1. science
2. medicine
3. genetics

13
DN.A.
STRUCTURE A N D F U N C T I O N S
CHAPTER REVIEW
Several experiments proved that DNA is the genetic material. Griffith ' s work revealed the presence of a transforming substance in pneumococcus infecting mice. A very
and associates working with the bacteria alone reported
that the transforming substance was DNA . The Hershey
and Chase experiments with aT virus offered more convincing evidence for the genetic ro le of DNA.
Several lines of investigation contributed to a
knowledge of DNA structure. Chargaff showed that the
amount of purine (adenine , guanine) equals the amount
of pyrimidine (cytosine, thymine) . Franklin ' s X-ray
diffraction analysis revealed the helical shape of the
molecule. Watson and Crick used the information gained
from these experiments to build a model of DNA. Alte rnating sugar-phosphate molecules form the sides
STUDY
of a ladder, with hydrogen-bonded base pairs composing the rungs. This ladder is twisted into a helix .
The Watson and Crick model also accurately predicted DNA replicates. As the helix unzips, each parental
strand serves as the template for the synthesis of a new
daughter strand. Through replication, each duplex produced is identical to the original double helix . This semiconservative method of replication was demonstrated
through the experiments of Meselson and Stahl.
Replication in prokaryotes and eukaryotes is bidirectional along the chromosome, altho ugh the details
of the process differ. Genetic mutations due to DNA
replication errors are infrequent because of proofreading,
and damage can be reversed by DNA repair enzymes.
EXERCISES
Study the text section by section as you answer the questions that follow .
ENETIC
ATE RIAL
• Early experiments showed that DNA is the ge netic material.
• Because DNA is the genetic material, its structure and functions constitute th e molec ular basis of
inheritance .
• DNA stores information that controls both the development and metabolism of a ce ll.
I . Check the requirements for a substance that is genetic material.
a. _ _ has the constancy to store information , and therefore to serve as a blueprint for each generation
b. _ _ can be replicated, and each new cell has a copy
c . _ _ can undergo mutations, res ultin g in variability between species
d. _ _ cannot be transmitted from generatio n to generation so that veracity exists
e . _ _ conducts photosynthesis, ens uring that each organism gets the energy it needs
2. Indicate whether these statements about Griffith's transformation experiments are true
(T) or false (F) .
a. _ _ The normalS strain was virulent.
b. _ _ The normal R strain was not virulent.
c. _ _ The heat-killed S strain was not virulent.
d. _ _ A mixture of heat-killed S strain and liveR strain was virulent.
3. Based on your answers to question 2, what was Griffith's conclusion?
107
4. Indicate whether these statements about A very's transformation experiments are true (T) or false (F).
a. _ _
Action of a DNase on the transforming substance prevented transformation.
b. _ _ The transforming substance had many nucleotides.
c.
Protein from S strain bacteria transformed R strain bacteria.
d. _ _
RNA was the tran sforming substance .
5. Based on your answers to question 4, what was Avery's concl usion?
In question 6, fill in the blanks.
and 35 S to
6. The following diagram shows that the two separate experiments used 32 P to label a.
label
of viruses. In each experiment, the viruses were allowed to infect bacteria, a nd then a
b.
blender was used to separate the caps id s from the bacteria . Radioactivity was found inside the cell only when
c.
was labeled. Replication of viruses followed , so the hypothesis that
is the
d.
genetic material was supported.
For e-i, write the location of radioactivity on the line s provided in the diagram.
}
}
Viruses in
liquid are not
radioactive.
Bacteria in
sediment are
radioactive.
centrifuge ____...
Culture bacteria and phage;
e. _ _ _ _ _ __
Agitation in blender; Empty capsids outside
f. _ _ _ _ __
capsids dislodged
Detect location of
radioactive DNA
g. _ _ _ _ __
a. Viral DNA is labeled.
}
}
centrifuge
Culture bacteria and phage ; radioactive
capsids stay outside bacterium
Agitate in blender:
capsids dislodged h._ _ _ _ __
Viruses in
liquid are
radioactive.
Bacteri a in
sediment
are not
radioactive.
~
Detect location of
radioactive capsids
b. Viral capsid is labeled.
• DNA is a double helix; each of the two strands is a polymer of four different nucleotides.
• Four nucleotides permit a great deal of variability , and the sequence of base pairs in DNA varies from gene to
gene.
• Hydrogen bonding between complementary bases joins the two strands.
7 . Four different nucleotides are found in DNA. Check the way(s) these nucleotides differ.
a.
They differ in their sugar content.
b. _ _ They differ in their phosphate content.
c. _ _ They differ in their base content.
8. What are the four different nucleotide bases in DNA ? - - - - - -- - - - -- - - - -- - - - -- -
108
9. Study the following table:
A
T
G
c
Homo sapiens
31.0
31.5
19.1
18.4
Drosophila melanogaster
27.3
27.6
22.5
22.5
Zea mays
25.6
25.3
24.5
24.6
Neurospora crassa
23.0
23.3
27.1
26.6
Escherichia coli
24.6
24.3
25.5
25.6
Bacillus subtilis
28.4
29.0
21.0
21.6
Species
a. Is the amount of each base constant between species? _ __ _ __
b. What is Chargaff's first rule?
c. With which requirement for the genetic material in question 1 (see page 107) do you associate this rule? _
d . What is constant within each species, as stated in Chargaff' s second rule?
e. With which requirement for the genetic material in qu est ion I (see page I 07) do you associate this rule? _
10. Examine the following diagram, which shows the ladder structure of DNA.
a. DNA is a polymer of _ _ _ _ __
b. Draw a box around one nucleotide.
c. The molecules making up the sides of the ladder are
d. Label a sugar and a phosphate .
e. What is meant by the phrase complementary base pairing?
f. Label the bases , which are complementary to those on the left.
g. What do you have to do to the ladder structure to have it match
the Watson and Crick model ? _ _ _ _ __
h. Explain what is meant by double helix. _ _ _ __ _ _ _ __ _
1.
