DNA - The Star of the Show

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Objective

SWBAT apply their understanding of DNA's structure and function to construct an explanation of how the DNA sequences determine the type of protein that is create.

Big Idea

Time to dig deeper than the double helix and construct ideas to explain the function of DNA.

Hook - DNA Brain Dump

5 minutes

Students will take the opportunity to record everything they know about DNA through an activity called a Brain Dump.  There is no pressure for assessment for this activity and no specific guidelines except that students need to write for at least one minute straight!  Students are encouraged to incorporate all details that are associated with DNA - structure, function, genetics, previous life science courses, ect.

After two minutes, students will turn to their partner and share their three most interesting facts from their paper.  Students are encouraged to record their partner's ideas to their Brain Dump list if they hear new or interesting information.

Student volunteers will be asked to share their best, most interesting fact with the class.

Brain Dumping is a strategy that encourages students to activate prior knowledge of the topic in an effort to build confidence and interest for the students either as an introduction or a closing activity for the lesson.

DNA Brain Dump - Student Work Samples - The work samples demonstrate a large range of student knowledge and understanding of the structure and function of DNA.  Some students appear to have read the textbook last night, while others are still trying to catch up and learn the content.

Students will watch this one-minute video as a quick review to make sure all students have the basics of DNA structure:

Direction Instruction - Introducing Our Star

15 minutes

Students will record the DNA Introduction Lecture Notes to solidify their understanding of how scientists analyzed laboratory data to create an accurate model of DNA's structure.  The Lecture Notes go into detail regarding the structure of DNA using base pairing and hydrogen bonds.

3 Investigations That Led To Our Understanding Of DNA

  • Griffith's Experiment: How do certain types of bacteria cause pneumonia?
  • Significant Outcome: Genetic information could be transformed from one bacterium to another. (transformation)
  • Avery's Experiment: Which molecule in the heat killed bacteria was needed for transformation?
  • Significant Outcome: The nucleic acid DNA stores and transmits the genetic information from one generation of an organism to the next.
  • Hershey –Chase Experiment: Which part of the virus enters the infected bacteria cell?
  • Significant Outcome: The genetic information in the bacteriophage is DNA (not protein)

 

Conclusions About DNA From Lab Data:

•Carries information from one generation to the next
•Put information to work by determining heritable characteristics
•Must be easily copied since almost every new cell gets a copy
The Structure of DNA

The DNA molecule is a double helix. Think of it as a ladder that has been twisted into a spiral. The outside of the ladder is made up of alternating sugar and phosphate groups. The sugar is called deoxyribose. The rungs of the ladder are made up of nitrogen-containing bases. There are four different nitrogen-containing bases in DNA:adenine (A), guanine (G), cytosine (C), and thymine (T). These four bases are of two types: purines and pyrimidines. Purines are large double-ring structures. Adenine and guanine are purines. Pyrimidines are smaller single-ring structures. Cytosine and thymine are pyrimidines.  Inside the DNA ladder, two bases pair up to make a "rung." One base sticks out from each sugar-phosphate chain toward the inside of the ladder. It forms a pair with a base sticking out from the opposite sugar-phosphate chain. Only three rings can fit between the two sugar-phosphate chains, so a pyrimidine (one ring) and a purine (two rings) form a pair. Because of the chemical structures of the bases, adenine always pairs with thymine, and cytosine always pairs with guanine.

 

DNA

 

 

 

 

Guided Practice - Building Our Star

20 minutes

DNA and Proteins 

DNA is called the blueprint of life. It got this name because it contains the instructions for making every protein in your body. Why are proteins important? Because they are what you muscles and tissues are made of; they synthesize the pigments that color your skin, hair, and eyes; they digest your food; they make (and sometimes are) the hormones that regulate your growth; they defend you from infection. In short, proteins determine your body's form and carry out its functions. DNA determines what all of these proteins will be.

Teacher Preparation: Copy the Constructing a Paper Helix Handout

Procedure:

  1. Working in pairs, cut out the model segments of deoxyribose, phosphate groups, and the bases provided. Color them according to the following color-code:
  • Deoxyribose: BLACK OR GRAY
  • Phosphate: WHITE
  • Adenine:   BLUE
  • Guanine:   GREEN
  • Thymine:   RED
  • Cytosine:   YELLOW

2.  Glue each nitrogenous base to a deoxyribose molecule by matching up the dots.

3. Glue each phosphate group to a deoxyribose molecule by matching up the stars.  **You have created 6 separate nucleotides.**

4. Now pair up the bases:  adenine with thymine  -   guanine with cytosine **Notice: to do this you have to turn some of the nucleotides "upside down" so that  the bases can pair up...this accounts for the fact that DNA is "ANTIPARALLEL" (one strand goes up, the other goes down)

5. Glue or tape the bases together and the nucleotides together...you end up with a small  piece of DNA that is 3 base pairs in length.

 Sample of Student WorkDNA Strand Up Close - each student color-coded and assembled 6 nucleotides which were then put together to create 3 base pairs.  The students' work was joined together to create a long strand and hung from the ceiling like party streamers.  It is fun to remind the students that the DNA strands are decorations for our "Daily Biology Party of Knowledge"!

Independent Practice - Analyzing Our DNA Models

10 minutes

Students will complete comprehension questions from the DNA Review Questions Handout that will require careful analysis of their DNA models and the lesson's Lecture Notes.  

  1. With what base does adenine (A) pair?
  2. With what base does guanine (G) pair?
  3. What is the smallest unit of DNA called?
  4. What is the shape of the DNA molecule?
  5. Which bases are purines? 
  6. Which bases are pyrimidines?
  7. Why must a purine pair with a pyrimidine?
  8. What is the name of the sugar in the DNA backbone?
  9. Suppose you know that the sequence of bases on one DNA strand is AGCTCAG.  What is the sequence of bases on the opposite strand?
  10. Assume that a 100-base pair DNA double helix contains 45 cytosines. How many adenines are there?

DNA Review Questions - Student Work Samples - Most students seem to have a strong understanding of the structure of DNA, but still need to practice the function of DNA as it relates to the structure of proteins that it creates.

 

Close - DNA's Next Role

5 minutes

The class will review student responses to the DNA Modeling activity in a whole group discussion.  Students will then turn to their partners and discuss if there the sequencing of base-pairs is important to the function of the cell/organism.  Students will have 2-3 minutes to reference their lecture notes, textbooks, and each other's creative thinking.

Students will respond to the following prompt as a 1-2 paragraph homework assignment: 

"How does the structure of the DNA model that the class assembled during today's lesson determine the structure of the proteins that will be created? 

Students are encouraged to look up the concepts of transcription and translation in their textbook for inspiration.  They are also reminded to use the introductory paragraph of today's activity: DNA is called the blueprint of life. It got this name because it contains the instructions for making every protein in your body. Why are proteins important? Because they are what you muscles and tissues are made of; they synthesize the pigments that color your skin, hair, and eyes; they digest your food; they make (and sometimes are) the hormones that regulate your growth; they defend you from infection. In short, proteins determine your body's form and carry out its functions. DNA determines what all of these proteins will be.