Molecular Biology and Genomics Practice Questions

Q: Which of the following represents a transition mutation?

A purine to a purine (e.g., A to G). **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.

Q: 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?

Both plants will glow. ### 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.

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

1. **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.

Q: 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?

CTC → CAC. ### 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.

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

DNA restriction. ### 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*).

Q: RNA polymerase recognizes which site on the DNA?

Promoter site. **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 1

Same amino acid is coded by multiple codons due to:

Accepted Answer

Degeneracy

Answer

Frame-shift mutation

Answer

Transcription

Answer

Mutation

Question 2

Which of the following represents a transition mutation?

Accepted Answer

A purine to a purine (e.g., A to G)

Answer

A purine to a pyrimidine (e.g., A to C)

Answer

A purine to a pyrimidine (e.g., A to T)

Answer

A pyrimidine to a purine (e.g., G to A)

Question 3

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?

Accepted Answer

Both plants will glow

Answer

Neither will glow

Answer

The first plant will glow

Answer

The second plant will glow

Question 4

What is site specific recombination?

Accepted Answer

Recombinase enzyme and ligase

Answer

Palindromic sequences

Answer

Ser-form Holliday intermediate

Answer

Inversion in the same orientation or non-precise

Question 5

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

Accepted Answer

1

Answer

19

Answer

4

Answer

9

Question 6

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?

Accepted Answer

CTC → CAC

Answer

GAG → GUG

Answer

CTC → CAC in RNA

Answer

GAG → CAC in RNA

Question 7

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

Accepted Answer

DNA restriction

Answer

tRNA restriction

Answer

mRNA restriction

Answer

rRNA restriction

Question 8

RNA polymerase recognizes which site on the DNA?

Accepted Answer

Promoter site

Answer

Initiation site

Answer

Regulator site

Answer

Stop site

Question 9

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

Accepted Answer

q and p arms

Answer

p and q arms

Answer

m and q arms

Answer

l and s arms

Question 10

Which statement regarding gene transcription in human cells is true?

Accepted Answer

Transcription reads the DNA template strand in the 3' to 5' direction.

Answer

Transcription always begins at an AUG codon, which codes for methionine.

Answer

Transcription does not require local unwinding of the two DNA strands.

Answer

Transcription occurs on only one of the two copies of each chromosome found in each cell.

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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