Nucleic Acid Biochemistry — MCQs

Nucleic Acid Biochemistry Indian Medical PG Practice Questions and MCQs

Question 161: Mitochondrial DNA inheritance is transmitted from:

A. Father
B. Grandmother
C. Grandfather
D. Mother (Correct Answer)

Explanation: ***Mother*** - **Mitochondrial DNA (mtDNA)** is exclusively inherited from the mother. - This is because the **egg cell** contributes the cytoplasm, including mitochondria, to the zygote, while the sperm primarily contributes nuclear DNA. *Father* - The father primarily contributes **nuclear DNA** during fertilization. - While sperm do contain mitochondria, these are typically **degraded** or excluded from the fertilized egg. *Grandmother* - This option confuses **lineage** with **direct transmission**. - The question asks about immediate parental transmission, which is from the **mother**, not the grandmother. - While mtDNA follows a maternal lineage (grandmother → mother → child), the direct source is always the mother. *Grandfather* - A grandfather does not transmit mtDNA to his offspring. - The inheritance pathway for mtDNA is strictly **maternal**.

Question 162: False statements are:

A. DNA replication proceeds in one direction
B. Lagging strand is synthesized by RNA primase
C. All of the options (Correct Answer)
D. Bacteria have multiple origins of replication

Explanation: ***All of the options*** - All statements are **false**. DNA replication proceeds **bidirectionally**, bacteria typically have a **single origin of replication**, and the lagging strand is synthesized by **DNA polymerase** after an RNA primer is laid down by **RNA primase**. *DNA replication proceeds in one direction* - This statement is **false** because **DNA replication** is a **bidirectional process**, meaning it proceeds in both directions from the origin of replication. - Replication forks move away from the **origin** on both sides, unraveling the DNA and synthesizing new strands. *Bacteria have multiple origins of replication* - This statement is **false**. Most **bacteria** (prokaryotes) have a **single origin of replication** (oriC) on their circular chromosome. - In contrast, **eukaryotes** have **multiple origins of replication** on their linear chromosomes to replicate their much larger genomes efficiently. - While rare exceptions exist in some bacterial species, the general rule for bacterial DNA replication is a single origin. *Lagging strand is synthesized by RNA primase* - This statement is **false**. The **lagging strand** is primarily synthesized by **DNA polymerase III** (in prokaryotes) or **DNA polymerase δ** (in eukaryotes). - **RNA primase** is responsible for synthesizing short **RNA primers** that provide a starting point for DNA polymerase, but it does not synthesize the entire lagging strand itself.

Question 163: DNA amplification is done by all, except:

A. DNA sequencing (Correct Answer)
B. Loop-mediated isothermal amplification (LAMP)
C. Ligase chain reaction
D. Polymerase chain reaction

Explanation: ***DNA sequencing*** - **DNA sequencing** determines the **nucleotide base order** in a DNA molecule but does not increase the amount of DNA. - While requiring a DNA template, it is an **analytical technique** rather than an amplification method. *Loop-mediated isothermal amplification (LAMP)* - **LAMP** is an **isothermal DNA amplification** technique that amplifies target DNA sequences at a constant temperature (60-65°C). - It uses a DNA polymerase with strand displacement activity and 4-6 primers to produce large amounts of DNA rapidly. *Ligase chain reaction* - **LCR** is an amplification method that detects specific **DNA sequences** by ligating adjacent probes. - It amplifies the signal from a target DNA sequence rather than the DNA itself by creating many copies of joined probes. *Polymerase chain reaction* - **PCR** is a widely used technique for **amplifying** a specific segment of DNA to produce many copies. - It involves cycles of **denaturation**, **annealing**, and **extension** using a DNA polymerase.

Question 164: Inosinic acid is biological precursor of ?

A. Purines and thymine
B. Orotic acid and uridylic acid
C. Adenylic acid and guanylic acid (Correct Answer)
D. Uracil and thymine

Explanation: ***Adenylic acid and guanylic acid*** - Inosinic acid (IMP) is a **key intermediate** in the **de novo purine synthesis pathway**. - It serves as the direct precursor for the synthesis of **adenylic acid (AMP)** and **guanylic acid (GMP)**, which are components of DNA and RNA. *Purines and thymine* - While inosinic acid is a precursor to purines, it is **not a precursor to thymine**. - Thymine is a **pyrimidine base** and is synthesized through a separate pathway. *Orotic acid and uridylic acid* - **Orotic acid** is an intermediate in **pyrimidine synthesis**, not purine synthesis. - **Uridylic acid (UMP)** is also a pyrimidine nucleotide, and its synthesis pathway involves orotic acid, not inosinic acid. *Uracil and thymine* - **Uracil** and **thymine** are pyrimidine bases, and their synthesis pathways are distinct from the purine synthesis pathway involving inosinic acid. - Inosinic acid is exclusively involved in the synthesis of **purine nucleotides**.

