DNA, the Genetic Material
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C HAPTER
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Chapter 1. DNA, the Genetic Material
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DNA, the Genetic Material
Explain the importance of DNA.
Define and describe a nucleotide.
Describe the shape of DNA.
Summarize the base-pairing rules.
Where’s the instructions?
How do your cells know what to do? Just like computers have lines of code to tell them what to do, your cells also
have instructions. Your cells’ instructions are molecules of DNA.
The DNA Pioneers
In the early 1950s, the race to discover DNA was on. At Cambridge University, graduate student Francis Crick
and research fellow James Watson had become interested in the field. Meanwhile, at King’s College in London,
Maurice Wilkins and Rosalind Franklin were also studying DNA. The Cambridge team’s approach was to make
physical models to narrow down the possibilities and eventually create an accurate picture of the molecule. The
King’s team took an experimental approach, looking particularly at x-ray diffraction images of DNA.
In 1951, Watson attended a lecture by Franklin on her work. She had found that DNA can exist in two forms,
depending on the relative humidity in the surrounding air. This had helped her understand that the phosphate part of
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FIGURE 1.1
James Watson and Francis Crick show off
their DNA Model in 1953.
the molecule was on the outside. Watson returned to Cambridge with this information and together with Crick made
a model that ultimately did not measure up to the scientific facts. It caused the head of their unit to tell them to stop
DNA research.
Meanwhile, Franklin, working mostly alone, found that her x-ray diffractions showed that the "wet" form of DNA
(in the higher humidity) had all the characteristics of a helix, or twisted ladder shape. She suspected that all DNA
was helical but did not want to announce this finding until she had sufficient evidence on the other form as well. Her
partner Wilkins was frustrated because he did not want to wait. In January, 1953, he showed Franklin’s results to
Watson, apparently without her knowledge or consent.
Watson and Crick took a crucial conceptual step, suggesting the molecule was made of two chains of nucleotides,
each in a helix as Franklin had found, but one going up and the other going down. Crick had just learned of
Chargaff’s findings about base pairs in the summer of 1952. He added that to the model, so that matching base pairs
interlocked in the middle of the double helix to keep the distance between the chains constant.
Watson and Crick showed that each strand of the DNA molecule was a template for the other. During cell division
the two strands separate and on each strand a new "other half" is built, just like the one before. This way DNA can
reproduce itself without changing its structure (except for occasional mutations).
The structure so perfectly fit the experimental data that it was almost immediately accepted. DNA’s discovery has
been called the most important biological work of the last 100 years, and the field it opened may be the scientific
frontier for the next 100. By 1962, when Watson, Crick, and Wilkins won the Nobel Prize for physiology/medicine,
Franklin had died from cancer. The Nobel Prize only goes to living recipients, and can only be shared among three
winners.
What is DNA?
DNA is the material that makes up our chromosomes and stores our genetic information. When you build a house,
you need a blueprint, a set of instructions that tells you how to build. The DNA is like the blueprint for living
organisms. The genetic information is a set of instructions that tell your cells what to do. (See video "Stated Clearly
- What is DNA")
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Chapter 1. DNA, the Genetic Material
MEDIA
Click image to the left or use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/156012
DNA is an abbreviation for deoxyribonucleic acid (de-oxy-ribo-nu-cleic acid). As you may recall, nucleic acids
are a type of molecule that store information. The deoxyribo part of the name refers to the name of the sugar that is
contained in DNA, deoxyribose. DNA may provide the instructions to make up all living things, but it is actually a
very simple molecule. DNA is made of a very long chain of nucleotides. In fact, in you, the smallest DNA molecule
has well over 20 million nucleotides.
Nucleotides
Nucleotides are composed of three main parts:
1. a phosphate group.
2. a 5-carbon sugar (deoxyribose in DNA).
3. a nitrogen-containing base.
FIGURE 1.2
The only difference between each nucleotide is the identity of the base. There are only four possible bases that make
up each DNA nucleotide: adenine (A), guanine (G), thymine (T), and cytosine (C).
