Molecular Biology and Genomics Practice Questions

Q: The test result of which investigation, for a child with developmental delay, is shown below?

DNA microarray. ***DNA microarray*** - **Array CGH** (comparative genomic hybridization) is the **first-line investigation** for unexplained developmental delay, detecting **copy number variations** (deletions/duplications) across the entire genome with high resolution. - The test result typically shows a **genomic plot** with **log2 ratios** displaying gains (duplications) and losses (deletions) as deviations from the baseline, making it easily identifiable. *Multiplex Ligation-dependent probe Amplification* - **MLPA** is a **targeted technique** that analyzes specific genomic regions or genes, not genome-wide screening like microarray. - Results are presented as **peak patterns** or **ratio plots** for specific probes, showing **dosage changes** in predetermined target sequences only. *Fluorescent in situ hybridisation* - **FISH** uses **fluorescent probes** to detect specific chromosomal regions and shows results as **colored signals** on metaphase chromosomes or interphase nuclei. - It's a **targeted approach** for known abnormalities rather than a genome-wide screening tool for developmental delay. *Chromosomal painting* - This technique uses **chromosome-specific probes** to "paint" entire chromosomes in different colors for detecting **large structural rearrangements**. - Results show **multicolored karyotypes** and are primarily used for complex rearrangements, not the subtle copy number changes typical in developmental delay.

Q: Which techniques are used to detect gene mutations?

All of the above. **Explanation:** Gene mutations are permanent alterations in the DNA sequence. Detecting these mutations is crucial for diagnosing genetic disorders, identifying predispositions to cancer, and personalized medicine. * **DNA Sequencing (Option C):** This is the **Gold Standard** for mutation detection. It allows for the base-by-base identification of the DNA sequence, making it capable of detecting point mutations, insertions, and deletions. * **Restriction Fragment Length Polymorphism (RFLP) (Option C):** This technique utilizes restriction enzymes that cut DNA at specific sequences. If a mutation occurs at a recognition site, the enzyme fails to cut, resulting in fragments of different lengths compared to the wild type. * **Denaturing Gradient Gel Electrophoresis (DGGE) (Option B):** This is a screening technique that separates DNA fragments based on their melting properties. Even a single base pair mutation changes the stability (melting temperature) of the DNA duplex, causing it to migrate differently in a gradient of denaturants. **Why "All of the above" is correct:** While the options provided represent different methodologies (sequencing is definitive, while DGGE and RFLP are screening/comparative methods), all are established laboratory techniques used to identify variations in the genetic code. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard for Mutation Detection:** DNA Sequencing (Sanger or Next-Generation Sequencing). * **RFLP Application:** Classically used for diagnosing **Sickle Cell Anemia** (loss of *MstII* restriction site). * **PCR (Polymerase Chain Reaction):** Often the prerequisite step for all these techniques to amplify the target gene. * **Southern Blotting:** Used for detecting large structural changes like gene deletions or duplications, but not typically for single-point mutations.

Q: All of the following require 5' capping except?

tRNA of Alanine. **Explanation:** The 5' capping process involves the addition of a 7-methylguanosine cap to the 5' end of a nascent RNA molecule. This modification is a hallmark of **RNA Polymerase II** transcripts. **Why tRNA of Alanine is the correct answer:** Transfer RNAs (tRNAs) are transcribed by **RNA Polymerase III**. Unlike mRNA, tRNA molecules do not undergo 5' capping. Instead, their 5' ends are processed by **RNase P**, which cleaves the leader sequence to generate the mature 5' terminus. Therefore, tRNA of Alanine does not require a 5' cap. **Analysis of Incorrect Options:** * **mRNA for Histone:** Although histone mRNAs are unique because they lack a poly-A tail, they are transcribed by RNA Polymerase II and **do require a 5' cap** for stability and translation initiation. * **siRNA (Small Interfering RNA):** These are typically derived from longer double-stranded RNA precursors or primary transcripts (pri-miRNA/shRNA) transcribed by RNA Polymerase II, which possess a 5' cap during their processing stages. * **U6 snRNA:** While most snRNAs (U1, U2, U4, U5) are transcribed by Pol II and have a trimethylguanosine cap, U6 is transcribed by **RNA Polymerase III**. However, U6 undergoes a unique modification where its 5' end is capped with a **γ-monomethyl phosphate cap**, technically making it a "capped" RNA, unlike tRNA. **High-Yield Facts for NEET-PG:** 1. **RNA Polymerase I:** Transcribes 45S pre-rRNA (precursor for 18S, 28S, and 5.8S rRNA). 2. **RNA Polymerase II:** Transcribes mRNA, miRNA, and most snRNAs (U1-U5). These typically have a 7-methylguanosine cap. 3. **RNA Polymerase III:** Transcribes tRNA, 5S rRNA, and U6 snRNA. 4. **Functions of the 5' Cap:** Protects against 5'→3' exonuclease degradation, facilitates nuclear export, and is essential for the binding of the eIF4F complex during translation initiation.

