Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics Indian Medical PG Practice Questions and MCQs

Question 691: Centromere is almost at the tip of the chromosome in which type?

A. Metacentric
B. Submetacentric
C. Acrocentric (Correct Answer)
D. Telocentric

Explanation: ### Explanation Chromosomes are classified based on the position of the **centromere**, which divides the chromosome into a short arm (**p arm**) and a long arm (**q arm**). **1. Why Acrocentric is Correct:** In **acrocentric** chromosomes, the centromere is located **very close to one end (the tip)**. This results in an extremely short p arm, which often contains repetitive DNA sequences coding for ribosomal RNA (rRNA), forming structures called **satellites**. **2. Analysis of Incorrect Options:** * **Metacentric (A):** The centromere is located exactly in the **middle**. The p and q arms are of equal length, giving the chromosome a 'V' shape during anaphase. * **Submetacentric (B):** The centromere is slightly off-center. This creates a distinct difference in length between the short (p) and long (q) arms, appearing 'L' shaped during anaphase. * **Telocentric (D):** The centromere is located at the **absolute tip** (telomere), meaning there is no p arm at all. While this fits the description of "at the tip," **telocentric chromosomes do not occur naturally in humans.** **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Human Acrocentric Chromosomes:** There are five pairs: **13, 14, 15, 21, and 22.** * **Robertsonian Translocation:** This specific type of mutation occurs **only in acrocentric chromosomes**. It involves the fusion of two long arms and the loss of the short arms. This is a common cause of familial Down Syndrome (translocation between 14 and 21). * **Nucleolar Organizer Regions (NORs):** In acrocentric chromosomes, the satellites on the p arms contain NORs, which are essential for the formation of the nucleolus.

Question 692: What is the action of topoisomerase III?

A. Remove positive supercoil
B. Remove negative supercoil
C. Form negative supercoil
D. Single strand break (Correct Answer)

Explanation: **Explanation:** Topoisomerases are essential enzymes that regulate the topological state of DNA during replication, transcription, and recombination. They are broadly classified into two types: **Type I** (cleave one strand) and **Type II** (cleave both strands). **Why Option D is Correct:** Topoisomerase III belongs to the **Type IA subfamily**. The fundamental mechanism of Type I topoisomerases involves creating a transient **single-strand break** (nick) in the DNA phosphodiester backbone. This allows the enzyme to pass another strand through the break or allow the DNA to rotate, thereby changing the linking number before resealing the nick. **Analysis of Incorrect Options:** * **Option A & B:** While Topoisomerase I and II are primarily known for removing positive and negative supercoils to relieve torsional stress, Topoisomerase III has a more specialized role. In eukaryotes, it is primarily involved in **DNA recombination** and resolving "intertwined" DNA structures (hemicatenanes) rather than general supercoil removal. * **Option C:** Forming negative supercoils is a specific function of **DNA Gyrase** (a Type II topoisomerase found in prokaryotes), which uses ATP to actively introduce negative supercoils. **High-Yield Clinical Pearls for NEET-PG:** * **Type I vs. II:** Type I (Topoisomerase I & III) are ATP-independent and make single-strand breaks. Type II (Topoisomerase II/Gyrase & IV) are ATP-dependent and make double-strand breaks. * **Bloom Syndrome:** This condition is caused by a mutation in the *BLM* gene (a helicase) which works in a complex with **Topoisomerase IIIα**. Deficiency leads to increased sister chromatid exchange and genomic instability. * **Pharmacology Link:** Topoisomerase I is inhibited by **Irinotecan/Topotecan**, while Topoisomerase II is inhibited by **Etoposide/Teniposide** and Fluoroquinolones (in bacteria).

Question 693: Which of the following enzymes participates in protein synthesis?

A. DNA ligase
B. DNA Helicase
C. Peptidase
D. Peptidyl transferase (Correct Answer)

Explanation: **Explanation:** **Peptidyl transferase** is the correct answer because it is the primary enzyme responsible for protein synthesis (translation). It catalyzes the formation of peptide bonds between adjacent amino acids. Crucially, in both prokaryotes and eukaryotes, this is not a protein-based enzyme but a **ribozyme** (catalytic RNA). In prokaryotes, it is located in the 23S rRNA of the 50S ribosomal subunit; in eukaryotes, it resides in the 28S rRNA of the 60S subunit. **Analysis of Incorrect Options:** * **DNA Ligase:** Functions in DNA replication and repair by joining Okazaki fragments or sealing nicks in the phosphodiester backbone. * **DNA Helicase:** Involved in the initiation of DNA replication; it unwinds the double helix by breaking hydrogen bonds between complementary bases. * **Peptidase:** These are proteolytic enzymes that catalyze the hydrolysis of peptide bonds (breaking down proteins), rather than synthesizing them. **High-Yield Clinical Pearls for NEET-PG:** * **Antibiotic Correlation:** Several antibiotics target the peptidyl transferase center. **Chloramphenicol** specifically inhibits this enzyme in the bacterial 50S subunit, preventing peptide bond formation. * **Ribozyme Nature:** Remember that the ribosome is a ribozyme. This is a frequent "exception to the rule" question in biochemistry (most enzymes are proteins, but this one is RNA). * **Energy Source:** The formation of the peptide bond by peptidyl transferase does not require external ATP/GTP; it utilizes the high-energy ester bond between the amino acid and the tRNA.

