Molecular Biology and Genomics Practice Questions

Q: Which of the following factors is responsible for deciding whether an antibody or immunoglobulin will remain membrane bound or get secreted?

Differential RNA Processing. **Explanation:** The decision of whether an immunoglobulin (Ig) is produced as a **membrane-bound receptor** (on B-cells) or a **secreted antibody** (by plasma cells) is determined by **Differential RNA Processing** (specifically, alternative polyadenylation and splicing). 1. **Why it is correct:** The heavy chain gene contains two potential polyadenylation sites. * If the first site is used, the mRNA excludes the hydrophobic transmembrane segment, resulting in a shorter protein that is **secreted**. * If the second site is used, the mRNA includes the hydrophobic C-terminal sequence that anchors the protein to the cell membrane. This process occurs *after* transcription but *before* translation, allowing a single B-cell to switch from a surface receptor to a secreted product without changing its DNA. 2. **Why other options are incorrect:** * **RNA Splicing (A):** While a component of processing, "Differential RNA Processing" is the more precise term as it encompasses the choice of polyadenylation sites which dictates the final splice form. * **Class Switching (B):** This involves DNA recombination to change the constant region (e.g., IgM to IgG). It changes the *isotype* and function, but not whether the Ig is membrane-bound or secreted. * **Allelic Exclusion (D):** This process ensures that a B-cell expresses an antigen receptor from only one of the two inherited alleles (parental or maternal), ensuring monoclonal specificity. **High-Yield Clinical Pearls for NEET-PG:** * **Membrane-bound Ig:** Found on naive B-cells (predominantly IgM and IgD). * **Secreted Ig:** Produced by terminal differentiation of B-cells into **Plasma cells**. * **Key Concept:** Somatic hypermutation and Class switching happen at the **DNA level**, whereas the choice between membrane-bound vs. secreted Ig happens at the **RNA level**.

Q: What is the function of single-strand binding proteins (SSBs) during DNA replication?

Prevents premature reannealing of strands. **Explanation:** **1. Why Option C is Correct:** During DNA replication, the enzyme **Helicase** unwinds the double helix, creating two single strands. These strands are naturally complementary and have a high affinity for one another. **Single-strand binding proteins (SSBs)** bind to these exposed strands to stabilize them and **prevent premature reannealing** (re-forming the double helix) or the formation of secondary structures like hairpins. This ensures the strands remain accessible as templates for DNA Polymerase. **2. Analysis of Incorrect Options:** * **Option A (Relieves torsional strain):** This is the function of **DNA Topoisomerases** (e.g., DNA Gyrase in prokaryotes). They create nicks in the DNA to resolve supercoiling caused by unwinding. * **Option B (Deoxynucleotide polymerization):** This is the primary catalytic function of **DNA Polymerase III** (in prokaryotes) or **DNA Polymerase $\delta$ and $\epsilon$** (in eukaryotes). * **Option D (Seals the nick):** This is the function of **DNA Ligase**, which catalyzes the formation of phosphodiester bonds between Okazaki fragments on the lagging strand. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Cooperativity:** SSB binding is "cooperative"—once one SSB molecule binds, it makes it easier for subsequent SSB molecules to bind to the adjacent DNA. * **Eukaryotic Equivalent:** In eukaryotes, the functional equivalent of the prokaryotic SSB is **Replication Protein A (RPA)**. * **Protection:** SSBs also protect the single-stranded DNA from being degraded by **nucleases** present in the cell. * **Drug Target:** Topoisomerase inhibitors (Option A) are clinically significant; for example, **Fluoroquinolones** inhibit bacterial DNA Gyrase, while **Etoposide/Teniposide** inhibit human Topoisomerase II.

Q: What does palindromic DNA imply?

