DNA, part 1-Structure and Function

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Objective

Students will be able to identify the structure and functions of DNA, and explain the role of DNA in storing and transmitting cellular information.

Big Idea

DNA contains a coded message that must be replicated.

Warm-Up

5 minutes

Warm-Up: “Have you ever received a message from a friend that was in secret code?  How did you figure out what the message said?” 

Allow students to share for 1-3 minutes.  Share a grade school experience you have had either sending or receiving coded messages.  This question serves two purposes.  One, it activate prior knowledge students have about codes and the usefulness of decoding messages.  Two, it allows for relationship building as students and teacher share experiences.  

Introduce New Material

20 minutes

 

Begin with a trailer from the movie, The Da Vinci Code.  Allow students to watch the clip before making any statements about the lesson.   After the clip ends, inform students that the lesson will focus on one of the most significant coded messages of all time, the DNA code.  Provide a brief summary of the movie in order to frame its relevance to the lesson on the DNA code.  I liken the Mona Lisa to living organisms that have unique codes that makes each a “one of a kind” masterpiece. 

Inform students that the student learning targets for this lesson are:

  • I understand the basic structure of DNA.
  • I can explain the events and products of DNA replication.

Display visual information as you teach and instruct students to take notes using a DNA & Replication Concept Map that you have provided or use a note-taking strategy that you have taught.  Guided notes provide greater support for the different learning styles of students.  Guided notes also better ensures that students will leave with the information I intend for them to capture. I vary the format of the guided notes from time to time to help students learn how to create notes using different formats. The notes format for this lesson is a graphic organizer.

Review the structure of the nucleotide as the monomer of the DNA molecule.  Look for students to recall this information from the lesson on macromolecules.  Use the terms monomer and polymer, reminding students that these terms both have prefixes which means that both terms should already be noted on their vocabulary maps.  After presenting information on the structure of the DNA molecule, share a brief video clip to help students visualize the DNA structure.

Make frequent checks for understanding throughout the teaching and note-taking process to identify gaps in students’ understanding of concepts.  For example:

  • What nitrogen bases make up the DNA “language”?
  • What molecules make up the sides of the DNA molecule?
  • What molecules make up the rungs of the DNA ladder?
  • What is the base pair rule?

After instruction on the structure and function of DNA, introduce the term, replication.  Explicit vocabulary instruction is important for building academic literacy and it includes instruction on not only content specific but non-content specific terms. Make sure that students know the meaning and synonyms of key terms. 

Before providing instruction on replication, give students 1-2 minutes to “turn and talk” about the reasons why DNA replication is needed.  Walk around and listen to students’ thoughts in order to gain a sense of their thinking and assess how well they are able to connect what they know about cell division from a previous lesson with the process of DNA replication.  At the end of the timed period, emphasize the DNA replication occurs so that cells are able to pass on the code or instructions to offspring cells. 

Use a non-science metaphor to help all learners gain a sense of the reasons why DNA code is replicated.  For example, use a recipe as an example.  Explain that if someone wants to make my famous German chocolate cake, that person will need a copy of the recipe from me first before attempting to replicate my famous German chocolate cake.  Show a video clip of DNA replication to reinforce students’ conceptual understanding of what occurs during this process.

Guided Practice

15 minutes

Inform students that they will now engage in a paper foldable activity that will allow them to model and understand DNA replication.  Provide both verbal and written instructions for completion of the foldable activity using a LCD projector for the different learner types.  Display a DNA template strand on a LCD projector. 

Distribute 8 1/2 x 11 sheets of paper and markers.  Narrate and model how to fold the paper:

1. Fold the paper into two equal parts, length-wise.  Students sometimes know this type of fold as "hotdog".

2. Hold the folded paper like an open book, and fold the top flap of paper back to the folded edge on the left.  Leave the bottom page unfolded.

Walk around to assist students who are not able to follow the verbal or visual modeling.  Because the folding of the paper may be difficult for some students, it may be useful to pre-fold the paper before distributing it so that folding of the paper correctly does not impede the completion of the activity. 

Using the “think aloud” strategy, narrate steps 1-6 and model how students will complete DNA replication foldable, using the Base-Pair Rule. 

Step #1

Instruct students to select a marker to write the sequence of nitrogen bases that are displayed on the board down the outer left flap of the paper.  Model by example so that students will be clear about where to write the letters (A,T,C or G).

Step #2

Pick a 2nd colored marker and draw a line from the letter just written on the left side across the paper to the outer edge of the right side of the paper, leaving enough space to write another letter.  Ask students what they think this line might represent.  Look for them to identify that this line represents the hydrogen bond that connect nitrogen bases.

Step #3

Write the complementary base for each nitrogen base next to the line just drawn across the page, connecting the two nitrogen bases. Remember the Base-Pair Rule.

Step #4

Now model replication by opening the paper to its full 8 ½ x 11 size.  Makre sure students know that is intended to model the unzipping of DNA.

Step #5

Add a nitrogen base to each of the exposed sides of the original template strand by writing the letter of the complementary  base on both sides of the two original sides.  Emphasis the importance of accurate replication so no mutations will occur. As students work, ask questions to help reinforce concepts.  For example, “What enzyme unzips the double helix?  Or, What enzyme adds nitrogen bases to the exposed nitrogen bases on the template strand?

Step #6

Model adding complementary bases to the first 2-3 nitrogen bases in the sequence. Walk around the room as you work using an interactive writing pad to make sure that students are following along and writing as you write.  Inform students that they will complete the addition of complementary bases for both double helixes on their own. Instruct them to label the completed task, DNA replication.

 

 

Independent Practice

20 minutes

Release students to complete the remaining base pairings on their own. Walk around to monitor that all students are on task and able to apply the complementary DNA base pair rule to complete the task. Expect that some students will be able to apply the rule more easily than others. Watch to see if students are able to note that they now have 2 exact replicas of the original template strand.  Ask students what they notice about the 2 strands.  Look for students to identify that the product of replication is two identical double helixes of DNA.

The student work sample shows what students are expected to be able to do; use the base pair rule to assign the correct nitrogen bases to the exposed sides to produce two identical strands of DNA.

Listen and observe students' ability to notice how the original strand is used to make an exact copy of the original strand.  Point out how the paper  is "unzipped"  when they open it and ask them to identify what enzyme is responsible for unzipping the double helix. Look for students to identify that helicase unzips the DNA molecule.  As they move into the process of adding nitrogen bases to the exposed side of the DNA strand, ask students to identify the name of the enzyme responsible for this function.  They should be able to name DNA polymerase as the enzyme.

Close

5 minutes

Display a “Ticket Out the Door Quickwrite” prompt and instruct students to respond to the prompt with a 3-5 sentence paragraph:

Explain the relevance of the movie, The DaVinci Code as it relates to DNA. Make at least two connections between your understanding of DNA and the movie plot.

Look for students to able to identify that our DNA is like a coded message that tells us our past.  Look for students to identify that each genetic code is unique to the organism, like the Mona Lisa is a unique portrait.

The students' Quickwrite responses indicate that students were able to convey that they have an understanding of the major concepts about DNA. Student 1 (page 1 of 3) conveys an understanding that DNA is a code that is one of kind for each individual. Student 2 (page 2 of 3) shows an understanding that DNA stored and sends coded information.  Student 3 (page 3 of 3) explains the DNA has a secret code "hidden within", like our genetic information.  

All three students were able to establish a connection between the movie's premise about the Mona Lisa containing a secret code and facts about DNA. The students' ability to to make a connection between the movie premise and DNA's code indicates that they possess a basic understanding of DNA and the genetic code.