DNA - The Double Helix
Name:
1. Read and ANNOTATE as you read!
Recall that the nucleus is a small spherical structure within a cell. It is often called the "control center"
because it controls all the activities of the cell including cell reproduction, and heredity. Chromosomes are
microscopic, threadlike strands composed primarily of the molecule DNA (short for deoxyribonucleic acid).
Because the DNA in chromosomes is so long and must frequently be accessed by the cell, it is highly organized
using special proteins called histones.
DNA is called a nucleic acid because it was first found in the nucleus, isolated from white blood cells in pus
way back in the 1860’s. In 1953, James Watson and Francis Crick “won” race to describe the structure of
DNA. Although these men were awarded with a Nobel Prize, many others contributed to this monumental
discovery.
The shape of DNA is a double helix, something like a twisted ladder. The sides of the ladder are made of
alternating sugar and phosphate molecules. The sugar is deoxyribose.
The rungs of the ladder are composed of nitrogen base pairs. The bases are known by the letters A, G, T, and
C. These bases always bond in a certain way. Adenine will only bond to thymine. Guanine will only bond
with cytosine. This is known as the "Base-Pair Rule".
The bases can occur in any order along a strand of DNA. The order of these bases is the code of DNA. Just as
in Greek or Morse code, a different sequence of symbols results in a different message. For instance
ATGCACATA is a different message than AATTACGGA. A strand of DNA contains millions of bases. (For
simplicity, the image below only contains a few.)
Note that that the bases attach to the sides of the ladder at the sugars and not the phosphate.
Each strand of the DNA helix is made of repeating units called nucleotides. Each nucleotide consists of three
molecules: a sugar (deoxyribose), a phosphate which links the sugars together, and one of the four bases. Two
of the bases are purines - adenine and guanine. The pyrimidines are thymine and cytosine. Note that the
pyrimidines are single ringed and the purines are double ringed.
2. NEATLY color the images on the following page. Color all the phosphates in a color of your choice (one
is labeled with a "p"). Color all the sugars (deoxyribose) in a different color of your choice (one is
labeled with a "D"). Add your chosen colors to the key below! Color each base a different color. Finally,
shade the hydrogen bonds a different color. Add this to your key.
Thymine
Adenine
Guanine
Cytosine
Deoxyribose
Phosphate
Hydrogen Bond
DNA - The Double Helix
3. List the DNA base sequences vertically
A
-
T
C
-
G
4. Read and Annotate as you read.
The two sides of the DNA ladder are held together loosely by hydrogen bonds. The DNA can actually "unzip"
when it needs to replicate - or make a copy of itself. DNA needs to copy itself every time a cell divides, so that
the new cells each contain a copy of the DNA. Without these instructions, the new cells wouldn't have the
information necessary for survival.
5. Color the image of DNA replication; use the colors from your key. You do not need to color the small
image in the box.
6. Read and annotate as you read
Every cell in your body has the same "blueprint" or the same DNA. Just as the blueprints of a house tell the
builders how to construct a house, the DNA "blueprint" tells the cell how to build the organism. Yet, how can
a heart be so different from a brain if all the cells contain the same instructions? Although much work
remains in genetics, it has become apparent that a cell has the ability to turn off most genes and only work
with the genes necessary to do a job. We also know that a lot of DNA apparently does not code for proteins.
Originally considered “junk DNA”, these large sections of DNA now appear to have the important job of
turning on and off genes. Current research at UW is just beginning to uncover the secrets of “junk DNA”.
7. Answer these questions.
a. Write out the full name for DNA. _________________________________________________________
b. Where in the cell are chromosomes located? _______________________________________________
c. What are chromosomes composed of?____________________________________________________
d. What two scientists established the structure of DNA? _______________________________________
e. What is the shape of DNA? ______________________________________________________________
f. What are the sides of the DNA ladder made of? _____________________________________________
g. What are the "rungs" of the DNA ladder made of? ___________________________________________
h. What sugar is found in DNA? ____________________________________________________________
i.
8. How do the bases bond together? A bonds with ______________
G bonds with _____________
j.
DNA is made of repeating units called _____________________________________________________
k. If one side of a DNA molecule has the sequence TACAAG; the other side would be _________________
l.
Why do some cells develop into brain cells while others develop into skin cells, when the DNA in ALL
the cells is exactly the same? In other words, if the instructions are exactly the same, how does one
cell become a brain cell and another a skin cell?