Molecular Biology and Genomics Practice Questions

Q: What is true about the coding strand of DNA?

Runs in the 5'-3' direction. **Explanation:** In molecular biology, DNA transcription involves two strands with distinct roles. The **coding strand** (also known as the **sense strand** or **plus (+) strand**) is the DNA strand whose base sequence corresponds directly to the sequence of the RNA transcript produced (with Thymine replaced by Uracil). **Why Option D is Correct:** By convention, the coding strand is always written in the **5' to 3' direction**. This matches the direction of the newly synthesized mRNA and the direction in which the ribosome reads the genetic code during translation. Because it "codes" for the protein sequence in the same orientation as the mRNA, it is designated as the 5'-3' strand. **Analysis of Incorrect Options:** * **Option A (Minus strand):** The coding strand is the **plus (+)** strand. The "minus" (-) strand refers to the template strand. * **Option B (Template strand):** The template strand (or antisense strand) is the one actually read by RNA polymerase to synthesize RNA via complementary base pairing. The coding strand is the non-template strand. * **Option C (Runs in the 3'-5' direction):** This describes the **template strand**. RNA polymerase moves along the template strand in a 3' to 5' direction so that it can synthesize the new RNA molecule in a 5' to 3' direction. **High-Yield Clinical Pearls for NEET-PG:** * **Directionality:** DNA synthesis (Replication), RNA synthesis (Transcription), and Protein synthesis (Translation) all occur in the **5' → 3' direction**. * **Sequence Identity:** The mRNA sequence is identical to the **coding strand** (except U for T) and complementary to the **template strand**. * **Promoters:** Regulatory sequences like the TATA box are described based on their position on the **coding strand**.

Q: What is an Okazaki fragment?

Synthesized by DNA polymerase delta in eukaryotes. **Explanation:** DNA replication is **semi-discontinuous**. While the leading strand is synthesized continuously, the **lagging strand** is synthesized in short, discontinuous segments known as **Okazaki fragments**. **Why Option B is correct:** In eukaryotes, DNA replication involves specific polymerases. While **Pol α** (alpha) initiates synthesis with an RNA primer, **DNA Polymerase δ (delta)** is the primary enzyme responsible for the elongation of the lagging strand (Okazaki fragments). It possesses high processivity and 3'→5' exonuclease activity for proofreading. **Analysis of Incorrect Options:** * **Option A:** Okazaki fragments are synthesized exclusively along the **lagging strand** (3' to 5' template direction) because DNA polymerase can only synthesize DNA in the 5' to 3' direction. * **Option C:** While Flap endonuclease I (FEN1) removes the RNA primers in eukaryotes, the actual "joining" or sealing of the nicks between fragments is performed by **DNA Ligase I**. * **Option D:** In prokaryotes (like *E. coli*), Okazaki fragments are synthesized by **DNA Polymerase III**. DNA Polymerase I is responsible for removing the RNA primer and filling the resulting gaps. **High-Yield Clinical Pearls for NEET-PG:** * **Polymerase Switching:** The transition from Pol α to Pol δ/ε is called polymerase switching. * **Eukaryotic Polymerases:** Remember **"α, δ, ε"**: * **α (Alpha):** Primase activity (starts the chain). * **δ (Delta):** Lagging strand synthesis. * **ε (Epsilon):** Leading strand synthesis. * **γ (Gamma):** Mitochondrial DNA replication. * **DNA Ligase:** Uses ATP in eukaryotes but NAD+ in some bacteria to form the phosphodiester bond.

Q: The disorder shown in the illustration is related to which DNA repair mechanism?

Nucleotide excision repair. ***Nucleotide excision repair*** - The illustration shows **Xeroderma Pigmentosum (XP)**, a disorder caused by defective **nucleotide excision repair (NER)** mechanism that removes **UV-induced DNA lesions** like pyrimidine dimers. - NER defects result in **extreme photosensitivity**, **premature aging**, and **high risk of skin cancers** due to inability to repair **bulky DNA adducts** caused by UV radiation. *Mismatch repair* - Defects in **mismatch repair** cause **Lynch syndrome** (hereditary nonpolyposis colorectal cancer), not the photosensitive skin disorder shown. - This mechanism corrects **base-base mismatches** and **insertion-deletion loops** during DNA replication, not UV-induced damage. *Base excision repair* - **Base excision repair (BER)** removes **small base modifications** like **8-oxoguanine** and **uracil** through glycosylases. - BER defects are not associated with **photosensitivity** or the characteristic skin manifestations seen in this illustration. *SOS repair* - **SOS repair** is an **error-prone DNA repair** system in bacteria activated under extreme DNA damage conditions. - This mechanism does not exist in **human cells** and is not related to any human genetic disorders or photosensitivity syndromes.

