Name: ______Date: ______

What is the Structure of DNA?

DNA has been referred to as the most exciting discovery of this century. DNA, or deoxyribonucleic acid is found in all living cells.

BACKGROUND INFORMATION

The DNA molecule is in the shape of a double helix. A helix is a spiral shape. The double helix forms the side of the molecule, which is like a “twisted ladder.” The sides are composed of alternating units of deoxyribose sugar and phosphates. The internal building blocks of the molecule, or the “rung of the ladder” are composed of nitrogen-containing bases that are joined to each other and the sides. The basic unit of DNA is called a nucleotide and is made of a deoxyribose sugar, a phosphate, and a nitrogen base. Draw a molecule of a single nucleotide below:

There are four types of nitrogen bases in DNA, which are separated into two different groups: purines and pyrimidines. The pictures below show the nitrogen bases and the letter used to abbreviate them. Write the full names of each of the bases underneath their pictures.

PURINES:

A: ______G: ______

PYRIMIDINES:

T: ______C: ______

The bases always pair in the same way, which is called complementary pairing, or Chargaff’s rule (after the scientist who discovered the ratio). What are the two pairings that always form?

______& ______always bond and ______& ______always bond.

Consider the base sequence of one chain of DNA. How would you fill in the corresponding portion of the other chain?

G C T G A C G A T G C A A C G T

______

In the diagram below, label each part of the helix. Be sure to include: phosphate, deoxyribose, hydrogen bonds, covalent bonds, nitrogen bases: adenine, thymine, guanine, & cytosine. Draw a box around and label a single nucleotide.

PROCEDURE

Using materials provided by your teacher, create a model of the DNA structure. Be sure to include all of the parts listed in the diagram above with a key identifying those parts. In addition, add a separate box with a model of a single nucleotide. Your model should be at least FOUR BASE PAIRS LONG and each type of nitrogen base should show up at least once. Once your are finished, complete the questions below.

ANALYSIS QUESTIONS

1. What is the name for the general structure of DNA? ______

2. What composes the backbone, or side pieces, of the DNA molecule?

3. What is the building block of DNA called? What are the three parts of this building block?

4. What are the purine bases found in DNA?

5. What are the pyrimidine bases found in DNA?

6. What is the structural difference between purines and pyrimidines?

7. What is meant by complementary pairing? What bases pair together?

8. Complete the chart below that lists the % of each type of base found in 3 different animals.

% A / % C / % G / % T
Chicken / 23.2 / 26.8
Mouse / 25.9 / 24.1
Human / 25.4