Molecular Biology and Genomics Practice Questions

Q: On the ribosome, mRNA binds?

to the small subunit. ### 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.

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

Uses thermolabile enzymes. **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).

Q: Which of the following activities increases in permissive chromatin?

Acetylation of histones. **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.

Q: Which of the following processes is NOT an function of the endoplasmic reticulum?

DNA synthesis. **Explanation:** The **Endoplasmic Reticulum (ER)** is a multifunctional organelle divided into two types: Rough ER (RER) and Smooth ER (SER). **Why DNA Synthesis is the Correct Answer:** DNA synthesis (Replication) occurs exclusively in the **Nucleus** (for genomic DNA) and the **Mitochondria** (for mitochondrial DNA). The ER lacks the necessary enzymes, such as DNA polymerase, and the structural environment required for the replication of the genetic code. Therefore, it is not a function of the ER. **Analysis of Other Options:** * **Protein Synthesis (Option A):** This is a primary function of the **Rough ER**. The presence of ribosomes on its surface allows for the translation and translocation of proteins destined for secretion, membrane integration, or lysosomal enzymes. * **Steroid Synthesis (Option B):** This is a hallmark function of the **Smooth ER**. Cells specialized in steroid hormone production (e.g., adrenal cortex, Leydig cells) have an abundance of SER. * **Triglyceride Synthesis (Option D):** The **Smooth ER** is the major site for lipid biosynthesis, including phospholipids, cholesterol, and triglycerides. **High-Yield Clinical Pearls for NEET-PG:** * **Sarcoplasmic Reticulum:** A specialized form of SER in muscle cells that acts as the primary storage site for **Calcium ions ($Ca^{2+}$)**, crucial for muscle contraction. * **Detoxification:** The SER in hepatocytes contains the **Cytochrome P450** enzyme system, responsible for the detoxification of drugs and toxins. * **Nissl Bodies:** These are large granules of RER found in neurons; they are responsible for high-rate protein synthesis. * **ER Stress:** Accumulation of misfolded proteins in the ER triggers the "Unfolded Protein Response" (UPR), which is linked to neurodegenerative diseases.

Q: Proteins are synthesized by:

Golgi bodies. The synthesis and processing of proteins is a multi-step pathway involving several organelles. While **Ribosomes** are the primary site of translation (assembling amino acids into polypeptide chains), the **Golgi apparatus** is essential for the final functional synthesis of complex proteins through **post-translational modifications**. [1] 1. **Why Golgi bodies are correct:** The Golgi apparatus is the "post office" of the cell. It is responsible for the biochemical modification of proteins received from the Rough Endoplasmic Reticulum (RER). This includes **glycosylation** (adding carbohydrates to form glycoproteins), **sulfation**, and **phosphorylation**. Without these modifications, many proteins remain non-functional; thus, the Golgi is integral to the synthesis of functional secretory proteins and membrane components. 2. **Why other options are incorrect:** * **Mitochondria:** Known as the "powerhouse of the cell," their primary role is ATP production via oxidative phosphorylation. * **Ribosomes:** These are the sites of protein *translation*. While they assemble the primary structure, they do not perform the complex modifications required for mature protein synthesis. [1] [2] * **Nuclear membrane:** This double-layered structure protects the genetic material and regulates nucleocytoplasmic transport; it does not synthesize proteins. **High-Yield NEET-PG Pearls:** * **I-Cell Disease:** A clinical correlation where a deficiency in phosphorylating enzymes in the Golgi leads to the failure of lysosomal enzyme targeting, causing skeletal abnormalities and restricted joint movement. * **Cis vs. Trans:** The *Cis-face* of the Golgi receives vesicles from the RER, while the *Trans-face* (Trans-Golgi Network) sorts and ships them. * **Protein Sorting:** The Golgi marks proteins destined for lysosomes with **Mannose-6-Phosphate**.

Q: In the human leukocyte antigen (HLA) system, where is the cluster of genes located?

Chromosome 6. ### Explanation **Correct Answer: A. Chromosome 6** The **Human Leukocyte Antigen (HLA)** system is the human version of the **Major Histocompatibility Complex (MHC)**. It consists of a large cluster of genes located on the **short arm (p-arm) of Chromosome 6**. These genes encode cell surface proteins essential for the immune system to distinguish "self" from "non-self." * **MHC Class I (HLA-A, B, C):** Present on all nucleated cells; recognized by CD8+ T-cells. * **MHC Class II (HLA-DP, DQ, DR):** Present on antigen-presenting cells (APCs); recognized by CD4+ T-cells. * **MHC Class III:** Encodes components of the complement system (C2, C4) and cytokines (TNF-α). --- ### Why the other options are incorrect: * **B. Chromosome 21:** This is the smallest autosome. Trisomy 21 results in **Down Syndrome**. It does not house the HLA cluster. * **C. X-Chromosome:** Associated with sex-linked disorders like Hemophilia A/B and Duchenne Muscular Dystrophy. * **D. Y-Chromosome:** Contains the **SRY gene**, responsible for male sex determination. --- ### High-Yield Clinical Pearls for NEET-PG: 1. **Inheritance:** HLA genes are highly polymorphic and inherited as a **haplotype** (one set from each parent) in a **codominant** fashion. 2. **Disease Associations:** * **HLA-B27:** Strongly associated with **Ankylosing Spondylitis**, Reiter’s syndrome, and Acute Anterior Uveitis. * **HLA-DR3/DR4:** Associated with **Type 1 Diabetes Mellitus**. * **HLA-DQ2/DQ8:** Associated with **Celiac Disease**. * **HLA-B*5701:** Screening required before starting **Abacavir** (to prevent hypersensitivity). 3. **Linkage Disequilibrium:** HLA alleles are often inherited together more frequently than expected by chance, a classic example of linkage disequilibrium.

