Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics Indian Medical PG Practice Questions and MCQs

Question 821: BRCA1 gene is located on which chromosome?

A. 17q21 (Correct Answer)
B. 13q12
C. 12q13
D. 21q17

Explanation: ***17q21*** - The **BRCA1 gene** is located on the long arm (q) of **chromosome 17** at region 21. - This gene plays a crucial role in **DNA repair** and is associated with an increased risk of breast and ovarian cancers. *13q12* - This location corresponds to the **BRCA2 gene**, which is also involved in DNA repair and cancer predisposition. - While both BRCA1 and BRCA2 are significant, they are distinct genes on different chromosomes. *12q13* - This is not the correct chromosomal location for either BRCA1 or BRCA2. - Various other genes are located on chromosome 12, but not BRCA1. *21q17* - This is an incorrect chromosomal designation; the "q" arm designation is typically followed by a band number, and 17 is not a standard band number for chromosome 21 in this context. - **Chromosome 21** is primarily associated with **Down syndrome** (trisomy 21).

Question 822: Which of the following statements about mitochondrial gene inheritance is true?

A. Mitochondrial gene mutations are inherited in a paternal pattern.
B. Mitochondrial genes are inherited from the mother. (Correct Answer)
C. Mitochondrial genes are inherited from the father.
D. Mitochondrial genes follow Mendelian inheritance.

Explanation: ***Mitochondrial genes are inherited from the mother.*** - During fertilization, the **ovum** contributes the vast majority of the cytoplasm, including all the mitochondria, to the zygote. - This results in a distinctive pattern of **maternal inheritance** for mitochondrial DNA (mtDNA); only mothers pass mitochondria to all their offspring. *Mitochondrial gene mutations are inherited in a paternal pattern.* - This statement is incorrect because mitochondrial inheritance is exclusively **maternal**, meaning mutations are passed from the mother, not the father. - Paternal DNA is primarily contained within the nucleus of the sperm head, and very few, if any, paternal mitochondria enter the oocyte during fertilization. *Mitochondrial genes are inherited from the father.* - This is incorrect as mitochondria are almost exclusively inherited from the **mother's ovum**. - The sperm provides its nuclear DNA but typically contributes negligible or no mitochondria to the zygote. *Mitochondrial genes follow Mendelian inheritance.* - Mitochondrial inheritance is **non-Mendelian**; it does not follow the classic dominant/recessive patterns or segregation expected with nuclear genes. - Due to maternal inheritance, all offspring of an affected mother will inherit the mitochondrial condition, regardless of sex.

Question 823: Which of the following is not a method for introducing a gene into target cells?

A. Electroporation
B. Transfection
C. Site directed recombination
D. FISH (Correct Answer)

Explanation: ***FISH*** - **FISH (Fluorescence In Situ Hybridization)** is a cytogenetic technique used to detect and localize the presence or absence of specific **DNA sequences** on chromosomes. - It is an imaging and diagnostic technique, not a method for introducing genetic material into cells. *Electroporation* - **Electroporation** is a physical method that uses an **electric pulse** to create temporary pores in cell membranes. - These temporary pores allow for the direct uptake of **exogenous DNA** (or other molecules) into the cell. *Transfection* - **Transfection** is a general term referring to the process of deliberately introducing **nucleic acids (DNA or RNA)** into eukaryotic cells. - It encompasses various methods, including chemical (e.g., lipid-based), physical (e.g., electroporation), and viral approaches. *Site directed recombination* - **Site-directed recombination** is a molecular biology technique used to introduce **targeted genetic changes** (insertions, deletions, or substitutions) at specific locations in a genome. - This process involves the direct introduction of modified genetic material that then recombines with the host genome.

Question 824: Which of the following statements is true regarding the role of enzymes in recombinant DNA technology?

A. Acid phosphatase is used
B. Reverse transcriptase needed
C. Restriction endonucleases are involved
D. DNA ligase is used (Correct Answer)

