Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics Indian Medical PG Practice Questions and MCQs

Question 631: Same amino acid is coded by multiple codons due to:

A. Degeneracy (Correct Answer)
B. Frame-shift mutation
C. Transcription
D. Mutation

Explanation: ### Explanation **Correct Option: A. Degeneracy** The genetic code consists of 64 codons, but only 20 standard amino acids. **Degeneracy** (or redundancy) refers to the phenomenon where a single amino acid is specified by two or more different codons. This occurs because the third position of the codon (the **Wobble position**) is often less specific, allowing one tRNA to recognize multiple codons. For example, Leucine is coded by six different codons. This redundancy acts as a protective mechanism against minor mutations. **Why Incorrect Options are Wrong:** * **B. Frame-shift mutation:** This is a genetic mutation caused by the insertion or deletion of nucleotides in a number not divisible by three. This shifts the reading frame, usually resulting in a completely different or non-functional protein, rather than explaining the coding mechanism. * **C. Transcription:** This is the biological process of copying a segment of DNA into RNA by the enzyme RNA polymerase. It is a step in gene expression, not a property of the genetic code itself. * **D. Mutation:** This is a general term for any change in the DNA sequence. While mutations can lead to changes in amino acids (missense) or premature stops (nonsense), they do not define the multi-codon relationship for a single amino acid. **High-Yield Clinical Pearls for NEET-PG:** * **Universal Code:** The genetic code is the same in almost all organisms, with minor exceptions (e.g., **Mitochondrial DNA**, where UGA codes for Tryptophan instead of a Stop codon). * **Non-overlapping & Commaless:** The code is read sequentially, three bases at a time, without skipping any nucleotides. * **Wobble Hypothesis:** Proposed by Francis Crick; it explains why we don't need 61 different tRNAs for 61 codons. The 5' base of the tRNA anticodon can form non-standard base pairs with the 3' base of the mRNA codon. * **Unambiguous:** While one amino acid can have many codons (Degeneracy), **one codon always codes for only one specific amino acid.**

Question 632: Which of the following represents a transition mutation?

A. A purine to a purine (e.g., A to G) (Correct Answer)
B. A purine to a pyrimidine (e.g., A to C)
C. A purine to a pyrimidine (e.g., A to T)
D. A pyrimidine to a purine (e.g., G to A)

Explanation: **Explanation:** In molecular biology, point mutations are classified into two main types based on the chemical nature of the substituted nitrogenous bases: **Transitions** and **Transversions**. **1. Why Option A is Correct:** A **Transition** mutation occurs when a nitrogenous base is replaced by another base of the same chemical class. * **Purine to Purine:** Adenine (A) $\leftrightarrow$ Guanine (G) * **Pyrimidine to Pyrimidine:** Cytosine (C) $\leftrightarrow$ Thymine (T) Since Option A describes a purine-to-purine change (A to G), it is the definition of a transition. These are more common in the genome than transversions because the molecular shape remains similar, causing less structural distortion to the DNA helix. **2. Why Incorrect Options are Wrong:** * **Options B and C:** These describe a **Transversion**. A transversion occurs when a purine (double-ring structure) is replaced by a pyrimidine (single-ring structure), or vice versa (e.g., A to C or A to T). * **Option D:** While this describes a change from G to A (which is a transition), the *text* of the option incorrectly labels it as "pyrimidine to purine." Guanine (G) is a purine, not a pyrimidine. **High-Yield Facts for NEET-PG:** * **Mnemonics:** * **PUR**e **A**s **G**old (**Pur**ines = **A**, **G**). * **CUT** the **PY** (**Py**rimidines = **C**, **U**, **T**). * **Transi**tion = **S**ame ring type (**S**ubstitution of like-for-like). * **Clinical Relevance:** Transition mutations at CpG islands (C to T) are the most frequent cause of spontaneous mutations in humans due to the deamination of 5-methylcytosine. * **Sickle Cell Anemia:** This is a classic example of a **Transversion** (GAG $\rightarrow$ GTG; Adenine to Thymine), leading to Glutamate being replaced by Valine.

