Molecular Biology and Genomics Practice Questions

Q: All of the following diseases are associated with trinucleotide repeat sequences except?

Hereditary motor and sensory neuropathy. **Explanation:** The correct answer is **Hereditary motor and sensory neuropathy (HMSN)**, also commonly known as **Charcot-Marie-Tooth (CMT) disease**. Unlike the other options, HMSN is not a trinucleotide repeat disorder. It is most commonly caused by a **duplication** of the PMP22 gene (CMT1A) on chromosome 17, or by point mutations affecting peripheral myelin proteins. **Trinucleotide Repeat Disorders** are characterized by the expansion of specific three-nucleotide sequences. When these repeats exceed a certain threshold, they cause gene instability and disease. * **Huntington’s Disease (Option A):** An autosomal dominant neurodegenerative disorder caused by **CAG** repeats (Polyglutamine disease) on Chromosome 4 [1]. * **Friedreich’s Ataxia (Option B):** An autosomal recessive spinocerebellar degeneration caused by **GAA** repeats in the Frataxin gene on Chromosome 9 [1]. * **Myotonic Dystrophy (Option C):** An autosomal dominant muscular dystrophy (Type 1) caused by **CTG** repeats in the DMPK gene on Chromosome 19. **High-Yield Clinical Pearls for NEET-PG:** 1. **Anticipation:** This is a hallmark of trinucleotide repeat diseases where the disease becomes more severe or has an earlier onset in successive generations due to further expansion of the repeats. 2. **Location of Repeats:** * **Exonic (Coding):** Huntington’s (CAG). * **Intronic (Non-coding):** Friedreich’s Ataxia (GAA). * **Untranslated Region (3' UTR):** Myotonic Dystrophy (CTG). * **Untranslated Region (5' UTR):** Fragile X Syndrome (CGG). 3. **Mnemonic for Friedreich's Ataxia:** **GAA** = **G**ait **A**taxia **A**lways.

Q: Which of the following statements regarding the Ames test is false?

Mutated bacteria fail to grow in the absence of histidine.. The **Ames Test** is a rapid biochemical assay used to screen chemicals for their potential to cause mutations (mutagenicity), which often correlates with carcinogenicity. ### **Explanation of the Correct Answer (Option B)** The statement in Option B is false because the test uses a **histidine-dependent (auxotrophic)** strain of *Salmonella typhimurium* that **already carries a mutation** preventing it from synthesizing histidine. * **Initial State:** These bacteria cannot grow on a histidine-free medium. * **The Mechanism:** When exposed to a mutagen, a **"reverse mutation" (back mutation)** occurs, restoring the bacteria's ability to synthesize histidine. * **The Result:** Therefore, it is the **reverted (newly mutated) bacteria** that **gain** the ability to grow in the absence of histidine. A high number of colonies indicates a strong mutagen. ### **Analysis of Other Options** * **Option A:** Correct. *Salmonella typhimurium* (specifically strains with pre-existing mutations in the *his* operon) is the standard organism used. * **Option C:** Correct. The primary purpose of the test is to identify substances that cause genetic damage (mutagens). * **Option D:** Correct. It was developed by Bruce Ames at UC Berkeley in the early 1970s. ### **High-Yield Clinical Pearls for NEET-PG** * **Rat Liver Extract (S9 Mix):** Many non-carcinogenic compounds become mutagenic only after metabolic activation in the liver. To simulate human metabolism, rat liver enzymes (S9 fraction) are added to the medium. * **Correlation:** Approximately 90% of known carcinogens test positive in the Ames test. * **Type of Mutation:** The test specifically detects **point mutations** (substitution, insertion, or deletion). * **Frameshift vs. Base-pair substitution:** Different strains of *Salmonella* are used to detect different types of mutations.

Q: Aminoacyl t-RNA is not required for which of the following?

