Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics Indian Medical PG Practice Questions and MCQs

Question 51: Which of the following is coding RNA?

A. snRNA
B. mRNA (Correct Answer)
C. Both snRNA and mRNA
D. miRNA

Explanation: **Explanation:** The classification of RNA is based on its function in protein synthesis. RNA is broadly divided into **Coding RNA** and **Non-coding RNA (ncRNA)**. **1. Why mRNA is the correct answer:** **mRNA (messenger RNA)** is the only type of RNA that carries the genetic blueprint from DNA to the ribosomes to be translated into a polypeptide chain. It contains **codons** that dictate the specific amino acid sequence of a protein. Therefore, it is the only "coding" RNA. **2. Why other options are incorrect:** * **snRNA (Small nuclear RNA):** These are non-coding RNAs found in the nucleus. They combine with proteins to form **snRNPs ("snurps")**, which are essential components of the **spliceosome**. Their role is structural and catalytic (removing introns), not coding. * **miRNA (microRNA):** These are short, non-coding RNA molecules (approx. 22 nucleotides) that function in **RNA interference (RNAi)**. They regulate gene expression post-transcriptionally by binding to mRNA and inhibiting translation or promoting degradation. * **tRNA and rRNA (Other non-coding RNAs):** Though not in the options, remember that Transfer RNA and Ribosomal RNA are also non-coding; they facilitate translation but do not provide the code themselves. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **RNA Polymerase II** is responsible for the synthesis of mRNA (the coding RNA). * **Post-transcriptional modifications** of mRNA include 5' capping (7-methylguanosine), 3' polyadenylation (Poly-A tail), and splicing. * **snRNA Deficiency:** Antibodies against snRNPs (specifically Anti-Smith antibodies) are highly specific for **Systemic Lupus Erythematosus (SLE)**. * **miRNA Clinical Link:** Dysregulation of miRNAs is frequently linked to oncogenesis (oncomiRs) and is a target for future cancer therapies.

Question 52: On the ribosome, mRNA binds?

A. between the subunits
B. to the large subunit
C. to the small subunit (Correct Answer)
D. none of these

Explanation: ### Explanation **1. Why the Correct Answer is Right:** In both prokaryotes and eukaryotes, the initiation of translation begins with the binding of mRNA to the **small ribosomal subunit** (30S in prokaryotes, 40S in eukaryotes). * In **prokaryotes**, the 16S rRNA of the 30S subunit recognizes the **Shine-Dalgarno sequence** on the mRNA. * In **eukaryotes**, the 40S subunit (along with initiation factors) binds to the **5' methylguanosine cap** and scans for the AUG start codon. The small subunit acts as the platform for mRNA decoding, ensuring the correct alignment of the mRNA codons with the tRNA anticodons. **2. Why the Other Options are Wrong:** * **Option A (Between the subunits):** While the mRNA is physically positioned between the two subunits once the full ribosome (70S or 80S) assembles, the initial and primary binding site for the mRNA strand is specifically on the small subunit. * **Option B (To the large subunit):** The large ribosomal subunit (50S or 60S) is primarily responsible for **peptidyl transferase activity** (catalyzing peptide bond formation) and contains the A, P, and E sites for tRNA binding. It does not have a high-affinity binding site for mRNA. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Aminoglycosides (e.g., Streptomycin):** These antibiotics act by binding to the **30S (small) subunit**, causing mRNA misreading and inhibiting the initiation complex. * **Tetracyclines:** Also bind to the **30S subunit**, preventing the attachment of aminoacyl-tRNA to the A-site. * **Ribozyme Activity:** The 23S rRNA (prokaryotes) or 28S rRNA (eukaryotes) in the **large subunit** acts as the peptidyl transferase; this is a classic example of a non-protein enzyme. * **Kozak Sequence:** In eukaryotes, the mRNA sequence (ACCAUGG) helps the small subunit identify the correct start codon.