Question 694: Which one of the following is a tyrosine tRNA analogue that causes premature chain termination?

A. Cycloheximide
B. Puromycin (Correct Answer)
C. Paromomycin
D. Erythromycin

Explanation: **Explanation:** **Puromycin** is a unique antibiotic that acts as a structural analogue of the 3' end of **aminoacyl-tRNA** (specifically tyrosinyl-tRNA). Due to this structural similarity, it enters the 'A' site of the ribosome during translation. The peptidyl transferase enzyme incorporates puromycin into the growing polypeptide chain. However, because puromycin contains an amide bond instead of an ester bond, it cannot bind the next incoming amino acid, leading to **premature chain termination** and the release of incomplete polypeptides. It affects both prokaryotes and eukaryotes. **Analysis of Incorrect Options:** * **Cycloheximide:** Inhibits the enzyme **peptidyl transferase** specifically in the 60S ribosomal subunit of eukaryotes, blocking elongation. It does not act as a tRNA analogue. * **Paromomycin:** An aminoglycoside that binds to the 30S subunit. it causes **misreading of mRNA** (translational inaccuracy) rather than acting as a structural analogue for termination. * **Erythromycin:** A macrolide that binds to the 50S subunit and inhibits **translocation** (movement of mRNA relative to the ribosome). **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Protein Synthesis Inhibitors:** "**Buy AT 30, CELL at 50**" * **30S inhibitors:** **A**minoglycosides (irreversible), **T**etracyclines (reversible). * **50S inhibitors:** **C**hloramphenicol, **E**rythromycin (Macrolides), **L**inezolid, **L**incosamides (Clindamycin). * **Diphtheria toxin** and **Pseudomonas Exotoxin A** inhibit eukaryotic translation by inactivating **Elongation Factor-2 (EF-2)** via ADP-ribosylation.

Question 695: What is the primary function of topoisomerases?

A. Deoxynucleotide polymerization
B. Relieve torsional strain (Correct Answer)
C. Initiate synthesis of RNA primers
D. Prevent premature reannealing of double-stranded DNA

Explanation: **Explanation:** **Topoisomerases** are essential enzymes that regulate the topological state of DNA during replication and transcription. As the DNA helicase unwinds the double helix, it creates "over-winding" or positive supercoiling ahead of the replication fork. This creates **torsional strain** that would eventually stall the replication machinery. Topoisomerases relieve this strain by creating transient nicks in the DNA backbone (Type I cuts one strand; Type II cuts both), allowing the DNA to rotate or pass through the break to dissipate the tension before resealing the phosphodiester bonds. **Analysis of Incorrect Options:** * **A. Deoxynucleotide polymerization:** This is the function of **DNA Polymerases**, which add dNTPs to the growing DNA strand. * **C. Initiate synthesis of RNA primers:** This is performed by **Primase** (an RNA polymerase), providing the 3'-OH group necessary for DNA polymerase to begin elongation. * **D. Prevent premature reannealing:** This is the role of **Single-Stranded Binding Proteins (SSBs)**, which stabilize the separated strands to keep the replication bubble open. **High-Yield Clinical Pearls for NEET-PG:** * **Type I Topoisomerase:** Relaxes DNA by cutting a single strand; does not require ATP. Inhibited by **Irinotecan** and **Topotecan** (used in colorectal and ovarian cancer). * **Type II Topoisomerase:** Cuts both strands; requires ATP. Inhibited by **Etoposide** and **Teniposide**. * **DNA Gyrase:** A bacterial Type II Topoisomerase that introduces negative supercoils. It is the specific target of **Fluoroquinolones** (e.g., Ciprofloxacin).

Question 696: Which type of RNA molecule is known to contain modified nucleotides?

