Nucleotide Structure
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All Questions for: Nucleotide Structure
Question 1: Which three components make up a nucleotide?
- A Phosphate, pentose sugar, and nitrogenous base
- B Phosphate, hexose sugar, and amino acid
- C Carboxyl group, amino acid, and lipid
- D Sugar, lipid, and phosphate
Explanation: A nucleotide consists of a phosphate group, a pentose sugar (ribose or deoxyribose), and a nitrogenous base. Together these form the monomeric unit of nucleic acids.
Question 2: A nucleoside differs from a nucleotide by lacking which component?
- A Phosphate group
- B Nitrogenous base
- C Pentose sugar
- D Hydroxyl group
Explanation: A nucleoside is a sugar-base complex. Adding one or more phosphate groups converts it to a nucleotide. Thus, the key difference is the absence/presence of the phosphate group.
Question 3: Which sugar is found in DNA?
- A Deoxyribose
- B Ribose
- C Glucose
- D Galactose
Explanation: DNA contains 2'-deoxyribose, which lacks a hydroxyl group at the 2' carbon. RNA instead contains ribose.
Question 4: Ribose and deoxyribose differ chemically at which carbon position?
- A 2' carbon
- B 3' carbon
- C 5' carbon
- D 1' carbon
Explanation: Ribose has a hydroxyl (–OH) group at the 2' carbon; deoxyribose has only hydrogen at that position, making it more stable and less reactive.
Question 5: Which of the following are purine bases?
- A Adenine and guanine
- B Cytosine and thymine
- C Adenine and thymine
- D Guanine and cytosine
Explanation: Purines (adenine and guanine) have a double-ring structure. Pyrimidines (cytosine, thymine, uracil) have a single-ring structure.
Question 6: Which of the following are pyrimidine bases?
- A Cytosine, thymine, and uracil
- B Adenine and guanine only
- C Adenine, guanine, and cytosine
- D Thymine and adenine
Explanation: Cytosine, thymine, and uracil are pyrimidines (single-ring structures). Adenine and guanine are purines.
Question 7: How many hydrogen bonds form between adenine and thymine in DNA?
- A 2
- B 1
- C 3
- D 4
Explanation: Adenine pairs with thymine via two hydrogen bonds. This is part of the complementary base-pairing rules described by Watson and Crick.
Question 8: How many hydrogen bonds form between guanine and cytosine?
- A 3
- B 2
- C 1
- D 4
Explanation: Guanine and cytosine form three hydrogen bonds, making G-C–rich regions of DNA more thermally stable than A-T–rich regions.
Question 9: Which nitrogenous base is found in RNA but not in DNA?
- A Uracil
- B Thymine
- C Cytosine
- D Adenine
Explanation: Uracil replaces thymine in RNA. Both pair with adenine.
Question 10: Which nitrogenous base is found in DNA but not in RNA?
- A Thymine
- B Uracil
- C Cytosine
- D Guanine
Explanation: Thymine is unique to DNA. RNA uses uracil in its place. This difference also contributes to DNA's greater chemical stability.
Question 11: The bond linking the 3' carbon of one nucleotide's sugar to the 5' phosphate of the next is called a:
- A Phosphodiester bond
- B Peptide bond
- C Glycosidic bond
- D Hydrogen bond
Explanation: Phosphodiester bonds form the sugar-phosphate backbone of DNA and RNA and are cleaved by nucleases during processes such as replication and repair.
Question 12: Chargaff's rules state that in double-stranded DNA:
- A Adenine equals thymine, and guanine equals cytosine
- B Adenine equals guanine, and thymine equals cytosine
- C Adenine equals cytosine, and thymine equals guanine
- D All four bases are present in equal amounts
Explanation: Chargaff's findings: the amount of adenine equals thymine (A=T) and guanine equals cytosine (G=C), reflecting their complementary base pairing.
Question 13: If a double-stranded DNA molecule contains 22% cytosine, what percentage of the bases is adenine?
- A 28%
- B 22%
- C 44%
- D 56%
Explanation: If C=22%, then G=22% (Chargaff). Remaining 56% is split equally between A and T, so A=28% and T=28%.
Question 14: Which nucleotide serves as the primary energy currency of the cell?
- A ATP
- B GTP
- C AMP
- D NAD+
Explanation: Adenosine triphosphate (ATP) is a nucleotide derivative whose phosphoanhydride bonds release energy upon hydrolysis, driving most cellular processes.
Question 15: Purine bases have how many rings in their structure?
- A Two rings
- B One ring
- C Three rings
- D Four rings
Explanation: Purines (adenine, guanine) contain a fused double-ring structure: a pyrimidine ring fused to an imidazole ring.
Question 16: Pyrimidine bases have how many rings in their structure?
- A One ring
- B Two rings
- C Three rings
- D Four rings
Explanation: Pyrimidines (cytosine, thymine, uracil) consist of a single six-membered ring containing two nitrogens.
Question 17: Which of the following is a nucleotide derivative acting as a second messenger in signal transduction?
- A cAMP
- B dATP
- C GTP
- D RNA
Explanation: Cyclic AMP (cAMP) is generated from ATP by adenylate cyclase and transmits signals in many hormone pathways.
Question 18: NAD+ and FAD function as electron carriers and are derived from which vitamins?
- A Niacin and riboflavin respectively
- B Thiamine and pyridoxine respectively
- C Cobalamin and folate respectively
- D Vitamin A and vitamin C respectively
Explanation: NAD+ is derived from niacin (vitamin B3) and FAD from riboflavin (vitamin B2). Both contain adenylate components and act as coenzymes.
Question 19: The 5' end of a DNA strand is characterized by:
- A A free phosphate group on the 5' carbon
- B A free hydroxyl group on the 5' carbon
- C A free amino group
- D A methylated base
Explanation: By convention, the 5' end of a nucleic acid strand carries a free phosphate group attached to the 5' carbon of the terminal sugar.
Question 20: The 3' end of a DNA strand is characterized by:
- A A free hydroxyl group on the 3' carbon
- B A free phosphate group on the 3' carbon
- C A capped triphosphate
- D A methylated guanine
Explanation: The 3' end of a nucleic acid strand has a free hydroxyl (–OH) group on the 3' carbon of the terminal sugar.
Question 21: Why is DNA more chemically stable than RNA?
- A Because DNA lacks the 2' hydroxyl group and is usually double-stranded
- B Because DNA contains uracil instead of thymine
- C Because DNA is single-stranded
- D Because DNA has more phosphate groups
Explanation: The absence of a 2' hydroxyl group in deoxyribose means DNA resists alkaline hydrolysis and ribonuclease activity far better than RNA.
Question 22: The sugar-phosphate backbone of DNA is:
- A Negatively charged at physiological pH
- B Positively charged at physiological pH
- C Neutral at all pH values
- D Variably charged depending on nucleotide sequence
Explanation: At physiological pH, each phosphodiester group carries a net negative charge, giving DNA its characteristic negative charge exploited in gel electrophoresis.
Question 23: Which bond attaches the nitrogenous base to the pentose sugar in a nucleoside?
- A N-glycosidic bond
- B Phosphodiester bond
- C Peptide bond
- D Ester bond
Explanation: A glycosidic (N-glycosidic) bond links the base's nitrogen to the anomeric C1' carbon of the sugar.
Question 24: Which of the following represents a nucleotide correctly?
- A Adenosine monophosphate (AMP)
- B Adenosine (nucleoside without phosphate)
- C Ribose alone
- D Adenine alone
Explanation: A nucleotide must include all three parts: a phosphate, a sugar, and a nitrogenous base. Adenosine monophosphate (AMP) contains all three.