Molecular Biology and Genomics Practice Questions

Q: In DNA replication, which enzyme synthesizes RNA primers?

Primase. ### ***Primase*** - **Primase** is an **RNA polymerase** that synthesizes short **RNA primers** (approximately 10-12 nucleotides long) to provide a free **3'-OH group** needed by **DNA polymerase** to initiate synthesis. - Without this primer, **DNA replication** cannot begin because **DNA polymerases** lack the ability to start synthesizing a new strand *de novo*. *Topoisomerase* - **Topoisomerases** (such as **DNA gyrase**) are responsible for relieving **supercoiling** and **torsional strain** in the DNA helix ahead of the replication fork. - They do not possess **primase** activity and do not participate in synthesizing the RNA primer. *DNA polymerase II* - **DNA polymerase II** is primarily involved in **DNA repair** mechanisms, specifically in replicating through damaged DNA segments. - It lacks the catalytic ability to synthesize RNA primers and is not a replication initiator. *DNA polymerase III* - **DNA polymerase III** is the primary enzyme responsible for the **elongation** of the leading and lagging strands, adding nucleotides to the existing primer. - While it is essential for synthesizing the bulk of the new DNA, it requires a preexisting primer to function and cannot generate one itself.

Q: Which molecular biotechnology is depicted in the image shown below:

CRISPR-Cas9 gene editing system. ***CRISPR-Cas9 gene editing system*** - This system utilizes a **guide RNA (gRNA)** to direct the **Cas9 endonuclease** to a specific complementary DNA sequence, creating a double-strand break. - It is a highly versatile and precise tool used for **genome editing**, enabling gene knockouts, insertions, or corrections by leveraging cellular repair mechanisms like **non-homologous end joining (NHEJ)** or **homology-directed repair (HDR)**. *Microarray analysis* - This technique involves hybridizing fluorescently labeled cDNA or cRNA samples to a chip containing thousands of **immobilized DNA probes**. - It is primarily used for **gene expression profiling** or genotyping, rather than inducing specific genetic alterations. *In situ hybridization* - This method uses labeled **DNA or RNA probes** to detect the presence and location of specific nucleic acid sequences within intact cells or tissues. - It is used for **diagnostic imaging of gene expression** or chromosomal abnormalities and does not involve gene editing activity. *DNA sequencing* - This process determines the exact **nucleotide order** of a DNA fragment, such as via the **Sanger method** or **next-generation sequencing (NGS)**. - It is a purely observational tool for reading genetic information and lacks the capacity to physically modify or cut genomic DNA.

Q: Which enzyme is responsible for unwinding the DNA double helix during replication?

Helicase. ***Helicase*** - **Helicase** is the specific enzyme responsible for **unwinding** the DNA double helix by breaking **hydrogen bonds** between complementary nitrogenous bases. - This action creates the **replication fork**, providing the single-stranded templates necessary for the replication process to proceed. *DNA polymerase* - Its primary role is the **synthesis** of new DNA strands by adding **deoxyribonucleotides** to a pre-existing primer. - It possesses **proofreading** capabilities but cannot physically separate the two parent strands of the DNA molecule. *Topoisomerase* - This enzyme functions to relieve **torsional strain** and prevent **supercoiling** of the DNA ahead of the replication fork. - It works by creating transient **single or double-stranded breaks** in the DNA backbone rather than unwinding the helix for template access. *DNA ligase* - Its main function is to join **Okazaki fragments** on the lagging strand by catalyzing the formation of **phosphodiester bonds**. - It acts as a molecular glue during the final stages of replication and **DNA repair**, not during the initial unwinding phase.

Q: Which type of RNA carries amino acids to the ribosome during protein synthesis?

Transfer RNA (tRNA). ***Transfer RNA (tRNA)*** - **tRNA** acts as an adapter molecule that matches specific **amino acids** to their corresponding **codons** on the mRNA template. - It contains an **anticodon loop** that base-pairs with mRNA and a 3' end that covalently binds the amino acid via **aminoacyl-tRNA synthetase**. *Ribosomal RNA (rRNA)* - **rRNA** is a structural and catalytic component of the **ribosome**, providing the scaffold for protein assembly. - It facilitates the formation of **peptide bonds** through its **peptidyl transferase** activity but does not transport amino acids. *Small nuclear RNA (snRNA)* - **snRNA** is primarily involved in the **splicing** of pre-messenger RNA within the nucleus. - It combines with proteins to form **snRNPs** (spliceosomes) and is not directly involved in the translation process at the ribosome. *Messenger RNA (mRNA)* - **mRNA** serves as the genetic blueprint that carries the **coding sequence** from DNA to the ribosome for translation. - It provides the **template** of codons that dictates the specific sequence of amino acids rather than transporting them.