All of the above. **Explanation:** **Palindromic DNA** refers to sequences of double-stranded DNA where the reading direction (5' to 3') is the same on both complementary strands. For example: 5'-GAATTC-3' 3'-CTTAAG-5' **Why Option D is Correct:** 1. **Short stretches of DNA:** Palindromic sequences are typically short, usually ranging from 4 to 8 base pairs in length. 2. **Recognized by specific restriction endonucleases:** This is the most significant functional feature. Restriction enzymes (molecular scissors) specifically bind to and cut DNA at these symmetrical palindromic sites. 3. **Codes for bacterial resistance:** In a clinical context, many antibiotic resistance genes (found on plasmids) are flanked by or contain palindromic sequences (transposons or "jumping genes"). These sequences allow the resistance genes to be excised and integrated into different parts of the bacterial genome or shared via horizontal gene transfer. **Analysis of Options:** * **Option A & B:** These are structural and functional definitions of palindromes in molecular biology. * **Option C:** While not all palindromes code for resistance, the mechanism of moving resistance genes (via transposons and integrons) relies heavily on these inverted repeat/palindromic sequences. Since A and B are definitively true, and C is true in the context of microbial genetics, "All of the above" is the most accurate choice. **High-Yield Clinical Pearls for NEET-PG:** * **Type II Restriction Endonucleases:** These are the most commonly used in recombinant DNA technology because they cut within the palindromic recognition site. * **Sticky vs. Blunt Ends:** Palindromic cuts can result in "sticky ends" (staggered cuts, e.g., *EcoRI*) or "blunt ends" (straight cuts, e.g., *SmaI*). * **Zinc Finger Motifs:** These are common DNA-binding proteins that often recognize palindromic sequences in the major groove of DNA.

Q: Steroids act as nuclear receptors which interact with DNA through which of the following structures?

Zinc finger motif. **Explanation:** **Mechanism of Steroid Hormone Action:** Steroid hormones (e.g., Glucocorticoids, Estrogen, Testosterone) are lipophilic and cross the cell membrane to bind with intracellular receptors. These receptors function as ligand-activated transcription factors. Once the hormone binds, the receptor-hormone complex undergoes a conformational change, translocates to the nucleus, and binds to specific DNA sequences called **Hormone Response Elements (HREs)**. The DNA-binding domain of these receptors contains a specific structural motif known as the **Zinc finger motif**. This motif consists of a zinc ion coordinated by cysteine and/or histidine residues, creating a "finger-like" projection that fits into the major groove of the DNA double helix to regulate gene expression. **Analysis of Incorrect Options:** * **Helix-turn-helix:** This is a common DNA-binding motif primarily found in **prokaryotic** transcription factors (e.g., lac repressor) and homeodomain proteins involved in development. * **Histidine:** While histidine can be a component of certain zinc fingers (Cys2His2), it is an amino acid, not a structural DNA-binding motif itself. * **Leucine zipper:** This motif is characterized by a leucine residue at every seventh position, forming an amphipathic helix. It is used for **protein dimerization** (e.g., AP-1, c-jun, and c-fos) rather than being the primary motif for steroid receptors. **High-Yield Clinical Pearls for NEET-PG:** * **Type I Receptors:** (Glucocorticoids, Mineralocorticoids, Sex steroids) are found in the cytoplasm bound to Heat Shock Proteins (HSP-90). * **Type II Receptors:** (Thyroid hormone, Vitamin D, Retinoic acid) are located constitutively in the nucleus. * **Zinc Finger Examples:** Steroid receptors, Vitamin D receptors, and Thyroid hormone receptors all utilize Zinc finger motifs. * **Mnemonic:** "Steroids use Fingers to touch DNA."

Q: Which of the following is NOT a circular DNA?