Q: Which biomedical tool used in DNA technology utilizes an oligomer with a single base pair substitution?

Restriction Fragment Length Polymorphism (RFLP). ### Explanation **Correct Option: B. Restriction Fragment Length Polymorphism (RFLP)** RFLP is a technique used to detect variations in DNA sequences. The underlying principle relies on the fact that a **single base pair substitution** (a Single Nucleotide Polymorphism or SNP) can either create or abolish a specific recognition site for a **restriction endonuclease**. When the DNA is digested with these enzymes, the resulting fragments differ in length between individuals. These fragments are then separated by electrophoresis and hybridized with a labeled **oligomer (DNA probe)** to visualize the specific polymorphic regions. This tool is classic for linkage analysis and identifying genetic disease carriers (e.g., Sickle Cell Anemia). **Analysis of Incorrect Options:** * **A. PCR:** This is an amplification technique using primers to exponentially increase DNA quantity. While it can be used *prior* to RFLP, the PCR process itself does not inherently rely on single base substitutions to function. * **C. Error coded mutation analysis:** This is a distractor term. While "Allele-Specific Oligonucleotide (ASO)" probes detect mutations, they are not the standard definition for the process described in the context of traditional RFLP. * **D. FISH:** This technique uses fluorescent probes to detect the presence, absence, or location of specific **large DNA sequences** or whole genes on chromosomes. It is used for gross chromosomal abnormalities (e.g., Trisomy 21, BCR-ABL translocation) rather than single base substitutions. **Clinical Pearls for NEET-PG:** * **Sickle Cell Anemia:** The classic RFLP example where a mutation in the $\beta$-globin gene destroys the *MstII* restriction site. * **VNTRs:** Variable Number Tandem Repeats are the basis for DNA fingerprinting, often analyzed via RFLP. * **Southern Blotting:** The laboratory technique required to visualize RFLP fragments.

Q: Which of the following methods is most suited to assess the function of a gene?

Gene knockout animals. To assess the **function** of a gene, we must observe the physiological consequences when that gene is absent. This is the principle of "Reverse Genetics." ### Why "Gene Knockout Animals" is Correct A **Gene Knockout** involves the deliberate inactivation or "deletion" of a specific gene within an organism’s genome. By comparing the phenotype of the knockout animal with a wild-type (normal) animal, researchers can determine exactly what biological processes that gene controls. For example, knocking out the *LDL receptor* gene in mice leads to hypercholesterolemia, confirming its function in cholesterol clearance. ### Why Other Options are Incorrect * **Southern Blot:** Used for the detection of a specific **DNA sequence** in a sample. It identifies the presence or size of a gene but does not provide information about its biological function. * **Northern Blot:** Used to study **gene expression** by detecting specific **RNA** molecules. While it shows if a gene is "active," it doesn't prove what the resulting protein actually does in the body. * **Transgenic Animals:** These animals have a **foreign gene inserted** into their genome (Gain-of-function). While useful, adding a gene can sometimes cause non-specific effects or overexpression artifacts, making "Knockout" (Loss-of-function) the gold standard for defining a gene's essential baseline function. ### High-Yield NEET-PG Pearls * **SNOW DROP Mnemonic:** **S**outhern-**D**NA, **N**orthern-**R**NA, **O**-**O**, **W**estern-**P**rotein. * **Knock-in:** A variation where a mutated gene is substituted for the normal gene to study specific diseases (e.g., Sickle Cell models). * **RNA Interference (RNAi):** Another method to study function by "silencing" mRNA, often called a "Gene Knockdown." * **CRISPR-Cas9:** The most modern and efficient tool used to create knockout models.

