Molecular Biology and Genomics Practice Questions

Q: In DNA replication, which enzyme fills gaps left by RNA primer removal?

DNA polymerase I. ***DNA polymerase I*** - This enzyme possesses **5' to 3' exonuclease activity** which is used to remove the RNA primers - It also has **5' to 3' polymerase activity** to synthesize DNA and fill the gaps left by primer removal - This dual function makes it uniquely suited for **"nick translation"** - the process of removing primers and filling gaps in Okazaki fragments *Primase* - **Synthesizes short RNA primers** (8-12 nucleotides) that provide a free 3'-OH group for DNA polymerase to initiate DNA synthesis - It does not fill gaps or remove primers during replication - Functions only at the initiation phase of DNA synthesis *DNA polymerase III* - The **primary replicative enzyme** responsible for synthesizing the bulk of new DNA in both the leading and lagging strands - It lacks the 5' to 3' exonuclease activity needed for primer removal and gap filling - Extends RNA primers but cannot remove them *Helicase* - Responsible for **unwinding the DNA double helix** at the replication fork by breaking hydrogen bonds between complementary base pairs - Creates the replication bubble but does not participate in primer processing or gap filling - Works ahead of the DNA polymerases

Q: Which enzyme unwinds DNA during replication?

Helicase. ***Helicase*** - **Helicase** is the enzyme responsible for **unwinding the double helix** of DNA by breaking the hydrogen bonds between complementary base pairs. - This action creates the **replication fork**, allowing other enzymes to access the single-stranded DNA templates. *Ligase* - **Ligase** is involved in joining DNA fragments, particularly **Okazaki fragments** on the lagging strand, by forming phosphodiester bonds. - It does not participate in the initial unwinding of the DNA helix. *Topoisomerase* - **Topoisomerase** enzymes relieve the **supercoiling** tension that builds up ahead of the replication fork as DNA unwinds. - While it's crucial for replication, it doesn't directly unwind the double helix itself; rather, it handles the topological stress. *Polymerase* - **DNA Polymerase** is responsible for synthesizing new DNA strands by adding nucleotides complementary to the template strand. - It plays a role in extending the DNA chain, not in the initial unwinding process.

Q: Which RNA modification at the 3' end stabilizes mRNA by preventing exonuclease degradation?

Polyadenylation. ***Polyadenylation*** - The addition of a **poly-A tail** (a long chain of adenine nucleotides) to the **3' end** of mRNA is the primary modification that protects it from degradation by **3' to 5' exonucleases**. - This tail also plays crucial roles in mRNA export from the nucleus, translation initiation, and determining mRNA half-life. - The poly-A tail is progressively shortened by deadenylases, and once critically shortened, the mRNA becomes susceptible to degradation. *Capping* - **5' capping** involves the addition of a **7-methylguanosine cap** to the **5' end** of mRNA. - While this protects mRNA from **5' to 3' exonuclease** degradation, the question specifically asks about the **3' end** modification, which is polyadenylation. - The cap structure is also essential for ribosome binding and translation initiation. *Splicing* - **Splicing** is the process of removing **introns** (non-coding regions) and joining **exons** (coding regions) in pre-mRNA. - Its main function is to produce a mature mRNA sequence that can be translated into a functional protein, not to directly prevent exonuclease degradation. *Methylation* - **Methylation** can occur on various nucleotides within RNA molecules (e.g., N6-methyladenosine or m6A). - While methylation can influence mRNA stability, translation efficiency, and splicing, it is primarily a regulatory modification rather than a direct structural protection against exonuclease degradation like polyadenylation.

Q: DNA Methylation is not related to?

Capping. ***Capping*** - **Capping** is a modification of messenger RNA (mRNA) that occurs during **mRNA processing** in eukaryotes, involving the addition of a 7-methylguanosine cap to the 5' end of the mRNA molecule. - This process is crucial for mRNA stability, translation initiation, and nuclear export, and is entirely **independent of DNA modifications** like DNA methylation. *DNA Replication* - DNA methylation plays a role in **DNA replication** to distinguish newly synthesized strands from parental strands during **DNA repair**. - In bacteria, methylation at specific sites (**Dam methylase**) helps in **mismatch repair** by identifying the parental strand. *Gene silencing* - **DNA methylation** of CpG islands in promoter regions is a well-established mechanism for **gene silencing** by altering chromatin structure and preventing transcription factor binding. - This epigenetic modification leads to stable transcriptional repression and is critical for processes like X-chromosome inactivation and genomic imprinting. *Mismatch repair* - In prokaryotes, **DNA methylation** marks the parental strand, which is used by the **mismatch repair system** to correct errors on the newly synthesized, unmethylated strand. - In eukaryotes, while not directly marking strands, DNA methylation can influence the efficiency of mismatch repair pathways by altering chromatin accessibility.

