Molecular Biology and Genomics Practice Questions

Q: Which of the following statements is true regarding DNA polymerases?

DNA polymerase I has significant repair activity.. ***DNA polymerase I has significant repair activity*** - DNA polymerase I plays a crucial role in **DNA repair**, including **excising RNA primers** and filling in the resulting gaps during replication. - Its **5' to 3' exonuclease activity** allows it to remove nucleotides ahead of its synthetic work, which is essential for correcting damage and removing primers. - This unique combination of activities makes DNA Pol I particularly important in **DNA repair mechanisms**. *Leading strand is synthesized by DNA polymerase I* - The **leading strand** in prokaryotes is primarily synthesized by **DNA polymerase III**, not DNA polymerase I. - DNA polymerase I is mainly involved in **primer removal** and filling gaps, not the bulk synthesis of new DNA strands. *Okazaki fragments are synthesized by DNA polymerase I* - **Okazaki fragments** are synthesized by **DNA polymerase III** in prokaryotes. - DNA polymerase I then replaces the RNA primers with DNA nucleotides within these fragments. *Only DNA polymerase III has proofreading activity* - This is **incorrect** - **DNA polymerases I, II, and III** all have **3' to 5' exonuclease proofreading activity**. - This proofreading function allows these enzymes to correct errors during DNA synthesis by removing incorrectly paired nucleotides.

Q: Which of the following statements about gene therapy is false?

Gene therapy is only used for genetic disorders.. ***Gene therapy is only used for genetic disorders.*** - This statement is **false** because gene therapy has applications beyond just genetic disorders. It is also being explored and used in the treatment of acquired diseases such as **cancer** and **infectious diseases**. - While it's a prominent approach for correcting genetic defects, its scope is much broader, involving the introduction or modification of genes to achieve a therapeutic effect in various conditions. *Gene also considered as drug* - This statement is **true**. Gene therapy products are often regulated as **drugs** or **biological products** by regulatory bodies like the FDA. - This is because they involve the delivery of genetic material that acts to modify gene expression or cell function to produce a therapeutic effect, similar to how traditional drugs work. *Has been tried in cystic fibrosis* - This statement is **true**. Gene therapy has been extensively investigated as a potential treatment for **cystic fibrosis (CF)**. - CF is caused by mutations in the **CFTR gene**, and researchers have attempted to deliver functional copies of this gene to the affected cells, particularly in the lungs, to correct the underlying defect. *Gene therapy can be used to treat some cancers.* - This statement is **true**. Gene therapy is an active area of research and treatment for various **cancers** [1]. - Approaches include introducing genes that make cancer cells more susceptible to chemotherapy, enhancing the immune system's ability to fight cancer, or directly killing cancer cells through gene delivery [1].

Q: If a sequence of 4 nucleotides codes for 1 amino acid, how many amino acids can be theoretically formed?

256. ***256*** - With **4 distinct nucleotides** and a code sequence of **4 nucleotides** per amino acid, the number of possible unique combinations is calculated as 4^4. - This results in 4 × 4 × 4 × 4 = **256 theoretically possible amino acids**. - This is a mathematical combinatorics calculation: with 4 choices at each of 4 positions, total combinations = 4^4 = 256. *64* - This number represents the combinations if **3 nucleotides** coded for one amino acid (4^3 = 64), which is the actual case in the **standard genetic code** (triplet codons). - However, the question specifies a hypothetical sequence of **4 nucleotides** per amino acid, making this option incorrect. *16* - This number would be correct if **2 nucleotides** coded for one amino acid (4^2 = 16). - The problem explicitly states that **4 nucleotides** code for each amino acid in this theoretical scenario. *4* - This would only be the case if each **single nucleotide** coded for one amino acid (4^1 = 4). - Given **4 distinct nucleotides** and a sequence length of 4, the potential for combinations is much higher.

Q: Phenotypic expression of a gene depending on the parent of origin is referred to as:

Genomic imprinting (parent-of-origin gene expression). ***Genomic imprinting (parent-of-origin gene expression)*** - **Genomic imprinting** is an epigenetic phenomenon where gene expression is dependent on whether the gene was inherited from the mother or the father. - This results in monoallelic expression of specific genes, with only one copy (maternal or paternal) being active. *Mosaic genetic variation* - **Mosaicism** refers to the presence of two or more populations of genetically different cells in one individual, all derived from a single zygote. - This typically arises from a somatic mutation during development, not from differential expression based on parental origin. *Nonpenetrance of genotype* - **Nonpenetrance** occurs when individuals carrying a disease-causing genotype do not express the associated phenotype. - This concept relates to the presence or absence of a phenotype, not the differential expression based on parental origin. *Genetic anticipation* - **Genetic anticipation** is the phenomenon where the symptoms of a genetic disorder become more severe and/or appear at an earlier age in successive generations. - This is commonly observed in disorders caused by expansions of trinucleotide repeats, such as Huntington's disease, and is distinct from parent-of-origin gene expression.