Question 633: Two transgenic plants are grown. One plant has a gene encoding a fluorescent pigment incorporated into its genome. The other plant has a firefly luciferase gene incorporated into its genome. Which of the two plants will glow in the dark?

A. Both plants will glow (Correct Answer)
B. Neither will glow
C. The first plant will glow
D. The second plant will glow

Explanation: ### Explanation **1. Why the Correct Answer (A) is Right:** The core concept here is **Bioluminescence** and **Fluorescence**, both of which result in the emission of light. * **Fluorescent Pigment (e.g., GFP):** These proteins absorb light at a specific wavelength and emit it at a longer wavelength. While they typically require an excitation source, in a biological context, they are often visualized as "glowing" when the appropriate light is present. * **Firefly Luciferase:** This is a classic example of bioluminescence. The enzyme luciferase catalyzes the oxidation of a substrate (luciferin) in the presence of ATP and oxygen, releasing energy in the form of light. This process does not require external light to initiate, making the plant glow inherently. * In the context of transgenic technology, both genes are used as **reporter genes** to confirm successful gene integration and expression. **2. Why Other Options are Wrong:** * **Option B:** Incorrect because both genes are functional light-emitting markers used specifically for visualization. * **Option C & D:** Incorrect because they assume only one mechanism (either fluorescence or bioluminescence) results in a visible glow, whereas both are established methods for creating "glowing" transgenic organisms. **3. NEET-PG Clinical Pearls & High-Yield Facts:** * **Reporter Genes:** GFP (Green Fluorescent Protein) and Luciferase are the two most common reporter genes used in molecular biology to study gene expression and promoter activity. * **GFP Source:** Originally isolated from the jellyfish *Aequorea victoria*. * **Luciferase Requirement:** Unlike GFP, Luciferase requires the addition of a substrate (**Luciferin**) and **ATP** to produce light. This makes it a sensitive marker for metabolic activity. * **Application:** In medicine, these techniques are used in **In-vivo Imaging Systems (IVIS)** to track tumor growth or the spread of infections in animal models.

Question 634: What is site specific recombination?

A. Palindromic sequences
B. Ser-form Holliday intermediate
C. Recombinase enzyme and ligase (Correct Answer)
D. Inversion in the same orientation or non-precise

Explanation: **Explanation:** **Site-specific recombination** is a type of genetic recombination where DNA strand exchange takes place between segments that possess at least a certain degree of sequence homology. Unlike homologous recombination, it occurs at specific, short DNA sequences recognized by specialized enzymes. **Why Option C is Correct:** The process is mediated by a specific class of enzymes called **site-specific recombinases** (e.g., Cre recombinase, Integrase). These enzymes function by breaking the DNA backbone at specific recognition sites and then rejoining the strands. This rejoining process requires **ligase** activity (often intrinsic to the recombinase mechanism) to seal the phosphodiester bonds, ensuring the integration or excision of DNA segments is complete and stable. **Analysis of Incorrect Options:** * **A. Palindromic sequences:** While palindromes are common in restriction enzyme sites and DNA protein-binding sites, site-specific recombination relies on specific **recombination sites** (like *att* sites in bacteriophages) which are not necessarily simple palindromes. * **B. Ser-form Holliday intermediate:** The Holliday junction is a hallmark of **homologous (general) recombination**, not typically the defining feature of the site-specific mechanism, which often involves a covalent protein-DNA intermediate. * **D. Inversion in the same orientation:** This is technically incorrect. If the recognition sites are in the **opposite** orientation, an inversion occurs; if they are in the **same** orientation, the segment is excised/deleted. **High-Yield Clinical Pearls for NEET-PG:** * **Phase Variation:** Bacteria use site-specific recombination to flip DNA segments (inversion), allowing them to switch surface antigens (e.g., *Salmonella* flagellar proteins) to evade the host immune system. * **V(D)J Recombination:** A specialized form of site-specific recombination mediated by **RAG-1 and RAG-2** enzymes, essential for generating antibody diversity in B and T cells. * **Lysogeny:** Bacteriophage lambda uses this mechanism to integrate its genome into the host *E. coli* chromosome.

