DNA AND REPLICATION
OBJECTIVES
1. What is the structure and function of DNA? (MCA-II science standard)
2. How does DNA replication work? (MCA-II science standard)
INTRODUCTION
DNA, or deoxyribonucleic acid, is sometimes called the blueprint of life. Like the blueprint for
a new home, DNA amazingly supplies all of the characteristics for every form of life on Earth.
How DNA can provide a chemical blueprint of the myriad of characteristics of life is a matter of
the complex structure of the molecule.
PROCEDURE
Part A
Watson and Crick described the shape of the DNA molecule as a double helix.
They called it this because it is shaped like a spiral ladder. The sides of the spiral
ladder are composed of two molecules: deoxyribose and phosphoric acid. Complete
the following questions based on the diagram below.
1. Deoxyribose is a sugar somewhat like the
table sugar you put in baked goods. Write the
molecular formula for deoxyribose by counting
the number of carbon, hydrogen, and oxygen
atoms: C __ H __ O __
2. Phosphoric acid is an acid somewhat like the
acid in your stomach. Write the molecular
formula for phosphoric acid: H __ P __ O __
Part B
The rungs of the spiral ladder are composed of nitrogenous bases.
Complete the following questions based on the diagram on the back of this page.
1. List the four different nitrogenous bases:
_________________, _________________, _________________, _________________
2. Look at the atoms that make up the nitrogenous bases. Why are they nitrogenous?
_________________________________________________________________
3. Think back to the work of Chargaff, Watson, and Crick. How do the bases pair?
______________ with ______________ and ______________ with ______________
Part C
The DNA molecule is built out of units called nucleotides.
Use the diagram below to answer the following questions.
1. A DNA nucleotide is built out of 3 different molecules:
a. _____________________________________
b. _____________________________________
c. _____________________________________
2. As shown, the nitrogenous base that forms a part of the
nucleotide might be cytosine. What are the three other
nitrogenous bases that might form part of a nucleotide?
a. _____________________________________
b. _____________________________________
c. _____________________________________
Part D
You will now be putting together a model of DNA. You will need the blue models
of DNA nucleotides found in your packet of materials.
1. Form a column of DNA nucleotides
in the order noted in the table.
2. Note that your column of DNA nucleotides
is only half of a ladder. What is the other
half of the ladder? Complete the table.
3. Form a matching column of DNA
nucleotides based on the completed table.
nucleotide
cytosine
thymine
guanine
adenine
guanine
4. Show your completed DNA model to
Your teacher and have him/her sign off.
_________________________________
cytosine
nucleotide
Part E
One of the wonders of life is that our cells can reproduce and make exact copies of
themselves. Through the process of replication, the DNA that forms a blueprint for
new cells is copied. In this way, the new cells receive the same DNA as the original
cells. But how does this work?
1. Open your DNA model along the point of attachment between nitrogenous base pairs.
In so doing, you will create two matching columns of DNA nucleotides.
2. Using the left half of your DNA model as a pattern, add new DNA nucleotides to form a
new right side to the DNA molecule. Take a look at the diagram below.
3. Using the right half of your DNA model as a pattern, add new DNA nucleotides to form
a new left side to the DNA molecule. You should now have two copies of DNA.
Show your copies to your teacher and have him/her sign off.
___________________
4. Look at the sequence of DNA nucleotides in the two copies of DNA. How do the
sequences of DNA nucleotides compare?
___________________________________
5. During replication, the original order of DNA nucleotides is carried over to the new
DNA molecule. Why do you think the order of DNA nucleotides is so important?
_______________________________________________________________________