Question 165: Apolipoprotein B-48 is made by which process?

A. DNA editing
B. RNA editing (Correct Answer)
C. RNA alternate splicing
D. RNA interference

Explanation: ***RNA editing*** - Apolipoprotein B-48 is synthesized from ApoB-100 mRNA through a process called **RNA editing** (specifically ApoB mRNA editing) - This involves a **cytidine deaminase enzyme (APOBEC-1)** that converts cytidine to uridine at position 6666, changing a glutamine codon (CAA) to a premature stop codon (UAA) in the small intestine - This results in a truncated protein that is 48% the length of ApoB-100 - ApoB-48 is produced in the **intestine**, while ApoB-100 (unedited) is produced in the **liver** *DNA editing* - DNA editing refers to permanent modifications in the DNA sequence itself - The ApoB gene remains unchanged; only the mRNA transcript is edited in intestinal cells - This is not the mechanism for producing ApoB-48 *RNA alternate splicing* - Alternative splicing involves selecting different combinations of exons from pre-mRNA to produce multiple mRNA isoforms - This process creates different protein variants through exon inclusion/exclusion - ApoB-48 production does not involve alternative splicing but rather direct nucleotide modification (C to U) within the coding sequence *RNA interference* - RNA interference (RNAi) is a biological process involving small RNA molecules (siRNA, miRNA) that silence gene expression - RNAi typically degrades mRNA or blocks translation - This process is not involved in generating a truncated protein like ApoB-48 from the same mRNA transcript

Question 166: Mark the false statement regarding mitochondrial DNA:

A. AGA and AGG are stop codons in mitochondrial DNA
B. Kearns-Sayre Syndrome is a large deletion in mitochondrial DNA
C. Does not show heteroplasmy (Correct Answer)
D. 1% of cellular DNA, 13 proteins of respiratory chain

Explanation: ***Does not show heteroplasmy*** - This statement is false because **mitochondrial DNA (mtDNA)** commonly exhibits **heteroplasmy**, meaning the presence of more than one type of mitochondrial genome within a cell or individual. - **Heteroplasmy** arises due to the presence of both normal and mutated mtDNA, which can be passed down from the mother. *AGA and AGG are stop codons in mitochondrial DNA* - This statement is true; in the **universal genetic code**, AGA and AGG code for **arginine**, but in **human mitochondrial DNA**, they serve as **stop codons**. - This is an example of the **differences** in genetic code interpretation between the nuclear genome and the mitochondrial genome. *Kearns-Sayre Syndrome is a large deletion in mitochondrial DNA* - This statement is true; **Kearns-Sayre Syndrome** is a well-known mitochondrial disorder caused by a **large single deletion** in the mitochondrial DNA. - This deletion often leads to chronic progressive **external ophthalmoplegia**, **retinal pigmentary degeneration**, and **cardiac conduction defects**. *1% of cellular DNA, 13 proteins of respiratory chain* - This statement is true; **mitochondrial DNA constitutes** approximately **1% of the total cellular DNA** by mass. - It codes for **13 essential proteins** that are part of the **electron transport chain** (respiratory chain) complexes in the mitochondrion, along with ribosomal RNAs (rRNAs) and transfer RNAs (tRNAs).

Question 167: If the percentage of thymine residues in DNA is 28%. What is the percentage of cytosine?

A. 36%
B. 44%
C. 22% (Correct Answer)
D. 28%

Explanation: ***22%*** - According to **Chargaff's rules**, in a DNA molecule, the amount of **adenine (A) is equal to the amount of thymine (T)**, and the amount of **guanine (G) is equal to the amount of cytosine (C)**. - If thymine (T) is 28%, then adenine (A) is also 28%, making a total of 56% for A+T. The remaining 44% (100% - 56%) is split equally between guanine and cytosine, so cytosine (C) is 22%. *36%* - This percentage would be plausible if the sum of adenine and thymine were 28%, which is incorrect as A and T are equal and their sum would thus be 56%. - This value does not adhere to the principle of **base pairing complementarity** and the total percentage of all bases summing to 100%. *44%* - This would be the combined percentage of guanine and cytosine, not the percentage of cytosine alone. - If cytosine were 44%, then guanine would also be 44%, leading to a total of 88% for G+C, which is inconsistent with T being 28%. *28%* - This is the percentage of thymine, and by **Chargaff's rules**, it would also be the percentage of adenine, not cytosine. - Cytosine percentages are derived from the remaining proportion of bases after accounting for adenine and thymine.

