Structure of DNA Lab

Introduction:

Deoxyribonucleic acid (DNA) is one of the two types of nucleic acids found in organisms and viruses. The structure of DNA determines which proteins particular cells will make. The general structure of DNA was determined in 1953 by James Watson and Francis Crick. The model of DNA that they constructed was made of two chains now referred to as the double helix. Each chain consists of linked deoxyribose sugars and phosphates units. The chains are complementary to each other. One of four nitrogen-containing bases connects the chains together like the rungs of a ladder. The bases are cytosine, guanine, thymine, and adenine. The DNA molecule looks like a spiral staircase. The structure of DNA is illustrated by a right handed double helix, with about 10 nucleotide pairs per helical turn.

DNA is a polymer. The monomer units of DNA are nucleotides. Each nucleotide consists of a 5-carbon sugar (deoxyribose), a nitrogen containing base attached to the sugar, and a phosphate group. (See Table 1.) There are four different types of nucleotides found in DNA, differing only in the nitrogenous base. Adenine and guanine are purines. Purines are the larger of the two types of bases found in DNA. They have two rings of carbons & nitrogens. Cytosine and thymine are pyrimidines and have a single carbon-nitrogen ring. (See Table 2.) The sequence of these bases encodes hereditary instructions for making proteins—which are long chains of amino acids. These proteins help build an organism, act as enzymes, and do much of the work inside cells.

Table 1

DNA Nucleotide
(Sugar + Phosphate + Base)

 Table 2

Pyrimidine
(single ring of C & N)

Purine
(double ring of C & N)

 

Materials:

Colored paper (any 5 different colors to run templates), scissors, transparent tape, coat hanger, hole punch, string or fishing line

Procedure:

  1. Use the section of DNA you have been assigned (Human hemoglobin or Chicken Hemoglobin), and figure out the sequence of bases present on the complementary strand of this molecule Table 1. 

Human Hemoglobin Chicken Hemoglobin
Left Strand Complementary Strand Left Strand Complementary Strand
TAA   GTT  
TGT   TGT  
CGA   CCG  
CCG   CCG  
CTG   CGA  
GTC   GTC  
CAA   TAT  
GTC   CGA  
CTT   TTG  
TGA   AGG  
  1. Count the number of bases (A, T, C, and G) you will need for both strands of the DNA model your group has been assigned, and cut out these bases. (60 total)

  2. Cut out a sugar and a phosphate for each of your DNA bases. (120 of each)

  3. Construct a nucleotide for each base that you have cut (sugar + phosphate + base) by taping these together. (20 total nucleotides)

  4. Using your assigned DNA sequence from Table 1, line up the nucleotides in the right order forming he left strand of your DNA molecule. (30 nucleotides)

  5. Add the other complementary nucleotides to form the right strand by taping the bases together (A bonds with T; C bonds with G).

  6. Once the strand is complete, secure it by adding more transparent tape or ask your teacher to laminate your model.

  7. Punch two holes at the top of your model, and attach the DNA model to a coat hanger with string.

  8. Carefully twist your model into a double helix (5 base pairs in a 1/2 turn and 10 in a complete turn). 

  9. Attach thin fishing line to the sides of the nucleotides to hold the turns in place.

  10. Hang your model from the ceiling using the top of your coat hanger.

TEMPLATES:

Adenine Thymine Cytosine Guanine Phosphate Sugar

Questions & Observations:

1. What 2 molecules make up the sides of the DNA molecule?

2. What nitrogen bases form the rungs of the DNA double helix?

3. What is meant by the complementary strand of DNA?

 

4. What sugar makes up DNA nucleotides?

5. How are nucleotides named?

 

6. DNA is the instructions for building what molecule in our cells?

7. What would happen if one or more bases on the DNA strand were changed?

 

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