Question 53: Which of the following is true about the function of Restriction Endonucleases?

A. They cut both strands of double-stranded DNA.
B. The cut ends produced are sticky.
C. The cut ends produced are blunt.
D. All of the above. (Correct Answer)

Explanation: **Explanation:** Restriction Endonucleases (REs), often called "molecular scissors," are enzymes derived primarily from bacteria that recognize and cut specific palindromic sequences in double-stranded DNA (dsDNA). **Why Option D is correct:** Restriction enzymes function by cleaving the phosphodiester backbone of **both strands** of the DNA helix (Option A). Depending on the specific enzyme and its cleavage site, the resulting DNA fragments can have two types of ends: 1. **Sticky (Cohesive) ends:** These have short, single-stranded overhanging sequences (e.g., *EcoRI*). These are highly useful in recombinant DNA technology as they can easily base-pair with complementary sequences (Option B). 2. **Blunt ends:** These occur when the enzyme cuts both strands at the same base pair position, leaving no overhangs (e.g., *SmaI* or *HaeIII*) (Option C). Since all three statements accurately describe the potential actions and outcomes of restriction endonuclease activity, **Option D** is the correct choice. **High-Yield Clinical Pearls for NEET-PG:** * **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*: *E. coli*, strain R, 1st enzyme). * **Type II REs:** These are the most commonly used in genetic engineering because they cut within or at fixed pointers near the recognition site and do not require ATP. * **Methylation:** Bacteria protect their own DNA from these enzymes by methylating their recognition sequences (Restriction-Modification System). * **Applications:** Essential for Restriction Fragment Length Polymorphism (RFLP) analysis, gene cloning, and DNA fingerprinting.

Question 54: Which of the following is NOT true about Polymerase Chain Reaction?

A. Requires a thermostable enzyme
B. Involves DNA denaturation, followed by primer annealing
C. Requires specific primers
D. Uses thermolabile enzymes (Correct Answer)

Explanation: **Explanation:** Polymerase Chain Reaction (PCR) is an *in vitro* technique used to amplify specific DNA sequences. The process relies on repeated cycles of high-temperature heating and cooling. **Why Option D is the Correct Answer:** PCR requires **thermostable** (heat-stable) enzymes, most notably **Taq Polymerase** (derived from the bacterium *Thermus aquaticus*). This enzyme can withstand the high temperatures (approx. 95°C) required for DNA denaturation without losing its catalytic activity. **Thermolabile** enzymes (heat-sensitive) would denature and become inactive during the first heating step, making them unsuitable for the PCR process. **Analysis of Other Options:** * **Option A:** True. As mentioned, a thermostable enzyme is essential to survive the denaturation phase. * **Option B:** True. Each PCR cycle consists of three distinct steps: **Denaturation** (separation of strands at ~95°C), **Annealing** (primers bind to target sequences at ~55°C), and **Extension** (DNA synthesis at ~72°C). * **Option C:** True. PCR requires two synthetic oligonucleotide **primers** that are complementary to the sequences flanking the target DNA region to initiate synthesis. **High-Yield Clinical Pearls for NEET-PG:** * **RT-PCR:** Used for RNA viruses (like SARS-CoV-2); it uses **Reverse Transcriptase** 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). * **Applications:** Diagnosis of genetic mutations (e.g., Sickle Cell Anemia), detection of infectious agents (TB, HIV), and forensic medicine (DNA fingerprinting).

Question 55: What does transcription refer to?

A. The process where an mRNA is used as a template for protein production.
B. The process where a DNA sequence is copied into RNA for the purpose of gene expression. (Correct Answer)
C. The process where DNA wraps around histones to form a nucleosome.
D. The process of replication of DNA prior to cell division.