Q: Gene silencing refers to a gene that would be expressed under normal circumstances being switched off by cell machinery. Which one of the following cellular components is not involved in gene silencing?

Ribosomal RNA. ***Micro-RNA*** - **Micro-RNAs** play a crucial role in gene regulation by binding to complementary sequences in mRNA, leading to gene silencing or degradation [1]. - They are involved in post-transcriptional regulation, effectively controlling gene expression at the **mRNA level** [1]. *Si RNA* - While **small interfering RNA (siRNA)** is also involved in gene silencing, it operates through a slightly different mechanism, primarily within RNA interference pathways. - SiRNAs typically require specific machinery for their function and are not the primary agents of gene silencing described in the question. *Double stranded RNA* - **Double-stranded RNA (dsRNA)** can induce silencing but serves primarily as a trigger for the RNA interference pathway rather than being the active component in gene silencing itself. - It is often utilized in research and therapeutic contexts but does not directly engage in the gene silencing process. *Ribosomal RNA* - **Ribosomal RNA (rRNA)** is primarily involved in protein synthesis and the structural component of ribosomes, with no role in gene silencing mechanisms. - It serves as a scaffold for ribosome assembly and is not implicated in the regulation of gene expression. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. With Illustrations By, pp. 16-18.

Q: When a gene is expressed exclusively from the allele inherited from one parent while the allele from the other parent is silenced, what is this phenomenon known as?

Genomic imprinting. ***Genomic imprinting*** - **Genomic imprinting** is an epigenetic phenomenon where certain genes are expressed in a **parent-of-origin-specific manner**. - This means that depending on whether the gene was inherited from the **mother or the father**, only one copy (maternal or paternal) is expressed, while the other is silenced. *Mosaicism* - **Mosaicism** describes the presence of **two or more cell lines** with different genotypes within a single individual, originating from a single zygote. - This typically arises from a **post-zygotic mutation** or chromosomal abnormality during early embryonic development. *Alleles* - **Alleles** are different forms of a **single gene** located at the same locus on homologous chromosomes. - An individual inherits **two alleles** for each gene, one from each parent, but both are usually expressed unless one is recessive. *Chimerism* - **Chimerism** refers to an individual composed of cells from **two or more different zygotes**, meaning the cells originate from different genetic lineages. - This can occur through processes like **fusion of two embryos** or organ transplantation.

Q: Preferential expression of a gene depending upon the parent of origin is called:

Genomic imprinting. ***Genomic imprinting*** * Genomic imprinting refers to the **epigenetic phenomenon** where the expression of certain genes is determined by whether they are inherited from the mother or the father. * This involves **DNA methylation** and histone modifications that lead to transcriptional silencing of either the maternal or paternal allele. *Mosaicism* * **Mosaicism** occurs when an individual has two or more genetically different cell lines originating from a single zygote. * It does not relate to parent-of-origin gene expression but rather to the presence of **different genotypes** within the same individual's cells. *Alleles* * **Alleles** are different forms of a gene located at the same locus on homologous chromosomes. * While alleles are involved in inheritance, their definition does not inherently describe the **parent-of-origin expression** pattern. *Chimerism* * **Chimerism** is the presence of cells from two or more different individuals in one individual, usually originating from the fusion of two or more zygotes. * This is different from genomic imprinting, which involves differential gene expression based on the **parental origin** of a gene within a single individual's genome.

Q: Which of the following statements about Taq DNA polymerase is correct?

Optimum temperature for chain elongation is 75°C. ***Optimum temperature for chain elongation is 75°C*** - **Taq polymerase** is a **thermostable enzyme** isolated from *Thermus aquaticus*, functioning optimally at high temperatures. - The optimal temperature for the **elongation step** in PCR, where Taq polymerase synthesizes new DNA strands, is typically around **72-78°C**, with 75°C falling within this optimal range. *Denatures at high temperatures* - While all proteins will eventually denature at extremely high temperatures, Taq polymerase is specifically known for its **thermostability** and **resistance to denaturation** at temperatures required for DNA strand separation in PCR (typically 94-98°C). - Its ability to withstand these high temperatures without significant loss of activity is its key advantage for use in **Polymerase Chain Reaction (PCR)**. *Provides high fidelity during DNA synthesis* - **Taq polymerase** is known for its relatively **low fidelity** due to the lack of 3' to 5' exonuclease activity (proofreading). - This low fidelity results in a higher error rate during DNA synthesis compared to other polymerases with proofreading capabilities, leading to more **mutations** during PCR. *Exhibits 3' to 5' exonuclease activity* - **Taq polymerase** typically **lacks 3' to 5' exonuclease activity**, meaning it does not have the ability to proofread and remove incorrectly incorporated nucleotides. - This absence of proofreading contributes to its relatively **lower fidelity** during DNA replication compared to other polymerases that possess this activity.

