Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics Indian Medical PG Practice Questions and MCQs

Question 241: Which of the following mutations is potentially lethal?

A. Substitution of adenine for cytosine
B. Substitution of methylcytosine for cytosine
C. Substitution of guanine for cytosine
D. Insertion of one base (Correct Answer)

Explanation: ### Explanation **1. Why "Insertion of one base" is the correct answer:** The insertion of a single nucleotide (or any number not divisible by three) results in a **Frameshift Mutation**. Since the genetic code is read in non-overlapping triplets (codons), adding one base shifts the entire reading frame downstream from the mutation site. This typically leads to: * **Garbled translation:** Every amino acid after the mutation is altered. * **Premature Stop Codons:** Most frameshifts result in a truncated, non-functional protein (Nonsense-mediated decay). Because this fundamentally destroys the protein's primary structure and function, it is far more likely to be **lethal** than a single amino acid substitution. **2. Why the other options are incorrect:** * **Options A and C (Substitution):** These are **Point Mutations** (specifically Transversion and Transition). These usually result in a **Missense mutation** (one amino acid change) or a **Silent mutation** (no change). While some (like Sickle Cell Anemia) cause disease, many are benign or non-lethal. * **Option B (Methylcytosine for Cytosine):** This is a form of **Epigenetic modification** rather than a traditional mutation. DNA methylation is a physiological process used for gene silencing and regulation. While abnormal methylation is linked to cancer, it is not a structural "lethal mutation" in the context of sequence disruption. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Frameshift Examples:** Duchenne Muscular Dystrophy (DMD) is often caused by frameshift deletions/insertions, whereas the milder Becker’s MD usually involves in-frame mutations. * **Tay-Sachs Disease:** Often caused by a 4-base pair insertion in the HEXA gene. * **Transition vs. Transversion:** Transitions (Purine to Purine) are more common than Transversions (Purine to Pyrimidine). * **Nonsense Mutation:** A point mutation that creates a premature stop codon (UAG, UAA, UGA).

Question 242: In which cellular component does protein biosynthesis primarily occur?

A. Cytoplasm
B. Endoplasmic Reticulum
C. Ribosomes (Correct Answer)
D. Mitochondria

Explanation: **Explanation:** **1. Why Ribosomes are the Correct Answer:** Protein biosynthesis, or **translation**, is the process where genetic information in mRNA is decoded to build polypeptide chains. The **ribosome** is the fundamental cellular machinery (ribonucleoprotein complex) where this occurs. It provides the structural framework and catalytic activity (peptidyl transferase) necessary to link amino acids via peptide bonds. While ribosomes can be free or membrane-bound, they are the specific site of synthesis in all living cells. **2. Analysis of Incorrect Options:** * **A. Cytoplasm:** This is the *location* where translation occurs, but it is too broad. The cytoplasm contains many organelles; the ribosome is the specific component responsible for the synthesis itself. * **B. Endoplasmic Reticulum (ER):** Only the **Rough ER** is involved in protein synthesis, and this is solely because it has ribosomes attached to its surface. The ER itself functions in protein folding, modification, and transport, not the primary assembly of the polypeptide chain. * **C. Mitochondria:** While mitochondria do have their own DNA and 70S ribosomes (mitoribosomes) to synthesize a small fraction of their own proteins, they are not the *primary* site for cellular protein synthesis. **3. NEET-PG High-Yield Clinical Pearls:** * **Eukaryotic vs. Prokaryotic Ribosomes:** Eukaryotes have **80S** (60S + 40S) ribosomes, while prokaryotes (and mitochondria) have **70S** (50S + 30S). This difference is the basis for the selective toxicity of many antibiotics. * **Antibiotic Targets:** * **30S Inhibitors:** Aminoglycosides, Tetracyclines. * **50S Inhibitors:** Chloramphenicol, Erythromycin (Macrolides), Clindamycin, Linezolid. * **Peptidyl Transferase:** In eukaryotes, this is a **ribozyme** (28S rRNA of the 60S subunit) that catalyzes peptide bond formation.

Question 243: Why do fetal cells continue to divide, while terminally differentiated adult cells do not?