Hydroxylysine. **Explanation:** The correct answer is **Hydroxylysine**. The fundamental concept here is the difference between **primary protein synthesis** (translation) and **post-translational modification**. Aminoacyl-tRNA is required only for the 20 standard amino acids that are coded by mRNA codons and incorporated into a polypeptide chain during translation on the ribosome. **Why Hydroxylysine is the correct answer:** Hydroxylysine is a **non-standard amino acid** found primarily in collagen. It does not have a specific codon in the genetic code and, therefore, does not have a corresponding aminoacyl-tRNA. Instead, it is formed by the **post-translational hydroxylation** of Lysine residues already incorporated into the polypeptide chain. This reaction is catalyzed by the enzyme *Lysyl hydroxylase*. **Why the other options are incorrect:** * **Proline, Lysine, and Methionine** are all standard (proteogenic) amino acids. * They are encoded by specific mRNA codons (e.g., AUG for Methionine). * Each requires a specific aminoacyl-tRNA synthetase to charge them onto their respective tRNAs for delivery to the ribosome during protein synthesis. **High-Yield Clinical Pearls for NEET-PG:** 1. **Vitamin C Dependency:** The hydroxylation of Proline and Lysine requires **Vitamin C (Ascorbic acid)** as a co-factor. Deficiency leads to **Scurvy**, characterized by defective collagen synthesis and capillary fragility. 2. **Enzymes:** *Prolyl hydroxylase* and *Lysyl hydroxylase* require molecular oxygen and $\alpha$-ketoglutarate, in addition to Vitamin C and $Fe^{2+}$. 3. **Function:** Hydroxyproline provides thermal stability to the collagen triple helix via hydrogen bonding, while Hydroxylysine is essential for the **cross-linking** of collagen fibers and the attachment of carbohydrate moieties (glycosylation).

Q: Which one of the following is a tyrosine tRNA analogue that causes premature chain termination?

Puromycin. **Explanation:** **Puromycin** is a unique antibiotic that acts as a structural analogue of the 3' end of **aminoacyl-tRNA** (specifically tyrosinyl-tRNA). Due to this structural similarity, it enters the 'A' site of the ribosome during translation. The peptidyl transferase enzyme incorporates puromycin into the growing polypeptide chain. However, because puromycin contains an amide bond instead of an ester bond, it cannot bind the next incoming amino acid, leading to **premature chain termination** and the release of incomplete polypeptides. It affects both prokaryotes and eukaryotes. **Analysis of Incorrect Options:** * **Cycloheximide:** Inhibits the enzyme **peptidyl transferase** specifically in the 60S ribosomal subunit of eukaryotes, blocking elongation. It does not act as a tRNA analogue. * **Paromomycin:** An aminoglycoside that binds to the 30S subunit. it causes **misreading of mRNA** (translational inaccuracy) rather than acting as a structural analogue for termination. * **Erythromycin:** A macrolide that binds to the 50S subunit and inhibits **translocation** (movement of mRNA relative to the ribosome). **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Protein Synthesis Inhibitors:** "**Buy AT 30, CELL at 50**" * **30S inhibitors:** **A**minoglycosides (irreversible), **T**etracyclines (reversible). * **50S inhibitors:** **C**hloramphenicol, **E**rythromycin (Macrolides), **L**inezolid, **L**incosamides (Clindamycin). * **Diphtheria toxin** and **Pseudomonas Exotoxin A** inhibit eukaryotic translation by inactivating **Elongation Factor-2 (EF-2)** via ADP-ribosylation.

Q: What is the primary function of topoisomerases?

Relieve torsional strain. **Explanation:** **Topoisomerases** are essential enzymes that regulate the topological state of DNA during replication and transcription. As the DNA helicase unwinds the double helix, it creates "over-winding" or positive supercoiling ahead of the replication fork. This creates **torsional strain** that would eventually stall the replication machinery. Topoisomerases relieve this strain by creating transient nicks in the DNA backbone (Type I cuts one strand; Type II cuts both), allowing the DNA to rotate or pass through the break to dissipate the tension before resealing the phosphodiester bonds. **Analysis of Incorrect Options:** * **A. Deoxynucleotide polymerization:** This is the function of **DNA Polymerases**, which add dNTPs to the growing DNA strand. * **C. Initiate synthesis of RNA primers:** This is performed by **Primase** (an RNA polymerase), providing the 3'-OH group necessary for DNA polymerase to begin elongation. * **D. Prevent premature reannealing:** This is the role of **Single-Stranded Binding Proteins (SSBs)**, which stabilize the separated strands to keep the replication bubble open. **High-Yield Clinical Pearls for NEET-PG:** * **Type I Topoisomerase:** Relaxes DNA by cutting a single strand; does not require ATP. Inhibited by **Irinotecan** and **Topotecan** (used in colorectal and ovarian cancer). * **Type II Topoisomerase:** Cuts both strands; requires ATP. Inhibited by **Etoposide** and **Teniposide**. * **DNA Gyrase:** A bacterial Type II Topoisomerase that introduces negative supercoils. It is the specific target of **Fluoroquinolones** (e.g., Ciprofloxacin).

Q: What is true about a silent mutation in a gene?

