Reasoning & Distinguish Questions

CHEExam Practice & Reasoning

These give-reason and short-distinguish items reward a single precise cause: a bond type, a base-pair count, or a missing enzyme. Most cluster around carbohydrate digestibility and DNA/RNA stability and pairing.

Complementary, antiparallel pairing

5'\text{-}ATGCTA\text{-}3' \ \longleftrightarrow\ 3'\text{-}TACGAT\text{-}5'

Pair

A

with

T

and

G

with

C

; the complement runs antiparallel.

Base-pair stability

G\text{-}C\ (3\ \text{H-bonds}) > A\text{-}T\ (2\ \text{H-bonds})

More hydrogen bonds = greater thermal stability of the pair.

Cellulose is indigestible to humans because its glucose units are joined by $\beta\text{-}(1\rightarrow4)$ glycosidic bonds, which human enzymes cannot hydrolyse; starch's $\alpha\text{-}(1\rightarrow4)$ links are digestible.
DNA is more stable than RNA because the 2-deoxyribose sugar lacks the C2 $\text{-}OH$ present in RNA's ribose, removing a reactive group prone to base-catalysed hydrolysis.
A G–C base pair is more stable than A–T because G–C is held by three hydrogen bonds whereas A–T has only two.
Nucleoside vs nucleotide: a nucleoside is base + sugar; a nucleotide adds a phosphate (a phosphorylated nucleoside).
To write a complementary DNA strand, pair A with T and G with C, and remember the two strands are antiparallel — read the complement $3'\rightarrow5'$ opposite the given $5'\rightarrow3'$ strand.
RNA is single-stranded and chemically more reactive than the double-stranded DNA helix, contributing to its lower stability and shorter cellular lifetime.
Guanine in DNA: the nucleoside is deoxyguanosine (guanine + deoxyribose) and the nucleotide is deoxyguanosine monophosphate (dGMP, with phosphate added).
Sucrose is non-reducing while glucose is reducing because sucrose's anomeric carbons are both locked in the glycosidic bond, leaving no free hemiacetal $\text{-}OH$ .
Amino acids have abnormally high melting points (for their mass) because they exist as ionic zwitterions, giving a strong salt-like crystal lattice.
Vitamin C must be supplied daily because it is water-soluble and not stored, being excreted in urine; fat-soluble vitamins are stored and need not be taken daily.
A DNA-rich-in-GC sample needs a higher temperature to 'melt' (separate the strands) than an AT-rich sample, a direct consequence of the extra hydrogen bond per GC pair.

Where the marks go

Attributing cellulose's indigestibility to glucose itself — the cause is the specific $\beta\text{-}1,4$ linkage that human enzymes cannot cleave (starch's $\alpha\text{-}1,4$ is digestible).
Giving DNA's stability reason as 'double-stranded' alone — the chemical reason expected is the missing C2-OH of deoxyribose.
Reversing the H-bond counts: A-T has 2 bonds, G-C has 3, so G-C is the more stable pair.
Writing the complementary strand in the same direction (parallel) — strands are antiparallel, so the complement is read $3'\rightarrow5'$ .
Pairing A with G or C with T — purine pairs only with the correct pyrimidine (A with T, G with C).

How the board asks it

Give reasonsglycosidic bond type as the single cause of digestibility
Give reasons: Cellulose is indigestible to humans whereas starch is readily digested.
Distinguishstructural definitions: nucleoside vs nucleotide, reducing vs non-reducing sugar
Distinguish between a nucleoside and a nucleotide, giving one example of each.
Assertion–Reasonthree h-bonds in g-c vs two in a-t and the resulting melting temperature
Assertion: A GC-rich DNA sample requires a higher temperature to melt than an AT-rich sample. Reason: A $G\text{-}C$ pair has three hydrogen bonds while an $A\text{-}T$ pair has only two. Select the correct option.
Predict the productcomplementary, antiparallel base-pairing rules
Write the complementary strand for the DNA sequence $5'\text{-}ATGCCG\text{-}3'$ , clearly indicating its $3'\rightarrow5'$ orientation.
Identify / classifynaming the nucleoside and nucleotide of a given base
Name the nucleoside and the nucleotide formed when guanine combines with deoxyribose in DNA.
Give reasonszwitterion ionic lattice raising the melting point
Account for the fact that amino acids have abnormally high melting points for their molecular mass.

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Written for Sublevo. Question text quoted anywhere in these notes is the Council’s and carries its year and paper; the board’s own diagrams are not reproduced.