Molecular Biology and Genomics Practice Questions

Q: Which enzyme is used to "flush" the sticky ends of DNA?

Klenow fragment. **Explanation:** In recombinant DNA technology, "flushing" or "blunting" refers to the process of converting cohesive (sticky) ends of DNA into blunt ends. **Why Klenow Fragment is correct:** The **Klenow fragment** is a large protein fragment produced when DNA Polymerase I from *E. coli* is enzymatically cleaved by subtilisin. It retains two critical activities: 1. **5' → 3' polymerase activity:** It fills in 5' overhangs (recessed 3' ends) by adding dNTPs. 2. **3' → 5' exonuclease activity:** It removes 3' overhangs. By performing these actions, it creates uniform, double-stranded "blunt" ends, making it the enzyme of choice for "flushing" DNA ends before ligation. **Why other options are incorrect:** * **Polynucleotide kinase:** This enzyme adds a phosphate group to the 5'-hydroxyl terminus of DNA or RNA. It is used for radio-labeling or preparing DNA for ligation, not for altering the physical structure of sticky ends. * **Alkaline phosphatase:** This enzyme removes 5' phosphate groups from DNA. It is used to prevent self-ligation of vector DNA, not for blunting ends. * **Primase:** This is a type of RNA polymerase that synthesizes short RNA primers during DNA replication *in vivo*. It has no role in the laboratory manipulation of DNA ends. **High-Yield Clinical Pearls for NEET-PG:** * **Klenow vs. DNA Pol I:** The Klenow fragment lacks the **5' → 3' exonuclease activity** (which is present in the intact DNA Pol I). This lack of 5' → 3' exonuclease activity is crucial because it prevents the degradation of the primer/template being synthesized. * **S1 Nuclease:** Another enzyme used for blunting DNA, but it works by degrading single-stranded protrusions. * **Taq Polymerase:** Unlike Klenow, Taq adds a single "A" overhang at the 3' end (TA cloning), which is the opposite of flushing.

Q: Which of the following is the smallest autosome?

Chromosome 21. **Explanation:** The human karyotype is traditionally arranged by size, from largest (Chromosome 1) to smallest. While **Chromosome 21** was originally thought to be larger than Chromosome 22 based on physical appearance under older microscopy, genomic sequencing has confirmed that **Chromosome 21 is the smallest autosome**, containing approximately 48 million base pairs (compared to ~51 million in Chromosome 22). **Analysis of Options:** * **Chromosome 21 (Correct):** It is the smallest autosome by DNA content. Its small size is clinically significant, as it is one of the few autosomes where a trisomy is compatible with survival (Down Syndrome), likely due to the relatively low number of genes it carries. * **Chromosome 1 (Incorrect):** This is the **largest** human autosome, containing nearly 249 million base pairs and the highest number of genes. * **Chromosome 22 (Incorrect):** Long considered the smallest, it is actually the second-smallest autosome. It was the first human chromosome to be fully sequenced. * **Y Chromosome (Incorrect):** While the Y chromosome is physically smaller than Chromosome 21 (containing only ~57 million base pairs), it is a **sex chromosome**, not an autosome. The question specifically asks for the smallest *autosome*. **High-Yield NEET-PG Pearls:** * **Largest Chromosome:** Chromosome 1. * **Smallest Chromosome overall:** Y Chromosome. * **Smallest Autosome:** Chromosome 21. * **Gene Density:** Chromosome 19 has the highest gene density; Chromosome 13 and the Y chromosome have the lowest. * **Clinical Correlation:** Trisomy 21 (Down Syndrome) is the most common viable autosomal trisomy.

Q: Which of the following is NOT a potential complication of gene therapy?

