Download Chapter 16: DNA

DNA
Deoxyribonucleic acid
A. Frederick Griffith identified DNA
as the genetic material in 1928
1. mixed two strains of the
bacterium Streptococcus
pneumoniae
a. the R strain was harmless
b. the S strain was pathogenic
2. heat-killed S strain was mixed
with live R strain and injected
into a mouse, which died of
pneumonia
a. transformation- a change
in genotype and phenotype
due to the assimilation of a
foreign substance (DNA) by
a cell
b. Avery, McCarty, and
MacLeod identified the
transforming substance as
DNA in 1944
B. T.H. Morgan identified that genes
are located on the chromosomes in
1940.
1. Chromosomes are composed of
DNA and proteins
C. Hershey and Chase showed that
DNA was the genetic material in
the bacteriophage (phage) T2
1. T2 infects E. coli
2. radio active trace experiment
a. radioactive sulfur identified
protein and ended up in the
supernatant
b. radioactive Phosphorus
identified DNA
(1) ended up inside the
bacterium  served as
the genetic material with
information to make new
viruses
D. Chargaff’s Rules
1. DNA composition varies from
species to species
2. The four bases (ATCG) are
found in charasteristic ratios for
a species
3. The % A = % T
The % G = % C
Ex. Human DNA
30.9% A, 29.4% T,
19.9% G, 19.8 % C
E. James D. Watson and Francis Crick
elucidated the three-dimensional
structure of DNA in 1953.
1. Rosalind Franklin and Maurice
Wilkins created diffraction
patterns of DNA with
X-ray crystallography
2. diffraction patterns yielded
structural data & measurements
a. helical shape- double helix
2nm wide
b. backbone of sugar phosphate
bonds
c. nitrogenous bases, one from
each strand, are connected
by H-bonds to form pairs in
the middle
(1) A=T
(2) G≡C
(3) the bases are 0.34 nm
apart
d. the helix twists every 10
bases at 3.4 nm
3. The linear sequence of the
four bases varies
a. each gene has a unique order
nitrogenous bases
F. DNA Replication
Semi conservative mechanism
When the double helix replicates
each of the daughter molecules will
have one old strand and one newly
synthesized strand
1. complimentary base pairing
enables existing DNA strands to
serve as templates for new
complimentary strands
2. the order of bases on one strand
is used to add complementary
bases and therefore duplicate the
pairs of bases exactly.
a. The double helix unwinds
and the strands separate
b. one at a time, nucleotides
line up along the template
strand according to the base
pairing rules.
c. Then the nucleotides are
linked to form new strands
3. Meselson & Stahl Experiment to
prove semiconservative
mechanism
a. labeled the nucleotides of
the original parental strands
with 15N
b. then transferred replicating
DNA to 14 N medium.
c. Separated replicated strands
by density in a centrifuge
G. Process of DNA Replication
1. starts at several origin of
replication sites along the DNA
a. the DNA strands separate
and form a replication
bubble with
replication forks at each
end
b. the replication bubble
elongate as replication
proceeds, the bubbles
eventually fuse
c. Helicase, a protein that
separates the template
strands at the replication
fork
d. Single-strand binding
Proteins keep template
strands apart.
2. To start synthesis an RNA
Primer is required.
a. Primase, an RNA
polymerase, that can start
synthesis from a single
strand, catalyses the
reaction to add the RNA
Primer leaving a 3' OH
end, to which DNA
Polymerase can add
nucleotides in a 5'3'
direction
b. Later, the RNA Primer is
replaced by DNA
3. DNA Polymerases catalyze the
elongation of new DNA at the
replication fork
a. nucleotides with
complimentary bases align
along the template strand
and are bonded by the DNA
Polymerase
b. strands of the double helix
are antiparallel. The sugarphosphate backbones run in
the opposite direction. One
strand runs from 3' OH →
5' PO4 , the other strand runs
from a 5' PO4 →3' OH
direction
(1) DNA Polymerase adds
nucleotides to the 3' end.
 the new DNA strand
elongates in the 5'  3'
(2) at the replication fork,
the parental strand 3'5'
is used by polymerase as
a template for the
synthesis of the leading
strand which elongates
continuously from
5'  3'
(3) the other parental strand
5'  3' is used by
polymerase as a template
for the synthesis of the
the lagging strand,
which is synthesized
from a 5'  3' direction,
in a serious of
discontinous Okazaki
fragments which are later
joined by enzyme DNA
Ligase.
4. Summary: