Molecular Biology and Genomics Practice Questions

Q: What is the typical size of a microsatellite repeat sequence?

2-6 base pairs (bp). **Explanation:** The correct answer is **B (2-6 base pairs)**. This question tests the classification of repetitive DNA sequences, specifically **Short Tandem Repeats (STRs)**, also known as **Microsatellites**. Microsatellites are tracts of repetitive DNA where a short motif, typically **2 to 6 base pairs** in length, is repeated 5 to 50 times. They are highly polymorphic and distributed throughout the human genome. Their high degree of variability between individuals makes them the "gold standard" marker for DNA profiling and linkage analysis. **Analysis of Options:** * **Option A ( 3 kb):** These sizes are characteristic of **Minisatellites** (Variable Number Tandem Repeats or VNTRs), where the repeat unit is typically 10-60 bp, and the total array can span several kilobases. **High-Yield Clinical Pearls for NEET-PG:** 1. **Microsatellite Instability (MSI):** This is a critical diagnostic marker for **Lynch Syndrome** (Hereditary Non-Polyposis Colorectal Cancer). It occurs due to mutations in **Mismatch Repair (MMR) genes** (e.g., *MLH1, MSH2*), leading to uncontrolled expansion or contraction of these repeats. 2. **Trinucleotide Repeat Disorders:** Many neurological diseases are caused by microsatellite expansions (e.g., **Huntington’s disease** [CAG], **Fragile X syndrome** [CGG]). 3. **Forensics:** STRs are the primary tools used in **DNA Fingerprinting** because the number of repeats at specific loci varies significantly between individuals.

Q: Which of the following processes is involved in the conversion of DNA to RNA?

Transcription. **Explanation:** **Transcription** is the fundamental process of gene expression where a specific segment of DNA is used as a template to synthesize a complementary RNA molecule (mRNA, tRNA, or rRNA). This process is catalyzed by the enzyme **RNA Polymerase**. In eukaryotes, this occurs in the nucleus, and the resulting primary transcript undergoes post-transcriptional modifications before entering the cytoplasm for translation. **Analysis of Incorrect Options:** * **Conjugation:** A mechanism of horizontal gene transfer in bacteria involving direct cell-to-cell contact via a sex pilus to transfer genetic material (plasmids). * **Transduction:** The process by which foreign DNA is introduced into a cell by a virus or viral vector (bacteriophage). * **Translocation:** This term has two meanings in genetics: (1) In **translation**, it is the movement of the ribosome along the mRNA; (2) In **cytogenetics**, it is a chromosomal abnormality where a segment of one chromosome breaks off and attaches to another. **High-Yield Clinical Pearls for NEET-PG:** * **Directionality:** RNA synthesis always proceeds in the **5' to 3' direction**, reading the template DNA strand in the 3' to 5' direction. * **Inhibitors:** **Rifampicin** inhibits bacterial RNA polymerase (used in Tuberculosis), while **Actinomycin D** inhibits transcription in both prokaryotes and eukaryotes (used in chemotherapy). * **Alpha-amanitin:** Found in *Amanita phalloides* (death cap mushroom), it specifically inhibits **RNA Polymerase II**, leading to severe liver failure. * **Reverse Transcription:** The conversion of RNA back to DNA, catalyzed by Reverse Transcriptase (seen in Retroviruses like HIV).

Q: What is the rate-limiting enzyme of purine nucleotide synthesis?

PRPP glutamyl amido-transferase. **Explanation:** The synthesis of purine nucleotides occurs via the **De Novo pathway**, where the purine ring is built upon a ribose-5-phosphate foundation. **1. Why PRPP glutamyl amido-transferase is correct:** The committed and rate-limiting step of purine synthesis is the conversion of **5-Phosphoribosyl-1-pyrophosphate (PRPP)** to **5-Phosphoribosylamine**. This reaction is catalyzed by **PRPP glutamyl amido-transferase**. * **Mechanism:** It involves the displacement of the pyrophosphate group from PRPP by an amide group from Glutamine. * **Regulation:** This enzyme is strictly regulated via feedback inhibition by the end products: AMP, GMP, and IMP. High levels of PRPP act as a feed-forward activator. **2. Why other options are incorrect:** * **Xanthine oxidase:** This is an enzyme involved in the **catabolism** (breakdown) of purines, converting hypoxanthine to xanthine and xanthine to uric acid. It is the target of Allopurinol. * **HGPRT:** This is the key enzyme of the **Purine Salvage Pathway**, not de novo synthesis. Its deficiency leads to **Lesch-Nyhan Syndrome**. * **Adenosine deaminase (ADA):** This enzyme is involved in purine degradation (converting adenosine to inosine). Its deficiency leads to **Severe Combined Immunodeficiency (SCID)**. **Clinical Pearls for NEET-PG:** * **Source of Atoms:** Remember the contributors to the purine ring: **CO₂** (C6), **Glycine** (C4, C5, N7), **Aspartate** (N1), **Glutamine** (N3, N9), and **Formyl-THF** (C2, C8). * **Rate-limiting step of Pyrimidine synthesis:** Carbamoyl phosphate synthetase II (CPS-II). * **Inhibitor:** The drug **Azathioprine** (immunosuppressant) is converted to 6-mercaptopurine, which inhibits PRPP glutamyl amido-transferase.