Genome integration. **Explanation:** The goal of gene therapy is to introduce genetic material into a patient’s cells to treat or prevent disease. In many therapeutic strategies, particularly those using viral vectors like Lentiviruses or Retroviruses, **Genome integration** is a **desired outcome** rather than a complication. Integration allows the therapeutic gene to be permanently incorporated into the host DNA, ensuring stable expression and transmission to daughter cells during cell division. **Why the other options are complications:** * **Genotoxicity (Option A):** This occurs when the integration of the transgene happens at an inappropriate site (insertional mutagenesis), potentially activating oncogenes or disrupting tumor suppressor genes, leading to cancer (e.g., leukemia). * **Gene silencing (Option C):** The host cell may recognize the foreign DNA and "switch it off" via epigenetic mechanisms like DNA methylation or histone modification, rendering the therapy ineffective. * **Immunotoxicity (Option D):** The patient’s immune system may mount an inflammatory response against the viral vector or the newly synthesized therapeutic protein, potentially leading to severe systemic inflammation or organ failure (e.g., the Jesse Gelsinger case). **High-Yield Clinical Pearls for NEET-PG:** * **Vectors:** Adeno-associated virus (AAV) is currently preferred for many therapies as it is non-integrating (remains episomal) and has low immunogenicity. * **Ex-vivo vs. In-vivo:** Ex-vivo therapy involves modifying cells outside the body (e.g., CAR-T cell therapy), while in-vivo involves direct injection of the vector. * **Severe Combined Immunodeficiency (SCID):** One of the first diseases treated with gene therapy; however, early trials were complicated by genotoxicity leading to T-cell leukemia.

Q: Which of the following enzymes does not require a primer for its activity?

RNA Polymerase. ### Explanation The fundamental difference between DNA and RNA synthesis lies in the requirement for a **3'-OH primer**. **Why RNA Polymerase is the Correct Answer:** RNA Polymerase is unique because it can initiate the synthesis of a polynucleotide chain *de novo*. It does not require a pre-existing primer; it simply binds to a specific DNA sequence called the **promoter** and begins joining ribonucleotides together. In fact, in DNA replication, a specialized RNA polymerase called **Primase** is responsible for creating the initial RNA primer that DNA polymerases then extend. **Why the Other Options are Incorrect:** * **DNA Polymerase I, II, and III:** All DNA polymerases (prokaryotic and eukaryotic) share a common limitation: they cannot start a new DNA strand from scratch. They can only add deoxyribonucleotides to an existing **3'-OH group** of a primer (usually an RNA primer). * **DNA Pol III** is the primary enzyme for elongation. * **DNA Pol I** is involved in removing RNA primers (5'→3' exonuclease activity) and filling the gaps. * **DNA Pol II** is primarily involved in DNA repair. **High-Yield Clinical Pearls for NEET-PG:** * **Primase:** A DNA-dependent RNA polymerase that synthesizes the 10-12 nucleotide RNA primer required for DNA replication. * **Reverse Transcriptase:** An RNA-dependent DNA polymerase (found in HIV) that **does** require a primer (specifically host tRNA). * **Directionality:** All polymerases (DNA and RNA) synthesize chains in the **5' to 3' direction**. * **Rifampicin:** A high-yield antibiotic that inhibits **bacterial RNA polymerase**, preventing the initiation of transcription.

Q: Which cyclin is associated with the G2/M transition in the cell cycle?

Cyclin B. **Explanation:** The cell cycle is regulated by a series of proteins called **Cyclins**, which bind to and activate **Cyclin-Dependent Kinases (CDKs)**. **Correct Option: B (Cyclin B)** Cyclin B is the primary cyclin involved in the **G2 to M phase transition**. It binds with **CDK1** to form the **Mitosis-Promoting Factor (MPF)**. This complex triggers the breakdown of the nuclear envelope, chromosome condensation, and the initiation of mitosis. **Incorrect Options:** * **Cyclin A:** Primarily associated with the **S phase** (DNA synthesis) and the transition into G2. It binds with CDK2 and CDK1. * **Cyclin C:** Associated with the **G0/G1 transition**, helping cells exit the quiescent state to enter the active cell cycle. * **Cyclin D:** The first cyclin produced in the cell cycle. It regulates the **G1 phase** and the G1/S transition by binding with CDK4/6 to phosphorylate the Retinoblastoma (Rb) protein. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic (ABCD):** * **D** comes first (**G1**) * **E** (Cyclin E) follows for **G1/S transition** * **A** is for **S phase** * **B** is for **B**eginning of Mitosis (**G2/M**) * **The "Restriction Point":** Controlled by Cyclin D. Once passed, the cell is committed to division regardless of growth factor presence. * **P53 Protein:** Known as the "Guardian of the Genome," it can arrest the cell cycle (usually at G1) by inducing **p21**, which inhibits CDK activity if DNA damage is detected. *Note: No references provided met the relevance threshold for citation.*

