Nucleic Acid Biochemistry Practice Questions

Q: Deamination of methylated cytosine forms which of the following?

Thymine. ***Thymine*** - Deamination of **5-methylcytosine** removes the amine group at the 4-position and replaces it with a keto group, forming **thymine**. - This reaction can lead to a common type of point mutation, as the DNA repair machinery might fail to distinguish this naturally occurring base from normal thymine. *Uracil* - **Uracil** is formed by the deamination of **unmethylated cytosine**, not methylated cytosine. - Uracil is a base found in RNA but not typically in DNA, so its presence in DNA signals a repair event. *Cytosine* - **Cytosine** is the original base before deamination occurs; deamination is a chemical modification that changes cytosine into another base. - If a base remains cytosine, it means deamination has not taken place. *Guanine* - **Guanine** is a purine base and is structurally unrelated to cytosine or its deamination products. - Deamination primarily affects pyrimidine bases like cytosine and uracil, not purines like guanine.

Q: Which bases are present in human DNA?

Adenine-guanine-cytosine-thymine. ***Adenine- guanine-cytosine-thymine*** - DNA (deoxyribonucleic acid) in humans, and most other organisms, is composed of four nitrogenous bases: **adenine (A), guanine (G), cytosine (C), and thymine (T)**. - These bases are paired specifically: **adenine with thymine (A-T)** and **guanine with cytosine (G-C)**, forming the rungs of the DNA double helix. *None of the above* - This option is incorrect because there is a definitive set of bases that make up human DNA, as described in the correct option. - The fundamental building blocks of human DNA are well-established and universally recognized in biology. *Adenine-guanine- cytosine-uracil* - This list of bases is characteristic of **RNA (ribonucleic acid)**, not DNA. - In RNA, **uracil (U)** replaces thymine (T), meaning RNA contains adenine (A), guanine (G), cytosine (C), and uracil (U). *Adenine-guanine-thiamine- uracil* - This option incorrectly includes **thiamine**, which is a **vitamin (B1)** and not a nitrogenous base found in nucleic acids. - It also incorrectly includes **uracil**, which is found in RNA, not DNA.

Q: Which organelle contains its own DNA apart from the nucleus?

Mitochondria. ***Mitochondria*** - Mitochondria contain their own **circular DNA (mtDNA)**, which is inherited maternally, and their own ribosomes. - This DNA encodes for some proteins essential for **cellular respiration** and its own replication, supporting the endosymbiotic theory. *RER* - The **Rough Endoplasmic Reticulum (RER)** is characterized by the presence of **ribosomes** on its surface. - It plays a crucial role in the **synthesis and modification of proteins** designated for secretion or insertion into membranes, but does not contain DNA. *Golgi complex* - The **Golgi complex** is involved in **modifying, sorting, and packaging proteins and lipids** for secretion or delivery to other organelles. - It consists of flattened sacs called cisternae but does not possess DNA. *SER* - The **Smooth Endoplasmic Reticulum (SER)** is involved in **lipid synthesis**, **detoxification** of drugs and poisons, and **calcium ion storage**. - Unlike mitochondria, it does not contain its own genetic material.

Q: Which is a product of purine metabolism?

Uric acid. ***Uric acid*** - **Uric acid** is the final breakdown product of **purine metabolism** in humans. - It's formed from the oxidation of **xanthine** by the enzyme **xanthine oxidase**. *Xanthine* - **Xanthine** is an intermediate compound in the purine metabolic pathway, but not the final product in humans. - It is converted to **uric acid** via the enzyme **xanthine oxidase**. *Beta alanine* - **Beta-alanine** is a **non-proteogenic amino acid** and is a breakdown product of **pyrimidine metabolism**, not purine. - It is involved in the synthesis of **carnosine**, a buffer in muscle tissue. *Urea* - **Urea** is the primary end product of **protein (amino acid) metabolism** in mammals, representing the main way the body excretes **nitrogen**. - It is produced in the **urea cycle** in the liver and is unrelated to purine breakdown.

Q: Which part of DNA is most susceptible to radiation?

Nucleotides. ***Nucleotides*** - Radiation primarily damages DNA at the **nucleotide level**, with the deoxyribose sugar component being most susceptible to ionizing radiation. - Radiation causes **hydroxyl radical formation** that attacks the sugar-phosphate backbone, leading to single-strand and double-strand breaks. - Purine and pyrimidine bases within nucleotides can also undergo radiation-induced modifications, causing **mutations** and loss of genetic information. *Nucleosides* - Nucleosides (base + sugar without phosphate) are not the functional unit within DNA strands. - While the sugar moiety is susceptible, nucleosides as isolated units are not the primary consideration when discussing **DNA strand damage**. - Radiation damage occurs to nucleotides as they exist in the DNA polymer, not to free nucleosides. *Double helix* - The double helix is the **overall structural configuration** of DNA, not a specific chemical component. - Radiation damages the double helix by affecting its constituent nucleotides, particularly through **sugar-phosphate backbone breaks**. - Double helix disruption is a consequence of nucleotide-level damage. *Phosphate groups* - Phosphate groups link nucleotides together but are relatively **less susceptible** to direct radiation damage compared to the deoxyribose sugar. - The phosphodiester bonds can be broken as a secondary effect of **sugar radical formation**, rather than being the primary target of radiation.

