Molecular Biology and Genomics Practice Questions

Q: Paternal disomy is found in which condition?

Angelman syndrome. ### Explanation The correct answer is **Angelman syndrome**. This question tests the concept of **Genomic Imprinting** and **Uniparental Disomy (UPD)**. #### 1. Why Angelman Syndrome is Correct Angelman syndrome is caused by the loss of the maternal expression of the **UBE3A gene** on chromosome 15 (15q11-q13). In a normal individual, the paternal copy of this gene is silenced (imprinted). Disease occurs when the maternal contribution is lost via: * Maternal deletion (70%) * **Paternal Uniparental Disomy (UPD):** The child inherits two copies of chromosome 15 from the father and none from the mother. Since both paternal copies are silenced, no functional UBE3A protein is produced. #### 2. Analysis of Incorrect Options * **A. Prader-Willi Syndrome:** This is the "opposite" of Angelman. It results from the loss of the *paternal* 15q11-q13 region. It is associated with **Maternal Disomy** (two copies from the mother, both of which are silenced). * **C. Fragile X Syndrome:** This is a **Trinucleotide Repeat Disorder (CGG)** involving the FMR1 gene on the X chromosome. It does not involve uniparental disomy. * **D. Hydatidiform Mole:** A **complete mole** involves paternal disomy of the *entire genome* (46,XX or 46,XY derived entirely from sperm), but in the context of specific clinical syndromes involving single chromosome pairs, Angelman is the classic association for paternal UPD. #### 3. High-Yield Clinical Pearls for NEET-PG * **Angelman Syndrome (Happy Puppet):** Characterized by inappropriate laughter, seizures, ataxia, and severe intellectual disability. * **Prader-Willi Syndrome:** Characterized by hyperphagia (obesity), hypogonadism, and hypotonia. * **Mnemonic:** **P**rader-Willi = **P**aternal deletion / **M**aternal disomy. **A**ngelman = **M**aternal deletion / **P**aternal disomy. (Remember: **P**ader-Willi has **P**aternal loss).

Q: What is the most important enzyme in DNA replication for chain elongation?

DNA polymerase III. **Explanation:** In prokaryotic DNA replication, **DNA Polymerase III** is the primary enzyme responsible for the synthesis of both the leading and lagging strands. It is highly processive, meaning it can add thousands of nucleotides without dissociating from the DNA template. Its high catalytic rate makes it the "workhorse" of **chain elongation**. It possesses 5' to 3' polymerase activity and 3' to 5' exonuclease activity (proofreading). **Analysis of Incorrect Options:** * **Helicase:** This enzyme is responsible for unwinding the DNA double helix at the replication fork by breaking hydrogen bonds. It initiates the process but does not elongate the chain. * **DNA Polymerase I:** While it has polymerase activity, its primary roles are removing RNA primers (via 5' to 3' exonuclease activity) and filling the resulting gaps. It is more involved in "clean-up" and repair than bulk elongation. * **Topoisomerase III:** Topoisomerases (like DNA Gyrase/Topoisomerase II) function to relieve torsional strain and supercoiling ahead of the replication fork. They do not synthesize DNA chains. **High-Yield Clinical Pearls for NEET-PG:** * **Eukaryotic Counterpart:** In humans, **DNA Polymerase δ (delta)** and **ε (epsilon)** perform the elongation roles equivalent to prokaryotic Pol III. * **Processivity:** The **Beta-clamp** (sliding clamp) is the subunit of Pol III that ensures it stays attached to the DNA, enabling high processivity. * **Drug Target:** Fluoroquinolones (e.g., Ciprofloxacin) target bacterial **DNA Gyrase** (Topoisomerase II) and Topoisomerase IV, preventing replication. * **Proofreading:** The 3' to 5' exonuclease activity is essential for maintaining high fidelity; defects in similar eukaryotic repair mechanisms lead to conditions like **HNPCC (Lynch Syndrome)**.

Q: Bromodeoxyuridine is structurally related to which of the following DNA components?