Q: Which of the following is not an example of a point mutation?

Frame-shift mutation. ***Frame-shift mutation*** - A **frame-shift mutation** involves the insertion or deletion of nucleotides not in multiples of three, altering the reading frame and typically leading to a completely different protein sequence or a premature stop codon. - While it can result from an insertion or deletion of a *single nucleotide*, its impact on the reading frame goes beyond a simple point alteration. *Silent mutation* - A **silent mutation** is a type of point mutation where a single nucleotide change in the DNA sequence does not change the amino acid sequence of the protein. - This is due to the **degeneracy of the genetic code**, where multiple codons can code for the same amino acid. *Nonsense mutation* - A **nonsense mutation** is a type of point mutation where a single nucleotide change results in a premature stop codon, leading to a truncated and often non-functional protein. - This significantly impacts protein synthesis by causing early termination of translation. *Missense mutation* - 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 protein's function, depending on the biochemical properties of the new amino acid and its location within the protein.

Q: Which test is commonly used to assess the mutagenic potential of carcinogens?

Ames test. ***Ames test*** - The **Ames test** uses specific strains of bacteria (e.g., *Salmonella typhimurium* and *E. coli*) to detect **mutagenic compounds** by reverse mutation. - It measures the ability of a chemical to induce **mutations** that restore the bacteria's ability to grow in a histidine-deficient medium. *Redox test* - A **redox test** measures the **oxidation-reduction potential** of a solution or system, indicating the balance between oxidizing and reducing agents. - While it can reflect cellular stress, it does not directly assess a substance's **mutagenic potential** or ability to cause DNA damage. *Bacteriophage* - A **bacteriophage** is a **virus that infects bacteria** and uses the bacterial cell's machinery to replicate itself. - While bacteriophages are used in genetic research as **vectors** or for studying gene expression, their primary role is not to directly assess the mutagenic potential of carcinogens. *Gene splicing* - **Gene splicing** is a molecular biology technique involving the **cutting and recombining of DNA segments**, often used in genetic engineering. - It is a method for creating new genetic combinations, not a test for directly evaluating the mutagenic potential of a compound.

Question 1

Which of the following statements is true regarding DNA polymerases?

Accepted Answer

DNA polymerase I has significant repair activity.

Answer

Leading strand is synthesized by DNA polymerase I.

Answer

Okazaki fragments are synthesized by DNA polymerase I.

Answer

Only DNA polymerase III has proofreading activity.

Question 2

Which of the following statements about gene therapy is false?

Accepted Answer

Gene therapy is only used for genetic disorders.

Answer

Gene also considered as drug

Answer

Gene therapy can be used to treat some cancers.

Answer

Has been tried in cystic fibrosis

Question 3

If a sequence of 4 nucleotides codes for 1 amino acid, how many amino acids can be theoretically formed?

Accepted Answer

256

Answer

4

Answer

64

Answer

16

Question 4

Phenotypic expression of a gene depending on the parent of origin is referred to as:

Accepted Answer

Genomic imprinting (parent-of-origin gene expression)

Answer

Mosaic genetic variation

Answer

Nonpenetrance of genotype

Answer

Genetic anticipation

Question 5

Gene silencing refers to a gene that would be expressed under normal circumstances being switched off by cell machinery. Which one of the following cellular components is not involved in gene silencing?

Accepted Answer

Ribosomal RNA

Answer

Double stranded RNA

Answer

siRNA

Answer

Micro-RNA

Question 6

When a gene is expressed exclusively from the allele inherited from one parent while the allele from the other parent is silenced, what is this phenomenon known as?

Accepted Answer

Genomic imprinting

Answer

Mosaicism

Answer

Alleles

Answer

Chimerism

Question 7

Preferential expression of a gene depending upon the parent of origin is called:

Accepted Answer

Genomic imprinting

Answer

Mosaicism

Answer

Alleles

Answer

Chimerism

Question 8

Which of the following statements about Taq DNA polymerase is correct?

Accepted Answer

Optimum temperature for chain elongation is 75°C

Answer

Denatures at high temperatures

Answer

Provides high fidelity during DNA synthesis

Answer

Exhibits 3' to 5' exonuclease activity

Question 9

Which of the following is not an example of a point mutation?

Accepted Answer

Frame-shift mutation

Answer

Nonsense mutation

Answer

Silent mutation

Answer

Missense mutation

Question 10

Which test is commonly used to assess the mutagenic potential of carcinogens?

Accepted Answer

Ames test

Answer

Redox test

Answer

Bacteriophage

Answer

Gene splicing

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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