Question 635: The gene for the Rh antigen is located on which chromosome?

A. 1 (Correct Answer)
B. 19
C. 4
D. 9

Explanation: **Explanation:** The Rh blood group system is the second most important system in transfusion medicine after ABO. It is encoded by two highly homologous genes: **RHD** (which determines Rh positivity) and **RHCE** (which determines C, c, E, and e antigens). Both of these genes are located on the **short arm of Chromosome 1 (1p36.11)**. **Analysis of Options:** * **A. Chromosome 1 (Correct):** This is the location of the *RHD* and *RHCE* genes. These genes are inherited as a linkage complex. * **B. Chromosome 19:** This chromosome carries the genes for the **Lewis (Le)** blood group and the **H substance** (FUT1 gene) and **Secretor** (FUT2 gene) status. * **C. Chromosome 4:** This is the location of the **MNS blood group** system (GYPA and GYPB genes). * **D. Chromosome 9:** This chromosome carries the **ABO gene** (at 9q34.2), which encodes the glycosyltransferases responsible for the A and B antigens. **High-Yield Clinical Pearls for NEET-PG:** 1. **Nature of Antigens:** Unlike ABO antigens (which are carbohydrates), Rh antigens are **transmembrane proteins**. 2. **Rh Null Phenotype:** A rare condition where individuals lack all Rh antigens; their RBCs show structural abnormalities (stomatocytosis) and mild hemolytic anemia. 3. **HDN:** Rh incompatibility is the most common cause of severe **Hemolytic Disease of the Newborn (HDN)**, typically occurring in the second pregnancy of an Rh-negative mother with an Rh-positive fetus. 4. **Inheritance:** Rh antigens are inherited in an autosomal dominant fashion. If a person has at least one *RHD* gene, they are Rh-positive.

Question 636: A 4-year-old female child complains of weakness and pain in joints. Her parents are known cases of sickle cell trait. Hemoglobin electrophoresis reveals an HbS pattern. This disease occurs because of which base alteration in DNA?

A. CTC → CAC (Correct Answer)
B. GAG → GUG
C. CTC → CAC in RNA
D. GAG → CAC in RNA

Explanation: ### Explanation **Concept:** Sickle Cell Anemia is caused by a **point mutation (missense mutation)** in the $\beta$-globin gene located on chromosome 11. Specifically, there is a substitution of **Glutamic acid** (polar) by **Valine** (non-polar) at the **6th position** of the $\beta$-polypeptide chain. **Why Option A is Correct:** The question specifically asks for the **base alteration in DNA**. * The normal DNA sequence (template strand) for the 6th codon is **CTC**, which codes for the mRNA codon **GAG** (Glutamic acid). * In Sickle Cell Anemia, the Thymine (T) in the DNA template is replaced by Adenine (A). * Therefore, the DNA sequence changes from **CTC to CAC**. **Analysis of Incorrect Options:** * **Option B (GAG → GUG):** This represents the change in **mRNA**, not DNA. While the mRNA does change from GAG to GUG, the question specifically asks for the DNA alteration. * **Option C & D:** These are incorrect because they describe alterations in RNA. Furthermore, DNA contains Thymine (T), while RNA contains Uracil (U); "CAC" is a possible RNA sequence, but the primary mutation occurs at the genomic DNA level. **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** Autosomal Recessive. * **Molecular Mechanism:** Substitution of a hydrophilic amino acid (Glutamate) with a hydrophobic one (Valine) creates a "sticky patch," leading to HbS polymerization under deoxygenated conditions. * **Diagnosis:** * **Screening:** Solubility test (Sodium dithionite). * **Confirmatory:** Hb Electrophoresis (HbS moves slowest towards the Anode due to loss of negative charge). * **Protective Effect:** Heterozygotes (Sickle cell trait) show resistance to *Plasmodium falciparum* malaria.