Q: Which type of mutation can act as a suppressor to restore the wild-type phenotype in organisms carrying a mutant gene?

Mutation of tRNA. ***Mutation of tRNA*** - A **tRNA suppressor mutation** can alter its anticodon, allowing it to recognize a **stop codon** (nonsense suppressor) or a missense codon, and insert an amino acid, thereby suppressing the original mutation. - This is a classic example of an **intergenic suppressor mutation** that acts at a different genetic locus from the original mutation. - These suppressors are particularly effective for **nonsense mutations** (premature stop codons) and certain missense mutations by correcting the decoding error during translation. *Frameshift mutation of coding gene* - A single frameshift mutation causes a shift in the **reading frame**, leading to a completely different protein sequence downstream and often a premature stop codon, which would worsen the phenotype. - While a **second compensating frameshift** mutation in the same gene could theoretically restore the reading frame (acting as an intragenic suppressor), this is context-dependent and less reliable than tRNA suppressors. - The question asks for mutations that "can act as a suppressor," and **tRNA mutations are the more universally recognized and reliable suppressor mechanism** in classical genetics. *Deletion of mutant gene* - **Deleting the mutant gene** removes the genetic information entirely but does not restore wild-type function; instead, it typically results in **loss of function** or complete absence of the protein. - This would lead to a **null phenotype** rather than restoration of wild-type phenotype, especially if the gene is essential. *Addition of another normal gene* - The **addition of another normal (wild-type) gene copy** provides a functional protein that can compensate for the mutant gene's deficiency. - While this can restore a wild-type phenotype, it represents **gene complementation** or gene therapy, not a true suppressor mutation that modifies the interpretation or expression of the existing mutant allele.

Q: Which of the following is not associated with post-transcription modification?

Glycosylation. ***Glycosylation*** - **Glycosylation** is a type of post-translational modification, which involves the enzymatic addition of carbohydrates to proteins or lipids, not RNA. - This process is crucial for protein folding, stability, and function in the cell, occurring after translation has been completed. *5' capping* - **5' capping** is a crucial post-transcriptional modification of eukaryotic pre-mRNA, involving the addition of a 7-methylguanosine cap to the 5' end. - This cap protects the mRNA from degradation, facilitates nuclear export, and is essential for translation initiation. *Methylation* - **Methylation** can occur as a post-transcriptional modification, affecting various RNA types including tRNA, rRNA, and mRNA. - For mRNA, internal methylation, particularly of adenosine residues (m6A), plays a role in mRNA stability, splicing, and translation regulation. *Endonuclease cleavage* - **Endonuclease cleavage** is a significant post-transcriptional modification, particularly in the maturation of rRNA and tRNA, where larger precursor molecules are cut into functional smaller units. - In mRNA processing, endonuclease cleavage is involved in the formation of the 3' end, signaling for the addition of the poly-A tail.

Q: Which molecular biology technique is used to amplify a specific DNA sequence and is essential for diagnosing infections by detecting pathogen DNA in clinical samples?

Polymerase chain reaction (PCR). ***Polymerase chain reaction (PCR)*** - PCR is a molecular biology technique that **amplifies specific DNA sequences** exponentially, allowing for the detection of even tiny amounts of pathogen DNA. - This high sensitivity and specificity make it an essential tool for **diagnosing infections** by molecular identification of pathogens. *Western blotting* - **Western blotting** is used to detect specific **proteins**, not DNA, in a sample. - It's mainly used for confirming the presence of antibodies to a pathogen or detecting specific pathogen proteins, not for DNA amplification. *ELISA* - **ELISA** (Enzyme-Linked Immunosorbent Assay) is an immunological assay used to detect and quantify **antigens** or **antibodies** in a sample. - While used in infection diagnosis, it does not involve the amplification of DNA sequences. *Flow cytometry* - **Flow cytometry** is a technique used to analyze the **physical and chemical characteristics of cells** or particles suspended in a fluid as they pass through a laser beam. - It is primarily used for **cell counting** and sorting, and does not amplify DNA.

Q: What are the potential implications of using CRISPR-Cas9 technology to correct mutations in the CFTR gene for the treatment of cystic fibrosis?

Ability to provide a permanent correction of CFTR mutations. ***Ability to provide a permanent correction of CFTR mutations*** - CRISPR-Cas9 directly edits the patient's own DNA, offering the potential for a **one-time, permanent genetic correction** of the underlying CFTR defect. - This contrasts with traditional therapies that only manage symptoms, as it targets the **root cause of cystic fibrosis**. *Possibility of off-target effects during gene editing* - **Off-target edits** occur when CRISPR-Cas9 cuts DNA at unintended sites, potentially leading to harmful mutations or cellular dysfunction. - These unintended edits are a significant safety concern and a major focus of ongoing research to improve the **specificity of gene editing tools**. *Limited effectiveness in reaching lung tissue* - Delivering CRISPR-Cas9 components effectively and safely to a sufficient number of cells in the **lung tissue**, particularly in the presence of mucus characteristic of CF, remains a significant challenge. - The mode of delivery (e.g., viral vectors, nanoparticles) needs to overcome these barriers to ensure the therapeutic agent reaches its target without triggering an excessive immune response. *Elimination of all risks associated with viral vector delivery* - This is **incorrect** as CRISPR-Cas9 therapy often utilizes viral vectors (AAV) for delivery, which carry inherent risks including **immunogenicity, limited cargo capacity, and potential insertional mutagenesis**. - While some non-viral delivery methods are being explored, the technology does **not eliminate delivery-associated risks**, making this statement false.

Question 1

Assertion: DNA methylation leads to gene silencing. Reason: Methylation prevents binding of transcription factors to promoter regions.

Accepted Answer

Both assertion and reason are true and reason is the correct explanation.

Answer

Assertion is true but reason is false.

Answer

Both assertion and reason are true but reason is not the correct explanation.

Answer

Both assertion and reason are false.

Question 2

In DNA replication, which enzyme fills gaps left by RNA primer removal?

Accepted Answer

DNA polymerase I

Answer

Primase

Answer

DNA polymerase III

Answer

Helicase

Question 3

Which enzyme unwinds DNA during replication?

Accepted Answer

Helicase

Answer

Ligase

Answer

Topoisomerase

Answer

Polymerase

Question 4

Which RNA modification at the 3' end stabilizes mRNA by preventing exonuclease degradation?

Accepted Answer

Polyadenylation

Answer

Capping

Answer

Splicing

Answer

Methylation

Question 5

DNA Methylation is not related to?

Accepted Answer

Capping

Answer

DNA Replication

Answer

Gene silencing

Answer

Mismatch repair

Question 6

Which type of mutation can act as a suppressor to restore the wild-type phenotype in organisms carrying a mutant gene?

Accepted Answer

Mutation of tRNA

Answer

Frameshift mutation of coding gene

Answer

Deletion of mutant gene

Answer

Addition of another normal gene

Question 7

Which of the following is not associated with post-transcription modification?

Accepted Answer

Glycosylation

Answer

5' capping

Answer

Methylation

Answer

Endonuclease cleavage

Question 8

How does the defective nucleotide excision repair (NER) mechanism in Xeroderma pigmentosum (XP) patients contribute to their increased risk of skin cancer?

Accepted Answer

Failure to repair UV-induced DNA damage leading to mutations.

Answer

Increased oxidative DNA damage resulting in breaks.

Answer

Impaired mismatch repair causing replication errors.

Answer

Defective base excision repair leading to abasic sites.

Question 9

Which molecular biology technique is used to amplify a specific DNA sequence and is essential for diagnosing infections by detecting pathogen DNA in clinical samples?

Accepted Answer

Polymerase chain reaction (PCR)

Answer

Western blotting

Answer

ELISA

Answer

Flow cytometry

Question 10

What are the potential implications of using CRISPR-Cas9 technology to correct mutations in the CFTR gene for the treatment of cystic fibrosis?

Accepted Answer

Ability to provide a permanent correction of CFTR mutations

Answer

Elimination of all risks associated with viral vector delivery

Answer

Possibility of off-target effects during gene editing

Answer

Limited effectiveness in reaching lung tissue

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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