MMSC491 Exam 1, Exam 2, Exam 3 – Complete Exam Study
Guide
Practice Questions with Verified ANSWER-s | 100%
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In an RNA duplex region, which bases can form stable pairs with more than one type of base?
a) Adenine and uracil – Incorrect: adenine pairs only with uracil in RNA helices.
b) Cytosine and uracil – Incorrect: cytosine pairs exclusively with guanine.
c) Guanine and uracil – ANS ✔ Guanine wobble-pairs with uracil, and uracil also pairs with adenine, so
both have two stable partners.
d) Adenine and guanine – Incorrect: adenine is limited to one partner (U).
Regarding base structure, which statement is INCORRECT?
a) Pyrimidines have one six-membered ring of 4 C + 2 N – Correct statement.
b) Purines have two fused rings, each containing nitrogen – Correct statement.
c) Thymine = 5-methyluracil – Correct statement.
d) Purines carry an amino group, unlike pyrimidines – ANS ✔ Incorrect: cytosine (a pyrimidine) also
carries an amino group.
Regarding the sugar residue in nucleic acids, which statement is INCORRECT?
a) The ring contains one oxygen and four carbons – Correct statement (furanose).
b) The 5′ carbon is exocyclic and bears the phosphate – Correct statement.
c) The ring contains five carbon atoms – ANS ✔ Incorrect: only four carbons are in the ring; the fifth is
exocyclic.
d) DNA lacks the 2′-OH present in RNA – Correct statement.
A nucleoside consists of:
a) Base + sugar – ANS ✔ A nucleoside is base linked to sugar with no phosphate.
b) Base + phosphate – Incorrect: that combination lacks the sugar.
c) Sugar + phosphate – Incorrect: that is just a sugar phosphate, no base.
,d) Base + sugar + phosphate – Incorrect: that is a nucleotide.
The bond joining the base to the sugar is:
a) Phosphodiester bond – Incorrect: that links nucleotides in the backbone.
b) N-glycosidic bond – ANS ✔ Formed at N9 of purines and N1 of pyrimidines.
c) Hydrogen bond – Incorrect: H-bonds join bases across strands.
d) Ester bond – Incorrect: ester chemistry applies to the phosphate backbone linkages.
At physiological pH, DNA migrates toward the anode in electrophoresis because:
a) Bases are positively charged – Incorrect: bases are essentially neutral at pH 7.
b) Phosphate groups carry a negative charge – ANS ✔ Each phosphodiester retains a free negative
charge.
c) Sugars are negatively charged – Incorrect: ribose/deoxyribose are uncharged.
d) It depends on the base sequence – Incorrect: charge is uniform per nucleotide.
Which is a feature of B-form DNA?
a) Left-handed helix – Incorrect: B-DNA is right-handed.
b) ~10.5 base pairs per turn with a wide major groove – ANS ✔ This is the classic Watson–Crick
physiological form.
c) Favored under dehydrating conditions – Incorrect: that describes A-DNA.
d) Single narrow groove – Incorrect: that describes Z-DNA.
Z-DNA is characterized by:
a) Right-handed helix with 11 bp/turn – Incorrect: that resembles A-DNA.
b) Left-handed zigzag helix favored by alternating G-C sequences – ANS ✔ Z-DNA forms in GC
alternation and negative supercoiling.
c) Only forms in RNA – Incorrect: Z-DNA is a DNA conformation.
d) Wide major groove rich in protein contacts – Incorrect: Z-DNA has one shallow groove.
The dominant force stabilizing the double helix is:
a) Hydrogen bonds between bases – Incorrect: they give specificity, not most stability.
b) Base stacking (hydrophobic/van der Waals) interactions – ANS ✔ Stacking of adjacent bases provides
the major stabilization.
c) Covalent bonds between strands – Incorrect: strands are held non-covalently.
d) Ionic bonds between sugars – Incorrect: sugars are uncharged.