Question 637: Which of the following occurs in the chromosome by the action of restriction enzymes?

A. tRNA restriction
B. DNA restriction (Correct Answer)
C. mRNA restriction
D. rRNA restriction

Explanation: ### Explanation **1. Why DNA Restriction is the Correct Answer** Restriction enzymes, also known as **restriction endonucleases**, are "molecular scissors" that recognize specific palindromic sequences within double-stranded **DNA**. Their primary function is to cleave the phosphodiester bonds of the DNA backbone. In nature, bacteria use these enzymes as a defense mechanism to "restrict" or cut up the DNA of invading bacteriophages. In the context of human chromosomes (which are composed of DNA and proteins), these enzymes target the DNA molecule specifically. **2. Why Other Options are Incorrect** * **tRNA, mRNA, and rRNA (Options A, C, and D):** These are all types of **RNA**. Standard restriction endonucleases are specific to double-stranded DNA. They do not recognize or cleave single-stranded RNA molecules. While there are enzymes that degrade RNA (RNases), they are not classified as "restriction enzymes" in the classical sense used in molecular biology and genomic mapping. **3. High-Yield Clinical Pearls for NEET-PG** * **Recognition Sites:** Most restriction enzymes recognize **palindromic sequences** (e.g., 5'-GAATTC-3' read the same way on the complementary strand). * **Types of Ends:** They can produce **"sticky ends"** (staggered cuts, e.g., *EcoRI*) or **"blunt ends"** (straight cuts, e.g., *HpaI*). Sticky ends are preferred in recombinant DNA technology for easier ligation. * **RFLP (Restriction Fragment Length Polymorphism):** This is a clinical application where restriction enzymes are used to detect genetic variations or mutations (e.g., diagnosing Sickle Cell Anemia by the loss of a *MstII* restriction site). * **Nomenclature:** The first letter comes from the Genus (*Escherichia*), the next two from the species (*coli*), and the Roman numeral indicates the order of discovery (*EcoRI*).

Question 638: RNA polymerase recognizes which site on the DNA?

A. Promoter site (Correct Answer)
B. Initiation site
C. Regulator site
D. Stop site

Explanation: **Explanation:** **Why the Correct Answer is Right:** Transcription begins with the binding of **RNA polymerase** to a specific DNA sequence called the **Promoter site**. This site is located upstream (5') of the gene to be transcribed. The promoter serves as the "recognition signal" that tells RNA polymerase where to land, which strand to read, and in which direction to move. In prokaryotes, the **Sigma ($\sigma$) factor** is specifically responsible for recognizing the promoter (Pribnow box), while in eukaryotes, various transcription factors assist RNA polymerase in binding to sequences like the **TATA box**. **Analysis of Incorrect Options:** * **Initiation site:** This is the specific nucleotide (usually a purine) where the first RNA nucleotide is actually incorporated (the +1 site). While it is part of the promoter region, the enzyme *recognizes* the broader promoter sequence to position itself correctly at the initiation site. * **Regulator site:** These are DNA sequences (like enhancers or silencers) where regulatory proteins (activators or repressors) bind to modulate the frequency of transcription, rather than being the primary docking site for RNA polymerase. * **Stop site (Terminator):** This is the sequence that signals the RNA polymerase to detach from the DNA template and cease transcription. **High-Yield NEET-PG Pearls:** * **Pribnow Box (TATAAT):** The prokaryotic promoter located at -10 position. * **Hogness/TATA Box:** The eukaryotic promoter located at -25 position. * **Rifampicin:** A key clinical correlation; it inhibits the $\beta$-subunit of bacterial DNA-dependent RNA polymerase, preventing the initiation of transcription (used in Tuberculosis). * **$\alpha$-Amanitin:** Found in *Amanita phalloides* mushrooms; it specifically inhibits **RNA Polymerase II**, leading to severe liver failure.

Question 639: The short and long arms of chromosomes are designated respectively as:

A. p and q arms
B. m and q arms
C. q and p arms (Correct Answer)
D. l and s arms

Explanation: **Explanation:** In human cytogenetics, chromosomes are divided into two segments by the centromere. The designation of these arms is based on their relative length: 1. **p arm (Short arm):** The letter **'p'** stands for the French word ***petit***, meaning small. This refers to the shorter segment of the chromosome. 2. **q arm (Long arm):** The letter **'q'** was chosen simply because it follows 'p' in the alphabet. This refers to the longer segment of the chromosome. The question asks for the **short and long arms respectively**, making **p and q** the standard designation. (Note: If the options provided list "q and p" as the correct answer for "short and long," it is likely a typographical error in the source material, as 'p' is universally defined as short and 'q' as long). **Analysis of Options:** * **Option A (p and q arms):** This is the standard scientific nomenclature (p = petit/short, q = long). * **Option B (m and q arms):** 'm' is not a standard designation for a chromosome arm. * **Option C (q and p arms):** This reverses the standard nomenclature (claiming q is short and p is long), which is technically incorrect in standard genetics. * **Option D (l and s arms):** While 's' for short and 'l' for long might seem logical, they are not the terms used in the International System for Human Cytogenomic Nomenclature (ISCN). **High-Yield Clinical Pearls for NEET-PG:** * **Metacentric:** Centromere is in the middle; p and q arms are of nearly equal length. * **Submetacentric:** Centromere is offset; p arm is distinctly shorter than the q arm. * **Acrocentric:** Centromere is near one end. In humans, chromosomes **13, 14, 15, 21, and 22** are acrocentric and contain **satellites** on their p-arms which code for ribosomal RNA (rRNA). * **Telocentric:** Centromere is at the very tip (not found in humans). * **Karyotype Notation:** A notation like **5p-** (Cri-du-chat syndrome) indicates a deletion on the short arm of chromosome 5.

Question 640: Which statement regarding gene transcription in human cells is true?

A. Transcription always begins at an AUG codon, which codes for methionine.
B. Transcription does not require local unwinding of the two DNA strands.
C. Transcription reads the DNA template strand in the 3' to 5' direction. (Correct Answer)
D. Transcription occurs on only one of the two copies of each chromosome found in each cell.

Explanation: **Explanation:** **1. Why Option C is Correct:** In human cells (eukaryotes), RNA polymerase synthesizes RNA in the **5' to 3' direction**. Because the RNA strand must be antiparallel to its template, the enzyme must read the **DNA template strand in the 3' to 5' direction**. This is a fundamental principle of nucleic acid synthesis: the template is read 3'→5' so the new strand can grow 5'→3'. **2. Why the Other Options are Incorrect:** * **Option A:** Transcription begins at a **Promoter site** (e.g., TATA box), not the AUG codon. The AUG codon is the *translation* start site on mRNA where protein synthesis begins. * **Option B:** Transcription **requires** local unwinding of the DNA double helix to expose the template strand. This forms a "transcription bubble," facilitated by RNA polymerase and general transcription factors (like TFIIH, which has helicase activity). * **Option C:** Transcription occurs on **both** copies of homologous chromosomes (except in specific cases like X-inactivation or genomic imprinting). Most genes are expressed biallelically. **3. NEET-PG High-Yield Clinical Pearls:** * **RNA Polymerase II:** The specific enzyme responsible for synthesizing mRNA in eukaryotes. It is inhibited by **α-amanitin** (found in *Amanita phalloides* mushrooms), leading to severe liver failure. * **Promoter Regions:** The **TATA box** (Hogness box) is located at -25 bp and is crucial for the assembly of the transcription initiation complex. * **Post-transcriptional Modifications:** Unlike prokaryotes, eukaryotic primary transcripts (hnRNA) must undergo 5' capping, 3' polyadenylation, and splicing (removal of introns) before leaving the nucleus.

Practice by Chapter

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