A. tRNA (Correct Answer)
B. rRNA
C. hnRNA
D. mRNA

Explanation: **Explanation:** **tRNA (Transfer RNA)** is the correct answer because it contains the highest percentage of modified nucleotides (approximately 10–15% of its residues). These modifications are essential for its unique "cloverleaf" secondary structure and "L-shaped" tertiary structure. Common modified bases include **Pseudouridine (ψ)**, **Dihydrouridine (D)**, **Ribothymidine (T)**, and **Inosine**. These modifications stabilize the molecule, facilitate proper folding, and ensure accurate codon-anticodon base pairing (wobble hypothesis). **Why other options are incorrect:** * **rRNA (Ribosomal RNA):** While rRNA does undergo some modifications (like methylation), the extent and variety are significantly lower than in tRNA. * **hnRNA (Heterogeneous nuclear RNA):** This is the primary transcript (pre-mRNA). While it undergoes processing (capping, tailing, splicing), it does not characteristically feature the diverse modified bases seen in tRNA. * **mRNA (Messenger RNA):** In eukaryotes, mRNA has a 7-methylguanosine cap, but it lacks the extensive internal base modifications (like D or ψ) that define tRNA. **High-Yield Clinical Pearls for NEET-PG:** * **TψC Loop:** Contains Ribothymidine and Pseudouridine; it is responsible for binding the tRNA to the ribosomal surface. * **DHU Loop:** Contains Dihydrouridine; it serves as the recognition site for the specific enzyme **Aminoacyl tRNA synthetase**. * **Smallest RNA:** tRNA is the smallest (4S), often called "Soluble RNA." * **Inosine:** Often found in the anticodon loop, allowing a single tRNA to recognize multiple codons (Wobble phenomenon).

Question 697: Why is DNA selected for genetic information storage over RNA?

A. DNA is more stable than RNA. (Correct Answer)
B. DNA is double-stranded, whereas RNA is typically single-stranded.
C. The two strands of DNA are complementary.
D. DNA contains genes.

Explanation: **Explanation:** The primary reason DNA is the preferred molecule for long-term genetic storage is its superior **chemical stability** compared to RNA. This stability is driven by two key structural differences: 1. **Absence of 2'-OH group:** DNA contains deoxyribose, which lacks a hydroxyl (-OH) group at the 2' carbon. In RNA, this 2'-OH group makes the molecule susceptible to **nucleophilic attack** and spontaneous hydrolysis, especially in alkaline conditions. 2. **Thymine vs. Uracil:** DNA uses Thymine instead of Uracil. Cytosine can spontaneously deaminate to form Uracil. If DNA used Uracil naturally, the repair machinery could not distinguish between a "natural" Uracil and one formed by damage. By using Thymine (5-methyluracil), the cell can easily identify and repair deaminated Cytosine. **Analysis of Incorrect Options:** * **Option B & C:** While double-strandedness and complementarity facilitate replication and repair, they are not the fundamental reasons for "selection." RNA can also form double-stranded structures (e.g., in some viruses) and exhibit complementarity, yet it remains chemically unstable. * **Option D:** This is a circular argument. Both DNA and RNA can carry genetic information (genes); for example, many viruses use RNA as their primary genome. **NEET-PG High-Yield Pearls:** * **Alkali Lability:** RNA is degraded by 0.1M NaOH due to the 2'-OH group; DNA is only denatured (separated into strands) but not degraded. * **Self-Repair:** The presence of Thymine in DNA is a "quality control" mechanism to prevent mutations from Cytosine deamination. * **Catalytic Activity:** Because of its reactivity, RNA can act as an enzyme (**Ribozymes**, e.g., Peptidyl transferase), whereas DNA is chemically inert, making it safer for storage.

Question 698: Which of the following cellular components contain circular DNA?

A. Virus
B. Chloroplast
C. Bacteria and Mitochondria
D. All of the above (Correct Answer)

Explanation: **Explanation:** The shape of DNA is a fundamental concept in molecular biology. While nuclear DNA in eukaryotes is linear, **circular DNA** is a hallmark of prokaryotic organisms and specific organelles, following the **Endosymbiotic Theory**. 1. **Bacteria:** Most prokaryotes possess a single, double-stranded circular chromosome located in the nucleoid. They may also contain smaller circular extrachromosomal DNA called **plasmids**. 2. **Mitochondria and Chloroplasts:** According to the endosymbiotic theory, these organelles originated from ancestral prokaryotes. Consequently, they retain their own genome (mtDNA and cpDNA), which is typically double-stranded and circular. Human mtDNA encodes 37 genes essential for the electron transport chain. 3. **Viruses:** Viral genomes are highly diverse. While many are linear, several viruses possess circular DNA (e.g., **Hepatitis B, Papillomavirus, and Bacteriophages like ΦX174**). **Why "All of the above" is correct:** Since bacteria, mitochondria, chloroplasts, and certain viruses all utilize circular DNA as their genetic blueprint, Option D is the most comprehensive answer. **Clinical Pearls for NEET-PG:** * **Mitochondrial Inheritance:** mtDNA is inherited exclusively from the **mother**. Mutations here lead to "Mitochondrial Myopathies" (e.g., MELAS, LHON). * **Replication:** Circular DNA often replicates via the **Theta ($\theta$) model** or **Rolling Circle mechanism**, unlike the linear replication seen in human nuclear DNA. * **Topoisomerases:** These enzymes (like DNA Gyrase in bacteria) are crucial for managing the supercoiling of circular DNA and are targets for antibiotics like **Fluoroquinolones**.

