Chapter 16 Study Guide
Courtesy of Julia Beamesderfer (2005)
DNA as the Genetic Material
I.
Discovering the Purpose of DNA
A. Until the 1940s, it was unknown as to whether the genetic material of chromosomes was
protein or DNA as the contents in the nucleus were both protein and DNA
B. In 1928, British medical officer Frederick Griffith was studying a bacterium that causes
pneumonia in mammals—streptococcus pneumoniae
1. Griffith had two strains, or varieties, of the bacteria—a pathogenic (disease-causing one)
and a variant that was harmless
2. After killing the pathogenic variety with heat and mixing it with living cells of the
harmless variety, Griffith injected the mixture into a mouse and noticed that some of the
harmless variant had been converted to the pathogenic form, and killed the mouse
3. Transformation- the phenomenon discovered by Griffith in which a chemical component
of a dead pathogenic disease causes a heritable change in other bacteria—in other words,
a change in genotype and phenotype due tot he assimilation by a cell of external DNA
4. By exposing the harmless bacteria to different chemicals from the heat-killed pathogenic
variety, Griffith noticed that only DNA caused transformation of the bacteria.
C. In 1952, Alfred Hershey and Martha Chase experimented with bacteriophages, or viruses that
infect bacteria.
1. Soon Hershey and Chase had discovered that DNA is the genetic material of the phage
T2, a phage that infects E. coli bacteria, which usually lives in mammal intestines
2. Since T2 could quickly turn E. coli into a virus-producing cell, the question was whether
protein or DNA was responsible for reprogramming the host cells
3. To test this, they tagged one batch of virus’ protein with radioactive sulfur and a different
batch with DNA containing radioactive phosphorus.
4. When he allowed these two different batches to infect E. coli cells, he noticed that only
the E. coli cells that had been exposed to tagged DNA had radioactivity in the actual
bacteria cell.
5. This showed that DNA was the hereditary material, and that when a virus attached to a
cell, its transported its DNA into that cell, causing it to produce new viral DNA and
proteins.
D. Chemist Erwin Chargaff added to evidence that DNA is genetic material for cells by
explaining the great diversity that can occur in DNA by the varying amounts of DNA’s four
nitrogenous bases within a species.
1. Remember, DNA consists of a nitrogenous base (which can be adenine (A), thymine (T),
guanine (G), or cytosine (C)), a pentose sugar called deoxyribose, and a phosphate group
2. Chargaff also discovered that the amount of A is always close to T, and G = C.
3. Hence, Chargaff’s rule: A pairs with T and C pairs with G.
E. Once it was known that DNA was the genetic material in cells, James Watson and Francis
Crick were the first scientists to discover its structure
1. When Watson saw an X-ray photograph of DNA taken by Rosalind Franklin, he and
Crick’s model was proposed. The X-ray showed three things: DNA was a double helix,
the distance between strands and the distance between nucleotides on the same strand.
2. They hypothesized that DNA was a double helix consisting of two strands of sugarphosphate backbones, held together by hydrogen bonds between the nitrogenous bases on
the interior of the helix
3. By trial and error, along with Chargaff’s information, Watson and Crick realized that
along the helix, A is paired with T, and C is paired with G.
a. These pairs were hypothesized by the fact that A and G are purines (with two organic
rings) and T and C were pyrimidines (with one organic ring) and that there must be a
uniform diameter of bases (ruling out A/G and T/C)
b. He then paired A/T and C/G by the fact that A could form 2 hydrogen bonds with T,
and C could form 3 hydrogen bonds with G.
c. With this in mind, the countless sequence of nucleotides along each DNA strand is
countless, representing the many different possible DNA strands.
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Chapter 16 Study Guide
Courtesy of Julia Beamesderfer (2005)
4.
They also discovered that the nitrogenous bases are stacked 0.34 nm apart, that the helix
makes one turn every ten layers of base pairs or 3.4 nm, and that the helix is 2 nm in
diameter
DNA Replication and Repair
I.
Base Pairing- due to the specific pairs that each DNA strands’ nitrogenous bases form, when the
two complementary strands are separated it is easy for each to replicate another
A. Watson and Crick’s three hypotheses for DNA replication
1. Conservative- the parental double helix remains intact and a second, all-new copy is
made
2. Semiconservative- The two strands of the parental molecule separate, and each functions
as a template for synthesis of a new complementary strand
3. Dispersive- each strand of both daughter molecules contains a mixture of old and newly
synthesized parts
B. Meselson-Stahl tested each of these hypotheses of DNA replication
1. Meselson and Stahl first cultured E. coli bacteria and exposed it to a medium containing a
heavy isotope of nitrogen, 15N, which was incorporated into the bacteria’s DNA
2. Then they transferred the bacteria to a medium containing lighter isotope of nitrogen,
14N, causing new DNA synthesized by the bacteria to be lighter than the previous DNA
3. They next spun the bacteria in a centrifuge
a. The first replication consisted of all hybrid DNA, ruling out conservative model
b. The next replication consisted of hybrid and light DNA, ruling out dispersive model
and supporting semiconservative
II.
