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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?
Hydroxyl group
Pentose sugar
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?
Galactose
Ribose
Deoxyribose
Glucose
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?
1' carbon
5' carbon
2' carbon
3' 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?
Cytosine and thymine
Guanine and cytosine
Adenine and thymine
Adenine and guanine
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?
Thymine and adenine
Cytosine, thymine, and uracil
Adenine, guanine, and cytosine
Adenine and guanine only
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
1
3
4
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?
Cytosine
Adenine
Thymine
Uracil
Explanation:
Uracil replaces thymine in RNA. Both pair with adenine.
Q.
Which nitrogenous base is found in DNA but not in RNA?
Thymine
Uracil
Cytosine
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:
Glycosidic bond
Peptide bond
Hydrogen bond
Phosphodiester 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 guanine, and thymine equals cytosine
Adenine equals thymine, and guanine equals cytosine
Adenine equals cytosine, and thymine equals guanine
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?
GTP
NAD+
AMP
ATP
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?
One ring
Two rings
Four rings
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?
Four rings
Three rings
Two rings
One ring
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?
RNA
GTP
dATP
cAMP
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?
Vitamin A and vitamin C respectively
Thiamine and pyridoxine respectively
Niacin and riboflavin respectively
Cobalamin and folate 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 amino group
A free hydroxyl group on the 5' carbon
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 free phosphate group on the 3' carbon
A methylated guanine
A capped triphosphate
A free hydroxyl group on the 3' carbon
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 lacks the 2' hydroxyl group and is usually double-stranded
Because DNA has more phosphate groups
Because DNA is single-stranded
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:
Negatively charged at physiological pH
Neutral at all pH values
Positively charged at physiological pH
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.
Q.
Which bond attaches the nitrogenous base to the pentose sugar in a nucleoside?
Ester bond
Peptide 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 (nucleoside without phosphate)
Ribose alone
Adenine alone
Adenosine monophosphate (AMP)
Explanation:
A nucleotide must include all three parts: a phosphate, a sugar, and a nitrogenous base. Adenosine monophosphate (AMP) contains all three.