Question 699: What is the function of endonucleases?

A. Cut DNA at specific DNA sequences (Correct Answer)
B. Enhancers
C. To identify antibiotic resistances
D. To identify coding regions

Explanation: **Explanation:** **1. Why Option A is Correct:** Endonucleases are enzymes that cleave the phosphodiester bonds within a polynucleotide chain. Specifically, **Restriction Endonucleases (REs)**, often called "molecular scissors," recognize and cut DNA at specific, often palindromic, nucleotide sequences known as **recognition sites**. This precision allows for the isolation of specific gene segments, which is the foundational step in Recombinant DNA technology. **2. Why Other Options are Incorrect:** * **Option B (Enhancers):** These are regulatory DNA sequences (not enzymes) that increase the rate of transcription of a particular gene by binding to transcription factors. * **Option C (Antibiotic Resistance):** While antibiotic resistance genes are used as **selectable markers** in cloning vectors to identify transformed cells, the act of identification is not a function of endonucleases. * **Option D (Coding Regions):** Coding regions (exons) are identified through sequencing or bioinformatics; endonucleases do not inherently distinguish between coding and non-coding regions unless a specific recognition site is present. **3. NEET-PG High-Yield Pearls:** * **Type II Restriction Endonucleases** are the most commonly used in labs because they cut at specific sites and do not require ATP. * **Blunt vs. Sticky Ends:** Some REs (like *EcoRI*) create staggered cuts (sticky ends), while others (like *SmaI*) create straight cuts (blunt ends). Sticky ends are preferred for ligation. * **Clinical Application:** Restriction Fragment Length Polymorphism (RFLP) uses these enzymes to detect mutations (e.g., in Sickle Cell Anemia, where a mutation abolishes a *MstII* recognition site). * **Nomenclature:** The first letter is the Genus, the next two are the species, and the Roman numeral denotes the order of discovery (e.g., *EcoRI* from *E. coli*).

Question 700: Reverse transcription involves which of the following processes?

A. RNA dependent DNA synthesis (Correct Answer)
B. DNA dependent RNA synthesis
C. DNA dependent DNA synthesis
D. RNA dependent RNA synthesis

Explanation: **Explanation:** **1. Why Option A is Correct:** Reverse transcription is the process by which genetic information flows "backward" from RNA to DNA, challenging the traditional Central Dogma. The enzyme responsible, **Reverse Transcriptase**, uses a single-stranded RNA template to synthesize a complementary DNA (cDNA) strand. Therefore, it is biochemically classified as an **RNA-dependent DNA polymerase**. **2. Why Other Options are Incorrect:** * **Option B (DNA-dependent RNA synthesis):** This describes **Transcription**, catalyzed by RNA Polymerase (e.g., synthesis of mRNA from a DNA template). * **Option C (DNA-dependent DNA synthesis):** This describes **Replication**, catalyzed by DNA Polymerase during the S-phase of the cell cycle. * **Option D (RNA-dependent RNA synthesis):** This describes **RNA Replication**, catalyzed by RNA-dependent RNA polymerase (RdRp), found in certain RNA viruses like Poliovirus or SARS-CoV-2. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Retroviruses:** The most notable example is **HIV**, which uses reverse transcriptase to integrate its genome into the host cell's DNA. * **Telomerase:** A specialized reverse transcriptase (containing its own RNA template) that maintains the ends of linear chromosomes. It is highly active in cancer cells and stem cells. * **Diagnostic Application:** **RT-PCR** (Reverse Transcription Polymerase Chain Reaction) utilizes this process to detect RNA viruses (like HIV or COVID-19) by first converting viral RNA into cDNA. * **Drug Target:** Nucleoside Reverse Transcriptase Inhibitors (**NRTIs**) like Zidovudine (AZT) are cornerstones of HAART therapy for HIV.

Practice by Chapter

DNA Replication and Repair Mechanisms

Practice Questions

Transcription Factors and Gene Regulation

Practice Questions

Epigenetics and DNA Methylation

Practice Questions

RNA Processing and Splicing

Practice Questions

miRNA and RNA Interference

Practice Questions

Protein Synthesis and Post-Translational Modifications

Practice Questions

Genomics and Human Genome Project

Practice Questions

Single Nucleotide Polymorphisms

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Gene Therapy Approaches

Practice Questions

CRISPR-Cas9 and Genome Editing

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DNA Fingerprinting and Forensics

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Molecular Basis of Genetic Diseases

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