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THE CHEMICAL
STRUCTURE OF
THE GENE: DNA
36-1 lntroduction
Page 63
Page 64
36-2 The Double Helix
36-3 Laboratory lnvestigation:
DNA Replication
Ch 36 The Chemical Struclure of the Gene: DNA
Page 70
62
Cfropter 36
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CfrEtnicof Stntcture
of tfi" $ene: DA(A
foa ra! not fraoe 6een responsi1{z for gwr fuitage,
6ut gou are responsi5te for your future,
Zig Ziglar
36-1 lntroduction
Obiective
You will be expected to know that the gene is the molecule known as DNA and
that its discovers were Watson and Crick.
You have learned that genes determine the color of your eyes, hair and skin. They
determine a wide variety of physical features and your potential intellectual ability,
among other things. How do they do it? How can a little spot on a chromosome that we
can't see exceBt through a microscope, determine almost every physical feature for all
species? Most of what is known about the gene has been discovered in the last half of
the Twentieth Century.
The gene is a molecule known as DNA.
lt is perhaps the most fascinating
molecule ever studied. lt is a large and complex molecule and contains four repeating
subunits. These subunits spelt out instructions to thb celljust as letters spell out words.
Recent fascinating research has made it possible to remove genes from one oiganism
and splice them into other organisms. This endeavor is referred to as genetic
engineering and you will learn about it in a later chapter.
DNA belongs to a class of compounds called nucleic acids because they are found
primarily in the nuclei of cells. The structure of DNA was worked out by Francis Crick of
England and James Watson of the United States. They received the Nobel prize for
their work in 1958.
An initial experiment in 1928 by Frederick Griffith showed that the gene was DNA. He
removed DNA from one type of bacteria and added it to a second strain of bacteria. The
second strain was transfoimed and took on genetic characteristics of the first strain. He
concluded that DNA must be the genetic material.
Ch 36 The Chemical Struclure ol the Gene: DNA
63
1. What contributions did Watson, Crick and Griffith make to the field of genetics?
36-2 The Double Helix
1. What is the significance of DNA to the cells and bodies of organisms?
Objectlve
You will be expected to describe the basic structure of DNA, its components and its
relationship to the chromosome. This activity is a prerequisite to all other labs and
reading topics on DNA and genetic engineering.
DNA is the molecule that makes up the chromosomes. The different segments along
the DNA molecule comprise the genes that govern body functions and physical
characteristics. The diagram on the next page shows the relationship of DNA to the
chromosomes in the nucleus. Study the diagram on the next page and be sure to read
all captions and then continue reading below. f l
Notice that the X-shaped chromosomes in the nucleus are composed of long tightly
coiled strands of DNA. The diagram is artificial in that the DNA is shown pulled out of a
chromosome and nucleus and progressively magnified. This is done to show details of
DNA and how it makes up the chromosome.
The DNA molecule itself is a long double stranded molecule. The two long strands are
twisted around each other to form what geneticists call a double helix (double spiral).
The two strands are held together by cross-piece subunits called nucleotide bases,
Examine these cross-pieces on the next page again. I I You will notice at the bottom
of the page, where the enlargement of DNA is its greatest, that the nucleotide bases are
molecule subunits symbolized by A, T, C and G. These subunits are arranged in a
different order in each specific gene. The arranged order of these 4 subunits spells out
the specifications lor gene expression. This is the genetic code found in all genes.
Gn
SO
The Chemical Structure ol the Gene: DNA
64
\,Y
The 46 chromosomss
in cell nuclei are
composed of DNA.
+.
nucleus
-.-
Enlarged section of the tightlY
crliled DNA that makes uP the
chromosome.
DNA Stnuctunc
As the DNA is enlarged lurther and
(
deoxynihonuelela
aald
)
uncoiled, the double helix structure
noticeable along with lhe nucleotide
is
bases'
E
I
lf the DNA in all 46 human chromosomes{rom 1
cell were uncoiled and stretched out end-lo-end,
h would be more than 6 feet long. ln the cell's
nucleus, this DNA is so small it is not viss.ible to
the unaided eya.
Thousands of genes are
contained on each DNA
molecule. These genes
regulate every
charactedstic and aspect
ol body function.
The genes on DNA are made of 4
nucleotide bases (or subunits), A, T,
As the strands of DNA separate in
c,&G.
replication, new bases lrom the
cell are incorporated inlo new
molecules of DNA
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o
P
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The ChemicalStructure ol the Gene: DNA
The genetic code, and thus a
genetic trait, is determined by
the order of the bases on DNA.