Thymidine. **Explanation:** **1. Why Thymidine is Correct:** Bromodeoxyuridine (BrdU) is a synthetic nucleoside that acts as a **structural analogue of Thymidine**. In its chemical structure, the methyl group at the 5th position of the uracil ring in thymidine is replaced by a **bromine atom**. Because of this structural similarity, DNA polymerase cannot distinguish between the two, and BrdU is incorporated into the DNA during the S-phase of the cell cycle in place of thymidine. This property makes it a vital tool in research to detect proliferating cells. **2. Why Other Options are Incorrect:** * **Uracil:** While BrdU is a derivative of uracil (5-bromouracil), it is specifically a **deoxyribonucleoside** (containing deoxyribose sugar). Uracil is a nitrogenous base found in RNA, not a DNA component. * **Adenosine:** This is a purine nucleoside. BrdU is a pyrimidine analogue; their ring structures (double vs. single ring) are entirely different. * **Cytosine:** Although cytosine is a pyrimidine, it lacks the specific substitution site at the 5th position that mimics the thymidine-adenine base pairing. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **BrdU Staining:** Used to measure **cell proliferation rates**. Incorporated BrdU is detected using fluorescently labeled monoclonal antibodies. * **Mechanism of Mutation:** 5-Bromouracil (the base in BrdU) can undergo a **tautomeric shift** from a keto form to an enol form. The enol form base-pairs with Guanine instead of Adenine, leading to **transition mutations** (T-A to C-G). * **Zidovudine (AZT):** Another high-yield thymidine analogue used in HIV treatment; it acts as a chain terminator during reverse transcription.

Q: Sickle cell anemia is the clinical manifestation of homozygous genes for an abnormal haemoglobin molecule. What event is responsible for the mutation in the b chain?

Point mutation. **Explanation:** Sickle cell anemia is a classic example of a **Point Mutation**, specifically a **missense mutation** resulting from a single nucleotide substitution. 1. **Why Point Mutation is correct:** The molecular basis of Sickle Cell Anemia involves a single base change in the DNA sequence of the **$\beta$-globin gene** located on chromosome 11. Specifically, there is a transversion where **Adenine is replaced by Thymine (GAG $\rightarrow$ GTG)** at the 6th codon. This results in the substitution of the amino acid **Glutamic acid** (polar/hydrophilic) with **Valine** (non-polar/hydrophobic) at the 6th position of the $\beta$-polypeptide chain. This single change causes the hemoglobin (HbS) to polymerize under deoxygenated conditions, leading to the characteristic "sickling" of RBCs. 2. **Why other options are incorrect:** * **Insertion/Deletion:** These involve the addition or loss of nucleotides, which usually cause a **frameshift mutation**, altering the entire downstream reading frame. This is seen in certain types of Thalassemia, not Sickle Cell Anemia. * **Non-disjunction:** This is a failure of homologous chromosomes or sister chromatids to separate during cell division, leading to **aneuploidy** (e.g., Trisomy 21/Down Syndrome), not single-gene point mutations. **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** Autosomal Recessive. * **Electrophoresis:** HbS moves **slower** than HbA toward the anode (+) because the loss of Glutamic acid reduces the negative charge of the molecule. * **Protective Effect:** Heterozygotes (Sickle cell trait) show resistance to *Plasmodium falciparum* malaria. * **Precipitating factors for sickling:** Hypoxia, acidosis, dehydration, and increased 2,3-BPG.

Q: In SCID, what type of DNA repair is defective?

End Joining Repair. **Explanation:** The correct answer is **End Joining Repair**, specifically **Non-Homologous End Joining (NHEJ)**. Severe Combined Immunodeficiency (SCID) can be caused by mutations in genes involved in **V(D)J recombination**, such as *RAG1/RAG2* or components of the NHEJ pathway (e.g., *Artemis*). During the development of B and T cells, the body intentionally creates double-strand breaks in DNA to rearrange antigen receptor genes. These breaks must be repaired by the NHEJ machinery. If this repair mechanism is defective, lymphocytes cannot mature, leading to a total lack of adaptive immunity (SCID). **Analysis of Options:** * **Option A (Double strand break repair):** While NHEJ is a *form* of double-strand break repair, "End Joining Repair" is the more specific and accurate term for the defect in SCID. General double-strand break repair via homologous recombination is defective in conditions like **Ataxia-telangiectasia** (ATM gene) or **BRCA1/2** mutations. * **Option B (Nucleotide excision repair):** This pathway repairs bulky lesions and pyrimidine dimers caused by UV light. Defects lead to **Xeroderma Pigmentosum**. * **Option D (Mismatch repair):** This pathway corrects errors during DNA replication. Defects lead to **Lynch Syndrome (HNPCC)**. **High-Yield Clinical Pearls for NEET-PG:** * **NHEJ** does not require a homologous template and is "error-prone." * **Adenosine Deaminase (ADA) deficiency** is the second most common cause of SCID (autosomal recessive), leading to the accumulation of dATP, which is toxic to lymphocytes. * **X-linked SCID** is the most common form, caused by a defect in the **IL-2 receptor gamma chain**. * **Radiosensitivity:** Patients with NHEJ-defective SCID are hypersensitive to ionizing radiation.

Q: Which of the following statements is NOT true about eukaryotic genes?

