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DNA - The Double Helix (Article)
Recall that the nucleus is a small, spherical, dense body in 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 of the chemical DNA (short for deoxyribonucleic acid). In simple
terms, DNA controls the production of proteins within the cell. These proteins in turn, form the structural units of
cells and control all chemical processes within the cell. Think of proteins as the building blocks for an organism:
proteins make up your skin, your hair, and parts of individual cells. How you look is largely determined by the
proteins that are made. The proteins that are made are determined by the sequence of DNA in the nucleus.
Chromosomes are composed of genes, which is a segment of DNA that codes for a particular protein which
in turn codes for a trait. Hence you hear it commonly referred to as “the gene for baldness” or “the gene for blue
eyes.” Meanwhile, DNA is the chemical that genes and chromosomes are made of. DNA is called a nucleic acid
because it was first found in the nucleus. We now know that DNA is also found in other organelles (the
mitochondria and the chloroplasts), though it is the DNA in the nucleus that actually controls the cell's workings.
In 1953, James Watson and Francis Crick established the structure of DNA. The shape of DNA is a double
helix, which is like a twisted ladder. The sides of the ladder are made up of alternating sugar and phosphate
molecules. The sugar in DNA is deoxyribose.
The rungs of the ladder are composed of pairs of 4 types of nitrogen bases. The bases are adenine, thymine,
guanine and cytosine. These bases are known by their coded letters: A, G, T, and C. These bases always bond in a
certain way. Adenine will only bond with 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
that contains the instructions. For instance, ATGCACATA would code for a different gene than AATTACGGA. A
strand of DNA contains millions of bases.
The two sides of the DNA ladder are held together loosely by hydrogen bonds. The DNA can actually
"unzip" when it needs to replicate (make a copy of itself). DNA needs to copy itself when the cell divides so that
the new cell contains an exact copy of the DNA. Without these instructions, new cells wouldn't have the correct
information. The hydrogen bonds are represented by small circles
The DNA helix is actually made of repeating units called nucleotides. Each nucleotide consists of three
molecules: a sugar (deoxyribose), a phosphate (which links the sugars together), and then one of the four bases (A,
T, C, or G).
Messenger RNA
So, now, we know the nucleus controls the cell's activities through the chemical DNA, but how? It is the
sequence of bases that determine which protein is to be made. The sequence is like a code that we can now
interpret. The sequence determines which proteins are made, and the proteins determine which activities will be
performed. That is how the nucleus is the control center of the cell. The only problem is that the DNA is too big
to go through the nuclear pores, so a chemical is used to read the DNA in the nucleus. That chemical is messenger
RNA (mRNA). The mRNA is small enough to go through the nuclear pores. It takes the message of the DNA to
the ribosomes and tells them what proteins are to be made. Recall that proteins are the body's building blocks. The
code taken to the ribosomes tells the ribosomes what is needed - like a recipe.
Messenger RNA is similar to DNA, but it is a single strand. Another difference is that it has no thymine.
Instead of thymine, mRNA contains the base uracil (U). In addition to that difference, mRNA has the sugar ribose
instead of deoxyribose. Ribose and deoxyribose are just different forms of sugar (like white sugar and brown
sugar). RNA stands for ribonucleic acid.
The Blueprint of Life
Every cell in your body has the same "blueprint," or the same DNA. Like the blueprints of a house tell the
builders how to construct a house, the DNA "blueprint" tells the cell how to build the organism. But how can a
heart be so different from a brain if all the cells contain the same DNA? 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 is nonsense and codes for nothing. These
regions of DNA that do not code for proteins are called "introns" or "junk DNA". The sections of DNA that do
actually code for proteins are called "exons."
DNA Replication
Each time a new cell is made, the cell must receive an exact copy of the parent cell’s DNA. The new cells
then receive the instructions and information needed to function. The process of copying DNA is called
replication. Replication occurs in a unique way – instead of creating a completely new strand of DNA, each strand
contains one of the original strands of DNA. For this reason, DNA replication is called “semi-conservative.” When
DNA is ready to replicate, or copy, the molecule “unzips” itself along the hydrogen bonds and new nucleotides are
added to each side. Nucleotides are floating around in the cell waiting to pair up with their match.
STOP
Now that you have finished reading the article, answer the questions on the student worksheet titled DNA-The
Double Helix (Student Worksheet). When you are done, follow all directions below in order to complete your
poster. Work in order!
Poster Directions
“The Genetic Code of Life: DNA”
Follow these coloring instructions for the packet of pictures
Pictures #2, 3, & 5:
-Color all the phosphates pink or red (one is labeled "P"). Color all sugar molecules (deoxyribose
& ribose) blue (on DNA, one is labeled "D"; on RNA, one is labeled “R”).
