Molecular Biology and Genomics Practice Questions

Q: A frameshift mutation does not occur in multiples of which number?

3. **Explanation:** The genetic code is **triplet-based**, meaning a sequence of three nucleotides (a codon) codes for a single amino acid. The "reading frame" is established at the start codon and continues sequentially. **Why 3 is the correct answer:** A **frameshift mutation** occurs when the number of nucleotides inserted or deleted is **not a multiple of three**. If exactly three nucleotides (or any multiple of 3, such as 6 or 9) are added or removed, it results in the gain or loss of entire amino acids without shifting the downstream reading frame. This is known as an **in-frame mutation**. For a mutation to be classified as a "frameshift," it must disrupt the triplet grouping, altering every subsequent codon and usually leading to a premature stop codon (nonsense mutation). **Analysis of Incorrect Options:** * **Options A, C, and D (2, 4, 5):** These numbers are not multiples of three. If 2, 4, or 5 nucleotides are inserted or deleted, the triplet alignment is broken. For example, deleting 2 bases shifts the frame by +1, while deleting 4 bases shifts it by +2. Therefore, these numbers **do** cause frameshift mutations. **Clinical Pearls & High-Yield Facts:** * **Cystic Fibrosis:** The most common mutation ($\Delta$F508) is an **in-frame deletion** of 3 nucleotides (phenylalanine), not a frameshift. * **Duchenne vs. Becker Muscular Dystrophy:** Duchenne is typically caused by **frameshift mutations** (severe phenotype), whereas Becker is often caused by **in-frame mutations** (milder phenotype). * **Consequence:** Frameshift mutations are generally more deleterious than point mutations because they alter the entire primary structure of the protein C-terminal to the mutation.

Q: All of the following are true regarding satellite DNA EXCEPT?

It is transcriptionally active.. ### Explanation **Satellite DNA** refers to highly repetitive DNA sequences that form a significant portion of the eukaryotic genome. Understanding its characteristics is crucial for molecular biology and forensic genetics. **Why Option D is the Correct Answer (The Exception):** Satellite DNA is primarily **transcriptionally inactive**. It is composed of non-coding sequences that do not translate into proteins. Structurally, it is categorized as **constitutive heterochromatin**, which is highly condensed and chemically modified (e.g., methylated) to prevent gene expression. Therefore, the statement that it is transcriptionally active is incorrect. **Analysis of Incorrect Options:** * **Option A:** Satellite DNA consists of short sequences (ranging from a few to hundreds of base pairs) that are **repeated in tandem** (head-to-tail). This distinguishes it from interspersed repeats like SINEs and LINEs. * **Option B:** Large blocks of satellite DNA (Macro-satellites) are characteristically found at the **centromeres**, where they play a structural role in spindle fiber attachment during mitosis. * **Option C:** Satellite DNA is also frequently clustered at the **telomeres** (the ends of chromosomes) and the secondary constrictions of acrocentric chromosomes, providing genomic stability. **High-Yield Facts for NEET-PG:** 1. **Buoyant Density:** The name "satellite" comes from the fact that these sequences have a different G-C content than the rest of the genome, causing them to form a separate "satellite" band during **cesium chloride (CsCl) density gradient centrifugation**. 2. **VNTRs and STRs:** Minisatellites (VNTRs) and Microsatellites (STRs) are types of satellite DNA used in **DNA Fingerprinting** due to their high degree of polymorphism. 3. **Trinucleotide Repeats:** Microsatellite expansions are responsible for diseases like **Huntington’s chorea** and **Fragile X syndrome**.

Q: Which molecular method is used to locate a known gene locus?

FISH (Fluorescence in situ hybridization). **Explanation:** **1. Why FISH is the correct answer:** Fluorescence in situ hybridization (FISH) is a cytogenetic technique used to detect and **locate a specific DNA sequence** on a chromosome. It utilizes fluorescently labeled DNA probes that are complementary to the known gene locus of interest. When the probe binds (hybridizes) to its target sequence on the patient's chromosome, it can be visualized under a fluorescence microscope. This makes it the gold standard for identifying the physical location of a known gene or detecting specific microdeletions/translocations. **2. Why the other options are incorrect:** * **CGH (Comparative Genomic Hybridization):** This technique is used to detect **copy number variations** (gains or losses of DNA) across the entire genome. It compares patient DNA with control DNA but does not pinpoint the specific locus of a single known gene as precisely as FISH. * **Chromosome Painting:** This is a variation of FISH that uses a cocktail of probes to color an **entire chromosome**. It is excellent for identifying complex structural rearrangements or identifying the origin of "marker chromosomes," but it is not used to locate a specific individual gene locus. * **RT-PCR:** This is a molecular technique used to measure **gene expression** by converting RNA into cDNA. It is used for quantification and detection of mRNA, not for locating a gene's physical position on a chromosome. **Clinical Pearls for NEET-PG:** * **FISH** is the investigation of choice for **microdeletion syndromes** (e.g., DiGeorge Syndrome - 22q11.2 deletion). * **Rapid Diagnosis:** FISH can be performed on interphase nuclei, meaning it does not require cell culture (unlike traditional karyotyping). * **HER2/neu status** in breast cancer is often confirmed via FISH if immunohistochemistry (IHC) results are equivocal (2+).