Q: What will happen to DNA if salt is added to it: (PGI Dec 2008)

Increase Melting point (Tm). ***Increase Melting point (Tm)*** - Adding **salt (ions)** to DNA stabilizes the **double helix structure** by shielding the negatively charged phosphate backbone. - This increased stability requires a **higher temperature** to break the hydrogen bonds and denature the DNA, thus increasing the melting point (Tm). *Not affect Tm* - This is incorrect because **ion concentration** is a significant factor influencing the stability of the DNA double helix. - Changes in salt concentration directly impact the electrostatic interactions within the DNA molecule. *Melting leads to denaturation of DNA* - While it's true that **melting refers to the denaturation** (separation of strands) of DNA, this option does not answer what happens *to the melting point* when salt is added. - DNA melting is a process, and the question asks about the effect of salt on the physical property of its melting temperature (Tm). *Decrease Tm* - This would occur if the DNA double helix were destabilized, for example, by reducing the **ionic strength** or adding denaturing agents. - Salt, by providing positive ions, actually helps stabilize the DNA structure, making it harder to denature.

Q: Inosinic acid is biological precursor of ?

Adenylic acid and guanylic acid. ***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**.

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

22%. ***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.

Q: True regarding mitochondrial DNA is:

Inherited from mother. ***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.

Q: A 3-month-old with recurrent infections and no thymus shadow has lymphopenia. What is the most likely enzyme deficiency?

Adenosine deaminase. ***Adenosine deaminase*** - **Adenosine deaminase (ADA)** deficiency is a hallmark cause of **severe combined immunodeficiency (SCID)**, characterized by profound B-cell and T-cell lymphopenia. - The absence of a **thymus shadow** on chest X-ray is a classic sign of T-cell developmental failure, highly suggestive of SCID, often due to ADA deficiency. *HGPRT* - Deficiency of **hypoxanthine-guanine phosphoribosyltransferase (HGPRT)** causes **Lesch-Nyhan syndrome**, which presents with **hyperuricemia**, neurological dysfunction, and self-mutilation. - It does not directly cause lymphopenia or thymic aplasia and is primarily a disorder of purine salvage rather than immune development. *Glucose-6-phosphatase* - **Glucose-6-phosphatase deficiency** leads to **Glycogen Storage Disease Type I (von Gierke disease)**, characterized by severe **fasting hypoglycemia**, lactic acidosis, hepatomegaly, and hyperlipidemia. - This enzyme deficiency is not associated with immunodeficiency, lymphopenia, or thymic abnormalities. *Pyruvate kinase* - **Pyruvate kinase deficiency** primarily affects red blood cells, causing **chronic hemolytic anemia** due to impaired glycolysis. - It is not associated with recurrent infections, lymphopenia, or T-cell deficiencies.

Question 1

Deamination of methylated cytosine forms which of the following?

Accepted Answer

Thymine

Answer

Uracil

Answer

Cytosine

Answer

Guanine

Question 2

Which bases are present in human DNA?

Accepted Answer

Adenine-guanine-cytosine-thymine

Answer

None of the above

Answer

Adenine-guanine-cytosine-uracil

Answer

Adenine-guanine-thiamine-uracil

Question 3

Which organelle contains its own DNA apart from the nucleus?

Accepted Answer

Mitochondria

Answer

RER

Answer

Golgi complex

Answer

SER

Question 4

Which is a product of purine metabolism?

Accepted Answer

Uric acid

Answer

Xanthine

Answer

Beta alanine

Answer

Urea

Question 5

Which part of DNA is most susceptible to radiation?

Accepted Answer

Nucleotides

Answer

Nucleosides

Answer

Double helix

Answer

Phosphate groups

Question 6

What will happen to DNA if salt is added to it: (PGI Dec 2008)

Accepted Answer

Increase Melting point (Tm)

Answer

Not affect Tm

Answer

Melting leads to denaturation of DNA

Answer

Decrease Tm

Question 7

Inosinic acid is biological precursor of ?

Accepted Answer

Adenylic acid and guanylic acid

Answer

Purines and thymine

Answer

Orotic acid and uridylic acid

Answer

Uracil and thymine

Question 8

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

Accepted Answer

22%

Answer

36%

Answer

44%

Answer

28%

Question 9

True regarding mitochondrial DNA is:

Accepted Answer

Inherited from mother

Answer

Linear double stranded

Answer

All respiratory proteins are synthesized within mitochondria itself

Answer

Low mutation rate

Question 10

A 3-month-old with recurrent infections and no thymus shadow has lymphopenia. What is the most likely enzyme deficiency?

Accepted Answer

Adenosine deaminase

Answer

HGPRT

Answer

Glucose-6-phosphatase

Answer

Pyruvate kinase

Nucleic Acid Biochemistry — MCQs

Nucleic Acid Biochemistry — MCQs

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