DNA Structure & Replication — Practice · Grade 10 Science

Chapter: DNA, Inheritance & Gene Expression

What this practice covers

By the end of DNA Structure & Replication you should be able to:

10 questions, multiple choice, numeric and written answer, each with a worked explanation.

Vocabulary used: nucleotide, double helix, complementary base pairing, semiconservative replication, DNA polymerase, antiparallel, semiconservative.

Problem 1

In DNA, the base adenine (A) always pairs with
  1. thymine (T)
  2. cytosine (C)
  3. guanine (G)
  4. another adenine (A)
Answer and explanation

thymine (T)

A-T and C-G are the complementary base pairs; adenine always bonds with thymine.

Problem 2

Which best describes the overall shape of a DNA molecule?
  1. A double helix — a twisted ladder
  2. A single straight line
  3. A flat triangle
  4. A solid sphere
Answer and explanation

A double helix — a twisted ladder

Two strands wind around each other to form the double-helix, or twisted-ladder, shape.

Problem 3

One strand of DNA contains 40 adenine (A) bases. On the strand that pairs with it, how many thymine (T) bases must there be to pair with them?
Answer and explanation

40 thymine bases

Adenine pairs only with thymine, so 40 A bases require 40 T partners.

Problem 4

Explain why DNA replication is described as 'semiconservative.'
Answer and explanation

Each new DNA molecule is made of one original (conserved) strand and one newly built strand, so half of every copy is 'old.' That is what 'semiconservative' means.

Problem 5

Why does complementary base pairing make DNA easy to copy accurately?
  1. Each strand acts as a template that determines exactly which bases must be added to build its partner
  2. The bases pair together completely at random
  3. Both strands are identical, so one can be thrown away
  4. DNA does not need to be copied before a cell divides
Answer and explanation

Each strand acts as a template that determines exactly which bases must be added to build its partner

Because A only pairs with T and C only with G, one strand's sequence dictates its partner's, so each old strand is a perfect template for building a new one.

Problem 6

An original DNA strand has bases A-C-G-T. Give its complementary sequence and explain the pairing rule. Explain the reasoning in your own words.
Answer and explanation

T-G-C-A in corresponding paired positions; actual strands have opposite orientation.

A pairs with T and C with G.

Apply the rule to each position in the supplied order.

Response criteria

  • Use the stated conditions.
  • Show the evidence or calculation.
  • Explain the conclusion and any stated limitation.

Problem 7

Write the complement of TTGC using base-pair rules. Show the reasoning, not just the conclusion.
Hints

Pairing rules provide a template mechanism; sequence order still matters.

Answer and explanation

AACG, pairing T with A and G with C.

Problem 8

Why can an unrepaired replication error become a mutation?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

A sequence change can persist and be copied into later DNA molecules.

Problem 9

Each copied DNA duplex retains one original strandEach copied DNA duplex retains one original strandATTAGCCGATTAGCCGATTAGCCGSemiconservative copying retains one old strand per productBlue: original strandsGreen: newly synthesized strands
Original strand-origin and pairing comparison. Colors identify strand history, not different base chemicals or all replication-fork proteins.
Interpret the supplied visual. After one ideal round, does either product contain two entirely old strands?
Answer and explanation

No. Each has one old and one new strand under the semiconservative model.

Semiconservative replication produces two duplexes, each with an original template strand and a newly synthesized partner in the basic model. Strand separation exposes templates, and pairing constrains new sequence. DNA polymerase builds in the 5′→3′ direction, with additional enzymes and mechanisms supporting copying and ligation. Replication is not splitting each base in half or preserving an entirely old double helix beside an entirely new one. One old strand per product is the central inheritance pattern; actual forks involve more detailed leading/lagging behavior.

No. Each has one old and one new strand under the semiconservative model.

Response criteria

  • Read the structures, axes, directions and units correctly.
  • Connect the evidence with the biological mechanism.
  • Respect stated model assumptions and distinguish an observation from a further inference.

Problem 10

Write the complementary strand opposite 5′-AACGTT-3′, then explain strand origin after one semiconservative round.
Answer and explanation

The directly paired complement is 3′-TTGCAA-5′. Each resulting duplex has one old template strand and one newly synthesized partner in the ideal model.

The directly paired complement is 3′-TTGCAA-5′. Each resulting duplex has one old template strand and one newly synthesized partner in the ideal model.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the biological mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Browse all courses