DoAssignment.ca

DoAssignment study guide

D3.1 · Explain DNA replication and repair

Learn to explain dna replication and repair through clear examples and targeted practice.

Ontario Grade 12 Biology

Molecular Genetics

How cells copy DNA and correct many errors

From SBI3U, you know that DNA carries information used by cells and that DNA is passed to new cells when cells divide. Before division, a cell needs to copy its DNA. The copy must preserve the order of its bases as closely as possible. A copying error or damage can change that order. Cells have processes that copy DNA and correct many problems. This lesson focuses on those processes. Replication makes a copy; repair fixes a problem in DNA. Neither process is perfectly error-free.

What you will learn

  • Explain why DNA replication occurs before cell division.
  • Describe how complementary base pairing guides DNA replication.
  • Identify the main roles of enzymes involved in replication.
  • Explain how proofreading and repair can correct DNA problems.

1. SBI3U bridge: DNA as a template

DNA is a molecule that stores genetic information. It consists of two strands twisted together. Each strand is built from smaller units that include one of four bases: adenine (A), thymine (T), cytosine (C), or guanine (G). The order of bases carries information.
The bases pair in a consistent way across the two strands. A pairs with T, and C pairs with G. This is complementary base pairing. For example, the strand ACGT matches with TGCA. A template is a strand that guides the formation of a matching strand.
These pairing rules make copying possible. If the two DNA strands separate, each can serve as a template. A new strand can form by matching bases to the exposed bases of that template.
A−T, C−G\mathrm{A-T,\ C-G}
  • DNA has two strands with complementary base sequences.
  • A pairs with T; C pairs with G.
  • A template guides the formation of a matching strand.

2. Replication: copying DNA

DNA replication is the process of making a DNA copy. It happens before cell division so each new cell can receive DNA. In the basic model, the two original strands separate. Each original strand guides the building of a new complementary strand.
Several enzymes, which are proteins that help cell processes happen, have distinct jobs. Helicase separates the two DNA strands by disrupting the attractions between paired bases. Primase makes a short RNA starting piece, called a primer, that gives DNA synthesis a place to begin. DNA polymerase adds DNA building blocks to the growing strand, following the template's base sequence. DNA polymerase can also proofread newly added bases and remove some that do not match.
DNA polymerase builds new DNA in sections on one of the templates. DNA ligase joins the sections of newly made DNA into a continuous strand. In this way, the enzymes help separate the strands, build complementary strands, and join sections. These roles are different, even though they work toward the same outcome.
The result is described as semiconservative replication. Semiconservative means that each resulting DNA molecule contains one original strand and one newly made strand. Evidence from experiments comparing DNA with different forms of nitrogen supported this model. This is evidence for the pattern of replication, not a claim that a simple classroom model shows every event inside a living cell.
Replication is not transcription or translation. Transcription makes an RNA copy of a DNA section. Translation uses information in RNA to help make a protein. Replication copies DNA so it can be passed to new cells.
ACGT→TGCA\mathrm{ACGT \rightarrow TGCA}
  • Helicase separates strands; primase makes a starting primer; DNA polymerase builds DNA; DNA ligase joins DNA sections.
  • Each original strand guides a complementary new strand.
  • Each DNA molecule after replication has one original strand and one new strand.

3. Repair: correcting DNA problems

DNA can be damaged, and bases can be paired incorrectly during copying. A mismatch is a pair of bases that does not follow the usual pairing rules. If a problem remains, it may change the DNA sequence. A lasting change in the sequence is called a mutation.
DNA polymerase proofreading checks newly made DNA as replication proceeds. If it detects a base that does not match the template, it can remove that base and add a more suitable one. This correction helps reduce copying errors.
Other repair processes can act after a problem appears. In mismatch repair, a repair system recognizes a mismatch and removes a short section of the newly made strand around it. DNA polymerase replaces that section using the other strand as a guide. DNA ligase then seals the remaining break in the strand.
Excision repair deals with a damaged section. Excision means removal. In a simplified model, repair proteins recognize the damaged area and remove a section containing it. DNA polymerase fills the gap using the undamaged strand as a template, and DNA ligase joins the repaired section to the rest of the strand. The intact strand helps indicate which bases belong in the replacement.
Repair can correct many problems, but it does not find or fix every one. If a problem is not corrected, it can remain in the DNA sequence. The pairing model explains how an intact strand can guide replacement, but it leaves out details about how repair proteins recognize particular problems.
damaged section→removed→replaced\mathrm{damaged\ section \rightarrow removed \rightarrow replaced}
  • Proofreading can remove an incorrectly added base during DNA synthesis.
  • Mismatch repair and excision repair remove and replace DNA sections.
  • DNA polymerase fills a gap; DNA ligase joins the strand.
  • Repair reduces errors but does not guarantee that every problem is fixed.

4. Using the model and its limits

For a sequence question, first identify the strand that is given. Apply the pairing rules one base at a time. Then explain what the new sequence represents. In replication, it is a new strand built beside a template. In repair, it may be a replacement for a removed section.
The same base-pairing rules apply in both processes, but their purposes differ. Replication copies the DNA molecule before cell division. Repair addresses damage or a copying problem in DNA. Naming the process helps prevent these ideas from being confused.
A short sequence is a useful model for practising base matching. It is not a picture of a whole DNA molecule or a full account of every enzyme and repair step in a cell. Keep conclusions limited to what the example shows: complementary matching, copying, or replacement.
  • Use base-pairing rules to find a complementary sequence.
  • State whether the example describes copying or repair.
  • A short sequence model shows the core idea, not every cellular detail.

