Molecular Biology and Genomics Practice Questions

Q: Which of the following statements about chimeric DNA is false?

Chimeric DNA does not require DNA ligases.. ***Chimeric DNA does not require DNA ligases.*** - This statement is **false** because **DNA ligases** are essential enzymes that catalyze the formation of a **phosphodiester bond** between the 3'-hydroxyl and 5'-phosphate ends of DNA fragments, sealing the nicks to create a stable recombinant DNA molecule. - Without DNA ligase, the inserted DNA fragment would only be held by hydrogen bonds between complementary sticky ends, making the **chimeric DNA molecule unstable** and prone to dissociation. *Sticky ends produced by restriction endonucleases facilitate the formation of chimeric DNA.* - **Restriction endonucleases** cleave DNA at specific recognition sites, often creating **sticky ends** (single-stranded overhangs) that are complementary to each other. - These sticky ends allow for specific and efficient **annealing** of DNA fragments from different sources, which is a crucial step in the formation of chimeric DNA. *The organism harboring chimeric DNA exhibits features of both itself and the properties of the insert.* - When chimeric DNA is **introduced into an organism** (e.g., through transformation), the inserted gene can be expressed, leading to the production of new proteins. - This expression can confer **new traits or functions** to the host organism, combining its original characteristics with those encoded by the foreign DNA. *Chimeric DNA is formed by linking DNA fragments of unrelated genomes.* - **Chimeric DNA**, also known as **recombinant DNA**, is precisely defined as a DNA molecule constructed in vitro by joining DNA segments from **two or more different organisms** or sources. - This process allows for the combination of genetic material that would not naturally occur together, creating novel gene combinations.

Q: What is the role of catabolite activator protein (CAP) in lac operon?

Positive regulator. ***Positive regulator*** - **Catabolite activator protein (CAP)**, also known as **cAMP receptor protein (CRP)**, binds to **cAMP** when glucose levels are low. - This complex then binds to the **promoter region** of the lac operon, enhancing the binding of **RNA polymerase** and thereby increasing **transcription** (positive regulation). *Negative regulator* - A negative regulator would **inhibit gene expression**, such as the **lac repressor** which binds to the operator in the absence of lactose. - CAP, in contrast, promotes gene expression when **glucose is absent** and **lactose is present**. *Attenuation* - **Attenuation** is a regulatory mechanism primarily observed in operons involved in amino acid synthesis, such as the **trp operon**. - It involves the premature termination of transcription due to the formation of specific **mRNA secondary structures**, which is not the primary role of CAP. *Constitutive expression* - **Constitutive expression** refers to the constant expression of genes regardless of environmental conditions. - The lac operon is a **regulated operon**, meaning its expression is turned on or off in response to cellular needs, not expressed constitutively.

Q: All of the following are true about Nucleic Acid Sequence Based Amplification except which of the following?

Denaturation is carried out at 94°C. ***Denaturation is carried out at 94°C*** - This statement is **incorrect** because **Nucleic Acid Sequence Based Amplification (NASBA)** is an **isothermal** amplification method, meaning it does not require high-temperature denaturation steps. - NASBA operates at a constant temperature (typically around **41°C**), making it distinct from PCR which involves thermal cycling with high-temperature denaturation at 94°C. - This is the **EXCEPT answer** - the only false statement among the options. *It is a specific amplification of RNA* - NASBA is indeed designed for the **specific amplification of RNA targets**, making it particularly useful for detecting RNA viruses or mRNA transcripts. - It utilizes **T7 RNA polymerase** to produce multiple copies of RNA from an RNA template. *It is not a replacement for reverse transcriptase PCR.* - While both detect RNA, NASBA is an **alternative to**, not a replacement for, **reverse transcriptase PCR (RT-PCR)**. - NASBA has advantages in certain settings, such as **isothermal operation** and continuous RNA amplification, but RT-PCR remains the gold standard for many applications. *It requires three enzymes: reverse transcriptase, RNase H, and T7 RNA polymerase.* - This statement is **correct**. NASBA employs **three key enzymes** in its amplification process: - **Reverse transcriptase (AMV-RT)**: Synthesizes cDNA from RNA template - **RNase H**: Degrades the RNA strand in RNA-DNA hybrids - **T7 RNA polymerase**: Generates multiple RNA copies from the DNA template

Q: Most commonly used vector for DNA cloning?

Plasmid. ***Plasmid*** - **Plasmids** are the most commonly used vectors for DNA cloning due to their small size, ease of manipulation, and ability to replicate independently within a host cell. - They contain essential features like an **origin of replication**, **multiple cloning sites**, and **antibiotic resistance genes**, facilitating the insertion, selection, and amplification of foreign DNA. *Virus* - While viruses can be used as vectors (e.g., **adenoviruses** and **retroviruses**), they are typically more complex to handle and are often reserved for specialized applications like gene therapy, not routine DNA cloning. - They also have **safety concerns** and can elicit immune responses in the host, making them less suitable for general cloning compared to plasmids. *Cosmid* - **Cosmids** are hybrid vectors that combine features of plasmids and bacteriophages, designed to carry larger DNA inserts (up to 45 kb). - While useful for cloning large DNA fragments, their larger size and more complex structure make them less versatile and less commonly used for everyday cloning compared to the smaller, more manageable plasmids. *Phage* - **Bacteriophages** (phages) like lambda phage are specifically designed to infect bacteria and can accommodate large DNA inserts (up to 20 kb). - Phage vectors are often employed for constructing **genomic libraries** due to their capacity for large inserts, but their application is more specialized than the broad utility of plasmids in basic DNA cloning.

Q: What term describes the observable traits of an organism resulting from gene expression?

Phenotype. **Phenotype** is the correct answer. ***Correct: Phenotype*** - Refers to the **observable characteristics** of an organism resulting from gene expression - Includes physical appearance, biochemical properties, physiological traits, and behavior - Results from the interaction between **genotype and environment** - Represents the **expressed traits** that can be directly observed or measured *Incorrect: Genotype* - Refers to the **genetic makeup** (DNA sequence/alleles) of an organism - Represents the **inherited instructions**, not the expressed traits - Cannot be directly observed without genetic testing *Incorrect: Genome* - The **complete set of genetic material** (all DNA) in an organism - Includes both coding and non-coding sequences - Represents the entire genetic blueprint, not specifically expressed traits *Incorrect: Morphology* - Refers to the **study of form and structure** of organisms - A descriptive term or field of study, not the term for expressed traits - More limited in scope than phenotype (which includes functional traits)

Q: At which cell cycle checkpoint is the cell cycle halted if the cell's DNA is damaged?

G2 - M. ***G2 - M*** - The **G2/M checkpoint** is the **primary checkpoint for DNA damage detection** in the cell cycle. - This checkpoint ensures that **all DNA is properly replicated and any DNA damage is repaired** before the cell enters mitosis. - If **DNA damage** is detected at the G2/M checkpoint, the cell cycle is halted through **p53-mediated pathways**, preventing damaged DNA from being passed to daughter cells. - This is the **most critical safeguard** against transmission of genetic errors. *G1 - S* - The **G1/S checkpoint** (restriction point) primarily monitors cell size, nutrient availability, and growth signals to determine if the cell should commit to DNA replication. - While it does perform **some DNA damage surveillance**, it is not the primary DNA damage checkpoint. - Its main function is ensuring the cell is ready to **initiate** DNA synthesis, not specifically to detect DNA damage. *S - G2* - The **intra-S checkpoint** monitors DNA replication during S phase and can detect replication errors. - However, this represents a monitoring process during synthesis rather than a major decision checkpoint for halting the cycle due to DNA damage. - The transition from S to G2 itself is not a recognized major checkpoint. *G0 - G1* - The **G0 phase** is a quiescent state where cells exit the active cell cycle. - The G0-G1 transition involves cells re-entering the cell cycle in response to growth signals, not a DNA damage checkpoint. - This is not a recognized checkpoint for DNA damage assessment.

Q: Which enzyme is responsible for synthesizing RNA during transcription?

RNA polymerase. ***RNA polymerase (Correct Answer)*** - **RNA polymerase** is the central enzyme responsible for synthesizing an **RNA strand** from a DNA template during transcription. - It unwinds the DNA helix, reads the nucleotide sequence, and adds complementary RNA nucleotides to form a new RNA molecule. - This is a fundamental process in **gene expression**, occurring in the nucleus of eukaryotic cells. *Primase (Incorrect)* - **Primase** is an enzyme involved in **DNA replication**, not transcription. - Its function is to synthesize short **RNA primers** (8-12 nucleotides) that provide a starting point for DNA polymerase. *Ligase (Incorrect)* - **Ligase** is an enzyme that joins DNA fragments together by forming **phosphodiester bonds**. - It is primarily involved in **DNA replication** and repair processes, connecting Okazaki fragments on the lagging strand or repairing nicks in DNA strands. *Topoisomerase (Incorrect)* - **Topoisomerases** are enzymes that regulate the **supercoiling** of DNA. - They relieve **torsional stress** that builds up during DNA replication and transcription by cutting and rejoining DNA strands, preventing tangling.

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

3. ***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.

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

Nonsense mutation. ***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 1

Which of the following statements about chimeric DNA is false?

Accepted Answer

Chimeric DNA does not require DNA ligases.

Answer

Sticky ends produced by restriction endonucleases facilitate the formation of chimeric DNA.

Answer

The organism harboring chimeric DNA exhibits features of both itself and the properties of the insert.

Answer

Chimeric DNA is formed by linking DNA fragments of unrelated genomes.

Question 2

What is the role of catabolite activator protein (CAP) in lac operon?

Accepted Answer

Positive regulator

Answer

Attenuation

Answer

Constitutive expression

Answer

Negative regulator

Question 3

All of the following are true about Nucleic Acid Sequence Based Amplification except which of the following?

Accepted Answer

Denaturation is carried out at 94°C

Answer

It is not a replacement for reverse transcriptase PCR.

Answer

It requires three enzymes: reverse transcriptase, RNase H, and T7 RNA polymerase.

Answer

It is a specific amplification of RNA

Question 4

Most commonly used vector for DNA cloning?

Accepted Answer

Plasmid

Answer

Virus

Answer

Cosmid

Answer

Phage

Question 5

What is the primary functional mechanism of small RNAs in cellular processes?

Accepted Answer

Regulate gene expression through RNA interference mechanisms

Answer

Typically less than 200 nucleotides in length

Answer

Involved in post-transcriptional regulation of gene expression

Answer

Always synthesized endogenously

Question 6

What term describes the observable traits of an organism resulting from gene expression?

Accepted Answer

Phenotype

Answer

Genotype

Answer

Genome

Answer

Morphology

Question 7

At which cell cycle checkpoint is the cell cycle halted if the cell's DNA is damaged?

Accepted Answer

G2 - M

Answer

G1 - S

Answer

S - G2

Answer

G0 - G1

Question 8

Which enzyme is responsible for synthesizing RNA during transcription?

Accepted Answer

RNA polymerase

Answer

Primase

Answer

Ligase

Answer

Topoisomerase

Question 9

What is the number of structural genes in the Lac operon?

Accepted Answer

3

Answer

4

Answer

5

Answer

2

Question 10

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

Accepted Answer

Nonsense mutation

Answer

Missense mutation

Answer

Termination mutation

Answer

Silent mutation

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

On this page

Practice by Chapter

Want unlimited practice?