Molecular Biology and Genomics Practice Questions

Q: All of the following are true statements about Cockayne syndrome, EXCEPT?

Increased risk of cancer. **Explanation:** Cockayne syndrome (CS) is a rare autosomal recessive disorder characterized by a defect in the **Transcription-Coupled Repair (TCR)** pathway, a sub-pathway of Nucleotide Excision Repair (NER). **Why "Increased risk of cancer" is the correct (Except) answer:** Unlike Xeroderma Pigmentosum (XP), which is also a defect in NER, Cockayne syndrome is **not** associated with an increased risk of skin cancer or internal malignancies. In CS, cells with damaged DNA are hypersensitive to UV light and undergo **apoptosis** (programmed cell death) rather than surviving with mutations. This high rate of cell death leads to growth failure and neurodegeneration but prevents the survival of pre-cancerous cells. **Analysis of other options:** * **A. Defective transcription-coupled repair:** This is the hallmark of CS. Mutations in *ERCC6* (CSB) or *ERCC8* (CSA) genes prevent the repair of DNA lesions on actively transcribed strands. * **B. Photosensitivity:** Patients exhibit extreme sensitivity to sunlight, resulting in severe sunburns even with minimal exposure. * **C. Mental retardation:** CS is characterized by progressive neurological degeneration, microcephaly, and developmental delays. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Triad:** Short stature (cachectic dwarfism), "bird-like" facies (sunken eyes), and premature aging (progeroid features). * **Differentiating CS from XP:** XP has a **high cancer risk** but usually lacks the severe developmental/progeroid features of CS. * **Key Feature:** "Mickey Mouse" appearance on imaging due to basal ganglia calcification. * **Pathophysiology:** TCR specifically repairs DNA damage that stalls RNA polymerase during transcription.

Q: A child develops blisters on exposure to sunlight. Irregular dark spots on the skin are also found. He is very likely to have a defect in which of the following mechanisms?

Nucleotide excision repair. ### Explanation The clinical presentation described—extreme photosensitivity (blisters on sun exposure) and irregular hyperpigmented spots (poikiloderma)—is classic for **Xeroderma Pigmentosum (XP)**. **1. Why Nucleotide Excision Repair (NER) is correct:** XP is an autosomal recessive disorder caused by a deficiency in the **Nucleotide Excision Repair** pathway. Normally, UV radiation causes the formation of **pyrimidine dimers** (specifically thymine dimers) in DNA. The NER mechanism identifies these bulky lesions, uses endonucleases to excise the damaged oligonucleotide strand, and fills the gap using DNA polymerase and ligase. In XP, this repair fails, leading to accumulated mutations, skin malignancies (Basal Cell Carcinoma, Squamous Cell Carcinoma, Melanoma), and the characteristic dermatological findings. **2. Why the other options are incorrect:** * **Mismatch Repair (MMR):** Defects here lead to **Lynch Syndrome** (Hereditary Non-Polyposis Colorectal Cancer). It corrects errors missed during DNA replication, not UV-induced damage. * **Recombination Repair:** Defects in homologous recombination repair are associated with **Ataxia-Telangiectasia** (ATM gene) or **Breast/Ovarian cancer** (BRCA1/2). * **Thymine Dimers:** This is the *result* of the damage (the lesion itself), not the *mechanism* of repair that is defective. **Clinical Pearls for NEET-PG:** * **Key Enzyme:** The most common defect in XP is a deficiency in **UV-specific endonuclease**. * **"Dry Skin":** The name *Xeroderma Pigmentosum* literally translates to "parched, pigmented skin." * **Neurological symptoms:** About 20-30% of patients may also exhibit progressive neurological degeneration. * **High-Yield Association:** NER is responsible for repairing "bulky adducts," while Base Excision Repair (BER) handles "spontaneous/deamination" insults.

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).

Question 1

All of the following are true statements about Cockayne syndrome, EXCEPT?

Accepted Answer

Increased risk of cancer

Answer

Defective transcription-coupled repair

Answer

Photosensitivity

Answer

Mental retardation

Question 2

A child develops blisters on exposure to sunlight. Irregular dark spots on the skin are also found. He is very likely to have a defect in which of the following mechanisms?

Accepted Answer

Nucleotide excision repair

Answer

Mismatch repair

Answer

Recombination repair

Answer

Thymine dimers

Question 3

What is the transition mutation of GATCCT?

Accepted Answer

GGTCCT

Answer

GTTCCT

Answer

GTCCCT

Answer

GGUGGT

Question 4

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 5

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

Accepted Answer

5'-UCAGACUGA-3'

Question 6

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 7

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 8

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 9

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 10

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

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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