Explanation: **Explanation:** **Transcription** is the first step of gene expression, occurring in the nucleus of eukaryotic cells. It involves the synthesis of an RNA molecule (mRNA, tRNA, or rRNA) from a DNA template, catalyzed by the enzyme **RNA Polymerase**. This process ensures that the genetic information stored in the stable DNA "blueprint" is converted into a portable RNA format that can be used to synthesize proteins. **Analysis of Options:** * **Option A (Incorrect):** This describes **Translation**, which occurs in the cytoplasm where ribosomes read mRNA to assemble amino acids into proteins. * **Option C (Incorrect):** This refers to **DNA Packaging**. DNA wrapping around an octamer of histone proteins forms a nucleosome, the basic unit of chromatin. * **Option D (Incorrect):** This describes **Replication**, the semi-conservative process of copying the entire genome using DNA Polymerase, occurring during the S-phase of the cell cycle. **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. * **Key Enzyme:** In eukaryotes, **RNA Polymerase II** is responsible for synthesizing mRNA. * **Inhibitors (Pharmacology Link):** * **Rifampicin** inhibits bacterial DNA-dependent RNA polymerase (used in TB). * **Alpha-amanitin** (from *Amanita phalloides* mushrooms) specifically inhibits RNA Polymerase II, causing severe liver failure. * **Post-transcriptional modifications:** Eukaryotic pre-mRNA undergoes 5' capping, 3' polyadenylation, and splicing (removal of introns) before leaving the nucleus.

Question 56: In a hybrid RNA-DNA molecule, the DNA sequence is 5'CTTAAG3'. What is the complementary RNA sequence?

A. 5'CTTAAG3'
B. 5'CUUAAG3' (Correct Answer)
C. 5'GAATTC3'
D. 5'GAAUUC3'

Explanation: ### Explanation **1. Why Option B is Correct** In molecular biology, "complementary" usually refers to the opposite strand (antiparallel). However, in the context of this specific question and its options, it refers to the **RNA transcript** or the **coding equivalent** in RNA form. The DNA sequence provided is **5'CTTAAG3'**. * In DNA-RNA hybridization (like during transcription), Adenine (A) pairs with Uracil (U), and Cytosine (C) pairs with Guanine (G). * If we look at the options provided, the question is testing the direct substitution of Thymine (T) with Uracil (U) while maintaining the same polarity. * Replacing 'T' with 'U' in the sequence 5'CTTAAG3' yields **5'CUUAAG3'**. This represents the RNA sequence that corresponds to that specific DNA segment. **2. Analysis of Incorrect Options** * **Option A (5'CTTAAG3'):** Incorrect because RNA does not contain Thymine (T); it contains Uracil (U). * **Option C (5'GAATTC3'):** This is the complementary DNA sequence in reverse-parallel orientation (3'GAATTC5'), but it uses DNA bases (T) instead of RNA bases. * **Option D (5'GAAUUC3'):** This would be the antiparallel RNA strand (the actual transcript if the given DNA was the template strand). While biologically logical, it does not match the intended answer pattern where the sequence's primary identity is preserved but converted to RNA format. **3. NEET-PG High-Yield Pearls** * **Chargaff’s Rule:** Applies to double-stranded DNA (A=T, G=C). It does **not** apply to single-stranded RNA or single-stranded DNA. * **RNA vs. DNA:** RNA has a hydroxyl (-OH) group at the 2' carbon of the ribose sugar, making it more chemically reactive and less stable than DNA. * **Directionality:** Always read sequences in the **5' to 3'** direction unless specified otherwise. This is the direction of polymerisation for DNA and RNA polymerases. * **Hybridization:** Occurs naturally during transcription (DNA-RNA) and during the action of Primase (RNA primer on DNA template).

Question 57: Which of the following activities increases in permissive chromatin?

