DNA Replication

Explaining the process of DNA Replication

The central dogma of molecular biology, described by Francis Crick, begins with DNA replication. It really is the essence of inheritance and perhaps even the essence of life itself. At SL there are just five key ideas to understand. This lesson leads students through the process of DNA replication and there is a short exercise about answering IB exam questions. There is also an online game from the Nobel Prize.

Lesson Description

Guiding Question

How do cells make a copy of their DNA before they divide into two cells?

How does a single strand form a template for the new nucleotides?

What are the roles of the two enzymes; DNA polymerase, and Helicase

Activity 1 - Understanding the basics

Watch the DNA Replication Visualization - wonderful to see but the detail is HL or beyond.

How does the cell do that?  Using a biology text book research the main functions of the enzymes Helicase and DNA polymerase.

This diagram shows the main points.

Complete the DNA Replication - students' worksheet to record your notes which outline the five main ideas about DNA Replication. You may need to consult your textbook to find out how to explain each of the diagrams.

Activity 2 The formation of new complementary DNA strands by DNA polymerase.

Watch this 40 second video to get the idea of complementary base pairing, then try the
DNA The Double Helix Game from the Nobel Prize Website

Activity 3 Improving Answers in IB Exam Questions

In twelve minutes, write an answer to the question below.

Explain how DNA is replicated within the cells of living organisms.

(Total 8 marks)

Here is an example of a typical student response.

In DNA replication the DNA is unzipped.

New DNA bases are added. The base A joins to T and C joins to G

DNA polymerase is the enzyme that builds these new strands.

Two new DNA molecules are produced in the process.

One of the DNA strands is conserved.

This exam answer achieves three marks, but it needs to be more precise in the detail for 8 marks.

The Marks Achieved are ....
  • the two DNA strands are separated;
  • bases pair A to T and C to G;
  • the enzyme DNA polymerase builds the new strands;
In DNA replication the DNA is unzipped.

New DNA bases (Too Vague) are added. The base A joins to T and C joins to G
DNA polymerase is the enzyme that builds these new strands.
Two new DNA molecules are produced in the process. (Too Vague)
One of the DNA strands is conserved (Too Vague)

.

What improvements could be made?

It is usually good to start the longer questions with a simple definition.

Always link the structures with their functions, especial enzymes in this answer.

Give precise details, 'bases' is a bit too vague.

Give an example to illustrate statements, if you can.

Avoid using a general phrase which sounds like the biological one, e.g. "DNA is conserved" is not the same as "The new DNA strands show semi-conservative replication"

This is an example of a much improved answer.

DNA replication is the production of two DNA molecules from one original DNA molecule.

In DNA replication the first step is to unwind the double helix and separate the two strands.
Helicase is the enzyme that does this.

New DNA nucleotides are added to each of the strands forming new double strands;

The bases are added using complementary base pairing. For example, the base Adenine joins to Thymine and Cytosine joins to Guanine

DNA polymerase is the enzyme that builds these new strands.

In each new DNA molecule there is complementary base pairing.

For each DNA molecule produced there is one of the original DNA strands and one new DNA strand.

Teachers notes

This activity should be used in conjunction with a text book or a teacher explanation on the board.

Students gradually build their knowledge of the enzymes names and function and the processes involved during activity 1 and Activity 2. The final activity should take about 20 minutes and students are asked to attempt an answer to an exam question, then to suggest improvements to the student answer on this page. The aim of this last activity is to improve the way students write their answers in exams and to encourage them to include names of the specific parts with descriptions of their functions.

A Suggested Lesson Plan.

Starter: 10 minutes

Main: 30 minutes

End: 20 minutes

  • Plenary - Students attempt to use their new knowledge to answer a difficult IB question, then improve their answers.
  • The example in Activity 3 is a common IB question and students who think they have understood the work are often disappointed that they are not achieving high marks, especially at this point in the course. The reasons for this are often that the way the students write the answers is not precise enough. For example "bases are added to the DNA strand" is not the same as "DNA nucleotides are added". This activity aims to help student to recognise this difference and use more precise language in their written answers, and in their revision.

Extra Resources

It would also be good to ensure that students can explain how Meselson and Stahl’s results support for the theory of semi-conservative replication of DNA.   This is covered well in the text books.

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