Molecular Biology and Genomics Practice Questions

Q: Gene therapy is most effectively given in which of the following conditions?

Severe combined immunodeficiency. **Explanation:** **Severe Combined Immunodeficiency (SCID)** is considered the "poster child" for successful gene therapy because it fulfills the ideal criteria for genetic intervention: it is caused by a single gene defect, affects hematopoietic stem cells (which are easily accessible), and provides a **selective survival advantage** to corrected cells. 1. **Why SCID is the Correct Answer:** The most common form treated via gene therapy is **ADA-SCID** (Adenosine Deaminase deficiency). When the functional ADA gene is inserted into the patient's autologous hematopoietic stem cells and re-infused, the corrected T-cells have a natural proliferative advantage over the defective ones. This allows for the restoration of the immune system without the high risks associated with allogeneic bone marrow transplants. 2. **Why Other Options are Less Effective:** * **Cystic Fibrosis:** Delivery is the main hurdle. The thick mucus in the lungs prevents viral vectors from reaching the target epithelial cells, and the turnover of these cells means the effect is often transient. * **Thalassemia & Sickle Cell Anemia:** These involve the hemoglobin molecule. Achieving the precise, high-level expression of the globin gene required to balance the alpha/beta chain ratio is biochemically complex compared to the simple enzyme replacement needed in SCID. **High-Yield Clinical Pearls for NEET-PG:** * **First Gene Therapy:** Performed in 1990 by William French Anderson on a 4-year-old girl (Ashanthi DeSilva) with **ADA-SCID**. * **Vectors:** Retroviruses and Lentiviruses are commonly used for SCID to ensure integration into the host genome. * **Recent Advance:** CRISPR-Cas9 is currently being explored for Sickle Cell Anemia, but SCID remains the classic historical and clinical success story in medical textbooks.

Q: Which enzyme is known to prevent cellular aging?

Telomerase. **Explanation:** **Telomerase** is the correct answer because it is a specialized ribonucleoprotein reverse transcriptase that maintains chromosomal stability. In somatic cells, DNA polymerase cannot replicate the extreme 3' ends of linear chromosomes (the **"End Replication Problem"**), leading to progressive shortening of telomeres with each cell division. When telomeres reach a critical minimum length, the cell enters **senescence** (cellular aging) or apoptosis. Telomerase prevents this by adding repetitive TTAGGG sequences to the ends of chromosomes, effectively acting as a "cellular fountain of youth." It is highly active in germ cells, stem cells, and cancer cells, but inactive in most mature somatic cells. **Incorrect Options:** * **DNA ligase:** Responsible for joining Okazaki fragments by creating phosphodiester bonds; it is essential for DNA repair and replication but does not prevent telomere shortening. * **DNA polymerase alpha:** Involved in the initiation of DNA replication by synthesizing an RNA primer; it cannot solve the end-replication problem. * **RNA polymerase II:** Responsible for the transcription of DNA into mRNA; it has no role in maintaining chromosomal length or preventing aging. **High-Yield Clinical Pearls for NEET-PG:** * **Composition:** Telomerase contains an RNA template (**TERC**) and a catalytic protein subunit (**TERT**). * **Cancer Link:** Approximately 85–90% of cancer cells upregulate telomerase to achieve **replicative immortality**. * **Shelterin Complex:** A protein complex that protects telomeres from being recognized as DNA double-strand breaks. * **Dyskeratosis Congenita:** A genetic disorder caused by telomerase deficiency, leading to premature aging signs, bone marrow failure, and mucosal leukoplakia.

Q: A mutation in which of the following sequences in eukaryotic mRNA will affect the process by which the poly-A tail is added to the 3' end of mRNA?

AAUAAA. **Explanation:** The correct answer is **AAUAAA**. This sequence is known as the **Polyadenylation Signal**, located approximately 10–30 nucleotides upstream of the cleavage site at the 3' end of eukaryotic pre-mRNA. **1. Why AAUAAA is correct:** In eukaryotes, the maturation of mRNA involves the addition of a poly-A tail (200–250 adenine residues). This process is triggered when the **Cleavage and Polyadenylation Specificity Factor (CPSF)** recognizes the highly conserved **AAUAAA** sequence. Once recognized, the pre-mRNA is cleaved, and the enzyme **Poly-A Polymerase (PAP)** adds the adenine residues. A mutation in this sequence prevents proper cleavage and polyadenylation, leading to unstable mRNA that is rapidly degraded. **2. Why other options are incorrect:** * **CCA:** This sequence is added to the **3' end of tRNA** (not mRNA) post-transcriptionally. it is the site where amino acids attach. * **CAAT:** This is the **CAAT box**, a conserved promoter sequence located upstream of the transcription start site that regulates the frequency of transcription initiation. * **GU...A...AG:** These are conserved sequences at the **splice sites**. GU is the 5' donor site, AG is the 3' acceptor site, and 'A' represents the branch point. They are essential for splicing, not polyadenylation. **Clinical Pearls for NEET-PG:** * **Poly-A Tail Function:** Increases mRNA stability, facilitates nucleocytoplasmic transport, and enhances translation efficiency. * **Poly-A Polymerase:** Unlike RNA polymerase, it does **not** require a DNA template. * **Clinical Correlation:** Mutations in the AAUAAA signal are associated with certain types of **α-thalassemia**, where failure to polyadenylate α-globin mRNA leads to its deficiency.