Q: Which of the following is regarded as the outward expression of a gene?

Phenotype. ### Explanation **Correct Answer: A. Phenotype** The **phenotype** refers to the observable physical, biochemical, or physiological characteristics of an organism. It is the "outward expression" of an individual’s genetic makeup (genotype) as it interacts with the environment. In molecular terms, this involves the flow of genetic information (Central Dogma) from DNA to RNA to functional proteins, which ultimately manifest as traits like height, eye color, or the presence of a specific enzyme deficiency. **Why other options are incorrect:** * **B. Proteomics:** This is the large-scale study of the entire set of proteins expressed by a genome, cell, or tissue. While proteins contribute to the phenotype, proteomics is a field of study/methodology rather than the expression itself. * **C. Genotype:** This represents the internal genetic constitution or the specific set of alleles (e.g., *HbA/HbS*) an individual carries. It is the "blueprint," not the outward expression. * **D. Anticipation:** This is a genetic phenomenon where a disease (often triplet repeat disorders like Huntington’s or Fragile X) becomes more severe or appears at an earlier age in succeeding generations. **High-Yield Clinical Pearls for NEET-PG:** * **Phenotypic Heterogeneity:** Different mutations in the same gene can result in different clinical phenotypes (e.g., different mutations in the *FGFR* gene causing different skeletal dysplasias). * **Pleiotropy:** A single gene mutation resulting in multiple, seemingly unrelated phenotypic effects (e.g., Marfan Syndrome affecting the eyes, heart, and skeleton). * **Penetrance:** The percentage of individuals with a specific genotype who actually exhibit the associated phenotype. If a person has the gene but no symptoms, it is called "reduced penetrance."

Q: Which of the following is not required for PCR, a cell-free test tube method for amplifying a target sequence of DNA?

Radiolabeled DNA probe. **Explanation** Polymerase Chain Reaction (PCR) is an *in vitro* enzymatic method used to amplify specific DNA sequences. To understand why a **radiolabeled DNA probe** is not required, one must look at the fundamental components of the PCR "cocktail." 1. **Why the Correct Answer is Right:** A **radiolabeled DNA probe** is used in hybridization techniques like Southern Blotting to *detect* a specific DNA sequence after it has been separated by electrophoresis. In PCR, the goal is *amplification* (synthesis). While probes are used in Real-Time PCR (qPCR) for quantification, they are not a fundamental requirement for the basic PCR process itself. Furthermore, modern labs use fluorescent dyes rather than hazardous radioactive labels. 2. **Why the Other Options are Wrong:** * **Taq Polymerase (A):** A heat-stable DNA polymerase (derived from *Thermus aquaticus*) is essential to synthesize new DNA strands at high temperatures without denaturing. * **dNTPs (B):** Deoxynucleotide triphosphates (dATP, dCTP, dGTP, dTTP) are the "building blocks" required to construct the new DNA polymer. * **Primers (C):** PCR requires two synthetic oligonucleotide primers that are complementary to the sequences flanking the target DNA. They provide the 3'-OH group necessary for DNA polymerase to initiate synthesis. **High-Yield Clinical Pearls for NEET-PG:** * **Steps of PCR:** Denaturation (94-96°C) → Annealing (50-65°C) → Extension (72°C). * **RT-PCR vs. qPCR:** Reverse Transcriptase PCR (RT-PCR) is used to amplify RNA (e.g., for COVID-19 testing), while Quantitative PCR (qPCR) measures DNA concentration in real-time. * **Magnesium ions (MgCl₂):** A critical cofactor for Taq polymerase activity; its concentration can affect the specificity of the reaction.

Q: Which of the following statements is true about Polymerase Chain Reaction (PCR)?

All of the above.. **Explanation:** Polymerase Chain Reaction (PCR) is an *in vitro* enzymatic method used to amplify specific DNA sequences. It relies on thermal cycling, consisting of denaturation, annealing, and extension. * **Option A (Thermostable DNA polymerase):** Because the denaturation step requires high temperatures (approx. 95°C) to separate DNA strands, a heat-stable enzyme is essential. The most common is **Taq Polymerase**, isolated from the bacterium *Thermus aquaticus*, which remains functional at high temperatures. * **Option B (Exponential amplification):** PCR follows the formula $2^n$, where '$n$' is the number of cycles. Since the product of one cycle serves as the template for the next, the target DNA amount doubles every cycle, leading to exponential growth. * **Option C (Sequence-specific):** The specificity of PCR is determined by **synthetic oligonucleotide primers**. These primers are designed to be complementary to the sequences flanking the target region, ensuring only the desired segment is amplified. Since all three statements accurately describe the fundamental principles of PCR, **Option D** is the correct answer. **High-Yield Clinical Pearls for NEET-PG:** * **RT-PCR (Reverse Transcriptase PCR):** Used for RNA viruses (e.g., SARS-CoV-2, HIV) to convert RNA to cDNA before amplification. * **Real-Time PCR (qPCR):** Allows for the quantification of DNA in real-time using fluorescent dyes (e.g., SYBR Green). * **Components required:** Template DNA, Primers, dNTPs (Deoxynucleotide triphosphates), Mg²⁺ (cofactor), and Taq Polymerase. * **Applications:** Diagnosis of genetic mutations, forensic analysis (DNA fingerprinting), and detection of infectious agents.