, Heating DNA increases absorbance at 260 nm. This is called:
a) Hypochromic effect – Incorrect: hypochromic is the decrease on renaturation.
b) Hyperchromic effect – ANS ✔ Unstacked single strands absorb more UV light.
c) Bathochromic shift – Incorrect: that is a wavelength shift, not intensity rise.
d) Melting anomaly – Incorrect: not a real term here.
The melting temperature (Tm) of DNA increases with:
a) Higher A-T content – Incorrect: A-T pairs have only two H-bonds.
b) Higher G-C content and higher salt concentration – ANS ✔ G-C pairs stack better with three H-
bonds; cations shield phosphates.
c) Lower ionic strength – Incorrect: low salt destabilizes the helix.
d) Presence of formamide – Incorrect: formamide is a denaturant that lowers Tm.
In a C₀t (renaturation) curve, the fast-renaturing fraction represents:
a) Unique single-copy genes – Incorrect: unique sequences renature slowly.
b) Repetitive DNA – ANS ✔ High copy number raises collision frequency, speeding reannealing.
c) Telomeric RNA – Incorrect: RNA is not measured in DNA C₀t.
d) Mitochondrial protein – Incorrect: irrelevant to renaturation kinetics.
RNA is hydrolyzed by alkali because:
a) Its bases are alkali-labile – Incorrect: bases are stable; the sugar reacts.
b) The 2′-OH attacks the phosphate forming a 2′,3′-cyclic intermediate – ANS ✔ DNA lacks 2′-OH and
resists alkali.
c) RNA lacks phosphate – Incorrect: RNA is a phosphate polymer.
d) RNA is always double-stranded – Incorrect: RNA is usually single-stranded.
The most abundant RNA species in a cell is:
a) mRNA – Incorrect: mRNA is ~5% of total RNA.
b) tRNA – Incorrect: tRNA is ~15%.
c) rRNA – ANS ✔ Ribosomal RNA is ~80% of cellular RNA.
d) miRNA – Incorrect: small RNAs are a minor fraction.
Amino acids are attached to tRNA at:
a) The 5′ cap – Incorrect: tRNA is not capped.
Guide
Practice Questions with Verified ANSWER-s | 100%
Guaranteed Value Pack GRADED A+
Latest 2026/2027 Update
In an RNA duplex region, which bases can form stable pairs with more than one type of base?
a) Adenine and uracil – Incorrect: adenine pairs only with uracil in RNA helices.
b) Cytosine and uracil – Incorrect: cytosine pairs exclusively with guanine.
c) Guanine and uracil – ANS ✔ Guanine wobble-pairs with uracil, and uracil also pairs with adenine, so
both have two stable partners.
d) Adenine and guanine – Incorrect: adenine is limited to one partner (U).
Regarding base structure, which statement is INCORRECT?
a) Pyrimidines have one six-membered ring of 4 C + 2 N – Correct statement.
b) Purines have two fused rings, each containing nitrogen – Correct statement.
c) Thymine = 5-methyluracil – Correct statement.
d) Purines carry an amino group, unlike pyrimidines – ANS ✔ Incorrect: cytosine (a pyrimidine) also
carries an amino group.
Regarding the sugar residue in nucleic acids, which statement is INCORRECT?
a) The ring contains one oxygen and four carbons – Correct statement (furanose).
b) The 5′ carbon is exocyclic and bears the phosphate – Correct statement.
c) The ring contains five carbon atoms – ANS ✔ Incorrect: only four carbons are in the ring; the fifth is
exocyclic.
d) DNA lacks the 2′-OH present in RNA – Correct statement.
A nucleoside consists of:
a) Base + sugar – ANS ✔ A nucleoside is base linked to sugar with no phosphate.
b) Base + phosphate – Incorrect: that combination lacks the sugar.
c) Sugar + phosphate – Incorrect: that is just a sugar phosphate, no base.
,d) Base + sugar + phosphate – Incorrect: that is a nucleotide.
