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KEY CONCEPT
DNA and RNA are required to
make proteins.
BEFORE, you learned
NOW, you will learn
• Traits pass from parents to offspring in predictable patterns
• Traits are passed on through
genes
• In sexual reproduction, offspring get half their genes
from each parent
• How the structure of DNA
stores information the cell
needs
• How DNA is copied
• How RNA uses the information
from DNA to make proteins
VOCABULARY
EXPLORE Templates
replication p. 137
RNA p. 138
How does a template work?
PROCEDURE
1
MATERIALS
Write a set of rules to describe how the
characters in line A relate to the characters
in line B.
• paper
• pencil
A
B
2 Place a piece of paper just under line C below. Use
the rules from step 1 to produce a template—the
corresponding pattern that goes with line C.
C
3 Give the rules and the template to a classmate to
produce a copy of line C.
WHAT DO YOU THINK?
What is a template and how does it differ from a copy?
DNA is the information molecule.
SUPPORTING MAIN IDEAS
Make a chart of information supporting the main
idea: DNA is the information molecule.
DNA is a molecule that stores information—that’s all it does. You
could compare the information in DNA to the books in your local
library. You might find a book describing how to bake a cake, make a
model sailboat, or beat your favorite computer game. The books, however, don’t actually do any of those things—you do. The “books” in
the DNA “library” carry all the information that a cell needs to function, to grow, and to divide. However, DNA doesn’t do any of those
things. Proteins do most of the work of a cell and also make up much
of the structure of a cell.
Chapter 5: DNA and Modern Genetics 135
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RESOURCE CENTER
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Learn more about DNA.
Proteins and Amino Acids
Proteins are large molecules that are made up of chains of amino
acids. Twenty different amino acids come together in enough combinations to make up the thousands of different proteins found in the
human body. Some proteins are small. For example, lysozyme is a
digestive protein that is made up of a sequence of 129 amino acids.
Some proteins are large. For example, dystrophin is a huge structural
protein that is made up of 3685 amino acids.
Check Your Reading
What is the relationship between proteins and amino acids?
DNA stores the information that enables a cell to put together the
right sequences of amino acids needed to produce specific proteins.
Scientists describe DNA as containing a code. A code is a set of rules
and symbols used to carry information. For example, your computer
uses a code of ones and zeroes to store data and then translates the
code into the numbers, letters, and graphics you see on a computer
screen. To understand how DNA functions as a code, you first need to
learn about the structure of the DNA molecule.
DNA molecule
DNA and the Genetic Code
The DNA molecule takes the shape of a double-stranded spiral,
which, as you can see from the diagram, looks something like a
twisted ladder. In Chapter 2, you read about different subunits that
make up the molecules found in cells. Nucleotide subunits make
up each of the two strands of the DNA molecule. One part of
the nucleotide forms the side rail of the DNA “ladder.”
The other part, the nucleotide base, forms the rung.
strand
Actually, two bases come together to form the rung,
as one nucleotide base attaches to another from the
opposite strand. You can see how the parts fit
together in the diagram to the left.
strand
nucleotide
bases
eukaryotic cell
DNA Base Pairs
Cytosine pairs with guanine.
Adenine pairs with thymine.
136 Unit 1: Cells and Heredity
nucleus
There are four different nucleotides in
DNA, identified by their bases: adenine (A),
thymine (T), cytosine (C), and guanine (G).
Because of differences in size and shape, adenine always pairs with thymine (A-T) and
cytosine always pairs with guanine
(C-G). The bases fit together like two pieces
of a jigsaw puzzle. These bases are often
referred to simply by their initials—A, T, C,
and G. The phrase “all tigers can growl” may
help you remember them.
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It is the sequence—the order—of bases in a strand of DNA that
forms the code for making proteins. Like a list of ingredients in a
recipe book, a set of bases specifies the amino acids needed to form a
particular protein. The cookbook uses just 4 bases—A, T, G, and C—
to code for 20 amino acids. A code of 2 bases to 1 amino acid gives
only 16 possible combinations. However, a code of 3 bases to 1 amino
acid gives 64 possible combinations.