Worked example

Predicting a complementary strand

A DNA template strand has the sequence GCTA. What sequence forms alongside it during replication?
  1. Match each base
    Use the pairing rules at each position. G matches C, C matches G, T matches A, and A matches T.
    GCTA→CGAT\mathrm{GCTA \rightarrow CGAT}
  2. Interpret the sequence
    The sequence on the right is the new strand. It is complementary to the given template strand.
Answer: The new strand is CGAT.
Check: Each base in CGAT is paired with its correct partner in GCTA.

Worked example

Following the strand composition

One DNA molecule contains two original strands. After semiconservative replication, describe the strands in each of the two DNA molecules.
  1. Use each original strand as a template
    The original strands separate. Each one guides the formation of a new complementary strand.
  2. Describe both products
    Each product receives one of the original strands and the new strand built beside it. The name semiconservative describes this retention of one original strand in each product.
    old+new, old+new\mathrm{old+new,\ old+new}
Answer: Each resulting DNA molecule contains one original strand and one newly made strand.
Check: There are two original strands, and each guides one new complementary strand.

Worked example

Explaining excision repair

A damaged section is removed from one DNA strand. The complementary strand remains undamaged. Explain how the gap can be repaired.
  1. Use the intact strand as a guide
    The undamaged strand acts as a template. Its bases indicate which complementary DNA building blocks should fill the gap.
  2. Fill and join the gap
    DNA polymerase adds replacement DNA by following the template. DNA ligase joins the replacement section to the rest of the strand.
    template→replacement→joined strand\mathrm{template \rightarrow replacement \rightarrow joined\ strand}
Answer: DNA polymerase replaces the missing section using the undamaged strand as a template, and DNA ligase joins the repaired section.
Check: The template strand supplies the pairing pattern, while the enzymes that build and join DNA have separate roles.

Common mistakes and how to avoid them

Saying that replication makes one molecule entirely from old DNA and one entirely from new DNA.
Correction: Semiconservative replication produces two molecules, each with one original strand and one newly made strand.
Treating helicase, DNA polymerase, and DNA ligase as if they do the same job.
Correction: Helicase separates strands, DNA polymerase builds DNA and can proofread, and DNA ligase joins DNA sections.
Calling repair another name for replication.
Correction: Replication copies DNA before cell division. Repair corrects damage or errors in DNA.
Claiming that repair prevents every mutation.
Correction: Repair corrects many problems, but some can remain and become lasting sequence changes.

Lesson summary

  • DNA replication makes a copy before cell division.
  • Complementary base pairing lets each original strand guide a new strand.
  • Helicase separates strands, primase makes a primer, DNA polymerase builds DNA and proofreads, and DNA ligase joins sections.
  • Semiconservative replication leaves one original and one new strand in each resulting molecule.
  • Proofreading and repair correct many problems, but they do not guarantee error-free DNA.

Check your understanding

Question 1

What sequence complements the DNA strand ATGC?
  1. TACG
  2. ATGC
  3. GCAT
  4. TAGC
Show answer and explanation
TACG
A pairs with T, T pairs with A, G pairs with C, and C pairs with G. The complementary sequence is TACG.

Question 2

Which enzyme separates the two DNA strands at the start of the basic replication model?
  1. Helicase
  2. DNA ligase
  3. DNA polymerase
  4. Primase
Show answer and explanation
Helicase
Helicase separates the strands. DNA polymerase builds DNA, DNA ligase joins DNA sections, and primase makes a starting primer.

Question 3

During excision repair, what does DNA polymerase do after a damaged section is removed?
  1. It fills the gap using the undamaged strand as a template.
  2. It separates the two original DNA strands.
  3. It joins the completed section to the rest of the strand.
  4. It changes the undamaged strand into RNA.
Show answer and explanation
It fills the gap using the undamaged strand as a template.
DNA polymerase adds replacement DNA by following the intact template. DNA ligase joins the repaired section.

Key terms

Base
One of the information-carrying units in DNA: A, T, C, or G.
Complementary base pairing
The matching pattern in DNA: A pairs with T, and C pairs with G.
Template
A strand that guides the formation of a matching DNA strand.
DNA replication
The process of copying DNA.
Helicase
An enzyme that separates the two DNA strands during replication.
Primase
An enzyme that makes a short RNA primer to provide a starting point for DNA synthesis.
DNA polymerase
An enzyme that adds DNA building blocks to a new strand and can proofread newly added bases.
DNA ligase
An enzyme that joins sections of DNA into a continuous strand.

Continue through SBI4U

View the complete SBI4U Ontario Grade 12 Biology curriculum and lessons

About this lesson and its review

Published by DoAssignment. This reviewed lesson follows Ontario Grade 12 Biology (SBI4U), expectation D3.1. It is a study resource, not an official curriculum publication.

Before publication, content is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability. Errors can still occur, so corrections are welcomed.

Official curriculum reference

Report a correction or ask a question