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