Q: What is the number of Barr bodies in XXY males?

1. ### Explanation **1. The Correct Answer (A):** The number of Barr bodies is determined by the formula: **n – 1**, where ‘n’ is the total number of X chromosomes present in the cell. This phenomenon is known as the **Lyon Hypothesis**. In a male with Klinefelter syndrome (47, XXY), there are two X chromosomes. Applying the formula (2 – 1 = 1), there is **one Barr body**. The Barr body represents a highly condensed, transcriptionally inactive form of the X chromosome called **facultative heterochromatin**. This inactivation occurs early in embryonic development to ensure **dosage compensation**, preventing females (or individuals with extra X chromosomes) from having double the amount of X-linked gene products compared to XY males. **2. Why Other Options are Incorrect:** * **Option B (2):** This would be seen in individuals with three X chromosomes, such as **47, XXX (Triple X syndrome)** or **48, XXXY**. * **Option C (3):** This would be seen in individuals with four X chromosomes, such as **48, XXXX** or **49, XXXXY**. * **Option D (0):** This is the normal finding in a **46, XY male** or a female with **45, XO (Turner syndrome)**, as they possess only one X chromosome. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Lyonization:** The process of X-inactivation is random, fixed, and incomplete (some genes on the short arm of the X chromosome escape inactivation). * **Xist Gene:** The *X-inactive specific transcript* (Xist) gene, located on the X inactivation center (Xic), produces a non-coding RNA that coats and silences the chromosome. * **Staining:** Barr bodies are typically visualized as dark-staining masses at the periphery of the nucleus in buccal smear preparations or as "drumsticks" in polymorphonuclear leukocytes (neutrophils). * **Rule of Thumb:** Regardless of the number of Y chromosomes, the cell always maintains only **one active X chromosome**; all others become Barr bodies.

Q: Which of the following statements regarding Ataxia Telangiectasia is TRUE?

It involves a defect in gene repair mechanisms.. **Explanation:** **Ataxia Telangiectasia (AT)** is a multisystem disorder caused by a mutation in the **ATM (Ataxia-Telangiectasia Mutated) gene** located on chromosome 11q22-23. The ATM protein is a serine-threonine kinase that plays a critical role in detecting **double-stranded DNA breaks**. When DNA damage occurs (e.g., from ionizing radiation), ATM activates cell cycle checkpoints (like p53) to stop the cell cycle for repair or trigger apoptosis. A defect in this gene leads to genomic instability, explaining why **Option C** is correct. **Analysis of Incorrect Options:** * **Option A:** AT is an **autosomal recessive** disorder, not dominant. Consanguinity is often a risk factor. * **Option B:** While AT patients have a high risk of malignancies, they are specifically predisposed to **lymphomas and leukemias** (due to defective V(D)J recombination) rather than squamous cell carcinoma. Skin cancers are more characteristic of Xeroderma Pigmentosum (defect in nucleotide excision repair). * **Option D:** AT is a **pediatric-onset** disease. Symptoms like progressive cerebellar ataxia usually manifest when the child begins to walk, and telangiectasias appear later (ages 3–6). **High-Yield Clinical Pearls for NEET-PG:** 1. **Clinical Triad:** Cerebellar ataxia, Oculocutaneous telangiectasia, and Recurrent sinopulmonary infections (due to IgA deficiency). 2. **Diagnostic Marker:** Elevated **Alpha-Fetoprotein (AFP)** levels after age 2 is a highly characteristic laboratory finding. 3. **Radiosensitivity:** Patients are hypersensitive to **ionizing radiation** (X-rays/CT scans); diagnostic imaging should be used sparingly. 4. **Other DNA Repair Defects:** Contrast with *Xeroderma Pigmentosum* (UV/Pyrimidine dimers) and *Lynch Syndrome* (Mismatch repair).

Q: Mitochondrial codons are an exception to the universality of codons. For example, the initiation codon in mammals is AUG, which codes for methionine. However, in mitochondria, methionine is coded by some other codon. Which among the following is that initiation codon?