To understand how genes express themselves and how scientists splice genes from one
organism into another, it's essential to understand specific details about the structure of
DNA. For this purpose, the DNA molecule has been redrawn on the next page in an
untwisted form to cause the molecule to appear as a ladder (lower right) . Study this
drawing carefully and return to the next paragraph. I_l
Notice that the vertical rails of the ladder are composed of alternating sugar-phosphate
subunits. I-]
The horizontal rungs or steps of the ladder are composed of the
nucleotide bases called Adenine (symbolized by A), Thymine (T), Cytosine (C), and
Guanine (G). Examine the DNA ladder. Certain nucleotide bases always pair with others.
What pairs with what? t_l You should have observed that A always pairs with T and C
with G. This is important to remember. The following chart shows some important
comparisons between the structures of DNA and RNA, another important cellular
molecule:
Component
Strand number
Nucleotide bases
Sugar
Name
DNA
double stranded helix
A = Adenine
T = Thymine
C = Cytosine
G. = Guanine
!=.e:gxyribose
_p9o_xyriboNucleic Acid
RNA
single stranded
A = Adenine
U = Uracil (lnsteao ol Thymine)
C = Cytosine
G = Guanine
Ribose
RiboNucleic Acid
HNA, as you will see, is an important helper nucleic acid[molecule needed in order for the
gene (DNA) to express itself. Study the above chart arld mentally note the differences
between RNA and DNA. t
l
j
You should remember that RNA has a different dncleotide base not found in DNA. ln
RNA, U (Uracil) substitutes for the T (Thymine) found in DNA. This rneans that when -'
base pairing occurs in RNA, C pairs witfr G, but that-UpaifS-Udtfu\. Some other
important differences are that RNA is single stranded, whereas DNA is double stranded
and wound in a helix (double spiral). The sugar in RNA is ribose rather than
deSxyribose which is reflected'in the differences in names; DeoxyriboNucleic Acid as
opposed to Ribonucleic Acid,
2.
3.
Chromosomes are made from what molecule?
lf the DNA found in all46 chromosomes were unwound and lined up end-to-end, how long would the DNA
be?
4. What are the iull chemical names of the 4 nucleotide bases found in DNA?
5. What are the full ihemical names of the 4 nucleotide bases fosF++n-RNA?
6. What is the genetic code? Explain.
7. How is one gene different from another gene?
8. What is the chemical name of the sugar found in DNA? ln RNA?
9. What is the complete chemical name of DNA? Of RNA?
10. Which nucleotide base is found in RNA but not DNA?
11. ln DNA, which bases pair with which other bases?
12. ln DNA, what bases makeup the vertical strands of the ladder?
13. Describe the 3-dimensional structure of the DNA molecule. How do all the parts fit together?
Ch 36 The Chemical Structure ol the Gene:
DNA
66
'
of D}$A
Tlhe
DNA
Key to the
NucI eotid
4
es
H.,W.E.-K = Guanine
lc
@ CI
= cytosine
q
= rnYmine
ffi
la
=
a\
= sugar
\-t
Adenine
(deoxyribose)
=
phosphate
Ch 36 The Chemical Siructure ol the Gene: DNA
67
36-3
{i'
Lab lnvestigation
DNA Replication
1. Name the four nucleotide
bases that make up DNA.
Objectlve
When you have finished this brief lab activity, you should be able to describe how
DNA makes exact copies of itself. (replicates)
DNA Replication
To replicate means to produce a copy of itself. DNA is the only known molecule that can
do this. When chromosomes double and pull apart in both mitosis and meiosis, the DNA
that makes up the chromosome must make an exact copy of itself (replicate). DNA is
able to make an exact replica of itself because of the base pairing characteristics
stressed earlier (A with T and C with G). When DNA makes a duplicate molecule of itself,
the two strands unwind. See how this is shown in the drawing on page 67. I I After
'as
the 2 strands have pulled apart, new bases (A, T, C & G) is well
new sugar and
phosphate units come into place according to the base pairing rules. A comes in
opposite the T and C opposite the G. When this has occuried, 2 identical DNA
molecules will be created. This has been an introduction to DNA replication. you will
demonstrate this with paper clips in the following lab investigation. This lab investigation
will help you to better understand the process.
DNA (Deoxyribonucleic acid) is a vital molecule in living organisms. The GENE is
made of DNA. DNA is one of the few molecules that can REPUCATE ITSELF. (To
replicate means to make.exact copies of itself.) Some of the most interesting reseaich
taking place today is being done with DNA. Before the student can understand genetic
engineering research, he or she must first understand the basic structures of DNA and
understand how this molecule replicates itself.