T – C –T
(DNA triplet)
codes for
arginine
The genetic code is, in fact, a triplet code. A specific sequence of
3 nucleotide bases codes for 1 amino acid. For example, the triplet
T-C-T on a strand of DNA codes for the amino acid arginine. Some
amino acids have two different codes. Others have three, and some
have four. A gene is the entire sequence of the bases that codes for all
the amino acids in a protein. Each gene is made up of a sequence of
bases at a particular location on the DNA.
Replication
When a cell divides into two cells, each daughter cell receives an
identical copy of the DNA. Before a cell divides, all of its DNA is copied,
a process referred to as replication. Let’s follow the process through for
one DNA molecule. First, the two strands of DNA separate, almost like
two threads in a string being unwound. Nucleotides in the area around
the DNA match up, base by base, with the nucleotides on each DNA
strand. C matches up with G, and A matches up with T. When replication is complete, there are two identical DNA molecules. Each molecule
has one strand of old DNA and one strand of new DNA.
(amino acid)
reading tip
Replicate includes the root
word meaning “to repeat.”
Replication
Replication produces two identical
molecules of DNA.
new strand
Original
DNA
molecule
original strand
New DNA
molecule
2
nucleotides
3
Nucleotides match
up and join to the
open DNA strands.
Two identical
DNA molecules
are formed.
1
The DNA molecule
opens up, separating
into two strands.
original strand
new strand
New DNA
molecule
Chapter 5: DNA and Modern Genetics 137
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DNA template strand
DNA template strand
new strand
new strand
parent DNA molecule
DNA template strand
DNA template strand
1
The parent DNA
molecule is made up
of two strands.
2
The two strands
separate and are
used as templates.
3
Two new identical
DNA molecules are
formed.
During replication, each strand of DNA is used as a template to
produce a copy of the other strand. A template is a pattern or shape
that produces a matching, or complementary, product. If you’ve ever
made a plaster model of your hand, you’ve worked with a template.
You press your hand into a soft material that leaves a mold of your
hand. You then pour liquid plaster into the mold to produce a copy of
your hand. The mold is a template. Its shape allows you to make a
complementary shape that matches your hand.
reading tip
Complementary has a root
that means “to complete.”
RNA is needed to make proteins.
DNA is not used to make proteins directly. Translating the genetic
code of DNA involves another type of molecule, RNA. RNA, or
ribonucleic acid, carries the information from DNA to a ribosome,
where the amino acids are brought together to form a protein. DNA
actually codes for RNA. Three different types of RNA are involved in
making proteins. They are named for their functions:
VOCABULARY
Remember to choose
strategies from an earlier
chapter or some of your
own to take notes on
the term RNA.
messenger RNA (mRNA)
• ribosomal RNA (rRNA)
• transfer RNA (tRNA)
In prokaryotic cells, RNA and proteins are both made in the cytoplasm. In eukaryotic cells, DNA is copied in the nucleus, then RNA
moves to the cytoplasm, where the proteins are made.
•
Transcription
The process of transferring information from DNA to RNA is called
transcription. The chemical structure of RNA is quite similar to the
structure of DNA. Both are made up of four types of nucleotide
subunits. Three of the bases that make up RNA are the same as in
DNA: guanine (G), cytosine (C), and adenine (A). However, the
fourth base is uracil (U), not thymine.
138 Unit 1: Cells and Heredity
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Transcription
Transcription produces a
single-stranded molecule
of RNA.
RNA Base Pairs
Cytosine pairs with guanine.
Adenine pairs with uracil.
1
The DNA molecule
opens up along a gene.
nucleotides
2
DNA
Nucleotides of RNA
match up and join to
the open DNA strand.
3
The completed RNA strand
is released and moves into
the cytoplasm.