AUA. ### Explanation **1. Why AUA is the Correct Answer** The genetic code is "nearly" universal, but human mitochondrial DNA (mtDNA) exhibits specific deviations. In the standard nuclear genetic code, **AUG** is the universal initiation codon (coding for Methionine). However, in mammalian **mitochondria**, both **AUG and AUA** function as codons for **Methionine** and can act as initiation codons. Additionally, **AUU** can sometimes serve as an initiator in mitochondria. This flexibility is a classic example of the non-universality of the mitochondrial genome. **2. Analysis of Incorrect Options** * **Option A (AGA):** In the standard code, AGA codes for Arginine. In human mitochondria, however, AGA (along with AGG) functions as a **Stop Codon**, rather than coding for an amino acid. * **Option B (AAG):** This codes for **Lysine** in both the standard genetic code and the mitochondrial code. It does not function as an initiation codon. * **Option D (AUG):** While AUG is the primary initiation codon in the nuclear genome (and also works in mitochondria), the question specifically asks for the *other* codon that codes for methionine/initiation in the context of mitochondrial exceptions. **3. NEET-PG High-Yield Clinical Pearls** * **Mitochondrial Exceptions to the Genetic Code:** * **UGA:** Standard = Stop; Mitochondrial = **Tryptophan** (Trp). * **AUA:** Standard = Isoleucine; Mitochondrial = **Methionine** (Met). * **AGA/AGG:** Standard = Arginine; Mitochondrial = **Stop Codons**. * **Maternal Inheritance:** mtDNA is inherited exclusively from the mother. * **Heteroplasmy:** The presence of a mixture of more than one type of organellar genome (normal and mutated) within a cell, explaining the variable severity of mitochondrial diseases (e.g., MELAS, LHON).

Q: Which of the following is contained within the nucleolus?

RNA. **Explanation:** The **nucleolus** is a non-membrane-bound, dense structure located within the nucleus. It is primarily known as the "ribosome factory" of the cell. **Why RNA is the correct answer:** The nucleolus is the site for the transcription of **ribosomal RNA (rRNA)** and the subsequent assembly of ribosomal subunits. It contains high concentrations of RNA and proteins. Specifically, it houses the genes for 5.8S, 18S, and 28S rRNA. While the question asks what is *contained* within it, RNA is the hallmark functional component that defines the nucleolus's density and purpose. **Analysis of Incorrect Options:** * **A & C. DNA and Chromatin material:** While the nucleolus forms around specific chromosomal regions called **Nucleolar Organizer Regions (NORs)**, the bulk of the cell's DNA and organized chromatin is located in the nucleoplasm *outside* the nucleolus. The nucleolus itself is a specialized sub-compartment, not the primary storage site for the genome. * **D. Protein molecules:** Although the nucleolus contains proteins (like nucleolin and fibrillarin) and ribosomal proteins imported from the cytoplasm, **RNA** is the more specific and characteristic answer in the context of standard medical physiology exams regarding the nucleolus's primary constituent and product. **High-Yield NEET-PG Pearls:** * **Nucleolar Organizer Regions (NORs):** In humans, these are located on the short arms of acrocentric chromosomes: **13, 14, 15, 21, and 22**. * **Size and Activity:** The size of the nucleolus is directly proportional to the protein-synthetic activity of the cell. Large, prominent nucleoli are seen in rapidly dividing cells (e.g., cancer cells) and protein-secreting cells (e.g., plasma cells). * **Membrane:** Remember, the nucleolus is **not** surrounded by a membrane; it is an aggregate of macromolecules.

Question 1

Gene therapy is most effectively given in which of the following conditions?

Accepted Answer

Severe combined immunodeficiency

Answer

Cystic fibrosis

Answer

Thalassemia

Answer

Sickle cell anemia

Question 2

Single gene defect causing multiple unrelated problems is known as:

Accepted Answer

Pleiotropism

Answer

Pseudodominance

Answer

Penetrance

Answer

Anticipation

Question 3

Which enzyme is known to prevent cellular aging?

Accepted Answer

Telomerase

Answer

DNA ligase

Answer

DNA polymerase alpha

Answer

RNA polymerase II

Question 4

A mutation in which of the following sequences in eukaryotic mRNA will affect the process by which the poly-A tail is added to the 3' end of mRNA?

Accepted Answer

AAUAAA

Answer

CCA

Answer

CAAT

Answer

GU...A...AG

Question 5

What is the number of Barr bodies in XXY males?

Accepted Answer

1

Answer

2

Answer

3

Answer

0

Question 6

Histone undergoes post-translational modification by which of the following mechanisms, except?

Accepted Answer

Glycosylation

Answer

Acetylation

Answer

Methylation

Answer

Phosphorylation

Question 7

Which of the following statements regarding Ataxia Telangiectasia is TRUE?

Accepted Answer

It involves a defect in gene repair mechanisms.

Answer

It is an autosomal dominant disorder.

Answer

It is associated with an increased risk of developing squamous cell carcinoma of the skin.

Answer

It is most commonly observed in adults.

Question 8

Which of the following is NOT a method used in gene therapy?

Accepted Answer

Fluorescence in situ hybridization (FISH)

Answer

Transfection

Answer

Electroporation

Answer

Bacteriophage transduction

Question 9

Mitochondrial codons are an exception to the universality of codons. For example, the initiation codon in mammals is AUG, which codes for methionine. However, in mitochondria, methionine is coded by some other codon. Which among the following is that initiation codon?

Accepted Answer

AUA

Answer

AGA

Answer

AAG

Answer

AUG

Question 10

Which of the following is contained within the nucleolus?

Accepted Answer

RNA

Answer

DNA

Answer

Chromatin material

Answer

Protein molecules

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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