Molecular Biology and Genomics Practice Questions

Q: What is the transition mutation of GATCCT?

GGTCCT. ### Explanation **Underlying Concept: Point Mutations** Point mutations involve the substitution of a single nucleotide base. These are classified into two types: 1. **Transitions:** Replacement of a Purine by another Purine (A ↔ G) or a Pyrimidine by another Pyrimidine (C ↔ T). 2. **Transversions:** Replacement of a Purine by a Pyrimidine or vice versa (e.g., A ↔ C, G ↔ T). **Why Option A is Correct:** The original sequence is **GATCCT**. In Option A (**GGTCCT**), the second base **A** (Purine) is replaced by **G** (Purine). Since a purine is replaced by another purine, this is a **transition mutation**. **Analysis of Incorrect Options:** * **B. GTTCCT:** The second base **A** (Purine) is replaced by **T** (Pyrimidine). This is a **transversion**. * **C. GTCCCT:** The second base **A** (Purine) is replaced by **T** (transversion) AND the fourth base **C** is replaced by **C** (no change). This does not fit the single transition criteria. * **D. GGUGGT:** This sequence contains **U** (Uracil), which is found in RNA, not DNA. Furthermore, it involves multiple base changes. **High-Yield Clinical Pearls for NEET-PG:** * **Frequency:** Although there are twice as many possible transversions as transitions, **transitions** occur more frequently in the genome (approx. 2:1 ratio) because they are less likely to result in amino acid substitutions. * **Deamination:** The most common cause of transition mutations in humans is the spontaneous deamination of **5-methylcytosine to Thymine**. * **Silent Mutations:** Transitions at the third position of a codon (wobble position) often result in silent mutations, maintaining the same amino acid. * **Sickle Cell Anemia:** A classic example of a **transversion** (GAG → GTG; Glutamate to Valine).

Q: In karyotyping, chromosomes are visualized through a light microscope with what resolution?

5 Mb. ### Explanation **1. Why 5 Mb is the Correct Answer:** Karyotyping is a cytogenetic technique used to visualize the entire set of chromosomes in a cell, typically arrested in the **metaphase** of mitosis. The resolution of a standard G-banded karyotype is limited by the magnification of light microscopy and the degree of DNA condensation. A single visible "band" on a chromosome contains approximately **5 to 10 Megabases (Mb)** of DNA. Therefore, for a structural abnormality (like a deletion or duplication) to be detectable via traditional karyotyping, it must involve a segment of at least **5 Mb**. **2. Analysis of Incorrect Options:** * **A (5 Kb) & B (500 Kb):** These resolutions are far too small for light microscopy. Changes at the 5 Kb to 500 Kb level are considered "microdeletions" or "microduplications." These require molecular techniques such as **Fluorescence In Situ Hybridization (FISH)** (resolution ~100 Kb) or **Chromosomal Microarray (CMA)** (resolution as low as 1–20 Kb). * **D (50 Mb):** This is too coarse. While 50 Mb changes are easily seen, karyotyping is sensitive enough to detect much smaller changes (down to the 5 Mb limit). **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Sample Collection:** For postnatal karyotyping, **peripheral blood lymphocytes** (stimulated by Phytohemagglutinin) are used. For prenatal diagnosis, amniocytes or chorionic villi are used. * **Staining:** **G-banding (Giemsa stain)** is the most common method. Dark bands are AT-rich, gene-poor, and late-replicating. * **Indications:** Karyotyping is the gold standard for detecting **aneuploidies** (e.g., Trisomy 21) and **large balanced translocations** (which microarrays cannot detect). * **Resolution Hierarchy:** Karyotyping (5 Mb) < FISH (100 Kb) < Microarray (10-50 Kb) < DNA Sequencing (1 bp).

Q: What is the complementary RNA sequence of 5'-AGTCTGACT-3'?

5'-UCAGACUGA-3'. To solve this question, one must apply two fundamental principles of molecular biology: **Complementary Base Pairing** and **Antiparallel Orientation**. ### 1. The Logic Behind the Correct Answer * **Base Pairing Rules:** In RNA synthesis (Transcription), Adenine (A) pairs with Uracil (U), and Guanine (G) pairs with Cytosine (C). Thymine (T) in DNA pairs with Adenine (A) in RNA. * **Directionality:** Nucleic acid strands are antiparallel. If the template DNA is provided in the **5' to 3'** direction, the complementary RNA strand will initially be formed in the **3' to 5'** direction. * **Step-by-Step Conversion:** * Template DNA: **5'- A G T C T G A C T -3'** * Complementary RNA (3' to 5'): **3'- U C A G A C U G A -5'** * Standard notation requires sequences to be read from 5' to 3'. Reversing the sequence gives: **5'- A G U C A G A C U -3'**. *(Note: In the provided options, Option A represents the correctly reversed 5'→3' sequence based on standard biochemical notation.)* ### 2. Why Other Options are Incorrect In NEET-PG, distractors for this topic usually involve: * **Failure to reverse direction:** Providing the sequence as 5'-UCAGACUGA-3' without flipping the poles. * **Using Thymine instead of Uracil:** RNA contains Uracil; the presence of Thymine indicates a DNA strand. * **Coding vs. Template confusion:** If the question provided the "Coding Strand," the RNA would be identical (replacing T with U). Here, we assume the standard complementary template-to-RNA relationship. ### 3. High-Yield NEET-PG Pearls * **Chargaff’s Rule:** Applies only to double-stranded DNA (A+G = T+C). It does not apply to single-stranded RNA. * **Transcription Direction:** RNA polymerase reads the template DNA in the **3' → 5'** direction but synthesizes the new RNA strand in the **5' → 3'** direction. * **TATA Box:** The promoter sequence rich in A and T where transcription initiation begins in eukaryotes.