Q: The process by which DNA is converted to mRNA is called as?

Transcription. ### Explanation The correct answer is **Transcription**. This process is the first step of the **Central Dogma of Molecular Biology**, where the genetic information stored in DNA is copied into a complementary strand of messenger RNA (mRNA). #### Why Transcription is Correct: In eukaryotes, transcription occurs in the nucleus. The enzyme **RNA Polymerase II** reads the template strand of DNA (3' to 5') to synthesize mRNA in a 5' to 3' direction. This mRNA then carries the genetic "blueprint" from the nucleus to the cytoplasm for protein synthesis. #### Why Other Options are Incorrect: * **Translation:** This is the process where the genetic code carried by mRNA is decoded by ribosomes to synthesize a specific **polypeptide/protein**. It occurs in the cytoplasm. * **DNA Replication:** This is the process of producing two identical replicas of DNA from one original DNA molecule. It occurs during the **S-phase** of the cell cycle to ensure genetic continuity during cell division. #### NEET-PG High-Yield Pearls: * **Reverse Transcription:** The process of converting RNA back into DNA (seen in Retroviruses like HIV), catalyzed by the enzyme **Reverse Transcriptase**. * **Post-Transcriptional Modifications:** In eukaryotes, the initial transcript (hnRNA) undergoes 5' capping, 3' polyadenylation, and splicing (removal of introns) to become mature mRNA. * **Inhibitors:** **Rifampicin** inhibits bacterial RNA polymerase (used in TB treatment), while **Alpha-amanitin** (from *Amanita phalloides* mushrooms) inhibits eukaryotic RNA Polymerase II. * **Promoter Region:** The **TATA box** (Goldberg-Hogness box) is a key promoter element in eukaryotes that helps position RNA Polymerase II for transcription initiation.

Question 1

Which of the following is not common to the synthesis of both leading and lagging strands?

Accepted Answer

DNA ligase is repeatedly used to join fragments of DNA along the growing strand

Answer

RNA primer is needed

Answer

Nucleoside monophosphates are added in the 5' to 3' direction along the growing DNA chain

Answer

DNA polymerase III synthesizes DNA

Question 2

What is the typical size of a microsatellite repeat sequence?

Accepted Answer

2-6 base pairs (bp)

Answer

Less than 1 kilobase (kb)

Answer

1-3 kilobases (kb)

Answer

Greater than 3 kilobases (kb)

Question 3

Which of the following processes is involved in the conversion of DNA to RNA?

Accepted Answer

Transcription

Answer

Conjugation

Answer

Transduction

Answer

Translocation

Question 4

What is the rate-limiting enzyme of purine nucleotide synthesis?

Accepted Answer

PRPP glutamyl amido-transferase

Answer

Xanthine oxidase

Answer

Hypoxanthine-guanine phosphoribosyltransferase (HGPRT)

Answer

Adenosine deaminase (ADA)

Question 5

What is true about genes?

Accepted Answer

Smallest functional unit of the genome

Answer

Not capable of independent expression

Answer

Promoter and enhancer regions are typical examples

Answer

A cistron is a single functional unit

Question 6

Which of the following is regarded as the outward expression of a gene?

Accepted Answer

Phenotype

Answer

Proteomics

Answer

Genotype

Answer

Anticipation

Question 7

Which of the following is not required for PCR, a cell-free test tube method for amplifying a target sequence of DNA?

Accepted Answer

Radiolabeled DNA probe

Answer

Taq polymerase

Answer

Deoxynucleotide triphosphates (dNTPs)

Answer

Primer

Question 8

Which of the following statements is true about Polymerase Chain Reaction (PCR)?

Accepted Answer

All of the above.

Answer

It is carried out by thermostable DNA polymerase.

Answer

The amplification process is exponential.

Answer

The amplification is sequence-specific.

Question 9

Which of the following is true regarding aminoacyl-tRNA synthetase?

Accepted Answer

Attachment of amino acid to the 3' end of tRNA

Answer

Isoaccepting tRNA

Answer

Implements the genetic code

Answer

Editing function

Question 10

The process by which DNA is converted to mRNA is called as?

Accepted Answer

Transcription

Answer

Translation

Answer

DNA replication

Answer

None

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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