Explanation: ***DNA ligase is used*** - **DNA ligase** plays a crucial role in **recombinant DNA technology** by forming **phosphodiester bonds** to join DNA strands, such as in inserting a gene into a plasmid. - This enzyme is essential for **recombinant DNA formation** by covalently linking compatible sticky or blunt ends of DNA fragments. - DNA ligase catalyzes the final step of **sealing nicks** in the DNA backbone, creating stable recombinant molecules. *Restriction endonucleases are involved* - **Restriction endonucleases** (restriction enzymes) are indeed crucial for **cutting DNA** at specific recognition sites, creating DNA fragments with sticky or blunt ends. - While absolutely essential to the overall process, this statement uses "are involved" which is less specific than "is used" for describing the ligase's direct role in joining fragments. - Both enzymes work sequentially: restriction enzymes cut, then ligase joins. *Acid phosphatase is used* - **Acid phosphatase** is an enzyme found in lysosomes and is involved in **phosphate group removal** from substrates. - It is not used in **recombinant DNA technology** for cutting or joining DNA fragments. *Reverse transcriptase needed* - **Reverse transcriptase** is used to synthesize **cDNA from an mRNA template**, a process known as reverse transcription. - This enzyme is essential for creating **cDNA libraries** but is not directly involved in the core steps of cutting and joining DNA fragments in standard recombinant DNA procedures.

Question 825: Transition from G2 to M phase of the cell cycle is controlled by which?

A. Retinoblastoma gene product
B. p53 protein
C. Cyclin E
D. Cyclin B (Correct Answer)

Explanation: ***Cyclin B*** - The transition from G2 to M phase is primarily controlled by the **maturation-promoting factor (MPF)**, which is a complex of **cyclin B** and **CDK1 (cyclin-dependent kinase 1)**. - **Cyclin B** levels rise during G2 and peak at M phase, activating CDK1 to initiate mitosis. *Retinoblastoma gene product* - The **retinoblastoma (Rb) protein** primarily regulates the G1/S transition by inhibiting the E2F transcription factor. - It acts as a **tumor suppressor**, preventing uncontrolled cell proliferation. *p53 protein* - The **p53 protein** is a critical **tumor suppressor** that monitors DNA integrity at various cell cycle checkpoints, particularly G1/S and G2/M. - If DNA damage is detected, p53 can induce cell cycle arrest and/or apoptosis, but it is not directly responsible for initiating M phase. *Cyclin E* - **Cyclin E** is essential for the **G1/S transition**, forming a complex with CDK2 to initiate DNA replication. - Its activity peaks during late G1 and early S phase, not during the G2/M transition.

Question 826: Gene amplification is achieved through

A. Polymerase Chain Reaction (Correct Answer)
B. DNA strand hybridization
C. In situ DNA hybridization
D. Ligase chain reaction (LCR)

Explanation: ***Polymerase Chain Reaction*** - **PCR** is the **gold standard** molecular biology technique that generates **millions to billions of copies** of a specific DNA segment over a short period. - It utilizes a cyclical process of **denaturation**, **annealing**, and **extension** with **thermostable DNA polymerase** to achieve exponential amplification. - **Most widely used** method for gene amplification in research and diagnostics. *DNA strand hybridization* - **DNA strand hybridization** is the process where two complementary single-stranded DNA molecules bind together to form a **double-stranded molecule**. - This process is fundamental to many molecular techniques but does not, in itself, achieve **amplification**; rather, it is a **binding event**. *In situ DNA hybridization* - **In situ hybridization** is a technique that localizes and detects specific **nucleic acid sequences** (DNA or RNA) within cells or tissues directly on a slide. - While it uses **hybridization**, its primary purpose is **detection and localization**, not the **amplification** of DNA sequences. *Ligase chain reaction (LCR)* - **LCR** is a molecular technique that does amplify DNA sequences exponentially using **DNA ligase** to join adjacent oligonucleotide probes. - However, it is **less commonly used** than PCR, has more **stringent requirements** (requires knowledge of both strands), and is primarily used for detecting **known point mutations** rather than general gene amplification. - **PCR remains the standard** technique when the question refers to gene amplification without additional qualifiers.

Question 827: Which of the following is the origin of the mitochondrial DNA of all human adult cells?

A. Paternal only
B. Maternal only (Correct Answer)
C. A combination of paternal and maternal DNA is inherited
D. Either paternal or maternal DNA is inherited

Explanation: ***Maternal only*** - **Mitochondria** and their DNA are almost exclusively inherited from the **mother's ovum**. - During fertilization, the sperm contributes only its nucleus to the zygote, while the ovum provides the bulk of the cytoplasm, including mitochondria. *Paternal only* - The **sperm's mitochondria** are typically found in its tail and are usually **destroyed** or degrade after entering the egg, or are actively excluded. - This prevents paternal mitochondrial DNA from being passed on to the offspring. *A combination of paternal and maternal DNA is inherited* - While nuclear DNA is a combination of paternal and maternal contributions, **mitochondrial DNA** inheritance is **uniparental** (from one parent only). - This is a fundamental aspect of human genetics and ancestry tracking. *Either paternal or maternal DNA is inherited* - This option incorrectly suggests variability, as **maternal inheritance** of mitochondrial DNA is the virtually universal rule in humans. - Rare exceptions of paternal mitochondrial DNA inheritance have been reported but are considered **anomalous** and not typical.