Q: Which of the following is an example of a reverse transcriptase?

Telomerase. **Explanation:** The correct answer is **Telomerase**. **Why Telomerase is a Reverse Transcriptase:** Reverse transcriptase is an enzyme that synthesizes DNA using an RNA template (RNA-dependent DNA polymerase). Telomerase is a specialized ribonucleoprotein complex responsible for maintaining the length of telomeres (the repetitive TTAGGG sequences at the ends of chromosomes). It contains an intrinsic RNA molecule that serves as a template to synthesize telomeric DNA, thereby preventing the "end-replication problem" and chromosomal shortening during cell division. **Analysis of Incorrect Options:** * **Gyrase (Topoisomerase II):** This enzyme relieves torsional strain (supercoiling) ahead of the replication fork by creating double-stranded breaks in DNA. * **Helicase:** This enzyme uses ATP to unwind the DNA double helix into single strands by breaking hydrogen bonds between nitrogenous bases. * **RNA Polymerase:** This enzyme performs transcription, synthesizing RNA from a DNA template (DNA-dependent RNA polymerase). **High-Yield Clinical Pearls for NEET-PG:** * **Cancer & Aging:** Telomerase activity is high in germ cells, stem cells, and **cancer cells** (conferring immortality), but low or absent in most somatic cells, leading to cellular senescence. * **Other Reverse Transcriptases:** Retroviruses like **HIV** utilize reverse transcriptase to integrate their viral genome into the host DNA. * **Zidovudine (AZT):** A common NEET-PG topic; it is a drug that inhibits reverse transcriptase, used in the treatment of HIV. * **Telomere Sequence:** In humans, the repeating hexanucleotide sequence is **5'-TTAGGG-3'**.

Q: Transcription is the synthesis of:

A single-stranded complementary copy of DNA. **Explanation:** Transcription is the fundamental process of gene expression where the genetic information stored in **DNA** is copied into **RNA**. **Why Option A is Correct:** During transcription, the enzyme **RNA polymerase** reads the template strand of DNA (3' to 5') and synthesizes a **single-stranded** RNA molecule in the 5' to 3' direction. This RNA transcript is **complementary** to the DNA template strand (with Uracil replacing Thymine) and identical in sequence to the coding (non-template) strand. **Why Other Options are Incorrect:** * **Option B:** DNA is double-stranded, but the resulting RNA transcript is always single-stranded. Double-stranded DNA synthesis occurs during **Replication**, not transcription. * **Option C:** Synthesis of a complementary copy of RNA is characteristic of **RNA-dependent RNA replication** (seen in certain RNA viruses) or **Reverse Transcription** (where DNA is made from RNA). * **Option D:** While rRNA is indeed produced via transcription, it is only one specific type of RNA. Transcription encompasses the synthesis of **all** RNA types, including mRNA and tRNA; therefore, "a complementary copy of DNA" is the more accurate general definition. **High-Yield Clinical Pearls for NEET-PG:** * **Directionality:** Transcription always proceeds in the **5' → 3' direction**. * **Enzymes (Eukaryotes):** * **RNA Pol I:** Nucleolus; synthesizes 28S, 18S, and 5.8S **rRNA**. * **RNA Pol II:** Nucleoplasm; synthesizes **mRNA** and snRNA (Inhibited by **α-amanitin** from *Amanita phalloides* mushrooms). * **RNA Pol III:** Nucleoplasm; synthesizes **tRNA** and 5S rRNA. * **Prokaryotes:** A single RNA polymerase (multimeric enzyme) synthesizes all types of RNA. **Rifampicin** inhibits the β-subunit of this bacterial RNA polymerase.

Q: A child develops skin tumors with blisters upon exposure to sunlight. Irregular dark spots on the skin were also found. The child most likely has a defect in which of the following DNA repair mechanisms?