No change in amino acid sequence in protein. ### Explanation **Concept Overview** A **silent mutation** is a type of point mutation where a single nucleotide base is substituted for another, but the resulting codon still codes for the same amino acid. This phenomenon is possible due to the **degeneracy (redundancy) of the genetic code**, where multiple codons can specify a single amino acid (e.g., GAA and GAG both code for Glutamic Acid). **Why Option B is Correct** The hallmark of a silent mutation is that while the DNA and mRNA sequences change, the **primary structure of the protein (amino acid sequence) remains identical**. Since the protein remains unchanged, its folding and function typically remain normal. **Analysis of Incorrect Options** * **Option A:** Inaccurate. A mutation by definition involves a change in the DNA sequence, which is then transcribed into a **changed mRNA sequence**. * **Option C:** Inaccurate. The gene is still transcribed and translated; the protein is expressed, just with the original amino acid sequence. * **Option D:** While often true, this is not the *definition* of a silent mutation. Some silent mutations can affect mRNA stability or splicing, potentially altering the *amount* of protein expressed, but the defining feature is the lack of change in the amino acid sequence itself. **High-Yield Clinical Pearls for NEET-PG** * **Wobble Hypothesis:** Silent mutations usually occur at the **third position** of the codon. * **Synonymous Mutation:** Another term for a silent mutation. * **Contrast with Missense:** A mutation that results in a *different* amino acid (e.g., Sickle Cell Anemia: Glutamic acid → Valine). * **Contrast with Nonsense:** A mutation that creates a premature **stop codon** (UAG, UAA, UGA), leading to a truncated, non-functional protein.

Q: Reverse transcription involves which of the following processes?

RNA dependent DNA synthesis. **Explanation:** **1. Why Option A is Correct:** Reverse transcription is the process by which genetic information flows "backward" from RNA to DNA, challenging the traditional Central Dogma. The enzyme responsible, **Reverse Transcriptase**, uses a single-stranded RNA template to synthesize a complementary DNA (cDNA) strand. Therefore, it is biochemically classified as an **RNA-dependent DNA polymerase**. **2. Why Other Options are Incorrect:** * **Option B (DNA-dependent RNA synthesis):** This describes **Transcription**, catalyzed by RNA Polymerase (e.g., synthesis of mRNA from a DNA template). * **Option C (DNA-dependent DNA synthesis):** This describes **Replication**, catalyzed by DNA Polymerase during the S-phase of the cell cycle. * **Option D (RNA-dependent RNA synthesis):** This describes **RNA Replication**, catalyzed by RNA-dependent RNA polymerase (RdRp), found in certain RNA viruses like Poliovirus or SARS-CoV-2. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Retroviruses:** The most notable example is **HIV**, which uses reverse transcriptase to integrate its genome into the host cell's DNA. * **Telomerase:** A specialized reverse transcriptase (containing its own RNA template) that maintains the ends of linear chromosomes. It is highly active in cancer cells and stem cells. * **Diagnostic Application:** **RT-PCR** (Reverse Transcription Polymerase Chain Reaction) utilizes this process to detect RNA viruses (like HIV or COVID-19) by first converting viral RNA into cDNA. * **Drug Target:** Nucleoside Reverse Transcriptase Inhibitors (**NRTIs**) like Zidovudine (AZT) are cornerstones of HAART therapy for HIV.

Q: Which of the following is NOT a component of tRNA?

Codon arm. **Explanation:** The correct answer is **C. Codon arm**. In molecular biology, tRNA (transfer RNA) acts as an adapter molecule that translates the genetic code from mRNA into a sequence of amino acids. While tRNA interacts with a codon, it does so via its **Anticodon arm**, which contains a triplet of bases complementary to the mRNA codon. There is no structural component known as a "Codon arm" in tRNA. **Analysis of Options:** * **A. D-loop:** This is a standard component of tRNA containing **Dihydrouridine**. It is primarily responsible for recognition by the enzyme aminoacyl-tRNA synthetase. * **B. TψC loop:** Also known as the T-loop, it contains **Ribothymidine** and **Pseudouridine** (ψ). This loop is essential for binding the tRNA to the ribosomal surface (specifically the 5S rRNA of the large subunit). * **D. Variable loop:** Located between the TψC loop and the Anticodon loop, its length varies among different tRNAs. It is used to classify tRNA into Class I (short loop) and Class II (long loop). **High-Yield Clinical Pearls for NEET-PG:** * **Structure:** tRNA has a **Cloverleaf** secondary structure and an **L-shaped** tertiary structure. * **3' End:** All tRNAs have a **CCA sequence** at the 3' hydroxy terminus, which is the attachment site for the specific amino acid (forming aminoacyl-tRNA). * **Wobble Hypothesis:** The 3rd base of the mRNA codon can form non-standard base pairs with the 1st base of the tRNA anticodon, allowing one tRNA to recognize multiple codons. * **Inosine:** Often found in the anticodon loop, it is a modified base that facilitates "wobbling."