A. Fetal cells possess numerous cyclin inhibitors that prevent adult cells from entering the S phase. (Correct Answer)
B. Phosphatase is absent in fetal cells.
C. Proteinase is absent in the fetus.
D. CDK (cyclin-dependent kinase) is absent in fetal cells.

Explanation: ### Explanation **1. Why Option A is Correct:** The cell cycle is strictly regulated by **Cyclin-Dependent Kinases (CDKs)** and their regulatory subunits, **Cyclins**. For a cell to transition from the G1 phase to the S phase (DNA replication), specific Cyclin-CDK complexes (like Cyclin D-CDK4/6) must be active. In **terminally differentiated adult cells** (e.g., neurons, cardiac myocytes), the cell cycle is halted in the **G0 phase**. This arrest is primarily maintained by **Cyclin-Dependent Kinase Inhibitors (CKIs)**, such as the **p21, p27, and p16** families. These inhibitors bind to and silence Cyclin-CDK complexes, preventing the phosphorylation of the Retinoblastoma (Rb) protein, thereby blocking entry into the S phase. **Fetal cells**, being highly proliferative, have low levels of these inhibitors or high levels of growth-promoting factors that override them, allowing continuous division. **2. Why Other Options are Incorrect:** * **Option B (Phosphatase):** Phosphatases (like CDC25) are actually essential for activating CDKs by removing inhibitory phosphate groups. Their absence would stop cell division, not promote it. * **Option C (Proteinase):** While proteasomal degradation (via ubiquitin ligases like APC/C) is vital for cycling through phases, "proteinase" is too non-specific and its absence is not the physiological reason for the difference in division potential. * **Option D (CDK):** CDKs are the "engines" of the cell cycle. If they were absent in fetal cells, the fetus could not grow or develop. **3. Clinical Pearls & High-Yield Facts:** * **G0 Phase:** Terminally differentiated cells are in G0. Stable cells (like hepatocytes) can re-enter the cycle from G0, while permanent cells (neurons) cannot. * **The "Restriction Point":** The transition from G1 to S phase is the most critical checkpoint in the cell cycle. * **Tumor Suppressors:** p53 induces p21 (a CKI), which halts the cell cycle to allow for DNA repair. Loss of these inhibitors is a hallmark of malignancy.

Question 244: The Major Histocompatibility Complex (MHC) gene is located in which chromosome?

A. Chromosome 12
B. Chromosome 6 (Correct Answer)
C. Chromosome 7
D. Chromosome 8

Explanation: ### Explanation **Correct Answer: B. Chromosome 6** The **Major Histocompatibility Complex (MHC)**, known in humans as the **Human Leukocyte Antigen (HLA)** system, is a large gene family located on the **short arm (p) of Chromosome 6** (specifically at 6p21.3). This region is one of the most gene-dense and polymorphic areas of the human genome. It encodes cell surface proteins essential for the acquired immune system to recognize foreign molecules. * **MHC Class I** (HLA-A, B, C) genes are located on Chromosome 6, but the $\beta_2$-microglobulin component is encoded on Chromosome 15. * **MHC Class II** (HLA-DR, DQ, DP) genes are also located on Chromosome 6. * **MHC Class III** genes (encoding complement components C2, C4, and TNF) are situated between Class I and Class II loci on Chromosome 6. **Why the other options are incorrect:** * **Chromosome 12:** Contains genes for the Homeobox (HOX) C cluster and Vitamin D receptor, but not MHC. * **Chromosome 7:** Houses the T-cell receptor (TCR) gamma and beta chain genes and the Cystic Fibrosis (CFTR) gene. * **Chromosome 8:** Contains the **c-myc** oncogene (relevant in Burkitt lymphoma translocations). **High-Yield Clinical Pearls for NEET-PG:** * **HLA-B27:** Strongly associated with Seronegative Spondyloarthropathies (e.g., Ankylosing Spondylitis). * **HLA-DR3/DR4:** Associated with Type 1 Diabetes Mellitus. * **HLA-DQ2/DQ8:** Associated with Celiac Disease. * **Inheritance:** MHC genes are inherited as a **haplotype** (a set of alleles from one parent) in a **codominant** fashion.