Q: Polypeptide chain termination is enhanced by:

Stop codon. ### Explanation **Why the Correct Answer is Right:** Translation termination occurs when the ribosome encounters one of the three **stop codons** (UAA, UAG, or UGA) on the mRNA. Unlike other codons, stop codons do not code for an amino acid and are not recognized by tRNA. Instead, they are recognized by **Release Factors (RFs)**. These factors trigger the peptidyl transferase to catalyze the addition of a water molecule instead of an amino acid, leading to the hydrolysis of the bond between the completed polypeptide chain and the tRNA, thereby releasing the protein. **Analysis of Incorrect Options:** * **B. Promoter:** This is a DNA sequence located upstream of a gene where RNA polymerase binds to initiate **transcription**, not translation. It has no role in polypeptide chain termination. * **C. Ribosomal unit:** While the ribosome is the machinery where translation occurs, the units themselves (40S/60S in eukaryotes) do not "enhance" termination; they dissociate *after* the stop codon is recognized by release factors. * **D. All:** Incorrect, as only the stop codon serves as the specific signal for termination. **NEET-PG High-Yield Pearls:** * **Mnemonic for Stop Codons:** **U** **A**re **A**way (UAA), **U** **A**re **G**one (UAG), **U** **G**o **A**way (UGA). * **Release Factors:** In Prokaryotes, RF1 (UAA, UAG) and RF2 (UAA, UGA) are used. In Eukaryotes, a single factor, **eRF1**, recognizes all three stop codons. * **Nonsense Mutation:** A point mutation that results in a premature stop codon, leading to a truncated, usually non-functional protein (e.g., in some forms of Beta-thalassemia). * **Energy Requirement:** Termination is an energy-dependent process requiring **GTP** hydrolysis.

Q: Which of the following methods cannot be used to detect gene expression?

Southern blot and Immunohistochemistry. **Explanation:** **Gene expression** refers to the process by which information from a gene is used to synthesize functional gene products—primarily **mRNA** (transcription) and **proteins** (translation). To detect gene expression, a technique must identify either the specific mRNA or the protein product. **Why Option D is Correct:** * **Southern Blot:** This technique is used to detect specific **DNA** sequences. Since DNA is present in almost every cell regardless of whether a gene is "turned on" or "off," Southern blotting identifies the presence or structure of a gene (e.g., deletions, insertions), but **not** its expression. * **Immunohistochemistry (IHC):** While IHC *does* detect protein expression in tissues, the combination in Option D makes it the best choice because Southern Blotting is strictly for DNA analysis, not expression. (Note: In many competitive exams, if a pair contains one definitely incorrect technique for the context, the entire option is considered the answer). **Why Other Options are Incorrect:** * **RT-PCR (Reverse Transcriptase PCR):** Converts mRNA into cDNA to quantify gene expression levels. It is the gold standard for detecting low-abundance mRNA. * **cDNA Microarray:** Allows for the simultaneous analysis of the expression of thousands of genes by measuring mRNA levels. * **Northern Blot:** Specifically used to detect and quantify **RNA** (mRNA), making it a classic method for studying gene expression. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic (SNOW DROP):** * **S**outhern = **D**NA * **N**orthern = **R**NA * **O** (ignore) = **O** (ignore) * **W**estern = **P**rotein * **Southwestern Blot:** Used to detect **DNA-binding proteins** (e.g., transcription factors like c-Jun or c-Fos). * **ELISA & Western Blot:** Both detect protein expression; Western blot is more specific (confirmatory test for HIV).