Q: Which of the following is NOT required for a standard polymerase chain reaction?

Dideoxynucleotides. ### Explanation **Core Concept: Polymerase Chain Reaction (PCR)** PCR is an *in vitro* enzymatic method used to amplify specific DNA sequences. The process mimics natural DNA replication but requires specific components to function in a thermal cycler. **Why Dideoxynucleotides (ddNTPs) are NOT required:** Dideoxynucleotides (ddNTPs) lack a **3'-OH group**, which is essential for forming phosphodiester bonds. When a ddNTP is incorporated, DNA synthesis terminates immediately. Therefore, ddNTPs are used in **Sanger Sequencing (Chain Termination Method)**, not in standard PCR, where the goal is to synthesize full-length DNA strands. **Analysis of Other Options:** * **Taq Polymerase:** A heat-stable DNA polymerase (derived from *Thermus aquaticus*) required to extend the primers by adding nucleotides at high temperatures. * **dNTPs (dATP, dCTP, dGTP, dTTP):** These are the "building blocks" or substrates required to synthesize the new DNA strand. * **Magnesium ions (Mg²⁺):** These act as a mandatory **cofactor** for DNA polymerase activity. They stabilize the negative charges on the phosphate backbone and facilitate the catalysis of phosphodiester bonds. **High-Yield NEET-PG Pearls:** 1. **Steps of PCR:** Denaturation (94-96°C) → Annealing (50-65°C) → Extension (72°C). 2. **Primers:** PCR requires two synthetic, short oligonucleotide primers that are complementary to the 3' ends of the target DNA. 3. **RT-PCR:** Uses Reverse Transcriptase to convert RNA into cDNA before amplification (Gold standard for COVID-19 diagnosis). 4. **Real-Time PCR (qPCR):** Used to quantify the amount of DNA/RNA in a sample using fluorescent dyes.

Q: What is the initial amino acid incorporated in prokaryotic protein synthesis?

Formyl-methionine. ### Explanation In prokaryotes (such as bacteria), protein synthesis is initiated by a specific modified amino acid: **N-formylmethionine (fMet)**. **Why Option C is Correct:** The initiation of translation in prokaryotes requires a specialized initiator tRNA ($tRNA_f^{met}$). This tRNA carries methionine, which is subsequently formylated by the enzyme **transformylase** using N10-formyl tetrahydrofolate as a donor. This formyl group mimics a peptide bond, helping the initiator tRNA bind directly to the **P-site** of the ribosome, whereas all subsequent aminoacyl-tRNAs enter at the A-site. **Why Other Options are Incorrect:** * **Option B (Methionine):** While methionine is the universal start codon (AUG) product, in prokaryotes, it must be formylated. Non-formylated methionine is the initiator for **eukaryotes**. * **Options A & D (Arginine & Alanine):** These are standard amino acids incorporated during the elongation phase but never serve as the primary initiator amino acid. --- ### High-Yield Clinical Pearls for NEET-PG * **Mitochondrial Connection:** Because mitochondria evolved from prokaryotic endosymbionts, human mitochondrial protein synthesis also starts with **N-formylmethionine**. * **Chemotaxis:** Human neutrophils have receptors (FPR1) that recognize fMet-containing peptides. This allows the immune system to identify and migrate toward sites of bacterial infection (fMet acts as a **PAMP** - Pathogen-Associated Molecular Pattern). * **Deformylation:** In many mature bacterial proteins, the formyl group (and sometimes the methionine itself) is removed post-translationally by peptide deformylase. * **Shine-Dalgarno Sequence:** In prokaryotes, the 16S rRNA of the 30S subunit recognizes this purine-rich sequence to correctly position the AUG start codon for fMet-tRNA binding.

