DNA structure and replication

Author(s): SEP staff

Lesson Overview

Grade level(s):

Middle School (6-8), High School (9-12), Grade 7, Grade 8, Grade 9


Biology/Life Science



Big ideas(s):

What is the structure of DNA?

How does the cell make a copy of its DNA to be passed on to mitotic daughter cells?

Vocabulary words:

DNA (deoxyribose nucleic acid), nucleotide, nitrogenous base, semi-conservative replication, complimentary base pairing

What you need:

  • Magnetic whiteboards (one for each pair of students)
  • Magnetic pieces representing sugar, phosphate, and the nitrogenous bases (Adenine, Thymine, Cytosine, Guanine)
  • 3-dimensional models of DNA molecule
  • Movie clip or animation showing the process of DNA replication

If you don't have white boards available, you can also print the attached DNA pieces on cardstock, have your students cut them out and glue them on paper. The simulation of the replication process won't work very well though....


Students will work in pairs.


Regular classroom

Time needed:

  • About 30 minutes intro presentation
  • About 20 minutes for the magnetic whiteboard activities (if you do the activity using the paper pieces, this part will take much longer)
  • 5 minute debrief
Author Name(s): 
SEP staff

Students assemble a DNA molecule, using magnetic pieces representing sugar, phosphate and the nitrogenous bases on magnetic whiteboards. Students then model the process of semi-conservative DNA replication applying the complementary base pairing rule.

Prerequisites for students: 

Students should be familiar with the process of cell division (mitosis) and understand that DNA replication has to occur prior to cell division in order to ensure that both daughter cells will receive a complete set of the genetic information.

Learning goals/objectives for students: 
  • Students will be able to name the building blocks that make up DNA.
  • Students will be able to describe the basic structure of the DNA molecule.
  • Students will be able to model the process of DNA replication.
Content background for instructor: 

DNA(deoxyribose nucleic acid) is a macro-molecule that consists of repeating units, called nucleotides. Nucleotides are made up of a sugar molecule, a phosphate and one out of four nitrogenous bases (Adenine, Thymine, Guanine, Cytosine). DNA is always found as double chains of nucleotides forming a structure that resembles that of a twisted ladder. Repeating sugar and phosphate molecules are forming the sides or backbone of the ladder and pairs of bases the rungs of the ladder. Adenine always pairs with Thymine and Guanine always pairs with Cytosine (A-T, C-G); they are complimentary base pairs. Consequently there are always as many "A's" than there are "T's" in a DNA molecule and equal numbers of "C's" and "G's" (Chargoff rule).

DNA carries the genetic information of the cell. When cells divide, they have to copy that genetic information and pass on one copy to each daughter cell. DNA replication is "semi-conservative": The DNA double helix unravels and unzips (the weak hydrogen bonds between the bases are broken by an enzyme (helicase). Each original DNA strand now serves as a template for the new. Free-floating nucleotides, that are made in other parts of the cell, match with their complimentary nucleotide "partner" and link up. In this way, a single DNA molecules becomes two. One strand of each of the two DNA molecules is the original (conserved), and one strand is freshly assembled (hence: Semi (half) conservative replication).

Getting ready: 
  • Check out materials at SEP

Lesson Implementation / Outline


Show PowerPoint presentation about

  • the discovery of the DNA structure by Watson and Crick using the famous x-ray crystallography picture by Rosalind Franklin and the findings of Chargoff (Chargoff's rule).
  • nucleotides, the building blocks of DNA
  • DNA replication

Show animation or video clip illustrating DNA replication

  1. Hand out magnetic whiteboards and bags with DNA pieces to each pair of students.
  2. Students assemble their DNA double strands.
  3. Point out the ladder structure and the complimentary base pairing.
  4. Have students count how many A's, T', G's and C's their DNA molecule contains. Refer back to the Chargoff rule.
  5. Direct students to "unzip" their DNA molecule by sliding the magnetic pieces of each strand apart.
  6. Students then assemble the new, complimentary DNA strands.
  7. Ask students to identify the original DNA strands and the new ones.
Wrap-up / Closure: 

Ask questions that require students to summarize what they learned.

"Why is the replication of DNA called semi-conservative?"; "Why is there always equal amounts of A's and T's in a molecule of DNA" etc.

dna_pieces.pdf1.09 MB
DNA strucutre and replication.ppt2.42 MB
NGSS Topics
High School Life Sciences: 
NGSS Disciplinary Core Ideas
NGSS Science and Engineering Practices
NGSS Science and Engineering Practices: 
NGSS Crosscutting Concepts
NGSS Crosscutting Concepts: 

Weblinks and References

Standards - Grade 7

Life Science - Cell Biology: 
1. All living organisms are composed of cells, from just one to many trillions, whose details usually are visible only through a microscope. As a basis for understanding this concept:
c. Students know the nucleus is the repository for genetic information in plant and animal cells.
e. Students know cells divide to increase their numbers through a process of mitosis, which results in two daughter cells with identical sets of chromosomes.
2. A typical cell of any organism contains genetic instructions that specify its traits. Those traits may be modified by environmental influences. As a basis for understanding this concept:
e. Students know DNA (deoxyribonucleic acid) is the genetic material of living organisms and is located in the chromosomes of each cell.

Standards - Grades 9-12 Biology

a. Students know the general structures and functions of DNA, RNA, and protein.
b. Students know how to apply base-pairing rules to explain precise copying of DNA during semiconservative replication and transcription of information from DNA into mRNA.


Installed PPT

My installed PPT version is not working, is there any issue with attachment or my software version is not compatible with your attachment file.