The Genetic Code
The various sequences of the four nucleotide bases make up the genetic code of your cells. It may seem strange that
there are only four letters in the “alphabet” of DNA. But since your chromosomes contain millions of nucleotides,
there are many, many different combinations possible with those four letters.
But how do all these pieces fit together? James Watson and Francis Crick won the Nobel Prize in 1962 for piecing
together the structure of DNA. Together with the work of Rosalind Franklin and Maurice Wilkins, they determined
that DNA is made of two strands of nucleotides formed into a double helix. Think of a spiral staircase or a twisted
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ladder, with the sugar and phosphate groups on the outside (frame), and the paired bases connecting the two strands
on the inside of the helix (the steps or rungs) ( Figure 1.3).
FIGURE 1.3
DNA’s three-dimensional structure is a
double helix. The hydrogen bonds between the bases at the center of the helix
hold the helix together.
Base-Pairing
The bases in DNA do not pair randomly. When Erwin Chargaff looked closely at the bases in DNA, he noticed
that the percentage of adenine (A) in the DNA always equaled the percentage of thymine (T), and the percentage
of guanine (G) always equaled the percentage of cytosine (C). Watson and Crick’s model explained this result by
suggesting that A always pairs with T, and G always pairs with C in the DNA helix. Therefore A and T, and G and
C, are "complementary bases," or bases that always pair together, known as a base-pair. The base-pairing rules
state that A will always bind to T, and G will always bind to C ( Figure 1.4). For example, if one DNA strand reads
ATGCCAGT, the other strand will be made up of the complementary bases: TACGGTCA.
Vocabulary
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base-pair: Bases in DNA that complement each other and are linked by hydrogen bonds.
chromosome: DNA wound around proteins; the chromosome forms during prophase of mitosis and meiosis.
DNA (deoxy-ribo-nucleic acid): Nucleic acid that is the genetic material of all organisms.
double helix: Shape of DNA; a double spiral, similar to a spiral staircase.
nucleic acid: Class of organic compound; nucleic acid includes DNA and RNA.
nucleotide: Units that make up DNA; the nucleotide consists of a 5-carbon sugar, a phosphate group, and a
nitrogen-containing base.
Summary
• DNA stores the genetic information of the cell in the sequence of its 4 bases: adenine, thymine, guanine, and
cytosine.
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Chapter 1. DNA, the Genetic Material
FIGURE 1.4
The chemical structure of DNA includes
a chain of nucleotides consisting of a 5carbon sugar, a phosphate group, and
a nitrogen base. Notice how the sugar
and phosphate form the backbone of DNA
(strands highlighted in pink), with the hydrogen bonds between the bases joining
the two strands.
• DNA is made of a long chain of nucleotides consisting of a 5-carbon sugar, a phosphate group, and nitrogencontaining base.
Explore More
Use the resources below to answer the questions that follow.
Explore More I
• What Is DNA? at http://learn.genetics.utah.edu/content/begin/dna/ . Click on the box "What Is DNA?" on
the left side of the page.
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2.
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4.
Where is nuclear DNA located in a eukaryotic organism?
Describe the structure of DNA.
What is another name for the "sentences" that DNA encodes?
What to genes tell a cell to do?
Explore More II
• Go to this link to build a DNA molecule: http://learn.genetics.utah.edu/content/begin/dna/builddna/ .
1. How long does it take a human cell to copy the DNA in its nucleus before it divides?
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2. How many new cells does your body produce every day?
3. How many hydrogen bonds form between guanine and cytosine? How many hydrogen bonds form between
adenine and thymine? Do you think this relationship helps minimize errors? Explain your reasoning.
4. What steps does the cell take to increase the rate of DNA replication?
Review
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4.
Describe the structure of DNA.
What does a nucleotide consist of?
What are the base-pairing rules?
If one DNA strand reads CCGTAATGCAT, what will be the sequence of the complimentary strand?
References
1. Image copyright ermess, 2014. The double helix of DNA. Used under license from Shutterstock.com
2. Mariana Ruiz Villarreal (LadyofHats). The chemical structure of DNA. CC BY-NC 3.0
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