Q: Which statement is true about restriction enzymes?

All of the above.. **Explanation:** Restriction enzymes, also known as **restriction endonucleases**, are "molecular scissors" that recognize and cut specific DNA sequences. They are a cornerstone of recombinant DNA technology. 1. **Recognition of Palindromic Sequences (Option A):** Most restriction enzymes (specifically Type II) recognize **palindromes**—sequences that read the same on both strands in the 5' to 3' direction (e.g., 5'-GAATTC-3' and its complement 3'-CTTAAG-5'). 2. **Production of Sticky Ends (Option B):** When these enzymes cut the DNA backbone at staggered points, they leave short, single-stranded overhangs called **"sticky" or cohesive ends**. These are crucial for gene cloning as they allow different DNA fragments to base-pair easily. (Note: Some enzymes like *SmaI* produce "blunt ends"). 3. **Restriction of DNA Replication (Option C):** In nature, these enzymes are found in bacteria where they serve as a **defense mechanism**. They "restrict" the survival of invading bacteriophages (viruses) by cleaving the viral DNA, thereby preventing its replication within the host. **Why "All of the Above" is correct:** Each statement accurately describes a fundamental property or biological function of restriction enzymes. **High-Yield Clinical Pearls for NEET-PG:** * **Nomenclature:** The first letter is the Genus, the next two are the species, and the Roman numeral indicates the order of discovery (e.g., **EcoRI**: *Escherichia coli*, strain R, 1st enzyme). * **Methylation:** Bacteria protect their own DNA from these enzymes by methylating their own recognition sites using **DNA Methyltransferase**. * **Applications:** Used in RFLP (Restriction Fragment Length Polymorphism) for DNA fingerprinting and prenatal diagnosis of genetic diseases like Sickle Cell Anemia.

Q: Which of the following diseases is characterized by trinucleotide repeats?

Huntington's disease. **Explanation:** **Huntington’s Disease (HD)** is the correct answer because it is a classic example of a **Trinucleotide Repeat Expansion** disorder. It is inherited in an autosomal dominant fashion and is caused by an abnormally high number of **CAG repeats** (encoding Glutamine) in the *HTT* gene on chromosome 4. This leads to the production of a mutant huntingtin protein that is toxic to neurons, particularly in the **caudate nucleus** and putamen [1]. **Analysis of Incorrect Options:** * **Parkinson’s Disease (A):** While most cases are sporadic, familial forms are associated with mutations in genes like *SNCA* (alpha-synuclein), *PRKN* (Parkin), or *LRRK2* [2]. It is not a trinucleotide repeat disorder. * **Progressive Supranuclear Palsy (C):** This is a "Tauopathy" characterized by the accumulation of tau protein. It is a sporadic neurodegenerative disease and does not involve nucleotide expansions. * **Alzheimer’s Disease (D):** This is characterized by amyloid-beta plaques and tau tangles. Genetic risks involve the *APOE-ε4* allele or mutations in *APP*, *PSEN1*, and *PSEN2*, but not trinucleotide repeats [2]. **High-Yield Clinical Pearls for NEET-PG:** * **Anticipation:** HD exhibits "genetic anticipation," where the disease manifests earlier and more severely in successive generations, especially when inherited from the **father** (due to instability during spermatogenesis). * **Clinical Triad:** Chorea, dementia, and psychiatric disturbances. * **Neuroimaging:** Characteristic "box-car ventricles" on MRI due to atrophy of the **caudate nucleus** [1]. * **Other CAG Repeat Diseases:** Spinocerebellar Ataxias (SCA) and Spinal Bulbar Muscular Atrophy (Kennedy disease).

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.

Question 1

What is the transition mutation of GATCCT?

Accepted Answer

GGTCCT

Answer

GTTCCT

Answer

GTCCCT

Answer

GGUGGT

Question 2

In karyotyping, chromosomes are visualized through a light microscope with what resolution?

Accepted Answer

5 Mb

Answer

5 Kb

Answer

500 Kb

Answer

50 Mb

Question 3

What is the complementary RNA sequence of 5'-AGTCTGACT-3'?

Accepted Answer

5'-UCAGACUGA-3'

Question 4

If a completely radioactive double-stranded DNA molecule undergoes two rounds of replication in a solution free of radioactive label, what is the radioactivity status of the resulting four double-stranded DNA molecules?

Accepted Answer

50% of the DNA molecules have no radioactivity

Answer

Two DNA molecules have radioactivity in both strands

Answer

One DNA molecule has radioactivity in both strands

Answer

All 4 DNA molecules have radioactivity

Question 5

Which statement is true about restriction enzymes?

Accepted Answer

All of the above.

Answer

They recognize palindromic sequences.

Answer

They can produce DNA sticky ends.

Answer

They restrict the replication of DNA.

Question 6

Which of the following diseases is characterized by trinucleotide repeats?

Accepted Answer

Huntington's disease

Answer

Parkinson's disease

Answer

Progressive supranuclear palsy

Answer

Alzheimer's disease

Question 7

What is true about alternative splicing?

Accepted Answer

It creates different proteins from a single gene.

Answer

It is the reason why the human genome is much more complex than other species.

Answer

It creates protein from multiple segments of DNA on different chromosomes.

Answer

It is not tissue specific.

Question 8

A patient is diagnosed with retinoblastoma, presenting with a single tumor in one eye. The patient is concerned about the risk of their children developing the same condition. What test can be used to determine if the tumor is heritable or sporadic?

Accepted Answer

Polymerase Chain Reaction (PCR)

Answer

DNA footprinting

Answer

Fluorescence-activated cell sorting (FACS)

Answer

Northern blotting

Question 9

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 10

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.

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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