Nuclear DNA. **Explanation:** The shape of DNA is primarily determined by the organism's complexity and the cellular compartment in which it resides. In eukaryotes, the genetic material is organized differently across various organelles. **1. Why Nuclear DNA is the Correct Answer:** In humans and other eukaryotes, **Nuclear DNA** is **linear**, not circular. It is organized into multiple chromosomes, each consisting of a single long linear molecule of double-stranded DNA associated with histone proteins. These linear strands have distinct ends called **telomeres**, which are essential for maintaining genomic stability. **2. Analysis of Incorrect Options:** * **Plasmid DNA:** These are small, extrachromosomal, **circular** DNA molecules found predominantly in bacteria and some eukaryotes (like yeast). They are widely used in recombinant DNA technology. * **Mitochondrial DNA (mtDNA):** According to the endosymbiotic theory, mitochondria originated from prokaryotes. Consequently, mtDNA is **double-stranded and circular**, resembling bacterial DNA. It lacks histones. * **Yeast DNA:** While yeast (Saccharomyces cerevisiae) has linear nuclear chromosomes, it is unique among eukaryotes for naturally harboring the **2-micron circle**, a stable **circular plasmid**. Since the question asks which is *not* circular, and yeast contains significant circular DNA elements, Nuclear DNA remains the most definitive answer. **Clinical Pearls for NEET-PG:** * **Prokaryotes:** Generally possess a single, circular chromosome. * **Telomerase:** This enzyme is unique to linear DNA (Nuclear DNA) to prevent the "end-replication problem." Circular DNA does not require telomerase. * **Maternal Inheritance:** Mitochondrial DNA is inherited exclusively from the mother; mutations here lead to "Mitochondrial Myopathies" (e.g., MELAS, LHON). * **Histones:** Present in Nuclear DNA but absent in Mitochondrial and Plasmid DNA.

Q: Which of the following is true about xeroderma pigmentosa?

DNA repair defect. **Explanation:** **Xeroderma Pigmentosum (XP)** is a rare genetic disorder characterized by an extreme sensitivity to ultraviolet (UV) radiation. 1. **Why the correct answer is right (DNA repair defect):** The fundamental defect in XP is a deficiency in **Nucleotide Excision Repair (NER)**. Normally, UV light causes the formation of **pyrimidine dimers** (specifically thymine dimers) in DNA. The NER pathway utilizes specific endonucleases to excise these damaged segments. In XP patients, these enzymes are defective, leading to the accumulation of mutations, which results in skin cancers at a very young age. 2. **Why the incorrect options are wrong:** * **A. Autosomal dominant:** XP is an **Autosomal Recessive** disorder. Both parents must be carriers for the offspring to be affected. * **B. Good long-term prognosis:** The prognosis is generally **poor**. Patients have a 1000-fold increased risk of developing cutaneous malignancies (Basal Cell Carcinoma, Squamous Cell Carcinoma, and Melanoma) and often die young due to metastasis. * **C. Purine dimers:** UV radiation specifically causes **Pyrimidine dimers** (Thymine-Thymine or Cytosine-Thymine), not purine dimers. **High-Yield Clinical Pearls for NEET-PG:** * **Enzyme involved:** UV-specific endonuclease (also known as XP proteins). * **Clinical Presentation:** Severe sunburn after minimal sun exposure, "parchment-like" skin, excessive freckling, and photophobia. * **Neurological involvement:** Some variants (e.g., De Sanctis-Cacchione syndrome) present with progressive neurological degeneration and dwarfism. * **Management:** Strict UV protection (sunscreen, protective clothing) is the only management.

Q: A hypothetical gene contains a coding strand with the sequence 5'-TGGAATTGTATG-3'. What would be the sequence of the resultant mRNA following transcription?

