Name: ________________________________________________________ Date: ________________ Period: ________
Standard: HE.5.B.1 Model the components of a DNA nucleotide and an RNA nucleotide & HE.5.B.2 Describe the WatsonCrick double helix model of DNA, using the base paring rule (adenine-thymine, cytosine-guanine).
Bell ringer: _________________________________________________________________________________________
__________________________________________________________________________________________________
The DNA Molecule – pg. 287-299
DNA is a very special molecule found in all living organisms. Why is DNA so important?
Genes carry information that determines your traits. Traits are characteristics you inherit from your parents; this
means your parents pass some of their characteristics on to you through genes. But how do genes relate to your hair
color, skin, and other physical traits? Genes are coded DNA instructions. DNA determines how amino acids are put
together to make protein molecules. The proteins you have, or the order they are put together gives you the physical
traits (characteristics) you have.
DNA is a long molecule made of units called nucleotides. Nucleotides are composed of a 5-carbon sugar called
Deoxyribose, a phosphate group, and a nitrogenous base. There are four kinds of nitrogenous bases in DNA. Two of
them are called Adenine and Guanine; they belong to the group called Purines. The other two are Cytosine and
Thymine, and they belong to the Pyrimidine group.
On the space below draw a Purine and a Pyrimidine. Label the 5-carbon sugar, phosphate group, and
nitrogenous base (textbook page 291).
Purine
Pyrimidine
How are purines and pyrimidines different? ______________________________________________________________
__________________________________________________________________________________________________
The Structure of DNA
When DNA is taken out of the cell and stretched out, it looks like a twisted ladder. This shape is called a double
helix. The sides of the DNA ladder are called the backbone, and it is composed of the phosphate group and the 5-carbon
sugar. The steps of the ladder are pairs of nitrogenous bases.
What makes up the backbone of the DNA helix?
What are the parts of a nucleotide?
What are the four DNA nitrogenous bases?
 DNA bases form pairs in specific ways. Chargaff’s rule: Adenine (A) always pairs with Thymine (T). Guanine (G)
always pairs with Cytosine (C). The bases are connected by hydrogen bonds.
Making DNA Models – PART I
Use colored marshmallows, Twizzlers and toothpicks to construct a DNA double helix. The Twizzlers will be the
backbone and the marshmallows the bases. The toothpicks are the hydrogen bonds that hold the bases together.
Materials:
-
2 pieces of Twizzlers, break them exactly in half.
-
20 marshmallows (10 for activity 1, and 10 for activity 2)
-
10 toothpicks
1) Let’s practice before we create our models
Using Chargaff’s rule, determine the sequence that complements the strand below:
T A C G T A T G A A
___________________________________
2) Swap papers with the group closest to you.
3) With your partner, create a nucleotide sequence using 5 bases for them. Write the sequence you created on the line
below (when you are done, swap papers back):
__________________________________________________________________________________________________
4) Write the complementary bases to the sequence above here:
__________________________________________________________________________________________________
5) Now follow the marshmallow key to create a model of the sequence above.
6) Twist your DNA model so that it looks like a double helix.
7) Have your DNA double helix model viewed by your teacher before moving on to the next step.
So far we have learned 2 of the 3 critical things genes can do:
I)
II)
Genes carry information from one generation to the next
Genes have to put that information to work by determining the heritable characteristics of organisms.
However, there is one more which we will investigate in the next part of this activity.
III)
Genes have to be easily copied because all of the cell’s genetic information is replicated every time a cell
divides.
Modeling DNA Replication – PART II
Replication means to make copies. In this part of the activity you will learn how DNA makes copies of itself.
Before a cell divides to make new ones, it has to make copies of its DNA. This process ensures that each new cell will
have a complete set of DNA molecules.
One of the amazing things Watson and Crick observed was that each strand of the DNA double helix has all the
information needed to reconstruct the other half. The strands are said to be complementary because each strand can
be used to make the other strand.
When DNA replicates, it “unzips” and each side is used as a template to make a new side of the DNA (following basepairing rules).
1) Break the hydrogen bonds between your marshmallows to separate both sides of your DNA.
The enzyme helicase is responsible for unzipping the DNA strands by breaking the hydrogen bonds between the
base pairs.
2) Separate the two sides of the DNA ladder by moving them to opposite sides of the paper towel.
3) Replicate each strand of DNA by using the base-pairing rules.
The enzyme DNA polimerase is responsible for hydrogen bonding the bases, and proof reading the strand to make
sure it has been copied correctly.
4) Fill out the results below, using letters to represent the bases.
5) Once this is complete Do NOT proceed until you have been given the ok to move on to the next task.
Old DNA
New DNA
New DNA
Old DNA
Post-lab Questions:
1. What do the whole toothpicks represent between the marshmallows?
________________________________________
2. What do the half toothpicks between the marshmallows and the Twizzlers represent?
________________________________________
3. DNA is composed of nucleotides. A nucleotide is made up of a _______________, __________________, and a
___________________.
4. What part of the molecules did the following candy represent?
DNA: Marshmallows: _____________________ and Twizzlers: ________________________________________
5. In ______________________ cells, DNA is found in the nucleus.
6. Use the following sequence to replicate DNA.
ATT
CGC
GCA
TCA
______ ______ ______ ______
7. Why was the discovery of DNA so important?
___________________________________________________________________________________________
___________________________________________________________________________________________
8. Explain the relationship between DNA and genes.
___________________________________________________________________________________________
___________________________________________________________________________________________
___________________________________________________________________________________________