A. Methylation of CpG islands
B. Phosphorylation
C. Acetylation of histones (Correct Answer)
D. Sumoylation

Explanation: **Explanation:** Chromatin exists in two functional states: **Heterochromatin** (condensed, transcriptionally inactive) and **Euchromatin** (relaxed, transcriptionally active/permissive). The transition between these states is regulated by post-translational modifications of histone tails. **Why Acetylation is Correct:** Histone Acetylation is mediated by **Histone Acetyltransferases (HATs)**. Histones are positively charged (rich in Lysine and Arginine), which allows them to bind tightly to the negatively charged DNA backbone. Acetylation neutralizes the positive charge on lysine residues, weakening the histone-DNA interaction. This results in a "relaxed" or **permissive chromatin** structure, allowing transcription factors and RNA polymerase access to the DNA. **Analysis of Incorrect Options:** * **A. Methylation of CpG islands:** DNA methylation (specifically at Cytosine-Guanine dinucleotides) is a hallmark of **transcriptional silencing**. It leads to chromatin condensation and is associated with genomic imprinting and X-inactivation. * **B. Phosphorylation:** While histone phosphorylation (e.g., H3) is involved in DNA repair and transcriptional activation, it is most characteristically associated with **chromosome condensation** during mitosis/meiosis. * **C. Sumoylation:** Histone sumoylation is generally a mark of **transcriptional repression**, acting as an antagonist to acetylation and promoting a more closed chromatin state. **NEET-PG High-Yield Pearls:** * **Mnemonic:** **A**cetylation **A**ctivates transcription; **M**ethylation **M**utes transcription (usually). * **HDACs:** Histone Deacetylases (HDACs) remove acetyl groups, leading to gene silencing. HDAC inhibitors (e.g., Valproic acid, Vorinostat) are used in psychiatry and oncology to re-activate silenced genes. * **CpG Islands:** Found in promoter regions; their hypermethylation is a common epigenetic mechanism in silencing tumor suppressor genes in cancer.

Question 58: In Eukaryotes, what produces mRNA?

A. RNA Polymerase I
B. RNA Polymerase II (Correct Answer)
C. RNA Polymerase III
D. RNA Polymerase IV

Explanation: ### Explanation In eukaryotes, gene transcription is specialized across three primary nuclear RNA polymerases. **RNA Polymerase II** is the enzyme responsible for synthesizing **messenger RNA (mRNA)**, which carries genetic information from DNA to ribosomes for protein synthesis. It also produces most microRNA (miRNA) and small nuclear RNA (snRNA). #### Analysis of Options: * **RNA Polymerase I (Option A):** Located in the nucleolus, it synthesizes the precursor of **ribosomal RNA (rRNA)**, specifically the 28S, 18S, and 5.8S subunits. (Memory aid: **R**NA Pol **I** = **R**ibosomal). * **RNA Polymerase II (Option B):** The correct answer. It transcribes all protein-coding genes into mRNA. It is uniquely sensitive to **$\alpha$-amanitin** (found in *Amanita phalloides* mushrooms). * **RNA Polymerase III (Option C):** Synthesizes small RNAs, primarily **transfer RNA (tRNA)** and the 5S rRNA subunit. (Memory aid: **T**RNA Pol **III** = **T**ransfer). * **RNA Polymerase IV (Option D):** Found only in plants, where it plays a role in siRNA synthesis and gene silencing; it is not present in humans. #### High-Yield Clinical Pearls for NEET-PG: 1. **Mushroom Poisoning:** $\alpha$-amanitin binds specifically to **RNA Polymerase II**, inhibiting mRNA synthesis and leading to severe hepatotoxicity and liver failure. 2. **Prokaryotes vs. Eukaryotes:** Unlike eukaryotes, prokaryotes have only **one** RNA polymerase (a multi-subunit complex) that synthesizes all types of RNA. 3. **Mitochondrial RNA Polymerase:** Mitochondria have their own distinct RNA polymerase, which resembles bacterial enzymes. 4. **Promoter Region:** RNA Pol II recognizes the **TATA box** (Hogness box) located approximately 25 base pairs upstream of the start site.

Question 59: Introns are not found in which type of DNA?