Question 245: Where are the cellular bearings of hereditary disease found?

A. DNA (Correct Answer)
B. Ribosome
C. RNA
D. Membrane

Explanation: **Explanation:** **Why DNA is the Correct Answer:** Hereditary diseases are caused by permanent alterations in the genetic material that can be passed from one generation to the next. **DNA (Deoxyribonucleic acid)** serves as the primary repository of genetic information in humans. It contains the specific sequences (genes) that encode for proteins. Mutations—such as point mutations, deletions, or insertions—within the DNA sequence lead to the production of defective proteins or the absence of essential proteins, which manifests as hereditary disease (e.g., Sickle Cell Anemia, Cystic Fibrosis). **Why Other Options are Incorrect:** * **Ribosome:** These are the cellular "workbenches" where protein synthesis (translation) occurs. While they read genetic instructions, they do not store hereditary information. * **RNA:** In humans, RNA acts as an intermediate messenger (mRNA) or functional component (tRNA, rRNA). While some viruses use RNA as genetic material, in human pathology, RNA is a transient molecule and not the primary "bearing" or permanent storage site of hereditary traits. * **Membrane:** Cell membranes provide structural integrity and cell signaling functions. They do not contain the genetic code required for inheritance. **NEET-PG High-Yield Pearls:** * **Central Dogma:** Information flows from DNA → RNA → Protein. Hereditary diseases originate at the DNA level. * **Mitochondrial DNA (mtDNA):** Remember that not all hereditary DNA is nuclear; mutations in mtDNA cause maternal inheritance patterns (e.g., LHON, MELAS). * **Epigenetics:** Changes in gene expression that do not involve alterations in the DNA sequence itself (e.g., DNA methylation) can also be heritable. * **Hotspots:** Certain areas of DNA (like CpG islands) are more prone to mutations, leading to a higher frequency of specific hereditary disorders.

Question 246: Binding of proteins to DNA is regulated by which of the following elements?

A. Copper
B. Zinc (Correct Answer)
C. Selenium
D. Nickel

Explanation: **Explanation:** The binding of proteins to DNA is a fundamental process in gene expression and regulation. The correct answer is **Zinc** because of its structural role in **Zinc Finger Motifs**. **1. Why Zinc is Correct:** Zinc finger motifs are the most common DNA-binding motifs found in eukaryotic transcription factors (e.g., Steroid hormone receptors, Vitamin D receptors). In these motifs, a zinc ion ($Zn^{2+}$) is coordinated by cysteine and histidine residues. This coordination stabilizes the protein’s "finger-like" structure, allowing it to fit precisely into the **major groove of the DNA** to regulate transcription. Without zinc, these proteins cannot maintain the conformation required to bind to specific DNA sequences. **2. Why Other Options are Incorrect:** * **Copper:** Primarily acts as a cofactor for redox enzymes (e.g., Cytochrome c oxidase, Superoxide dismutase). While it is essential for cellular respiration and iron metabolism, it does not form structural DNA-binding motifs. * **Selenium:** Essential for the function of antioxidant enzymes like **Glutathione peroxidase** and the conversion of T4 to T3 via deiodinases. It is incorporated into proteins as selenocysteine but does not regulate DNA binding. * **Nickel:** While required in trace amounts for certain bacterial enzymes (like Urease), it has no established physiological role in human DNA-protein interactions. **High-Yield Clinical Pearls for NEET-PG:** * **Zinc Finger Proteins:** Examples include the **Glucocorticoid receptor**, **Estrogen receptor**, and **TFIIIA**. * **Other DNA-binding motifs:** Leucine zipper (e.g., c-fos, c-jun), Helix-turn-helix, and Helix-loop-helix. * **Clinical Correlation:** Zinc deficiency can lead to **Acrodermatitis enteropathica**, characterized by dermatitis, alopecia, and diarrhea, partly due to the impairment of various zinc-dependent transcription factors and enzymes.

Question 247: Which of the following processes in a vector is used to increase the yield of protein produced in recombinant protein synthesis?