Thymidine dimer repair. ### Explanation The clinical presentation of **photosensitivity, blistering, and irregular hyperpigmented spots** (poikiloderma) upon sun exposure is classic for **Xeroderma Pigmentosum (XP)**. **1. Why Thymidine Dimer Repair is Correct:** Ultraviolet (UV) radiation causes the formation of **pyrimidine dimers** (specifically thymidine dimers), which create kinks in the DNA strand. In healthy individuals, these are repaired by the **Nucleotide Excision Repair (NER)** pathway. XP is caused by an autosomal recessive deficiency in the **UV-specific endonuclease** (or other NER proteins), leading to an inability to excise these dimers. This accumulation of mutations leads to early-onset skin cancers (Basal Cell Carcinoma, Squamous Cell Carcinoma, and Melanoma). **2. Why the Other Options are Incorrect:** * **Base Excision Repair (BER):** Repairs "small" lesions like deaminated bases (e.g., Cytosine to Uracil) or oxidized bases using Glycosylases. It is not the primary mechanism for UV-induced bulky adducts. * **Mismatch Repair (MMR):** Corrects errors that escape proofreading during DNA replication (e.g., C-A mismatches). Defects in MMR lead to **Lynch Syndrome** (HNPCC). * **Double-Strand Break Repair:** Involves Non-Homologous End Joining (NHEJ) or Homologous Recombination. Defects in these pathways lead to **Ataxia-Telangiectasia** or **BRCA1/2** related cancers. **Clinical Pearls for NEET-PG:** * **Key Enzyme Defect:** UV-specific endonuclease (NER pathway). * **Hallmark:** 1000-fold increased risk of skin cancer; "Children of the Night" (must avoid all sunlight). * **Differential:** If the question mentions "premature aging" and "bird-like facies" without a high cancer risk, think **Cockayne Syndrome** (also an NER defect).

Question 1

In DNA replication, which enzyme synthesizes RNA primers?

Accepted Answer

Primase

Answer

Topoisomerase

Answer

DNA polymerase II

Answer

DNA polymerase III

Question 2

Which molecular biotechnology is depicted in the image shown below:

Accepted Answer

CRISPR-Cas9 gene editing system

Answer

Microarray analysis

Answer

In situ hybridization

Answer

DNA sequencing

Question 3

A research team is developing a gene therapy approach using CRISPR-Cas9 to correct a point mutation causing sickle cell disease. They must decide between two strategies: (A) correcting the mutation in hematopoietic stem cells ex vivo, or (B) in vivo correction in bone marrow. Considering molecular physiology principles, what is the most significant advantage of strategy A over strategy B?

Accepted Answer

Strategy A allows for screening and selection of successfully edited cells before transplantation, minimizing off-target effects

Answer

Strategy A requires lower doses of viral vectors

Answer

Strategy A produces faster clinical improvement

Answer

Strategy A is less expensive to implement

Question 4

How does alternative splicing contribute to protein diversity in humans despite having approximately 20,000 genes?

Accepted Answer

By allowing different combinations of exons to be included in mature mRNA

Answer

By modifying amino acids after translation

Answer

By changing the sequence of codons in the mRNA

Answer

By producing different tRNA molecules

Question 5

Why does DNA replication occur in a 5' to 3' direction only?

Accepted Answer

Because DNA polymerase can only add nucleotides to the 3'-OH group

Answer

Because primase can only synthesize primers in that direction

Answer

Because the template strand is read in 3' to 5' direction only

Answer

Because helicase unwinds DNA in only one direction

Question 6

Which enzyme is responsible for unwinding the DNA double helix during replication?

Accepted Answer

Helicase

Answer

DNA polymerase

Answer

Topoisomerase

Answer

DNA ligase

Question 7

Which type of RNA carries amino acids to the ribosome during protein synthesis?

Accepted Answer

Transfer RNA (tRNA)

Answer

Ribosomal RNA (rRNA)

Answer

Small nuclear RNA (snRNA)

Answer

Messenger RNA (mRNA)

Question 8

Which of the following is an example of a reverse transcriptase?

Accepted Answer

Telomerase

Answer

Gyrase

Answer

Helicase

Answer

RNA polymerase

Question 9

Transcription is the synthesis of:

Accepted Answer

A single-stranded complementary copy of DNA

Answer

A double-stranded complementary copy of DNA

Answer

A complementary copy of RNA

Answer

A complementary copy of rRNA

Question 10

A child develops skin tumors with blisters upon exposure to sunlight. Irregular dark spots on the skin were also found. The child most likely has a defect in which of the following DNA repair mechanisms?

Accepted Answer

Thymidine dimer repair

Answer

Base excision repair

Answer

Mismatch repair

Answer

Double-strand break repair

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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