Question 828: What term describes the external manifestations of the genome?

A. Genotype
B. Phenotype (Correct Answer)
C. Allele
D. Polymorphism

Explanation: ***Correct: Phenotype*** - The **phenotype** refers to the **observable characteristics** or traits of an organism, which result from the interaction of its genotype with the environment. - These external manifestations can include physical appearances (e.g., eye color), physiological traits (e.g., blood pressure), and even measurable behaviors. - This is the classic definition of phenotype - the external/outward expression of the genome. *Incorrect: Genotype* - The **genotype** describes the **genetic makeup** of an individual, specifically the set of alleles possessed for a particular gene or genes. - It represents the internal genetic code, not its outward expression. - Genotype is the "blueprint," while phenotype is the "building." *Incorrect: Allele* - An **allele** is one of two or more alternative forms of a gene that arise by mutation and are found at the same place on a chromosome. - It is a specific variation of a gene, not the observable manifestation of the entire genome. - Alleles are components of the genotype. *Incorrect: Polymorphism* - A **polymorphism** is a common variation in the DNA sequence among individuals in a population. - It refers to differences in the genetic code itself, not the resulting physical or functional traits. - Polymorphisms contribute to genetic diversity at the DNA level.

Question 829: Xeroderma pigmentosum is due to:

A. Nucleotide excision defect (Correct Answer)
B. Base excision repair defect
C. Cross-linking repair defect
D. SOS repair mechanism defect

Explanation: ***Nucleotide excision defect*** - **Xeroderma pigmentosum (XP)** is a genetic disorder characterized by a defect in the **nucleotide excision repair (NER)** pathway. - This pathway is crucial for repairing **DNA damage**, particularly **pyrimidine dimers** caused by **ultraviolet (UV) radiation**. *Base excision repair defect* - **Base excision repair (BER)** is a distinct DNA repair pathway that primarily deals with **small, non-helix-distorting base lesions** and single-strand breaks. - Defects in BER are associated with different conditions, such as some forms of **cancer susceptibility** but not XP. *Cross-linking repair defect* - **Cross-linking repair** mechanisms address formations where two strands of DNA are covalently linked, or DNA is linked to proteins, often by agents like **alkylating agents**. - Disorders like **Fanconi anemia** are associated with defects in **DNA interstrand crosslink repair**, which is different from the UV-induced damage seen in XP. *SOS repair mechanism defect* - The **SOS response** is a global bacterial mechanism dealing with extensive DNA damage, primarily involving the induction of various DNA repair enzymes. - There isn't a direct human disease called "SOS repair mechanism defect" that is analogous to XP; human repair mechanisms are more complex and distinct from the bacterial SOS response.

Question 830: Which of the following statements about polymerase chain reaction (PCR) is true?

A. It is used for the separation of protein fragments.
B. It is a method for recombinant DNA synthesis.
C. It is a method for enzymatic amplification of DNA. (Correct Answer)
D. None of the options.

Explanation: ***It is a method for enzymatic amplification of DNA.*** - PCR is a laboratory technique used to make **millions to billions of copies** of a specific DNA segment. - This amplification is carried out enzymatically by **DNA polymerase**, which synthesizes new DNA strands. *It is a method for recombinant DNA synthesis.* - **Recombinant DNA synthesis** involves combining DNA from different sources, often for genetic engineering, which is a broader application than the core function of PCR. - While PCR can be a *tool* used within recombinant DNA technology (e.g., to amplify a gene for cloning), its primary definition is not synthesizing recombinant DNA itself. *It is used for the separation of protein fragments.* - The separation of protein fragments is typically achieved through techniques like **electrophoresis** (e.g., SDS-PAGE), which separates proteins based on size and charge. - PCR is specifically designed for manipulating and amplifying **DNA**, not proteins. *None of the options.* - This option is incorrect because the statement "It is a method for enzymatic amplification of DNA" accurately describes the fundamental function of PCR.

Practice by Chapter

DNA Replication and Repair Mechanisms

Practice Questions

Transcription Factors and Gene Regulation

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Epigenetics and DNA Methylation

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RNA Processing and Splicing

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miRNA and RNA Interference

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Protein Synthesis and Post-Translational Modifications

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Genomics and Human Genome Project

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Single Nucleotide Polymorphisms

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Gene Therapy Approaches

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CRISPR-Cas9 and Genome Editing

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DNA Fingerprinting and Forensics

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Molecular Basis of Genetic Diseases

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