Polycistronic mRNA. **Explanation:** In eukaryotic genetics, the organization and expression of genes differ significantly from prokaryotes. The correct answer is **A (Polycistronic mRNA)** because this is a characteristic feature of **prokaryotes**, not eukaryotes. **1. Why Polycistronic mRNA is NOT true for Eukaryotes:** Eukaryotic genes are typically **monocistronic**, meaning one mRNA molecule carries the genetic information to encode only a **single polypeptide**. In contrast, prokaryotic mRNA is often polycistronic, where one mRNA strand contains multiple open reading frames (ORFs) that code for several different proteins (e.g., the Lac Operon). **2. Analysis of Incorrect Options:** * **B. Noncoding introns:** Eukaryotic genes are "split genes." They contain **exons** (coding regions) and **introns** (non-coding regions). Introns are removed during splicing. * **C. Nuclear genes and pseudogenes:** Eukaryotes contain functional nuclear genes and **pseudogenes** (sequences that resemble functional genes but have lost their protein-coding ability due to mutations). * **D. Modification of mRNA:** Eukaryotic pre-mRNA undergoes extensive **post-transcriptional modification** (5' capping, 3' polyadenylation, and splicing) within the nucleus before being exported to the cytoplasm for translation. **High-Yield Clinical Pearls for NEET-PG:** * **Splicing Errors:** Mutations at splice sites are responsible for diseases like **β-Thalassemia**. * **Cap & Tail:** The 7-methylguanosine cap (5') and Poly-A tail (3') protect mRNA from exonucleases and are essential for translation initiation. * **Exception:** While most eukaryotic mRNA is monocistronic, some viruses (like Poliovirus) produce a single polyprotein that is later cleaved, mimicking a polycistronic-like effect.

Q: Sigma (σ) subunit is present in which of the following?

RNA polymerase. **Explanation:** The **Sigma (σ) subunit** is a critical component of the **Prokaryotic RNA polymerase holoenzyme**. In bacteria, the core enzyme (consisting of subunits α₂ββ'ω) can synthesize RNA but lacks the ability to recognize specific promoter sequences. The addition of the sigma factor converts the core enzyme into the **holoenzyme**, which specifically recognizes and binds to the promoter region (Pribnow box/-10 and -35 sequences), thereby initiating transcription. Once the RNA chain reaches about 10 nucleotides in length, the sigma factor dissociates, and the core enzyme continues elongation. **Analysis of Options:** * **80S Ribosomes (Option A):** These are eukaryotic ribosomal complexes involved in translation. They consist of 40S and 60S subunits, not sigma factors. * **Initiation of DNA replication (Option C):** This process involves proteins like DnaA (recognition), Helicase (unwinding), and Primase, but not the sigma subunit. * **DNA Polymerase (Option D):** This enzyme is responsible for DNA synthesis. While it requires a primer and various accessory proteins (like the sliding clamp), it does not utilize a sigma subunit for initiation. **High-Yield Clinical Pearls for NEET-PG:** * **Rifampicin:** This key antitubercular drug acts by inhibiting the **β-subunit** of bacterial DNA-dependent RNA polymerase, preventing transcription initiation. * **Promoter Recognition:** In eukaryotes, the function of the sigma factor is analogous to **General Transcription Factors (GTFs)** like TFIID. * **Alpha-amanitin:** Found in *Amanita phalloides* mushrooms, it specifically inhibits **RNA Polymerase II**, leading to severe hepatotoxicity.

Q: What constitutes the driving force for the transport of proteins into and out of the nucleus?

GTP hydrolysis within the cytosol. ### Explanation The transport of proteins across the nuclear envelope (through Nuclear Pore Complexes) is an active process regulated by the **Ran-GTPase cycle**. This cycle provides the directionality and energy required for both nuclear import and export. **1. Why Option C is Correct:** The driving force for nuclear transport is the **hydrolysis of GTP to GDP**, which occurs specifically in the **cytosol**. * **The Mechanism:** The enzyme **Ran-GAP** (GTPase Activating Protein) is localized in the cytosol. It triggers Ran-GTP to hydrolyze its bound GTP into GDP. * **The Result:** This hydrolysis maintains a high concentration of Ran-GDP in the cytosol and Ran-GTP in the nucleus. This steep **Ran-GTP gradient** is what powers the release of cargo into the nucleus (import) or the movement of cargo out of the nucleus (export). **2. Why Other Options are Incorrect:** * **Options A & B (ATP):** While many cellular transport mechanisms use ATP, nuclear transport specifically utilizes the energy from **GTP**. * **Option D (GTP hydrolysis in the nucleus):** The nucleus contains **Ran-GEF** (Guanine Nucleotide Exchange Factor), which replaces GDP with GTP. Therefore, the nucleus is a site of **GTP loading**, not hydrolysis. Hydrolysis in the nucleus would destroy the gradient necessary for transport. **3. High-Yield NEET-PG Pearls:** * **Ran-GTP Gradient:** High in the Nucleus; Low in the Cytosol. * **Importins:** Bind cargo in the cytosol (where Ran-GTP is low) and release it in the nucleus (where Ran-GTP is high). * **Exportins:** Bind cargo in the nucleus only when Ran-GTP is present, forming a ternary complex. * **NLS & NES:** Proteins destined for the nucleus have a **Nuclear Localization Signal** (rich in basic amino acids like Lysine and Arginine), while those leaving have a **Nuclear Export Signal**.