Q: Which of the following is an inhibitor of RNA polymerase in eukaryotes?

Alpha-amanitin. The correct answer is **Alpha-amanitin**. This question tests the fundamental understanding of transcription inhibitors and their specificity toward eukaryotic versus prokaryotic systems. **1. Why Alpha-amanitin is correct:** Alpha-amanitin is a potent cyclic peptide toxin found in the *Amanita phalloides* (Death Cap) mushroom [1]. It specifically inhibits **RNA Polymerase II** in eukaryotes, which is responsible for synthesizing mRNA. By binding to the enzyme, it prevents the translocation of DNA and RNA, effectively halting protein synthesis and leading to cell death (primarily in the liver). **2. Why the other options are incorrect:** * **Rifampicin:** This is a bactericidal antibiotic that inhibits **Bacterial (Prokaryotic) RNA Polymerase**. It binds to the beta-subunit of the bacterial enzyme, preventing the initiation of transcription. It does not inhibit eukaryotic RNA polymerase, which is why it is used clinically to treat infections like Tuberculosis and Leprosy without killing human cells. * **Options C and D:** These are incorrect because the inhibition is specific to the domain of life (Eukaryote vs. Prokaryote). **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Differential Sensitivity:** RNA Polymerase II is highly sensitive to Alpha-amanitin; RNA Polymerase III is moderately sensitive; RNA Polymerase I is resistant. * **Mushroom Poisoning:** Clinical presentation of *Amanita* ingestion involves a latent period, followed by severe GI distress, and eventually **fulminant hepatic failure** [1]. * **Actinomycin D:** Another high-yield inhibitor that inhibits transcription in **both** prokaryotes and eukaryotes by intercalating into DNA. * **Rifampicin Side Effect:** Remember the classic "orange-colored secretions" (urine, sweat, tears) associated with this drug.

Q: ABO blood groups inheritance is an example of?

Co-dominance. ### Explanation **Correct Option: D. Co-dominance** The ABO blood group system is a classic example of **Co-dominance** and **Multiple Allelism**. In co-dominance, both alleles in a heterozygote are fully expressed, and neither is dominant over the other. * The ABO system is governed by the *I* gene, which has three alleles: $I^A$, $I^B$, and $i$. * While $I^A$ and $I^B$ are both dominant over $i$ (complete dominance), they are **co-dominant** to each other. * In an individual with the genotype $I^AI^B$, both A and B antigens are expressed equally on the red blood cell surface, resulting in the AB blood group. **Why other options are incorrect:** * **A. Mitochondrial inheritance:** This refers to traits passed only from the mother to all offspring (e.g., LHON, MELAS). ABO genes are located on **Chromosome 9** (autosomal). * **B. Allelic exclusion:** This is a process where only one allele of a gene is expressed while the other is silenced (common in B-lymphocytes for immunoglobulin synthesis). In ABO, both alleles are expressed. * **C. Sex-linked inheritance:** These traits are carried on X or Y chromosomes (e.g., Hemophilia, Color blindness). ABO inheritance is **autosomal**. **High-Yield Clinical Pearls for NEET-PG:** * **Bombay Phenotype:** A rare condition where the individual lacks the **H-substance** (genotype *hh*). Even if they possess $I^A$ or $I^B$ genes, they phenotypically test as O-group because the precursor H-antigen is missing. * **Universal Donor/Recipient:** O negative is the universal donor (no antigens); AB positive is the universal recipient (no antibodies). * **Linkage:** The ABO gene locus is linked to the gene for **Nail-Patella Syndrome**.

Question 1

On the ribosome, mRNA binds?

Accepted Answer

to the small subunit

Answer

between the subunits

Answer

to the large subunit

Answer

none of these

Question 2

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

Accepted Answer

Uses thermolabile enzymes

Answer

Requires a thermostable enzyme

Answer

Involves DNA denaturation, followed by primer annealing

Answer

Requires specific primers

Question 3

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

Accepted Answer

5'CUUAAG3'

Answer

5'CTTAAG3'

Answer

5'GAATTC3'

Answer

5'GAAUUC3'

Question 4

Which of the following activities increases in permissive chromatin?

Accepted Answer

Acetylation of histones

Answer

Methylation of CpG islands

Answer

Phosphorylation

Answer

Sumoylation

Question 5

Which of the following processes is NOT an function of the endoplasmic reticulum?

Accepted Answer

DNA synthesis

Answer

Protein synthesis

Answer

Steroid synthesis

Answer

Triglyceride synthesis

Question 6

Proteins are synthesized by:

Accepted Answer

Golgi bodies

Answer

Mitochondria

Answer

Ribosomes

Answer

Nuclear membrane

Question 7

In the human leukocyte antigen (HLA) system, where is the cluster of genes located?

Accepted Answer

Chromosome 6

Answer

Chromosome 21

Answer

X-Chromosome

Answer

Y-chromosome

Question 8

At which of the following levels does the predominant regulation of gene expression take place in both prokaryotes and eukaryotes?

Accepted Answer

Transcription

Answer

DNA replication

Answer

Translation

Answer

Post-translational modification

Question 9

Which of the following is an inhibitor of RNA polymerase in eukaryotes?

Accepted Answer

Alpha-amanitin

Answer

Rifampicin

Answer

Both

Answer

None

Question 10

ABO blood groups inheritance is an example of?

Accepted Answer

Co-dominance

Answer

Mitochondrial inheritance

Answer

Allelic exclusion

Answer

Sex-linked inheritance

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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