Q: Which of the following cellular bodies is NOT found in the nucleus?

P-bodies. **Explanation:** The correct answer is **A. P-bodies (Processing bodies)**. **1. Why P-bodies are the correct answer:** P-bodies are distinct, membrane-less granules found exclusively in the **cytoplasm**. They are composed of translationally repressed mRNAs and enzymes involved in mRNA turnover. Their primary functions include mRNA decay (decapping and degradation), mRNA storage, and translational repression. They serve as "processing centers" for mRNA before it is either degraded or returned to the translation machinery. **2. Why the other options are incorrect:** * **Nucleolus:** The most prominent structure in the nucleus; it is the site of ribosomal RNA (rRNA) synthesis and ribosome biogenesis. * **Cajal bodies:** Small sub-nuclear organelles involved in the maturation of small nuclear ribonucleoproteins (snRNPs) and telomerase assembly. * **Interchromatin granule clusters (Speckles):** These are nuclear domains enriched in pre-mRNA splicing factors. They serve as storage and modification sites for splicing machinery. **3. High-Yield Clinical Pearls for NEET-PG:** * **P-bodies vs. Stress Granules:** Both are cytoplasmic. While P-bodies are involved in mRNA decay, **Stress Granules** form specifically during cellular stress to protect mRNA and initiate survival pathways. * **Spinal Muscular Atrophy (SMA):** This condition is linked to the loss of the SMN protein, which is localized in **Gems** (Gemini of Cajal bodies), highlighting the clinical importance of nuclear bodies. * **Nucleolus and Cancer:** Hypertrophy of the nucleolus is a classic histopathological marker of malignant cells due to increased protein synthesis requirements.

Question 1

A frameshift mutation does not occur in multiples of which number?

Accepted Answer

3

Answer

2

Answer

4

Answer

5

Question 2

All of the following are true regarding satellite DNA EXCEPT?

Accepted Answer

It is transcriptionally active.

Answer

It consists of repeated DNA sequences arranged in tandem.

Answer

It is typically clustered around the centromeres.

Answer

It is often found clustered around the telomeres.

Question 3

Which molecular method is used to locate a known gene locus?

Accepted Answer

FISH (Fluorescence in situ hybridization)

Answer

CGH (Comparative genomic hybridization)

Answer

Chromosome painting

Answer

RT-PCR (Reverse transcription polymerase chain reaction)

Question 4

Hemoglobin electrophoresis of erythrocytes from a young child with anemia reveals an unstable hemoglobin, Hb Cranston (HbCr), with an abnormal b-globin chain. Given the nucleotide sequences of the normal b-globin gene (HbNl) and the HbCr b-chain, which of the following would account for the development of HbCr?

Accepted Answer

A two base pair addition resulted in the elimination of a stop codon in the b-chain.

Answer

A frameshift mutation resulted in the deletion of several amino acid residues in the b-chain.

Answer

A mutation in the stop codon resulted in elongation of the b-chain.

Answer

A point mutation resulted in the insertion of a stop codon in the b-chain.

Question 5

Which of the following is NOT required for a standard polymerase chain reaction?

Accepted Answer

Dideoxynucleotides

Answer

Taq polymerase

Answer

Deoxynucleotide triphosphates (dNTPs)

Answer

Magnesium ions

Question 6

What is the initial amino acid incorporated in prokaryotic protein synthesis?

Accepted Answer

Formyl-methionine

Answer

Arginine

Answer

Methionine

Answer

Alanine

Question 7

Which of the following is an example of a non-coding RNA?

Accepted Answer

siRNA

Answer

miRNA

Answer

tRNA

Answer

mRNA

Question 8

Which of the following cellular bodies is NOT found in the nucleus?

Accepted Answer

P-bodies

Answer

Nucleolus

Answer

Cajal bodies

Answer

Interchromatin granule clusters

Question 9

Chromosomal mutations can be identified by all of the following methods except:

Accepted Answer

Agarose gel electrophoresis

Answer

Single nucleotide polymorphism

Answer

Denaturing gradient gel electrophoresis

Answer

Dideoxynucleotide chain termination sequencing

Question 10

Which of the following statements is true regarding Okazaki fragments?

Accepted Answer

Several Okazaki fragments must be sequentially synthesized for each replication fork

Answer

Segments of RNA attached to an RNA initiator component

Answer

Related to the leading strand

Answer

Helicase acts on the leading strand to unwind dsDNA

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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