Explain what is meant by antiparallel strands. _ _ _ __ __ _
109
• DNA is able to replicate, and in this way genetic information is passed from one cell generation to the next.
11 . Study the following diagram of replication.
a. The bases in parental DNA are held together by what type of bond (not shown)? _ _ _ _ __
b. What happens to these bonds for replication to take place? - - - -- - - -- - - - - - - - - c. During replication , new nucleotides move into proper position by what methodology? _______
d. Elongation of DNA is catalyzed by an enzyme called
will be
. When replication is finished, there
DNA molecules.
e. Each new double helix consists of an _______ strand and a _ _ _____ strand. Therefore, the
process is called _ _ _ _ __
f. Each double helix has _ _ _ _ _ _ (the same, a different) sequence of comp lementary paired bases.
12. Use the following diagram to help you fill in the blanks, and complete the explanation of the Meselso n and
Stahl experiment.
14N
DNA in-
tr--- light DNA
hybrid DNA
CsCl
solution in
centrifuge
tubes
l-heavy DNA
centrifuge for 2-3 days
a. Possible results when DNA is centrifuged in CsC!
-hybrid
DNA
heavy
DNA
DNA when cells are grown in
JSN
b. Steps in Meselson and Stahl experiment
110
DNA after one division in 14N
DNA after two divisions in
14N
First, Meselson and Stahl grew bacteria in a medium with a.
step, only
molecules of DNA were in the cells. When they switched the medium to
b.
c ._ _ _ _ _ _ _ ,
(see lower left of diagram). By this
then
d. _ _ _ _ _ _ _ .DNA
divisions, half of the DNA molecules were
molecules were in the cell after one cell division. After two
e. _ _ _ _ _ _ _ ,
and the other half were
f. _ _ _ _ _ __
g. Color the DNA molecules to be consistent with your descr iption in the paragraph .
13. Mese lson and Stahl concluded that DNA replication is
mo lecule contains a(n)
b. _ _ _ __ __
a.
,
meaning that each daughter
strand and a(n) c. _______ strand.
Prokaryotic Vers us E u karyotic Replication (P. 233)
14. Place the appropriate letter(s) next to each statement to indicate DNA replication in:
P-prokaryotes E-eukaryotes P ,E-both
a. _ _ Cells complete replication in a matter of hours .
b. _ _ Cells complete replication in a matter of minutes.
c. _ _ It begins at numerous origins.
d. _ _ It begins at one origin .
e.
It is bidirectional.
f. _ _ A rep lication fork is involved.
Replication Errors (P . 233 )
• Mutations occ ur whe n there are errors during the replication process .
15 . Indicate whether these statements are true (T) or false (F). Rewrite any false statements to make them true.
a. _ _ A genetic material must be able to undergo rare changes called mutations. Rewrite:
b. _ _ Mutations introduce variations that can possibly cause evolution to occur. Rewrite:
c. _ _ M utations are rare because RNA polymerase checks to make sure that complementary base pairing
occ urs correctly . Rewrite:
d. _ _ This process is called "proofreading." Rewrite:
Ill
KEYWORD CROSSWORD
Review key terms by completing this crossword puzzle, using the following alphabetized li st of term s:
adenine
bacteriophage
cytosine
DNA
DNA polymerase
guanine
mutation
nucleic acid
purine
pyrimidine
RNA
thymine
1
2
3
4
5
7
6
I
8
9
10
11
I
Across
2 One of four nitrogen-conta in ing bases in nucleotides
compos ing the structure of DNA and RNA ; pairs with
thy min e
3 One offo ur nitrogen-containing bases in nucleotides
composi ng the structure of DNA and RNA; pairs with
guanine
4 Nucleic acid po lymer produced fro m covale nt bonding of nucleotide monomers that co nta in the sugar
ribose; he lp s carry o ut protein sy nth esis
5 Type of nitrogen-containing base, suc h as ade nin e
and guani ne, havin g a doub le rin g str ucture
7 Durin g replication, an enzyme that joins the nucleotides comp le mentary to a DNA temp late
I 0 One of fo ur nitrogen-containing bases in nucleotides
composing the structure of DNA; pairs with adenine
II
Polymer of nucleotides; both DNA a nd RNA are
this (two words)
112
Down
Virus that parasitizes a bacter ial ce ll as its host ,
often de stroyin g it by lytic action
6 One of four nitroge n-conta inin g bases in nucleotides
compos in g the structure of DNA and RNA; pairs with
cytosi ne
7 Nuc leic acid polymer produc e d from cova le nt
bonding of nucleotide monomers that contain the
sugar deoxyribose; the ge net ic material of nearly
all o rgan is ms
8 Type of nitrogen-containing base, such as cytosine,
thymine, and uracil , havi ng a single -ring structure
9 Alteration in chromo so me stru cture or number and
a lso an alteration in a ge ne due to a change in DNA
compos iti o n