Q: Which of the following is NOT a feature of the genetic code?

Ambiguous. The genetic code is a set of rules used by living cells to translate information encoded within genetic material into proteins. Understanding its properties is fundamental for medical biochemistry. ### **Why "Ambiguous" is the Correct Answer** The genetic code is **unambiguous**, not ambiguous. This means that **one specific codon always codes for one specific amino acid**. For example, the codon UUU always codes for Phenylalanine and nothing else. If the code were "ambiguous," a single codon could code for multiple different amino acids, which would lead to the synthesis of unpredictable and non-functional proteins. ### **Explanation of Incorrect Options** * **B. Non-overlapping:** The code is read sequentially, three bases at a time. A single base is part of only one codon and is not shared between adjacent codons. * **C. Commaless:** There are no "punctuation marks" or spacers between codons. Once translation begins at the start codon (AUG), the mRNA is read continuously until a stop codon is reached. * **D. Specific:** This is a synonym for "unambiguous." It reinforces that a particular codon is dedicated to a specific amino acid. ### **High-Yield Clinical Pearls for NEET-PG** * **Degeneracy (Redundancy):** While one codon codes for only one amino acid (unambiguous), **one amino acid can be coded by multiple codons** (e.g., Leucine has six codons). This provides protection against silent mutations. * **Universality:** The code is the same in almost all organisms. **Exception:** Mitochondrial DNA (e.g., UGA codes for Tryptophan in mitochondria, but is a Stop codon in the cytosol). * **Wobble Hypothesis:** Proposed by Francis Crick; it explains why the third base of a codon can sometimes vary without changing the amino acid, allowing one tRNA to recognize multiple codons.

Question 1

In E. coli, the structural genes of the lac operon are stimulated under which condition?

Accepted Answer

Presence of lactose and absence of glucose

Answer

Presence of glucose only

Answer

Presence of lactose only

Answer

Presence of glucose and absence of lactose

Question 2

The test result of which investigation, for a child with developmental delay, is shown below?

Accepted Answer

DNA microarray

Answer

Multiplex Ligation-dependent probe Amplification

Answer

Fluorescent in situ hybridisation

Answer

Chromosomal painting

Question 3

Which of the following best describes the inheritance pattern of an X-linked recessive disease?

Accepted Answer

50% of sons are affected if the mother is a carrier and the father is normal

Answer

Vertical transmission from father to son

Answer

50% of daughters are carriers if the father is affected and the mother is normal

Answer

50% of sons are carriers if the mother is affected and the father is normal

Question 4

Which techniques are used to detect gene mutations?

Accepted Answer

All of the above

Answer

This option seems to be a placeholder or incorrectly formatted.

Answer

Denaturing gradient gel electrophoresis

Answer

DNA sequencing and Restriction fragment polymorphism (RFLP)

Question 5

All of the following require 5' capping except?

Accepted Answer

tRNA of Alanine

Answer

mRNA for Histone

Answer

siRNA

Answer

U6 snRNA

Question 6

Which biomedical tool used in DNA technology utilizes an oligomer with a single base pair substitution?

Accepted Answer

Restriction Fragment Length Polymorphism (RFLP)

Answer

Polymerase Chain Reaction (PCR)

Answer

Error coded mutation analysis

Answer

Fluorescence In Situ Hybridization (FISH)

Question 7

Which of the following methods is most suited to assess the function of a gene?

Accepted Answer

Gene knockout animals

Answer

Southern blot

Answer

Northern blot

Answer

Transgenic animals

Question 8

Polypeptide chain termination is enhanced by:

Accepted Answer

Stop codon

Answer

Promoter

Answer

Ribosomal unit

Answer

All

Question 9

Which of the following methods cannot be used to detect gene expression?

Accepted Answer

Southern blot and Immunohistochemistry

Answer

RT-PCR

Answer

cDNA microarray

Answer

Northern blot

Question 10

Which of the following is NOT a feature of the genetic code?

Accepted Answer

Ambiguous

Answer

Non-overlapping

Answer

Commaless

Answer

Specific

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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