DNA, part 2 -DNA Modeling

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Objective

Students will use a model to build a molecule of DNA and show how it replicates.

Big Idea

DNA is a ladder-like structure in shape.

Warm-Up

5 minutes

Warm-Up: How are DNA helicase and DNA polymerase alike and different?

As a review of the content covered the previous lesson, DNA, part 1, ask students to compare and contrast these two molecules that play a role in replication.  This question allows students to use their knowledge to draw connections that will help them deepen their understanding.  Allow students to share their responses and engage in academic discourse.  Listen to all responses before communicating that both are enzymes that act on DNA and that that they have different functions.   

Note: Warm-Up questions are a great way to activate prior knowledge right at the start of class.  When writing warm-up questions, I write questions that push students’ thinking beyond basic recall or reciting of facts.  I like questions that offer students an opportunity to engage in academic discourse, which builds their ability to articulate and defend their thinking.

Content Review

20 minutes

Distribute sets of four colored cards to students: red, blue, yellow and green.  The cards will be used in a review activity to assess how much students recall from the previous lesson; DNA, part 1.  Display the DNA review which consists of a set of ten questions.  Each question has four answer choices.  Each multiple choice response is written in one of four colors, red, yellow, blue, and green.

Explain how the review activity works:

  1. Each of ten questions will be read to the group. 
  2. After each question is read, give students 10 seconds to choose their answer. 
  3. Count 1-2-3 and tell students to raise the color card that represents their answer choice for the question:
  • A = RED
  • B = YELLOW
  • C = BLUE
  • D = GREEN

Explain the rules:

  1. Students cannot talk about the answer choices before cards are raised.  This is an independent activity that is used to assess individual knowledge.
  2. Students cannot change their card once they raise it, even when they are the only person raising a particular color card.

After students raise their cards, look around to see how many correct and incorrect color cards are raised and use the information as a formative assessment.  For example, in the first picture most students holding up a blue card, which is the color card for the correct answer choice B.  However, two students are holding up other colors, green and yellow.  This shows that the students who chose the incorrect answer have different misconceptions about which answer is correct.  

In the second picture, students are holding up green cards, with one student holding up a yellow card.  In this case, the student holding the yellow card is correct and the other students are incorrect.  This shows that there is a shared misconception about the content that needs to be addressed in a spiral review.  

Whenever there is not one hundred percent agreement about the correct answer, take the time to explain the "why" of the correct answer after students raise their cards. Or, select students to give an explanation of the why the correct answer is the correct answer.

Guided Practice

10 minutes

Display the DNA structure lab using a digital overhead and LCD projector so that students will be able to follow along as you preview the lab.  Distribute copies of the DNA Structure Lab.  Distribute envelopes that contain the nuclotide parts. 

Explain that there are several pieces in each envelope. Open an envelope and instruct students to do the same. Instruct students to place each of the 6 nucelotide parts on the desk.  Instruct students to examine the pieces. As a check for understanding, ask students to identify the monomer that makes up DNA.  Students should be able to identify that a nucleotide is the monomer of DNA.  

Ask a student to read the part A instructions and instruct others to follow along silently as the information is read aloud.  Model how to answer question 1, using the think aloud strategy.  

For example, say: The nucelotide of DNA is made of three molecules.  When I think of the name, deoxyribonucleic acid, I see ribose in the namer.  I know that DNA is named for its sugar, so a sugar is one of the molecules in the monomer and second, I think a nitrogen base is another molecule and a phosphate group is the third molecule of the monomer.

Instruct students to take a few minutes to work together to discuss and respond to questions 2 and 3.  Allow students to share their responses with other groups.  The response to question 2 should indicate that the nucleotides are alike because they all are made of the same 3-part structure.  Question 3 should note that each is different because each has a different nitrogen base (A,T,C or G).

Independent Practice

20 minutes

Instruct students to work in groups of two to complete the two remaining parts of the lab; parts B and C. 

Walk around to observe students as they create models of the DNA structure.  Look for students to be able to correctly join the nitrogen bases as the "rungs" of the ladder, with the sugars and phosphates as the sides of the ladder.  Look specifically for students to demonstrate a working knowledge of the "Base-pair" rule as they correctly join the nitrogen bases of the left and right sides of the ladder

Allow students to use their notes as a resource as they answer the questions.  Provide guidance or assistance, as needed.  It’s important to listen to students comments as they work together in groups.  Listening serves as a formative assessment of students' understanding of concepts.  If it is observed that there is a question that most students are not able to answer, stop and perform a quick re-teach of a particular point of instruction.  

Close

5 minutes

Display a 3-2-1 Ticket Out the Door. Ask students to respond to the following:

  • 3 things I feel like I understand better….
  • 2 things I really liked about the lesson today…
  • 1 thing I still don’t understand….

If time permits, allow students to share the 1 thing that is still not understood.  Doing so before class ends, allows for a quick spiral back and clarification before students leave the classroom.