Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics Indian Medical PG Practice Questions and MCQs

Question 801: What is the number of structural genes in the Lac operon?

A. 3 (Correct Answer)
B. 4
C. 5
D. 2

Explanation: ***3*** - The Lac operon contains **three structural genes**: *lacZ*, *lacY*, and *lacA*. - These genes encode enzymes essential for the metabolism of **lactose**: **β-galactosidase**, **lactose permease**, and **thiogalactoside transacetylase**, respectively. *4* - This option is incorrect as the Lac operon consists of **three structural genes**, not four. - Adding a regulatory gene (like *lacI*) to the structural genes would still not result in a total of four, as *lacI* is distinct from the structural genes. *5* - This option is incorrect; the Lac operon is defined by its **three structural genes** and specific regulatory elements. - There are no typical configurations of the Lac operon that would include five structural components. *2* - This option is incorrect because the Lac operon contains **three distinct structural genes**, each with a specific function. - Omitting *lacA* would leave out the gene for thiogalactoside transacetylase, an important, although less critical, enzyme in lactose metabolism.

Question 802: What is the term for a single mutation in a nucleotide base pair that results in a termination codon?

A. Missense mutation
B. Nonsense mutation (Correct Answer)
C. Termination mutation
D. Silent mutation

Explanation: ***Nonsense mutation*** - A **nonsense mutation** occurs when a single nucleotide base pair change leads to the formation of a **premature stop codon**, which results in a truncated and often non-functional protein. - The term "nonsense" refers to the fact that the new codon signals an early termination of protein synthesis. *Missense mutation* - A **missense mutation** involves a single nucleotide change that results in a codon coding for a **different amino acid**, potentially altering protein function but not necessarily terminating it. - This type of mutation can have varying effects on protein function, from benign to severe, depending on the amino acid substitution. *Termination mutation* - While a nonsense mutation does result in **premature termination**, "termination mutation" is not the standard or most precise scientific term used to describe this specific type of genetic alteration. - The more accurate and widely accepted terminology is **nonsense mutation** for a change leading to a stop codon. *Silent mutation* - A **silent mutation** is a type of point mutation that changes a single nucleotide, but does not change the amino acid sequence of the protein due to the **degeneracy of the genetic code**. - These mutations have **no observable effect** on the organism's phenotype as the protein produced remains unchanged.

Question 803: Which of the following statements is true regarding the telomerase theory of aging?

A. Decreased telomere length is associated with aging (Correct Answer)
B. Abnormal telomerase activation is associated with cancer
C. Telomere stability directly maintains chromosomal integrity
D. Increased telomere length is associated with prolonged cellular lifespan

Explanation: ***Decreased telomere length is associated with aging*** - Telomeres are protective DNA-protein caps at chromosome ends that **shorten with each cell division** - Progressive telomere shortening triggers **cellular senescence** (Hayflick limit) and apoptosis - This mechanism directly contributes to aging and **age-related diseases** - The telomere theory of aging (Olovnikov hypothesis) states that telomere attrition is a primary driver of biological aging *Abnormal telomerase activation is associated with cancer* - **Telomerase is reactivated in ~85-90% of cancers**, enabling unlimited replicative potential - Normal adult somatic cells have low/absent telomerase activity - While telomerase can extend cellular lifespan, its aberrant activation leads to malignancy, not healthy aging *Telomere stability directly maintains chromosomal integrity* - Telomeres prevent chromosome degradation, end-to-end fusions, and DNA damage responses - This is a **protective function**, not the basis of the telomere theory of aging - The aging theory focuses on **consequences of telomere shortening**, not stability maintenance *Increased telomere length is associated with prolonged cellular lifespan* - Longer telomeres do correlate with younger biological age and extended replicative capacity - However, this describes the **inverse relationship** rather than the core aging theory - The telomere theory specifically explains aging through **progressive shortening**, not length extension

Question 804: Which chromosome is responsible for the production of MIF?