A. Promoter induction (Correct Answer)
B. Origin of Replication
C. Translation initiation
D. Translation or transcription inhibition

Explanation: **Explanation:** In recombinant DNA technology, the goal of protein synthesis is to maximize the expression of a specific gene of interest. This is primarily achieved through **Promoter Induction**. **1. Why Promoter Induction is Correct:** A promoter is a DNA sequence that initiates transcription. In expression vectors, "inducible promoters" (like the *lac* promoter) are used. These promoters are normally "off" to prevent the metabolic burden on the host cell during its growth phase. Once the bacterial population reaches a high density, an inducer (e.g., IPTG) is added. This "turns on" the promoter, leading to massive transcription of the target gene into mRNA, which subsequently increases the **yield of the protein**. **2. Analysis of Incorrect Options:** * **Origin of Replication (ori):** This sequence determines the **copy number** of the plasmid within the host cell. While it ensures the plasmid is replicated, it does not directly control the rate of protein synthesis from those plasmids. * **Translation Initiation:** While essential for starting the synthesis of a polypeptide chain (involving the Shine-Dalgarno sequence in prokaryotes), it is a qualitative requirement for protein production rather than a primary mechanism used to "increase yield" in a controlled industrial or laboratory setting. * **Translation/Transcription Inhibition:** These processes would decrease or stop protein production, which is the opposite of the desired outcome. **High-Yield Clinical Pearls for NEET-PG:** * **Expression Vectors vs. Cloning Vectors:** Expression vectors must contain a promoter, a ribosome-binding site, and a termination signal, whereas cloning vectors only require an *ori*, a selectable marker, and a multiple cloning site. * **IPTG (Isopropyl β-D-1-thiogalactopyranoside):** A classic inducer used in labs because it mimics lactose but is not metabolized by *E. coli*, ensuring constant induction. * **Post-translational modifications:** Bacteria cannot perform glycosylation; therefore, human proteins requiring sugar moieties (like Erythropoietin) must be produced in eukaryotic systems (e.g., CHO cells).

Question 248: Which of the following is an amber codon?

A. UAA
B. UAG (Correct Answer)
C. UGA
D. UGG

Explanation: ### Explanation In molecular biology, translation is terminated when the ribosome encounters a **stop codon** (also known as a nonsense codon) on the mRNA. There are three specific stop codons, each historically assigned a "color" name based on the laboratory strains in which they were discovered. **1. Why UAG is Correct:** **UAG** is known as the **Amber codon**. It does not code for any amino acid; instead, it signals the termination of polypeptide synthesis by recruiting release factors. **2. Analysis of Incorrect Options:** * **A. UAA (Ochre):** This is the most frequently used stop codon in *E. coli*. It is referred to as the Ochre codon. * **C. UGA (Opal):** Also known as the Umber codon. Interestingly, in mitochondria, UGA is not a stop codon but instead codes for **Tryptophan**. * **D. UGG:** This is a sense codon that codes for the amino acid **Tryptophan**. It is unique because, along with Methionine (AUG), it is one of only two amino acids coded by a single codon. **3. NEET-PG High-Yield Pearls:** * **Mnemonic to remember stop codons:** * **UAA** (U Are Away) – **Ochre** * **UAG** (U Are Gone) – **Amber** * **UGA** (U Go Away) – **Opal** * **Nonsense Mutation:** A point mutation that changes a sense codon into one of these three stop codons, leading to premature termination and a truncated, usually non-functional, protein. * **Universal Genetic Code Exceptions:** While the code is nearly universal, human **mitochondria** use UGA for Tryptophan and AUA for Methionine, which differs from the nuclear genetic code.

Question 249: A 5-yr old boy is diagnosed with Duchenne muscular dystrophy, with a mutation in the promoter site for the dystrophin gene. Which of the following statements is true?

A. Initiation of dystrophin protein synthesis is affected. (Correct Answer)
B. Capping of dystrophin mRNA is affected.
C. Poly-A tail of dystrophin mRNA is affected.
D. Termination of dystrophin protein synthesis is affected.