Q: What enzyme is responsible for synthesizing cDNA from mRNA?

Reverse transcriptase. **Explanation:** **1. Why Reverse Transcriptase is Correct:** Reverse transcriptase (also known as RNA-dependent DNA polymerase) is the enzyme responsible for synthesizing **complementary DNA (cDNA)** using an mRNA template. This process reverses the "Central Dogma" of molecular biology (DNA → RNA). In the laboratory, this is a crucial step in **RT-PCR**, where mRNA is converted to cDNA to study gene expression, as cDNA lacks the non-coding introns present in genomic DNA. **2. Why Other Options are Incorrect:** * **DNA Polymerase:** This enzyme synthesizes DNA using a **DNA template** during replication. It requires a primer and works in the 5' to 3' direction but cannot utilize an RNA template. * **RNA Polymerase:** This enzyme performs **transcription**, synthesizing RNA from a DNA template. * **DNA Ligase:** Known as the "molecular glue," this enzyme joins DNA fragments (like Okazaki fragments) by forming phosphodiester bonds. It does not synthesize new DNA strands from templates. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Source:** Reverse transcriptase was first discovered in retroviruses (e.g., **HIV**). It is encoded by the *pol* gene. * **Telomerase:** A specialized reverse transcriptase (TERT) that maintains chromosomal ends (telomeres). Its activity is high in cancer cells and stem cells. * **Diagnostic Use:** RT-PCR is the gold standard for diagnosing RNA virus infections, such as **COVID-19 (SARS-CoV-2)** and monitoring **HIV viral load**. * **Inhibitors:** Nucleoside Reverse Transcriptase Inhibitors (NRTIs) like **Zidovudine (AZT)** are key drugs in HAART therapy for HIV.

Q: A change from UAC to UAG represents which type of mutation?

Nonsense mutation. **Explanation:** The correct answer is **Nonsense mutation**. **1. Why it is correct:** A nonsense mutation occurs when a single nucleotide substitution results in a **premature stop codon** (UAA, UAG, or UGA) within the mRNA sequence. In this case, the codon **UAC** (which codes for the amino acid Tyrosine) is changed to **UAG** (the "Amber" stop codon). This leads to the premature termination of protein synthesis, resulting in a truncated, usually non-functional protein. **2. Why the other options are incorrect:** * **Frameshift mutation:** This occurs due to the **insertion or deletion** of a number of nucleotides not divisible by three. This shifts the entire reading frame downstream, altering all subsequent amino acids. A single base substitution (like UAC to UAG) does not shift the reading frame. * **Deletion:** This involves the physical removal of one or more nucleotides from the DNA sequence. While a deletion can cause a frameshift, the question describes a specific base substitution (C to G), not a loss of genetic material. * **Missense mutation:** This is a point mutation where a single nucleotide change results in a codon that codes for a **different amino acid** (e.g., UAC to UAU is silent, but UAC to UCC would be missense). Since UAG is a stop codon and not an amino acid, it is specifically classified as nonsense. **High-Yield Clinical Pearls for NEET-PG:** * **Stop Codons:** Remember them as **U** **A**re **A**way (UAA), **U** **A**re **G**one (UAG), and **U** **G**o **A**way (UGA). * **Transition vs. Transversion:** UAC to UAG is a **transversion** (Pyrimidine 'C' replaced by Purine 'G'). * **Clinical Example:** Nonsense mutations are frequently seen in severe forms of **Beta-thalassemia** (β⁰) and **Duchenne Muscular Dystrophy (DMD)**. * **Nonsense-mediated decay:** The cell often recognizes and degrades mRNA containing premature stop codons to prevent the accumulation of toxic truncated proteins.