A. Y chromosome
B. Chromosome 22 (Correct Answer)
C. Chromosome 16
D. X Chromosome

Explanation: ***Chromosome 22*** * The **macrophage migration inhibitory factor (MIF)** gene is located on **chromosome 22q11.2**, making it responsible for MIF production. * MIF is a crucial **pro-inflammatory cytokine** involved in immune responses and inflammation. *Chromosome 16* * Chromosome 16 contains genes like those for **alpha-globin** and **CDH1 (E-cadherin)**, not MIF. * Disorders associated with chromosome 16 include **alpha-thalassemia** and certain types of **hereditary diffuse gastric cancer**. *X Chromosome* * The X chromosome contains genes primarily involved in **sex determination** and various X-linked disorders. * Examples include genes like **DMD (Duchenne muscular dystrophy)** and **F8 (hemophilia A)**, but not MIF. *Y chromosome* * The Y chromosome contains genes, such as **SRY (sex-determining region Y)**, that are critical for male sexual development. * It plays a role in male-specific traits and conditions like **infertility** but does not carry the gene for MIF.

Question 805: Which gene is responsible for the production of COX type 3?

A. COX 3 gene
B. COX 2 gene
C. None of the above
D. COX I gene (Correct Answer)

Explanation: ***COX I gene*** - COX-3 is an **alternatively spliced variant** of the **COX-1 gene** (specifically, a splice variant of the COX-1 mRNA that retains intron 1). - While it was initially thought to be a distinct gene, research has shown that it arises from the same genetic locus as COX-1. *COX 2 gene* - The COX-2 gene encodes for the **inducible cyclooxygenase enzyme**, which is responsible for prostaglandin synthesis during inflammation. - It is a separate gene from COX-1 and has distinct regulatory mechanisms and physiological roles. *COX 3 gene* - There is currently **no distinct gene in humans** specifically identified as "COX-3". - COX-3 refers to a protein isoform derived from the COX-1 gene, not a separate genetic locus. *None of the above* - This option is incorrect because COX-3 is indeed derived from the **COX-1 gene** through alternative splicing. - The existence of COX-3 as a distinct protein product has been demonstrated, although its precise physiological role in humans is still under investigation.

Question 806: What is a key similarity between the processes of replication and transcription?

A. Use RNA primers for initiation.
B. Use ribonucleotides as precursors.
C. Are semi-conservative events.
D. Involve phosphodiester bond formation with elongation occurring in the 5' - 3' direction. (Correct Answer)

Explanation: ***Involve phosphodiester bond formation with elongation occurring in the 5' - 3' direction.*** - Both DNA replication and RNA transcription synthesize nucleic acid polymers by forming **phosphodiester bonds** between incoming nucleotides. - The new strand in both processes is always elongated in the **5' to 3' direction**, as new nucleotides are added to the 3' hydroxyl group of the growing strand. *Use RNA primers for initiation.* - **DNA replication** requires **RNA primers** to initiate synthesis of new DNA strands, as DNA polymerase cannot start a new strand *de novo*. - **Transcription (RNA synthesis)** does not require a primer; **RNA polymerase** can initiate transcription *de novo* at a promoter sequence. *Use ribonucleotides as precursors.* - **Transcription** uses **ribonucleotides** (ATP, UTP, CTP, GTP) as precursors to synthesize RNA. - **Replication** primarily uses **deoxyribonucleotides** (dATP, dTTP, dCTP, dGTP) to synthesize DNA, although it temporarily uses ribonucleotides for RNA primers. *Are semi-conservative events.* - **DNA replication** is a **semi-conservative process**, meaning each new DNA molecule consists of one original strand and one newly synthesized strand. - **Transcription** is **not semi-conservative**; it involves synthesizing an RNA molecule from a DNA template, leaving the original DNA template unchanged.

Question 807: By which enzyme is cDNA synthesized from RNA?

A. Helicase
B. DNA-dependent DNA polymerase
C. Topoisomerase
D. Reverse transcriptase (Correct Answer)

Explanation: ***Reverse transcriptase*** - **Reverse transcriptase** is a unique enzyme that synthesizes a **complementary DNA (cDNA)** strand from an **RNA template**. - This process, known as **reverse transcription**, is crucial in retroviruses and molecular biology techniques like RT-PCR. *Helicase* - **Helicase** enzymes are responsible for **unwinding nucleic acid double helices**, separating DNA strands during replication and transcription. - It does not synthesize DNA from an RNA template. *DNA-dependent DNA polymerase* - **DNA-dependent DNA polymerase** synthesizes new **DNA strands using an existing DNA template** during DNA replication. - It cannot use RNA as a template to synthesize DNA. *Topoisomerase* - **Topoisomerase** enzymes are involved in **managing DNA supercoiling** by creating transient breaks in the DNA backbone. - They do not synthesize DNA from any template.