Explanation: ### Explanation **1. Why Option A is Correct:** The **promoter** is a specific DNA sequence located upstream (at the 5' end) of a gene. Its primary function is to serve as the binding site for **RNA polymerase II** and various transcription factors. This binding is the critical first step in gene expression. If the promoter is mutated, the recruitment of the transcriptional machinery is impaired, leading to a failure in the initiation of mRNA synthesis (transcription). Since no mRNA is produced (or it is produced in insufficient quantities), the subsequent **initiation of protein synthesis** (translation) cannot occur. In the context of Duchenne Muscular Dystrophy (DMD), a promoter mutation results in a near-total absence of the dystrophin protein. **2. Why the Other Options are Incorrect:** * **Option B (Capping):** Capping occurs at the 5' end of the mRNA transcript during elongation. It is regulated by the C-terminal domain of RNA polymerase, not the promoter sequence itself. * **Option C (Poly-A tail):** Polyadenylation occurs at the 3' end of the mRNA and is directed by the polyadenylation signal sequence (AAUAAA) located at the end of the gene, not the promoter. * **Option D (Termination):** Termination of translation is governed by **stop codons** (UAA, UAG, UGA) within the coding sequence of the mRNA, which are unrelated to the promoter site. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **DMD Genetics:** Most commonly caused by **large deletions** (65%) leading to a **frameshift mutation**, resulting in a truncated, non-functional protein. Promoter mutations are a rarer cause. * **Promoter Examples:** The **TATA box** (Hogness box) in eukaryotes and the **Pribnow box** (TATAAT) in prokaryotes are classic promoter elements. * **Dystrophin Gene:** It is the largest known human gene, making it highly susceptible to spontaneous mutations. * **Becker Muscular Dystrophy (BMD):** Unlike DMD, BMD usually involves **in-frame mutations**, resulting in a shortened but partially functional dystrophin protein (milder phenotype).

Question 250: Termination of protein synthesis is performed by all except?

A. Releasing factor
B. Stop codon
C. Peptidyl transferase (Correct Answer)
D. UAA codon

Explanation: **Explanation:** The termination of protein synthesis (translation) is a highly regulated process that occurs when the ribosome encounters a termination signal. **Why Peptidyl Transferase is the correct answer:** Peptidyl transferase is an enzyme (specifically a ribozyme located in the 28S rRNA of the large ribosomal subunit) primarily responsible for **peptide bond formation** during the **elongation** phase. While it does play a role in the final hydrolysis of the bond between the peptide chain and tRNA during termination, it is fundamentally an elongation enzyme. In the context of this question, it is the "odd one out" because it does not act as a termination signal or a specific termination factor. **Analysis of other options:** * **Stop Codons (UAA and Option B):** There are three termination codons: **UAA** (Ochre), **UAG** (Amber), and **UGA** (Opal). These do not code for any amino acid. When a ribosome reaches these codons, translation halts because there are no corresponding tRNAs. * **Releasing Factors (RF):** Since no tRNA recognizes stop codons, proteins called Releasing Factors (RF1, RF2, RF3 in prokaryotes; eRF in eukaryotes) bind to the ribosome. They mimic the structure of tRNA and trigger the release of the completed polypeptide chain. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Stop Codons:** **U** **A**re **A**way (UAA), **U** **A**re **G**one (UAG), **U** **G**o **A**way (UGA). * **Energy Requirement:** Termination is an energy-dependent process requiring **GTP hydrolysis**. * **Diphtheria Toxin:** Inhibits eukaryotic translation by ADP-ribosylation of **Elongation Factor-2 (EF-2)**, preventing translocation. * **Puromycin:** An antibiotic that causes premature chain termination by acting as an analog of aminoacyl-tRNA.

Practice by Chapter

DNA Replication and Repair Mechanisms

Practice Questions

Transcription Factors and Gene Regulation

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Epigenetics and DNA Methylation

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RNA Processing and Splicing

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miRNA and RNA Interference

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Protein Synthesis and Post-Translational Modifications

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Genomics and Human Genome Project

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Single Nucleotide Polymorphisms

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Gene Therapy Approaches

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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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