Q: The attachment of eukaryotic mRNA to the ribosome is mediated through which of the following?

Guanyl cap. **Explanation:** In eukaryotes, the initiation of translation begins with the recognition of the **5' Guanyl cap** (7-methylguanosine cap) by the eukaryotic initiation factor 4E (eIF4E). This cap-binding complex then recruits the 40S ribosomal subunit to the mRNA. This process is essential for the ribosome to scan the mRNA for the start codon (AUG). **Analysis of Options:** * **Guanyl cap (Correct):** It serves as the primary recognition signal for the ribosome in eukaryotes. It also protects mRNA from exonuclease degradation. * **Poly-A tail:** Located at the 3' end, it enhances stability and translation efficiency but is not the primary site for initial ribosomal attachment. * **tRNA:** These are adapter molecules that carry amino acids to the ribosome; they do not mediate the initial attachment of the mRNA strand to the ribosome itself. * **Shine-Dalgarno sequence:** This is a **prokaryotic** feature. It is a purine-rich sequence located upstream of the start codon that aligns the 16S rRNA of the 30S ribosomal subunit. Eukaryotes use the **Kozak consensus sequence** for a similar purpose, but the initial binding is cap-dependent. **Clinical Pearls for NEET-PG:** * **Kozak Sequence:** In eukaryotes, the ribosome identifies the correct AUG start codon by recognizing the Kozak sequence (5'-ACCAUGG-3'). * **Cap-Independent Translation:** Some viral and cellular mRNAs can bypass the guanyl cap requirement using an **IRES (Internal Ribosome Entry Site)**. * **eIF4F Complex:** This is the "cap-binding complex" consisting of eIF4E (binds cap), eIF4A (helicase), and eIF4G (scaffold). Overexpression of eIF4E is often linked to cancer progression.

Q: A mutation in a codon that causes a change in the coded amino acid is known as what?

Missense mutation. **Explanation:** **1. Why Missense Mutation is Correct:** A **missense mutation** is a type of point mutation where a single nucleotide change results in a codon that codes for a **different amino acid**. This can alter the tertiary structure and function of the resulting protein. A classic clinical example is **Sickle Cell Anemia**, where a point mutation (GAG → GTG) causes Glutamate to be replaced by Valine at the 6th position of the β-globin chain. **2. Why the Other Options are Incorrect:** * **Mitogenesis (A):** This refers to the induction of mitosis (cell division), typically by a mitogen. It is a physiological process, not a type of genetic mutation. * **Somatic Mutation (B):** This describes *where* a mutation occurs (in non-germline body cells) rather than the *effect* on the genetic code. Somatic mutations are not passed to offspring but can lead to cancer. * **Recombination (D):** This is the process by which DNA strands are broken and repaired to produce new combinations of alleles (e.g., crossing over during meiosis). It is a mechanism for genetic diversity, not a specific point mutation. **3. High-Yield Clinical Pearls for NEET-PG:** * **Silent Mutation:** A nucleotide change that codes for the *same* amino acid (due to the degeneracy of the genetic code). * **Nonsense Mutation:** A change that results in a **premature stop codon** (UAA, UAG, UGA), leading to a truncated, usually non-functional protein (e.g., β-thalassemia). * **Frameshift Mutation:** Insertion or deletion of nucleotides (not in multiples of three), altering the entire reading frame downstream (e.g., Duchenne Muscular Dystrophy). * **Transition vs. Transversion:** Transition is Purine to Purine (A↔G) or Pyrimidine to Pyrimidine (C↔T); Transversion is Purine to Pyrimidine or vice versa.

Q: All are steps of PCR EXCEPT?

Transformation. **Explanation:** The **Polymerase Chain Reaction (PCR)** is an *in vitro* enzymatic technique used to amplify specific DNA sequences. It operates through a repetitive thermal cycling process consisting of three fundamental steps: 1. **Denaturation (Option A):** The double-stranded DNA template is heated (typically to **94–96°C**) to break the hydrogen bonds between bases, resulting in two single strands. 2. **Annealing (Option B):** The temperature is lowered (typically **50–65°C**) to allow synthetic oligonucleotide primers to bind (hybridize) to their complementary sequences on the single-stranded DNA. 3. **Extension/Elongation (Option C):** The temperature is raised (typically **72°C**), and a thermostable DNA polymerase (e.g., **Taq polymerase**) synthesizes a new DNA strand by adding dNTPs to the 3' end of the primers. **Why Transformation is the Correct Answer:** **Transformation (Option D)** is a process used in *in vivo* molecular cloning where a host cell (usually a bacterium like *E. coli*) takes up foreign extracellular DNA from its environment. It is not a step in the PCR thermal cycle. **High-Yield Clinical Pearls for NEET-PG:** * **Taq Polymerase:** Derived from the bacterium *Thermus aquaticus*; it is heat-stable, which is essential for surviving the denaturation phase. * **RT-PCR:** Uses Reverse Transcriptase to convert RNA into cDNA before amplification (Gold standard for diagnosing **COVID-19**). * **Real-Time PCR (qPCR):** Allows for the quantification of DNA in real-time using fluorescent dyes (e.g., SYBR Green). * **Components required:** Template DNA, Primers, dNTPs, Taq Polymerase, and $Mg^{2+}$ ions (cofactor).