DNA Replication
A. Vocabulary Related to Replication
1. Origin of Replication- special sites along DNA where replication begins; it consists of a
specific sequence of nucleotides
2. Replication Fork- the Y-shaped region at the end of a replication bubble where new
strands of DNA elongate
3. Proteins Functioning in DNA Synthesis
a. DNA Polymerase- enzymes which catalyze the elongation of new DNA at the
replication fork by adding nucleotides one by one to the template strand of DNA to
form new strand. This is driven by nucleoside triphosphates (nucleotides with 3
phosphate groups) (like ATP yet with deoxyribose sugar instead of ribose in ATP)
b. DNA Ligase- enzyme that joins Okazaki fragments of a lagging strand into a single
DNA strand
c. Primase- since DNA polymerases can only add nucleotides to existing
polynucleotides paired to a complementary strand, the enzyme primase is needed to
initiate chains (primers) to serve as basis for chain. It does this by joining RNA
nucleotides to make a primer.
d. Helicase- an enzyme that untwists the double helix at the replication fork and
separates the two old DNA strands
e. Single-Strand Binding Protein- protein which lines up along the unpaired DNA
strands and holds them apart while they serve as templates for the synthesis of new
complementary strands
4. Antiparallel DNA Strandsa. The double helix strands are antiparallel (their sugar-phosphate backbones run in
opposite directions). One strand runs from the end were the fifth carbon is bonded to
phosphate towards the end were the third carbon is bonded, and vice versa.
b. Since DNA polymerases can only add nucleotides to the free 3’ end of the growing
DNA strand ( in a 5’ > 3’ direction) there must be adjustments made to replication
(below)
c. Leading Strand- the DNA strand that can synthesize continually due to it replicating
in the mandatory direction of 5’ > 3’
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Chapter 16 Study Guide
Courtesy of Julia Beamesderfer (2005)
d.
III.
IV.
Lagging Strand- the DNA strand that must be built discontinually in the opposite
direction by making creating small fragments from 5’ > 3’ (like backstitch in sewing
machine) called Okazaki fragments and then connecting them together
B. Step by Step Replication
1. Starting at origins of replication along DNA molecules, Helicase enzymes untwist the
double helix at its replication forks, separating the two old DNA strands
2. Then molecules of single-strand binding protein line up along the unwound DNA strands
to hold apart and stabilize them as each strand serves as a template for the synthesis of a
new complementary strand
3. The leading strand is initiated by an RNA primer and synthesized continuously in the 5’
> 3’ direction by DNA polymerase
4. The lagging strand is synthesized discontinuously. Primase synthesizes short RNA
primers, which are extended by DNA polymerase to form an Okazaki fragment
5. RNA primers are replaced by DNA with help from DNA polymerase
6. DNA ligase joins the Okazaki fragments of the lagging strand
7. **Remember- this occurs in BOTH directions of the origin of replication, each direction
containing a leading and lagging strand
DNA Repair- while replication occurs, many repair mechanisms correct mistakes along the way
A. Mismatch Repair- this DNA repair mechanism fixes mistakes that are made when DNA is
copied.
1. During DNA replication, this is carried out by DNA polymerase, which proofreads each
nucleotide against its template as it is added to a strand. When mistake is found,
polymerase removes it and resumes synthesis
B. Genetic maintenance is also required in DNA
1. Often DNA molecules are subjected to harmful physical and chemical agents such as
reactive chemicals, radioactive emissions, X-rays, and Ultraviolet light.
2. These changes are corrected by DNA-repair enzymes who fix problem
3. Excision Repaira. Nuclease, a DNA-cutting enzyme, removes the damaged segment of the strand
b. The resulting gap is filled with nucleotides paired to undamaged strand with use of
DNA polymerase and ligase
c. Very Important! EX- organisms with xeroderma pigmentosum have an inherited
defect in the excision-repair enzymes, and are consequently hypersensitive to
sunlight due to uncorrected mutations of skin cells from UV light, causing cancer
Problems with DNA-End Replication
A. Because DNA polymerase can only add nucleotides to the 3’ end of a preexisting
polynucleotide, primers that begin replication on a leading strand cannot be replaced by DNA
B. Consequently, repeated replication would produce shorter and shorter DNA molecules
C. Telomeres- these special nucleotide sequences on the end of DNA molecules consist of
multiple repetitions of a short nucleotide sequence, which contain no genes. These sections of
DNA protect real genes from being eroded through successive founds of replication.
1. Due to the ever-shortening of telomeres, it is possible that they are a limiting factor in the
lifespan of an organism
D. Telomerase- an enzyme that catalyzes the lengthening of telomeres (by using a molecule of
RNA along with its protein which contains a nucleotide sequence to serve as the template for
new telomere segments)
1. Telomerase is not present in most cells of multicellular organisms, but is present in cells
that give rise to gametes, making sure offspring have long telomeres
2. Telomerase has often been found in somatic, cancerous cells that need to lengthen their
unusually short telomeres (caused my numerous divisions)
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