It is assumed that you now already know the basic structure of DNA. Flecall that DNA is
the GENE and is a double-stranded nucleic acid molecule composed of two strands,
twisted into a double helix (double spiral). Each DNA strand is composed of four
molecular sub-units called NUCLEOTIDE BASES. ln this activity, you will construct a
DNA molecule out of colored paper clips. You will then learn and review how the
molecule replicates itself to fornr an exact copy of itself. Each different colored paper clip
will represent a different nucleotide base. Use the following colored clips to ieiresent
the four nucleotide bases as follows:
Ch 36 The Chemical Struclure of the Gene: DNA
68
EACH PAIR OF STUDENTS WILL NEED THE FOLLOWING:
The time needed for
this activity is about
35 - 40 minutes.
14 Black clips = Adenine (A)
14 White clips= Thymine (T)
Red clips = Cytosine (C)
9 Gr:een clips = Guanine (G)
I
Remember that each DNA molecule has a unique structure that makes it different from
other DNA molecules (or genes) in that the sequence of A, T, C, and G vary from one
molecule (gene) to another. You will be making a shoft sequence of a human gene that
controls thJbody's production of growth hormone which causes growth during childhood
and adolescence. benetic engineers call this gene the hGH (human Growth Hormone)
gene. This gene is actually made of 573 nucleotide base pairs. You will only construct
the first ten bases in the gene.
WORK IN TEAMS OF TWO STUDENTS
STEp ONE: Use the colored paper clips according to the key above and construct the
top strand of the hGH gene according to the diagram of the gene below. Link the 10
appropriate colored clips for the top chain shown below' I l
12345 6 78I
10
A-A-G-C-T-T-A-T-G-G
T-T-C-G-A-A-T-A-C-C
Notice that the bottom strand of the DNA molecule follows the "rule of complementarity",
which means that A bonds with T, and C bonds with G.
STEP TWO: Now construct the bottom strand of the hGH gene by linking 10 more clips
into a chain according to the pattern above. The entire sequence of this gene is known.
Your DNA model should resemble the following :
t
1
STEp THREE: Set the two chains side-by-side as shown in the drawing above so that
A bonds with T and C with G.
You now have a model of the hGH gene' (first 10 bases only).
Compare the 2 chains within each other side-by-side to verify that C bonds with G and A
with T. When this gene replicates in the nucleus of a cell, the double-strand begins to
separate at one end and as it separates, new nucleotide bases are moved into place by
enzymes so as to form the beginning of 2 new identical molecules. These A,T, C, and
G bases are present in the nulleus of each cell and come from food molecules. When
these new bases are brought into place, the A bonds with T and the C bonds with G.
Ch 36 The Chemical Structure of the Gene: DNA
69
STEP FOUH: Open your hGH DNA molecule as shown below:
STEP FIVE: Now use the other available clips to create the beginning of 2
strands. Remember A with T and C with G. Connect the clips as follows:
new
m-1
STEP
SIX:
Continue separating the strands and bring in appropriate new bases (clips)
to create 2 complete new double-stranded hGH gene molecules. Remember that A
bonds opposite to T and C is opposite to G. You should have 6 clips left. Save them for
later.
Answer the questions that follow.
2.
Examine tne Z double-stranded DNA molecules. Are they identical or different in any
way?
3.
lf you were asked to replicate each of the 2 DNA molecules on your table to create 4
identical DNA molecules, how would you go about doing this?
4. You now have 2 copies
of a segment of the hGH gene on your table. During periods
of growth and cell division, the chromosomes, which are made-up of genes, must
divide. What features about DNA replication causes each new DNA molecules to be
exactly like the original?
Cells can divide, along with their DNA, in this manner without any errors for thousands of
replications. On occasion an error can occur. When this type of error occurs in a cellthat
is destined to become an egg or a sperm cell, it is called a MUTATION
WHAT IS A GENE MUTATION?
To demonstrate a gene mutation, place one of your paper clip hGH DNA strands in front
ldentify the second nucleotide base called Adenine (A) which is black. To
of you.
cause a mutation. remove this 2nd black clip and replace it with a red cytosine (C) clip.
t I
Ch 36 The Chemical Structure of the Gene: DNA
70
I I
You have just demonstrated how a mutation occurs. This replacement usually
occurs when the DNA is replicating.
5. When this mutated DNA molecule replicates, will the resulting new DNA be similar
or
ditferent from the hGH gene? Explain.
This type of mutation can be caused by a variety of circumstances including radiation,
chemical exposure or it can occur spontaneously without known cause.
6.
Draw structures of the new double-stranded mutated gene you created by using the
letters A, T, C, and G in a manner similar to that shown earlier.
Since the sequence of bases is different in this molecule, the oenetic code is
different. This gene CANNOT direct the cell to produce normal human growth
hormone.
7.
ln your own words, explain how mutations can occur in cells and how this might affect
the new organism with the mutation.
Please separate all the paper clips and organize them by color in appropriate
containers, t I Count them. You need to have 14 black and 14 white, 9 red and 9
green. I I Call your teacher to check your table. I l
Ch 36 The Chemical Structure ol the Gene:
DNA
71