Q: Which cation is commonly used in PCR?

Magnesium. **Explanation:** **1. Why Magnesium ($Mg^{2+}$) is the Correct Answer:** Magnesium ions (usually added as $MgCl_2$) are an essential **cofactor** for **Taq DNA polymerase**. The enzyme requires divalent cations to function. $Mg^{2+}$ ions facilitate the formation of the phosphodiester bond by stabilizing the transition state of the enzyme-substrate complex and neutralizing the negative charges on the phosphate backbone of the DNA and the incoming dNTPs. This reduces electrostatic repulsion, allowing the primer to anneal to the template and the polymerase to catalyze DNA synthesis. **2. Why the Other Options are Incorrect:** * **Calcium ($Ca^{2+}$):** While a divalent cation, $Ca^{2+}$ actually inhibits DNA polymerase activity by competing with $Mg^{2+}$ for the binding site, leading to reduced efficiency. * **Lithium ($Li^+$) and Sodium ($Na^+$):** These are monovalent cations. While monovalent cations (like $K^+$ or $Na^+$) are sometimes used in PCR buffers to neutralize the DNA backbone and affect the melting temperature ($T_m$), they cannot replace the catalytic role of a divalent cofactor like Magnesium. **3. NEET-PG High-Yield Facts & Clinical Pearls:** * **Concentration Matters:** Too little $Mg^{2+}$ results in low yield or no product; too much $Mg^{2+}$ decreases the specificity of the reaction, leading to **non-specific amplification** (spurious bands). * **Chelating Agents:** If a sample contains **EDTA** (a chelator), it can sequester $Mg^{2+}$ and inhibit the PCR reaction. This is why DNA for PCR is often stored in low-EDTA buffers. * **Other Components:** Remember the "Ingredients" of PCR: Template DNA, Primers (forward and reverse), dNTPs, Taq Polymerase, and $Mg^{2+}$ buffer. * **Taq Polymerase:** Derived from the thermophilic bacterium *Thermus aquaticus*; it is heat-stable, which is crucial for the denaturation step ($94\text{--}96^\circ\text{C}$).

Question 1

What is true about the coding strand of DNA?

Accepted Answer

Runs in the 5'-3' direction

Answer

Minus strand

Answer

Template strand

Answer

Runs in the 3'-5' direction

Question 2

What is an Okazaki fragment?

Accepted Answer

Synthesized by DNA polymerase delta in eukaryotes

Answer

Synthesized along the leading strand

Answer

Joined by Flap endonuclease I

Answer

Synthesized by DNA polymerase I in prokaryotes

Question 3

Which of the following is true about DNA polymerase in eukaryotes?

Accepted Answer

Components include alpha, beta, TS, delta, and epsilon subunits.

Answer

Beta subunit is associated with DNA repair.

Answer

TS subunit is associated with DNA repair.

Answer

Delta subunit is associated with the synthesis of mitochondrial DNA.

Question 4

Senescent cells are deficient in what?

Accepted Answer

Telomerase

Answer

RNA polymerase

Answer

DNA polymerase

Answer

Helicase

Question 5

Aminoacyl t-RNA is required for all except?

Accepted Answer

Hydroxyproline

Answer

Methionine

Answer

Cystine

Answer

Lysine

Question 6

The disorder shown in the illustration is related to which DNA repair mechanism?

Accepted Answer

Nucleotide excision repair

Answer

Mismatch repair

Answer

Base excision repair

Answer

SOS repair

Question 7

The attachment of eukaryotic mRNA to the ribosome is mediated through which of the following?

Accepted Answer

Guanyl cap

Answer

Poly-A tail

Answer

tRNA

Answer

Shine-Dalgarno sequence

Question 8

A mutation in a codon that causes a change in the coded amino acid is known as what?

Accepted Answer

Missense mutation

Answer

Mitogenesis

Answer

Somatic mutation

Answer

Recombination

Question 9

All are steps of PCR EXCEPT?

Accepted Answer

Transformation

Answer

Denaturation

Answer

Annealing

Answer

Extension

Question 10

Which cation is commonly used in PCR?

Accepted Answer

Magnesium

Answer

Calcium

Answer

Lithium

Answer

Sodium

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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