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Key Ideas

  • Chromosomes are made of two chemical substances, deoxyribonucleic acid (DNA) and protein.
  • A gene is a region of the DNA of a chromosome that can be copied (replicated) and that codes for a specific protein.
  • DNA is composed of four different nucleotides-adenine, thymine, guanine, and cytosine.
  • Each nucleotide is composed of a sugar molecule (deoxyribose), a phosphate molecule, and one of four nitrogen bases.
  • Nucleotides are arranged in pairs, guanine with cytosine, and adenine with thymine, to form DNA.

Overview

Building on their knowledge of the structure and function of chromosomes from Section 2, students learn about the scientific discoveries leading to the Watson and Crick model of the DNA double helix. They perform two laboratory experiments with DNA. They first remove and spool purified DNA from solution to examine and record its physical properties. Students then isolate DNA from the nuclei of mammalian thymus cells to observe its appearance, and then they design an alternative procedure for isolating DNA from these cells. Students construct a paper model of DNA to examine the structure of the double helix and then they use the model to simulate the replication of DNA in preparation for cell division.

Objectives

Students:

\checkmark describe the scientific discoveries leading to the model of the structure of DNA.

\checkmark identify the scientific contributions of Franklin, Watson, and Crick.

\checkmark precipitate, spool, and observe purified DNA.

\checkmark isolate and examine mammalian DNA.

\checkmark construct a model of the DNA double helix.

\checkmark simulate the replication of DNA.

Vocabulary

deoxyribonucleic acid, (DNA), double helix, gene, nucleotide, replication

Students Materials

Activity 3-1: Precipitation and Spooling of DNA

  • Activity Report

Per team of 2-4 students

  • Test tube containing 2\;\mathrm{ml} DNA in solution
  • Test tube containing 4\;\mathrm{ml} alcohol
  • 50\;\mathrm{ml} beaker containing strong salt (\mathrm{NaCl}) solution and an eye dropper
  • Wooden skewer

Activity 3-2: Removing DNA from Thymus Cells

  • Activity Report
  • Safety goggles
  • Sample of fresh thymus cells in a beaker; Sand; Liquid soap, clear; Alcohol; Cheesecloth square (several layers, 15 \times 15\;\mathrm{cm}); Mortar and pestle; Test tube; Small funnel; Test tube rack; Wooden skewer; Forceps; Eyedropper; Permanent marking pen; Paper towels; Black construction paper, 4 \times 4\;\mathrm{cm}; Transparent tape; Microscope, slides, and cover slips; Tap water in beaker

Activity 3-3: Building and Using a DNA Model

  • Resource
  • Activity Report
  • Scissors; Paper; 6 sets of different colored paper; Tape

Teacher Materials

Activity 3-1: Precipitation and Spooling of DNA

  • Teacher Resource
  • Optional: DNA visuals, including models and/or posters, lab materials

Note: You can substitute glass for the stirring rod.

Activity 3-2: Removing DNA from Thymus Cells

  • Activity Report Answer Key
  • Serrated knife for cutting the thymus tissue
  • Extra student materials, especially cheesecloth, skewers, test tubes, and fresh thymus cells Note: You can substitute glass for the stirring rod.
  • Methylene blue stain can be used to stain the thymus nuclei in Step 5
  • Optional: DNA visuals, including models and/or posters
  • Model of a cell with a large, distinct nucleus
  • Picture of a human torso to show the location of the thymus gland

Activity 3-3: Building and Using a DNA Model

  • Activity Report Answer Key
  • Models and diagrams of DNA molecules and nucleotides
  • Extra supply of colored paper (6 different colors)

Advance Preparation

See Activities 3-1, 3-2 and 3-3 in the student edition.

Activity 3-1: Precipitation and Spooling of DNA

  • Order DNA for delivery at least a week before use. Dissolve DNA in buffer or distilled water and store several days in refrigerator.
  • If you are going to do the DNase extension (Resource) you will need additional materials indicated in Resource.

Activity 3-2: Removing DNA from Thymus Cells

  • Purchase fresh thymus tissue, also called sweet-breads from your local butcher. You can freeze the thymus tissue if you do not plan to use it right away.
  • Cut the thymus tissue into 2-\mathrm{cm} cubes using a clean knife.
  • Keep all solutions cold prior to activity.

Activity 3-3: Building and Using a DNA Model

  • You can have students color the sugars, phosphates, and nitrogen bases as follows, or you can copy them on me indicated colors of paper.
  • 60 deoxyribose sugars (white)
  • 60 phosphates (orange)
  • 15 of each of the four nitrogenous bases: adenine (red), thymine (blue), cytosine (yellow), and guanine (green)
  • Allow ample time to precut the template pieces.

Interdisciplinary Connections

Art Make a drawing or 3-D model of a DNA molecule.

Math Research how many base pairs make up a typical chromosome or a DNA molecule. Discuss how it is possible to have so many traits and variations considering that there are only 4 nucleotides which make up DNA.

Language Arts Assign the book The Double Helix by James Watson.

Social Studies Research and write about the history and the awarding of the Nobel Prize for medicine.

Image Attributions

Description

Authors:

Grades:

6 , 7 , 8

Date Created:

Feb 23, 2012

Last Modified:

Feb 23, 2012
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