The bond joining the base to the sugar is:
a) Phosphodiester bond – Incorrect: that links nucleotides in the backbone.
b) N-glycosidic bond – ANS ✔ Formed at N9 of purines and N1 of pyrimidines.
c) Hydrogen bond – Incorrect: H-bonds join bases across strands.
d) Ester bond – Incorrect: ester chemistry applies to the phosphate backbone linkages.
At physiological pH, DNA migrates toward the anode in electrophoresis because:
a) Bases are positively charged – Incorrect: bases are essentially neutral at pH 7.
b) Phosphate groups carry a negative charge – ANS ✔ Each phosphodiester retains a free negative
charge.
c) Sugars are negatively charged – Incorrect: ribose/deoxyribose are uncharged.
d) It depends on the base sequence – Incorrect: charge is uniform per nucleotide.
Which is a feature of B-form DNA?
a) Left-handed helix – Incorrect: B-DNA is right-handed.
b) ~10.5 base pairs per turn with a wide major groove – ANS ✔ This is the classic Watson–Crick
physiological form.
c) Favored under dehydrating conditions – Incorrect: that describes A-DNA.
d) Single narrow groove – Incorrect: that describes Z-DNA.
Z-DNA is characterized by:
a) Right-handed helix with 11 bp/turn – Incorrect: that resembles A-DNA.
b) Left-handed zigzag helix favored by alternating G-C sequences – ANS ✔ Z-DNA forms in GC
alternation and negative supercoiling.
c) Only forms in RNA – Incorrect: Z-DNA is a DNA conformation.
d) Wide major groove rich in protein contacts – Incorrect: Z-DNA has one shallow groove.
The dominant force stabilizing the double helix is:
a) Hydrogen bonds between bases – Incorrect: they give specificity, not most stability.
b) Base stacking (hydrophobic/van der Waals) interactions – ANS ✔ Stacking of adjacent bases provides
the major stabilization.
c) Covalent bonds between strands – Incorrect: strands are held non-covalently.
d) Ionic bonds between sugars – Incorrect: sugars are uncharged.
, Heating DNA increases absorbance at 260 nm. This is called:
a) Hypochromic effect – Incorrect: hypochromic is the decrease on renaturation.
b) Hyperchromic effect – ANS ✔ Unstacked single strands absorb more UV light.
c) Bathochromic shift – Incorrect: that is a wavelength shift, not intensity rise.
d) Melting anomaly – Incorrect: not a real term here.
The melting temperature (Tm) of DNA increases with:
a) Higher A-T content – Incorrect: A-T pairs have only two H-bonds.
b) Higher G-C content and higher salt concentration – ANS ✔ G-C pairs stack better with three H-
bonds; cations shield phosphates.
c) Lower ionic strength – Incorrect: low salt destabilizes the helix.
d) Presence of formamide – Incorrect: formamide is a denaturant that lowers Tm.
In a C₀t (renaturation) curve, the fast-renaturing fraction represents:
a) Unique single-copy genes – Incorrect: unique sequences renature slowly.
b) Repetitive DNA – ANS ✔ High copy number raises collision frequency, speeding reannealing.
c) Telomeric RNA – Incorrect: RNA is not measured in DNA C₀t.
d) Mitochondrial protein – Incorrect: irrelevant to renaturation kinetics.
RNA is hydrolyzed by alkali because:
a) Its bases are alkali-labile – Incorrect: bases are stable; the sugar reacts.
b) The 2′-OH attacks the phosphate forming a 2′,3′-cyclic intermediate – ANS ✔ DNA lacks 2′-OH and
resists alkali.
c) RNA lacks phosphate – Incorrect: RNA is a phosphate polymer.
d) RNA is always double-stranded – Incorrect: RNA is usually single-stranded.
The most abundant RNA species in a cell is:
a) mRNA – Incorrect: mRNA is ~5% of total RNA.
b) tRNA – Incorrect: tRNA is ~15%.
c) rRNA – ANS ✔ Ribosomal RNA is ~80% of cellular RNA.
d) miRNA – Incorrect: small RNAs are a minor fraction.
Amino acids are attached to tRNA at:
a) The 5′ cap – Incorrect: tRNA is not capped.