Q: Which of the following statements regarding telomerase is true?

It is a reverse transcriptase.. **Explanation:** **1. Why Option A is Correct:** Telomerase is a specialized **ribonucleoprotein enzyme** that functions as a **RNA-dependent DNA polymerase**, also known as **reverse transcriptase**. It contains an internal RNA template (hTR) which it uses to synthesize repetitive DNA sequences (TTAGGG in humans) at the 3' ends of chromosomes (telomeres). This mechanism counteracts the "end-replication problem," where DNA polymerase cannot fully replicate the lagging strand, preventing the loss of vital genetic information. **2. Why Other Options are Incorrect:** * **Option B:** In humans, telomerase is highly active in **germ cells, stem cells, and cancer cells**. However, it is **inactive or has very low activity in most differentiated somatic cells**, leading to progressive telomere shortening and cellular senescence (the Hayflick limit). * **Option C:** Telomerase is involved in **DNA replication/maintenance**, not protein translation. Translation is the process of synthesizing proteins from mRNA via ribosomes. * **Option D:** Telomerase **prevents** the shortening of DNA. It is the *absence* of telomerase activity that leads to the progressive shortening of chromosomes during each cell division. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Cancer Link:** Approximately 85–90% of cancer cells upregulate telomerase to achieve **replicative immortality**. * **Shelterin Complex:** A group of proteins that protects telomeres from being recognized as double-stranded DNA breaks. * **Dyskeratosis Congenita:** A genetic disorder caused by mutations in telomerase components, leading to premature aging and bone marrow failure. * **Sequence:** Remember the human telomeric repeat sequence: **5’-TTAGGG-3’**.

Question 1

Which enzyme is used to "flush" the sticky ends of DNA?

Accepted Answer

Klenow fragment

Answer

Polynucleotide kinase

Answer

Alkaline phosphatase

Answer

Primase

Question 2

Which of the following is the smallest autosome?

Accepted Answer

Chromosome 21

Answer

Chromosome 1

Answer

Y chromosome

Answer

Chromosome 22

Question 3

Which of the following is NOT a potential complication of gene therapy?

Accepted Answer

Genome integration

Answer

Genotoxicity

Answer

Gene silencing

Answer

Immunotoxicity

Question 4

Which of the following enzymes does not require a primer for its activity?

Accepted Answer

RNA Polymerase

Answer

DNA Polymerase I

Answer

DNA Polymerase II

Answer

DNA Polymerase III

Question 5

Which cyclin is associated with the G2/M transition in the cell cycle?

Accepted Answer

Cyclin B

Answer

Cyclin A

Answer

Cyclin C

Answer

Cyclin D

Question 6

Which of the following contributes to polypeptide synthesis?

Accepted Answer

Enhancer

Answer

Leader sequence

Answer

tRNA

Answer

ncRNA

Question 7

What enzyme is responsible for synthesizing cDNA from mRNA?

Accepted Answer

Reverse transcriptase

Answer

DNA polymerase

Answer

RNA polymerase

Answer

DNA ligase

Question 8

What is a frame shift mutation?

Accepted Answer

Deletion or insertion of nucleotides not in multiples of three

Answer

Substitution of an amino acid

Answer

Change in an mRNA base at the 3rd nucleotide of a codon

Answer

Resulting in a premature stop codon

Question 9

A change from UAC to UAG represents which type of mutation?

Accepted Answer

Nonsense mutation

Answer

Frameshift mutation

Answer

Deletion

Answer

Missense mutation

Question 10

Which of the following statements regarding telomerase is true?

Accepted Answer

It is a reverse transcriptase.

Answer

It is active in most somatic cells.

Answer

It is involved in protein translation.

Answer

It contributes to the shortening of DNA.

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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