A:** Eukaryotic Topoisomerase I relaxes supercoils by cutting a single strand of DNA and does not require ATP. * **Option B:** Prokaryotic Topoisomerase III is a Type I topoisomerase involved primarily in recombination and resolving "knots" in DNA. * **Option C:** Prokaryotic Topoisomerase I (also known as Omega protein) relaxes negative supercoils but cannot introduce them. #### Clinical Pearls for NEET-PG: * **Pharmacological Target:** DNA Gyrase is the primary target of **Quinolones and Fluoroquinolones** (e.g., Ciprofloxacin, Nalidixic acid). These drugs inhibit the A-subunit of the enzyme, leading to bacterial cell death. * **Subunit Structure:** It is a tetramer composed of two **GyrA** subunits (involved in DNA breakage/rejoining) and two **GyrB** subunits (possessing ATPase activity). * **Eukaryotic Counterpart:** Humans possess Topoisomerase II, but it lacks the ability to introduce negative supercoils; it only relaxes them. This structural difference allows for selective toxicity of antibiotics.

Q: What does the amber codon refer to?

Stop codon. **Explanation:** The correct answer is **B. Stop codon**. In molecular biology, translation termination is signaled by specific nucleotide triplets called **Stop codons** (or nonsense codons). There are three stop codons in the universal genetic code: 1. **UAG (Amber)** 2. **UGA (Opal)** 3. **UAA (Ochre)** These codons do not code for any amino acid. Instead, they are recognized by release factors that trigger the hydrolysis of the ester bond between the tRNA and the polypeptide chain, effectively terminating protein synthesis. The name "Amber" was a whimsical designation given by researchers (specifically Caltech's Richard Epstein and Charles Steinberg) based on the German translation of the last name of their colleague, Harris Bernstein. **Analysis of Incorrect Options:** * **A. Mutant codon:** While a mutation can create a stop codon (known as a **nonsense mutation**), the term "Amber" specifically refers to the UAG sequence itself, not the process of mutation. * **C. Initiating codon:** The universal initiation codon is **AUG**, which codes for Methionine (in eukaryotes) or N-formylmethionine (in prokaryotes). * **D. Codon for more than one amino acid:** This describes **ambiguity**. However, the genetic code is **unambiguous**, meaning one codon always codes for only one specific amino acid. **NEET-PG High-Yield Pearls:** * **Mnemonic to remember stop codons:** **U** **A**re **G**one (UAG), **U** **G**o **A**way (UGA), **U** **A**re **A**way (UAA). * **Nonsense Mutation:** A point mutation that changes an amino acid codon into a stop codon, leading to a truncated, usually non-functional protein. * **Exceptions:** In human mitochondria, UGA codes for Tryptophan rather than acting as a stop codon. * **Selenocysteine:** Known as the 21st amino acid, it is encoded by the UGA codon when a specific insertion sequence (SECIS) is present.

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**.

Question 1

DNA Gyrase is:

Accepted Answer

Prokaryotic DNA Topoisomerase II

Answer

Eukaryotic DNA Topoisomerase I

Answer

Prokaryotic DNA Topoisomerase III

Answer

Prokaryotic DNA Topoisomerase I

Question 2

What does the amber codon refer to?

Accepted Answer

Stop codon

Answer

Mutant codon

Answer

Initiating codon

Answer

Codon for more than one amino acid

Question 3

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 4

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 5

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 6

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 7

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 8

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

Accepted Answer

Streptomycin

Answer

Penicillin

Answer

Sulphonamides

Answer

Cephalosporins

Question 9

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 10

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

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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