A. Nuclear DNA
B. Mitochondrial DNA (Correct Answer)
C. B DNA
D. Z DNA

Explanation: **Explanation:** The correct answer is **Mitochondrial DNA (mtDNA)**. **1. Why Mitochondrial DNA is the correct answer:** Human mitochondrial DNA is a small, circular, double-stranded molecule that is highly "gene-dense." Unlike nuclear DNA, mtDNA is characterized by a lack of **introns** (non-coding intervening sequences). Almost the entire mitochondrial genome consists of coding sequences, with the exception of a small non-coding region known as the D-loop (displacement loop), which functions in replication and transcription control. This lack of introns reflects the **Endosymbiotic Theory**, suggesting mitochondria evolved from prokaryotic ancestors (bacteria), which also typically lack introns. **2. Analysis of Incorrect Options:** * **Nuclear DNA:** This is the primary site where introns are found. Eukaryotic nuclear genes are "split genes," containing exons (coding) and introns (non-coding). Introns are removed via splicing during post-transcriptional modification. * **B DNA:** This refers to the most common right-handed helical conformation of DNA under physiological conditions. It describes the *structure* of DNA, not its genomic organization. B DNA can contain both introns and exons. * **Z DNA:** This is a left-handed zig-zag conformation of DNA often found in regions with alternating purine-pyrimidine sequences. Like B DNA, it is a structural variant and does not dictate the presence or absence of introns. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Maternal Inheritance:** mtDNA is inherited exclusively from the mother. * **Genetic Code Exceptions:** Mitochondria use a slightly different genetic code (e.g., **UGA** codes for Tryptophan instead of a Stop codon). * **Mutation Rate:** mtDNA has a 10x higher mutation rate than nuclear DNA due to a lack of protective histones and proximity to reactive oxygen species (ROS). * **Heteroplasmy:** The presence of a mixture of more than one type of organellar genome (normal and mutated) within a cell.

Question 60: The anticodon region is an important part of the:

A. r-RNA
B. mRNA
C. t-RNA (Correct Answer)
D. hn-RNA

Explanation: **Explanation:** The **anticodon region** is a specific sequence of three nucleotides located on the **tRNA (transfer RNA)** molecule. Its primary function is to recognize and base-pair with a complementary **codon** on the mRNA strand during translation. This interaction ensures that the correct amino acid, which is covalently attached to the 3' end (CCA tail) of the tRNA, is incorporated into the growing polypeptide chain. **Why the other options are incorrect:** * **mRNA (Messenger RNA):** Contains **codons**, not anticodons. Codons are the three-letter genetic codes transcribed from DNA that dictate the amino acid sequence. * **rRNA (Ribosomal RNA):** Forms the structural and catalytic core of the ribosome (e.g., peptidyl transferase activity). It does not contain anticodons. * **hnRNA (Heterogeneous nuclear RNA):** This is the primary transcript (pre-mRNA) found in the nucleus before splicing and processing. It contains introns and exons but lacks an anticodon region. **High-Yield Clinical Pearls for NEET-PG:** * **Wobble Hypothesis:** Proposed by Francis Crick, it states that the 3rd base of the mRNA codon and the 1st base of the tRNA anticodon can have non-traditional base pairing, allowing one tRNA to recognize multiple codons. * **Aminoacyl-tRNA Synthetase:** The enzyme responsible for "charging" tRNA with its specific amino acid. It is the true "translator" of the genetic code. * **Structure:** tRNA has a **Cloverleaf** secondary structure and an **L-shaped** tertiary structure. * **Inosine:** Often found in the first position of the anticodon, it can pair with A, U, or C, facilitating the "wobble" effect.

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

Practice Questions

Gene Therapy Approaches

Practice Questions

CRISPR-Cas9 and Genome Editing

Practice Questions

DNA Fingerprinting and Forensics

Practice Questions

Molecular Basis of Genetic Diseases

Practice Questions

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