Q: What is the primary function of microRNA?

Gene Regulation. **Explanation:** **MicroRNAs (miRNAs)** are small, non-coding RNA molecules (typically 21–25 nucleotides long) that play a pivotal role in **post-transcriptional gene regulation**. They function by binding to the 3' untranslated region (3' UTR) of specific target messenger RNAs (mRNAs). This binding usually leads to either **translational repression** or **mRNA degradation**, effectively "silencing" the gene expression. **Analysis of Options:** * **A (Correct):** miRNAs regulate approximately 30–60% of human genes, acting as rheostats to fine-tune protein levels within the cell. * **B (Incorrect):** Splicing of pre-mRNA is primarily the function of **snRNAs** (small nuclear RNAs) which form the spliceosome complex. * **C (Incorrect):** Initiation of translation is mediated by eukaryotic initiation factors (eIFs) and the 40S ribosomal subunit. miRNAs generally *inhibit* rather than initiate this process. * **D (Incorrect):** Carrying messages for protein synthesis is the primary function of **mRNA** (messenger RNA). **High-Yield Clinical Pearls for NEET-PG:** * **Biogenesis:** miRNAs are transcribed by **RNA Polymerase II** as primary-miRNA (pri-miRNA), processed in the nucleus by the **Drosha** enzyme, and further cleaved in the cytoplasm by the **Dicer** enzyme. * **RISC Complex:** To function, miRNA must be loaded into the **RNA-induced silencing complex (RISC)**, which contains the **Argonaute** protein. * **OncomiRs:** miRNAs that are dysregulated in cancer. For example, *miR-21* often acts as an oncogene, while *let-7* acts as a tumor suppressor. * **Therapeutics:** miRNA mimics and antagomirs (antisense oligonucleotides) are being researched as targeted molecular therapies.

Question 1

Paternal disomy is found in which condition?

Accepted Answer

Angelman syndrome

Answer

Prader-Willi syndrome

Answer

Fragile X syndrome

Answer

Hydatiform mole

Question 2

What is the most important enzyme in DNA replication for chain elongation?

Accepted Answer

DNA polymerase III

Answer

Helicase

Answer

DNA polymerase I

Answer

Topoisomerase III

Question 3

Bromodeoxyuridine is structurally related to which of the following DNA components?

Accepted Answer

Thymidine

Answer

Uracil

Answer

Adenosine

Answer

Cytosine

Question 4

Sickle cell anemia is the clinical manifestation of homozygous genes for an abnormal haemoglobin molecule. What event is responsible for the mutation in the b chain?

Accepted Answer

Point mutation

Answer

Insertion

Answer

Deletion

Answer

Non-disjunction

Question 5

In SCID, what type of DNA repair is defective?

Accepted Answer

End Joining Repair

Answer

Double strand break repair

Answer

Nucleotide excision repair

Answer

Mismatch repair

Question 6

Which of the following statements is NOT true about eukaryotic genes?

Accepted Answer

Polycistronic mRNA

Answer

Noncoding intron

Answer

Contain nuclear gene and pseudogene

Answer

Modification of mRNA before transportation from the nucleus

Question 7

Sigma (σ) subunit is present in which of the following?

Accepted Answer

RNA polymerase

Answer

80S ribosomes

Answer

Initiation of DNA replication

Answer

DNA polymerase

Question 8

What constitutes the driving force for the transport of proteins into and out of the nucleus?

Accepted Answer

GTP hydrolysis within the cytosol

Answer

ATP hydrolysis within the cytosol

Answer

ATP hydrolysis within the nucleus

Answer

GTP hydrolysis within the nucleus

Question 9

What is the primary function of microRNA?

Accepted Answer

Gene Regulation

Answer

Splicing of pre mRNA

Answer

Initiation of Translation

Answer

Carrying messages for protein synthesis

Question 10

Which of the following is NOT true about the genetic code?

Accepted Answer

Overlapping

Answer

Degenerate

Answer

Ambiguous

Answer

Universal

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

On this page

Practice by Chapter

Want unlimited practice?