-Color each thymine orange, and label all of these “T” on pictures #2 & #5
-Color each adenine green, and label all of these “A” on pictures #2, #3, & #5
-Color each guanine purple, and label all of these “G” on pictures #2, #3, & #5
-Color each cytosine yellow, and label all of these “C” on pictures #2, #3, & #5
-Color each uracil brown and label all of these “U” on picture #3
-Color the hydrogen bonds gray (picture #2)
Picture #6
-Color the nucleotides. Phosphates (P) are pink or red, deoxyribose sugars (D) are blue, T is
orange, A is green, G is purple, and C is yellow. Color the hydrogen bonds (
) gray.
Assembling the FRONT of the poster
1.
2.
3.
4.
Lay the poster paper in the “portrait” (up and down) position.
Across the top, write the title in large letters: “The Genetic Code of Life: DNA”
Put your name, class period, and date in the lower right hand corner.
Refer to the “Sample Layout” on the back of your worksheet titled “DNA – The Double
Helix.” Cut out the pictures, correctly position them, and glue the diagrams onto the poster.
PICTURE #1:
1. Beside enlarged chromosome (a large “X”), write this caption:
 Chromosomes are located inside the nucleus of the cell. Chromosomes are made up of
thousands of genes. If you unravel a chromosome, you would unravel thousands of genes.
2. Beside the strand of DNA coming off of the chromosome, write this caption:
 If the chromosome were stretched out, it would form many strands in the shape of a
double helix called DNA.
3. Draw an arrow to the twisted DNA (at the bottom of the picture). Label it “Double Helix”
PICTURE #2
1. Beside the strand of enlarged DNA, write this caption:
 DNA is made up of many nucleotides which each contain a phosphate, a sugar, and a base.
There are four different bases: A, T, C, and G. A always bonds with T, and C always
bonds with G.
PICTURE #5
1. Beside the replicating, or “unzipping,” DNA, write this caption:
 When DNA is ready to replicate itself, the first thing that happens is it begins to unzip
at the hydrogen bonds. Once the strands have unzipped, each adds a new nucleotide to
each side. The new DNA is given to new cells and is an exact copy of the parent cell’s
DNA.
PICTURE #4
1. Beside the diagram of the traits, write this caption:
 The genes (alleles) found on the strands of DNA code for individual traits.
2. Choose 3 colors to represent each of the 3 traits on your poster. Do the following:
1. Underline each trait with one of the 3 colors (each trait should have a different
color)
2. Draw a band on each side of the large chromosome (in picture #1) for each of the
traits. This shows where the genes are located on the homologous chromosomes.
Remember: These are homologous chromosomes, so the genes should be drawn in the
same order and same position on each side (see the example if you need to).
3. Write this caption next to the colors on your enlarged chromosome in Picture #1:
 Each band represents genes (alleles) that code for individual traits
PICTURE #3
1. Beside the messenger RNA, write this caption:
 The DNA tells the messenger RNA what proteins it needs made. The DNA is too large
to fit through the pores of the nuclear membrane, so the messenger RNA carries the
message out of the nucleus to the ribosomes. mRNA then instructs the ribosomes about
what proteins to make.
PICTURE #6
1. Write this caption next to the nucleotides:
 Nucleotides are the subunits of DNA made up of a phosphate, a sugar, and a base.
Repeating nucleotides form the double helix which forms the “blueprint” for making
proteins.
KEY (PICTURE #7)
1. Color in the key with the appropriate color used for each part of the DNA diagram.
Assembling the BACK of the poster
1. Across the top, write the title in large letters: “DNA Mutations”
2. Cut out all 4 DNA strands of DNA
DNA Strand #1 - Title this strand: “Original Sequence”
1. Paste strand #1 below this title (see sample in front of class)
DNA Strand #2 – Title this strand “Deletion Mutation”
1. Cut along the dotted lines to cut out the A-T pair of bases. Take away the A-T pair of
bases, but keep it because you will need it for strand #3.
2. Glue the two large remaining sides of the strand together.
3. Below the strand, write this caption: “In a deletion mutation, a base is left out. In this case,
A-T was deleted.”
DNA Strand #3 - Title this strand “Substitution Mutation”
1. Cut along the dotted lines to cut out the C-G pair of bases. Take away the C-G pair of bases,
but keep it because you will need it for strand #4.
2. Using the A-T pair of bases that you cut out from the deletion mutation, substitute that
pair of bases in for the C-G base that you took out. Glue down all 3 pieces of the strand.
3. Below the strand, write this caption: “In a substitution mutation, the wrong base is used. In
this case, A-T was used instead of C-G.”
DNA Strand #4 – Title this strand “Insertion Mutation”
1. Cut the strand along the dotted line so it becomes two pieces.
2. Using the C-G pair of bases that you cut out for the substitution mutation, insert this base
in between the two pieces. Glue down all 3 pieces of the strand.
3. Below the strand, write this caption: “In an insertion mutation, an extra base is added. In
this case, C-G was added.”
BOTTOM OF POSTER – Title this section “Consequences of Mutations”
1. Beneath the title, list the three consequences of mutations below:
1) Improved Trait
2)No Change
3)Harmful Trait