RNA
During transcription, DNA is again used as a template, this time
to make a complementary strand of RNA. Only individual genes are
transcribed, not a whole DNA molecule. The DNA again opens up, just
where the gene is located. As shown in the diagram above, RNA bases
match up to complementary bases on the DNA template. Adenine
pairs with uracil (A-U) and cytosine pairs with guanine (C-G).
reminder
DNA base pairs:
C-G, A-T
RNA base pairs:
C-G, A-U
Transcription is different from replication in some important
ways. Only one strand of DNA is transcribed, which means just a single strand of RNA is produced. When transcription is complete, the
RNA is released, it does not stay attached to DNA. This means that
many copies of RNA can be made from the same gene in a short
period of time. At the end of transcription, the DNA molecule closes.
DNA
Translation
Replication and transcription involve passing along information that
is coded in the language of nucleotide bases. To make proteins, cells
have to translate this language of nucleotide bases into the language of
amino acids. Three specific bases equal one amino acid. The actual
assembly of the amino acids in their proper sequence is the translation. Translation takes place in the cytoplasm of a cell. It involves all
three types of RNA.
Transcription
RNA
Translation
Protein
Chapter 5: DNA and Modern Genetics 139
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Translation
The assembling of amino acids to form
a protein occurs in the cytoplasm.
2
amino acid
Transfer RNA (tRNA)
matches up and and joins
to the mRNA strand.
1
A ribosome attaches
to an mRNA molecule
at the beginning of a
coding region.
ribosome
tRNA with an
amino acid attached
mRNA
molecule
3
As the ribosome moves along
the mRNA strand, it attaches
one amino acid to another.
chain of amino acids
4
The tRNA molecule is
released after the amino
acid has been attached.
chain of amino acids
is released
5
Once the ribosome reaches
the end of the coding region,
the completed chain of
amino acids is released.
140 Unit 1: Cells and Heredity
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Proteins are made on ribosomes, structures that are made up of
ribosomal RNA and proteins. If you think of DNA as a cookbook for
making different proteins, and mRNA as a recipe for making a protein, then the ribosome is the place where the cooking gets done. In
this analogy, tRNA gathers the ingredients, which are amino acids.
A tRNA molecule is shaped in such a way that one end of it can
attach to a specific amino acid. The other end of tRNA has a triplet
of bases that is complementary to a triplet of bases on mRNA.
Transfer RNA does the actual translation of bases to amino acid
when it matches up with mRNA. The diagram on page 140 shows
the whole process.
reading tip
Refer to the diagram on
page 140 as you read the
text. The numbers in the
text match the numbers in
the diagram.
1
Translation begins when a ribosome attaches to the beginning end
of an mRNA molecule.
2
A tRNA molecule carrying an amino acid matches up to a complementary triplet on mRNA on the ribosome.
3
The ribosome attaches one amino acid to another as it moves
along the mRNA molecule.
4
The tRNA molecules are released after the amino acids they carry
are attached to the growing chain of amino acids.
5
The ribosome completes the translation when it reaches the end of
the mRNA strand. The newly made protein molecule, in the form
of a chain of amino acids, is released.
Check Your Reading
Describe how the three different types of RNA work
together in protein synthesis.
VISUALIZATION
CLASSZONE.COM
The process of making proteins is basically the same in all cells. The
flow of information in a cell goes from DNA to RNA to protein.
Watch an animation of
how proteins are made.
KEY CONCEPTS
CRITICAL THINKING
.
1. Describe the shape of the DNA
molecule and how nucleotide
bases fit into that structure.
4. Infer What might happen if
the wrong amino acid is put
on a tRNA molecule?
2. What is a protein and what is
it made up of?
5. Apply Copy the following
sequence of DNA bases:
A-T-C-A-G-G. Write the complementary mRNA and tRNA
sequences for this.
6. Synthesize Study the
sequences you wrote for
question 5. How does the
tRNA sequence compare to the
original DNA sequence?
3. Identify three types of RNA
involved in protein synthesis
and briefly describe what
they do.
CHALLENGE
Chapter 5: DNA and Modern Genetics 141