Q: Which of the following statements about the genetic code is FALSE?

It involves punctuation.. ### Explanation The genetic code is the set of rules by which information encoded in genetic material is translated into proteins. Understanding its properties is fundamental to molecular biology and medical genetics. **Why "It involves punctuation" is FALSE:** The genetic code is **commaless**. Once the translation begins at the start codon (AUG), the "reading frame" is established, and the mRNA is read continuously, three nucleotides at a time, without any skipped bases or "punctuation" marks between codons. There are no internal spacers; if a base is added or deleted, it results in a **frameshift mutation**, altering every subsequent codon. **Analysis of Other Options:** * **A. Degenerate:** Most amino acids are coded by more than one codon (e.g., Leucine has six). This provides "wobble" protection against some point mutations. * **B. Universal:** The code is the same across almost all organisms, from bacteria to humans. *Exception:* Minor variations exist in 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 like ABCDEF, the codons are ABC and DEF, never BCD. **High-Yield Clinical Pearls for NEET-PG:** * **Initiation Codon:** **AUG** (codes for Methionine in eukaryotes and N-formylmethionine in prokaryotes). * **Stop Codons (Nonsense Codons):** **UAA** (Ochre), **UAG** (Amber), and **UGA** (Opal). These do not code for any amino acid. * **Wobble Hypothesis:** Proposed by Francis Crick; it explains why the third base of a codon can often be changed without changing the amino acid, allowing one tRNA to recognize multiple codons. * **Frameshift Mutations:** These occur due to the "commaless" nature of the code. Examples include certain forms of **Duchenne Muscular Dystrophy** and **Tay-Sachs disease**.

Question 1

All of the following diseases are associated with trinucleotide repeat sequences except?

Accepted Answer

Hereditary motor and sensory neuropathy

Answer

Huntington's disease

Answer

Friedreich's ataxia

Answer

Myotonic dystrophy

Question 2

Which of the following statements regarding the Ames test is false?

Accepted Answer

Mutated bacteria fail to grow in the absence of histidine.

Answer

Salmonella typhimurium is used in the test.

Answer

It is used for testing mutagens.

Answer

It was developed by Bruce N. Ames in the 1970s.

Question 3

Aminoacyl t-RNA is not required for which of the following?

Accepted Answer

Hydroxylysine

Answer

Proline

Answer

Lysine

Answer

Methionine

Question 4

Which one of the following is a tyrosine tRNA analogue that causes premature chain termination?

Accepted Answer

Puromycin

Answer

Cycloheximide

Answer

Paromomycin

Answer

Erythromycin

Question 5

What is the primary function of topoisomerases?

Accepted Answer

Relieve torsional strain

Answer

Deoxynucleotide polymerization

Answer

Initiate synthesis of RNA primers

Answer

Prevent premature reannealing of double-stranded DNA

Question 6

What is true about a silent mutation in a gene?

Accepted Answer

No change in amino acid sequence in protein

Answer

No change in mRNA

Answer

No expression of protein

Answer

No change in the expression of protein

Question 7

Reverse transcription involves which of the following processes?

Accepted Answer

RNA dependent DNA synthesis

Answer

DNA dependent RNA synthesis

Answer

DNA dependent DNA synthesis

Answer

RNA dependent RNA synthesis

Question 8

Which of the following is NOT a component of tRNA?

Accepted Answer

Codon arm

Answer

D-loop

Answer

TpsC loop

Answer

Variable loop

Question 9

Which of the following statements about the genetic code is FALSE?

Accepted Answer

It involves punctuation.

Answer

It is degenerate.

Answer

It is universal.

Answer

It is non-overlapping.

Question 10

Which statement about mitochondrial DNA is true?

Accepted Answer

Mitochondrial DNA codes for less than 20% of the proteins involved in the respiratory chain.

Answer

Mitochondrial DNA has fewer mutations compared to nuclear DNA.

Answer

Mitochondrial DNA has 3x10^9 base pairs.

Answer

Mitochondrial DNA receives 23 chromosomes from each parent.

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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