Question 168: True regarding mitochondrial DNA is:

A. Linear double stranded
B. All respiratory proteins are synthesized within mitochondria itself
C. Inherited from mother (Correct Answer)
D. Low mutation rate

Explanation: ***Inherited from mother*** - **Mitochondrial DNA (mtDNA)** is exclusively inherited from the mother because the sperm's mitochondria are typically destroyed after fertilization or do not enter the oocyte. - This **maternal inheritance pattern** makes mtDNA useful for tracing lineage and studying human population movements. *Linear double stranded* - **Mitochondrial DNA** is typically **circular**, not linear, and double-stranded, similar to a plasmid in bacteria. - **Linear DNA** is characteristic of nuclear chromosomes in eukaryotes. *All respiratory proteins are synthesized within mitochondria itself* - While mitochondria contain their own ribosomes and synthesize some proteins, the majority of **respiratory chain proteins** are encoded by **nuclear DNA** and imported into the mitochondria. - The **mitochondrial genome** encodes only a small fraction of the proteins necessary for mitochondrial function, primarily components of the electron transport chain. *Low mutation rate* - **Mitochondrial DNA** has a **higher mutation rate** compared to nuclear DNA due to a less robust DNA repair system and exposure to reactive oxygen species generated during oxidative phosphorylation. - The high mutation rate can contribute to mitochondrial diseases and can also be used in evolutionary studies.

Question 169: Klenow fragment is formed by loss of fragment having which activity?

A. 5'- 3' polymerase
B. 3'- 5' polymerase
C. 3'- 5' exonuclease
D. 5'- 3' exonuclease (Correct Answer)

Explanation: ***5'- 3' exonuclease*** - The **Klenow fragment** is obtained by proteolytic cleavage of **DNA Polymerase I** from *E. coli*. - This process removes the domain responsible for the **5' → 3' exonuclease activity**, leaving the polymerase and 3' → 5' exonuclease activities intact. *5'- 3' polymerase* - The **Klenow fragment retains** the **5' → 3' polymerase activity**, which is essential for DNA synthesis. - This activity synthesizes new DNA strands in the **5' to 3' direction**. *3'- 5' polymerase* - **DNA Polymerase I** (and thus the Klenow fragment) does not possess **3' → 5' polymerase activity**. - DNA polymerases always synthesize DNA in the **5' to 3' direction**. *3'- 5' exonuclease* - The **Klenow fragment retains** the **3' → 5' exonuclease activity**, which is crucial for **proofreading**. - This activity removes incorrectly paired nucleotides from the **3' end** of the growing DNA strand.

Question 170: Which of the following binds mRNA with ribosome in prokaryotes?

A. 7 methyl guanosine capping
B. tRNA
C. Poly A tail
D. Shine Dalgarno sequence (Correct Answer)

Explanation: ***Shine Dalgarno sequence*** - The **Shine-Dalgarno sequence** is a **ribosome binding site** in prokaryotic mRNA, typically located 3-9 nucleotides upstream of the start codon (AUG). - It base-pairs with a complementary sequence in the **16S rRNA** of the 30S ribosomal subunit, correctly positioning the ribosome for translation initiation. *7 methyl guanosine capping* - **7-methylguanosine capping** occurs at the 5' end of **eukaryotic mRNA** and is crucial for ribosome binding, protection from degradation, and export from the nucleus. - This modification is **absent in prokaryotic mRNA**, which does not require nuclear export or 5' capping for translation. *tRNA* - **tRNA (transfer RNA)** molecules are responsible for carrying specific **amino acids** to the ribosome and recognizing corresponding codons on the mRNA. - While essential for protein synthesis, tRNA itself **does not bind mRNA to the ribosome**; instead, it mediates the amino acid incorporation based on the mRNA sequence once the ribosome is bound. *Poly A tail* - The **poly-A tail** is a long stretch of adenine nucleotides added to the **3' end of eukaryotic mRNA**, important for stability and translation efficiency. - It is **largely absent in prokaryotic mRNA** and does not play a role in ribosome binding in prokaryotes.

Practice by Chapter

Nucleotide Structure and Function

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DNA Structure and Replication

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RNA Structure and Types

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Transcription: RNA Synthesis

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Post-Transcriptional Modifications

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Translation: Protein Synthesis

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Genetic Code and Codon Usage

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Regulation of Gene Expression

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Mutations and DNA Repair

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Purine Metabolism and Disorders

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Pyrimidine Metabolism and Disorders

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Nucleotide Degradation and Salvage Pathways

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