Question 808: Location of gene on chromosome is identified by

A. Karyotyping
B. Genetic mapping (Correct Answer)
C. Microarray
D. Genomic imprinting

Explanation: ***Genetic mapping*** - **Genetic mapping** (also called chromosome mapping) uses various techniques to determine the **physical location (locus)** of genes on a chromosome. - Techniques include **linkage analysis**, **FISH (Fluorescence In Situ Hybridization)**, chromosomal banding, and analysis of **inheritance patterns** of traits and genetic markers. - This identifies both the **relative positions** between genes and their **absolute chromosomal addresses**. *Karyotyping* - **Karyotyping** is a technique that visualizes the entire set of chromosomes in an organism. - While it can identify **large chromosomal abnormalities** like aneuploidy or major deletions/insertions, it does not pinpoint the exact location of a specific gene. *Microarray* - **Microarray** technology is used to study the expression levels of thousands of genes simultaneously or to detect specific genetic variations. - It does not directly map the physical location of a gene on a chromosome. *Genomic imprinting* - **Genomic imprinting** is an epigenetic phenomenon where certain genes are expressed in a **parent-of-origin-specific manner**. - It describes a mechanism of gene regulation rather than a method for identifying the location of a gene on a chromosome.

Question 809: What is the most important tool used in genetic engineering?

A. Topoisomerase
B. DNA Ligase
C. Restriction endonuclease (Correct Answer)
D. Helicase

Explanation: ***Restriction endonuclease*** - **Restriction endonucleases** are crucial for genetic engineering as they specifically cut DNA at particular recognition sites, allowing the insertion or deletion of genes. - This precise cutting ability is fundamental for creating **recombinant DNA** molecules. *Helicase* - **Helicase** is primarily involved in unwinding the DNA double helix during processes like DNA replication and transcription. - While essential for cellular functions, it does not directly manipulate DNA for gene insertion or modification in the way restriction enzymes do. *Topoisomerase* - **Topoisomerase** enzymes are responsible for managing DNA supercoiling, preventing tangling during DNA replication and transcription by cutting and rejoining DNA strands. - It plays a role in DNA structure but is not directly used for targeted gene editing or insertion. *DNA Ligase* - **DNA ligase** is essential for joining DNA fragments, which is a critical step in genetic engineering after restriction endonucleases have cut the DNA. - However, while it acts as a "molecular glue" to seal nicks and re-form phosphodiester bonds, it cannot initiate the precise cutting required to isolate genes.

Question 810: Which type of RNA contains codons for specific amino acids?

A. Transfer RNA (tRNA)
B. Messenger RNA (mRNA) (Correct Answer)
C. Small nuclear RNA (snRNA)
D. Ribosomal RNA (rRNA)

Explanation: ***Messenger RNA (mRNA)*** - **mRNA** carries the genetic information from **DNA** in the nucleus to the **ribosomes** in the cytoplasm. - This information is encoded in sequences of three nucleotides called **codons**, each specifying a particular amino acid. *Transfer RNA (tRNA)* - **tRNA** molecules are responsible for **carrying specific amino acids** to the ribosome during protein synthesis. - Each **tRNA** has an **anticodon** that base-pairs with a complementary **codon** on the **mRNA** strand. *Small nuclear RNA (snRNA)* - **snRNA** is primarily involved in **RNA splicing**, a process that removes introns from pre-mRNA. - It forms part of the **spliceosome** complex, which is crucial for mature mRNA formation but does not contain codons itself. *Ribosomal RNA (rRNA)* - **rRNA** is a major component of **ribosomes**, the cellular machinery responsible for protein synthesis. - While it plays a critical structural and catalytic role in translation, it does not carry genetic code in the form of codons.

Practice by Chapter

DNA Replication and Repair Mechanisms

Practice Questions

Transcription Factors and Gene Regulation

Practice Questions

Epigenetics and DNA Methylation

Practice Questions

RNA Processing and Splicing

Practice Questions

miRNA and RNA Interference

Practice Questions

Protein Synthesis and Post-Translational Modifications

Practice Questions

Genomics and Human Genome Project

Practice Questions

Single Nucleotide Polymorphisms

Practice Questions

Gene Therapy Approaches

Practice Questions

CRISPR-Cas9 and Genome Editing

Practice Questions

DNA Fingerprinting and Forensics

Practice Questions

Molecular Basis of Genetic Diseases

Practice Questions

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