5'-UGGAAUUGUAUG-3'. ### Explanation **1. Why the Correct Answer is Right:** In molecular biology, transcription involves synthesizing RNA from a DNA template. To understand the relationship between DNA and mRNA, one must distinguish between the two DNA strands: * **Coding Strand (Sense Strand):** This strand has the same sequence and polarity (5' to 3') as the resultant mRNA, with the sole exception that **Thymine (T)** in DNA is replaced by **Uracil (U)** in RNA. * **Template Strand (Antisense Strand):** This is the strand actually read by RNA polymerase (3' to 5') to create a complementary RNA molecule. Since the question provides the **Coding Strand (5'-TGGAATTGTATG-3')**, the mRNA sequence will be identical in sequence and direction, simply substituting 'T' for 'U'. Thus, the mRNA is **5'-UGGAAUUGUAUG-3'**. **2. Why Incorrect Options are Wrong:** * **Option A:** This represents the complementary sequence to the coding strand (the template strand) in DNA form, but in the wrong orientation. * **Option B:** This contains an incorrect sequence length and incorrect base substitutions. * **Option D:** This is a random sequence that does not follow the rules of complementarity or transcription. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Directionality:** RNA polymerase always synthesizes mRNA in the **5' → 3' direction**, reading the template strand in the **3' → 5' direction**. * **The "T to U" Rule:** In any exam question, if the "Coding Strand" is given, do not perform complementary base pairing; just swap T for U. If the "Template Strand" is given, you must find the complement. * **Promoter Region:** Transcription starts upstream of the coding sequence at the promoter site (e.g., TATA box in eukaryotes, Pribnow box in prokaryotes). * **Post-transcriptional modification:** In eukaryotes, the initial transcript (hnRNA) undergoes 5' capping, 3' polyadenylation, and splicing before becoming mature mRNA.

Question 1

Which of the following factors is responsible for deciding whether an antibody or immunoglobulin will remain membrane bound or get secreted?

Accepted Answer

Differential RNA Processing

Answer

RNA Splicing

Answer

Class Switching

Answer

Allelic Exclusion

Question 2

What is the function of single-strand binding proteins (SSBs) during DNA replication?

Accepted Answer

Prevents premature reannealing of strands

Answer

Relieves torsional strain

Answer

Deoxynucleotide polymerization

Answer

Seals the nick between Okazaki fragments

Question 3

Which of the following statements is true about protein translation?

Accepted Answer

Protein translation involves three main steps: initiation, elongation, and termination.

Answer

IF-2 (Initiation Factor 2) prevents the reassociation of ribosomal subunits.

Answer

IF-3 and 1A are involved in the binding of the initiating codon to the ribosome.

Answer

None of the above statements are true.

Question 4

What does palindromic DNA imply?

Accepted Answer

All of the above

Answer

Short stretches of DNA

Answer

Recognised by specific restriction endonuclease

Answer

Codes for bacterial resistance

Question 5

Which of the following functions is performed by RNA interference (RNAi) in gene regulation?

Accepted Answer

Gene expression reduction (knockdown)

Answer

Gene insertion (knock-in)

Answer

Gene deletion (knockout)

Answer

Gene expression enhancement (knock-up)

Question 6

Steroids act as nuclear receptors which interact with DNA through which of the following structures?

Accepted Answer

Zinc finger motif

Answer

Helix turn helix

Answer

Histidine

Answer

Leucine zipper

Question 7

Which of the following is NOT a circular DNA?

Accepted Answer

Nuclear DNA

Answer

Plasmid DNA

Answer

Mitochondrial DNA

Answer

Yeast DNA

Question 8

Which of the following is true about xeroderma pigmentosa?

Accepted Answer

DNA repair defect

Answer

Autosomal dominant

Answer

Good long-term prognosis

Answer

Purine dimers

Question 9

Which of the following is a strategy used to increase the yield of protein produced in recombinant protein synthesis?

Accepted Answer

Genes for protease inhibitors

Answer

Promoter induction

Answer

Translation initiation

Answer

Translation and transcription termination

Question 10

A hypothetical gene contains a coding strand with the sequence 5'-TGGAATTGTATG-3'. What would be the sequence of the resultant mRNA following transcription?

Accepted Answer

5'-UGGAAUUGUAUG-3'

Answer

3'-ACCTTAACATAC-5'

Answer

5'-ACCUUTTAACAUAC-3'

Answer

3'-UCCTTUUCAUAC-5'

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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