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-Chapter 11
DNA: The Carrier of Genetic Information
Lecture Outline
I. Most genes carry information for making proteins
A. 1908, Archibald Garrod proposed that genes are
responsible for enzymes
“Inborn Errors of Metabolism”
B. George Beadle and Edward Tatum in the 1940s
worked on Neurospora
1. One mutant gene affected 1 single step in a
metabolic pathway
2. The “one gene, one enzyme hypothesis”
a. Now known that some genes encode
proteins that are not enzymes;
work of Pauling and others
b. Also known that one gene is responsible
for one polypeptide chain
3. The “one gene, one polypeptide hypothesis”
II. Evidence that DNA was the hereditary material was first
found in microorganisms
A. Early biologist believed that the genetic material
was a protein
B. Frederick Griffith in 1928 studied pneumococcus in
mice
1. Smooth (S) strain was virulent
2. Rough (R) strain was not virulent
3. Heat-killed S strain was not virulent
4. A combination of heat-killed S and R strain
was virulent
5. Something caused the R strain to become
virulent (i.e. transformation)
C. This “transforming principle” was later identified
by Avery and colleagues in 1944 as DNA
D. Alfred Hershey and Martha Chase’s experiments
on bacteriophages
1. Labeled the protein coat with radioactive
sulfur, the DNA with radioactive phosphorus
2. Found that only the phosphorus was found in
the bacterium after infection with the
bacteriophages; providing further evidence that
DNA is the genetic material
III. The structure of DNA allows it to carry information and
to be faithfully duplicated
A. Nucleotides can be covalently linked in any order
to form long polymers
1. DNA is a nucleotide-based molecule
containing deoxyribose, phosphate, and
a nitrogen-containing base
2. The bases of DNA are purines (adenine and
guanine) and pyrimidines (thymine and cytosine)
3. Nucleotides are covalently bonded with a
sugar-phosphate backbone
a. The linkage forms a 3’, 5’ phosphodiester
linkage
b. One end of the molecule has a free 5’
carbon; the other end has a free 3’ carbon
B. DNA is made of two polynucleotide chains
intertwined to form a double helix
1. Rosalind Franklin and Maurice Wilkins did xray diffraction analysis on DNA
a. Results showed that the molecule was
helical, and had several repetitive
elements at 0.34 nm, 3.4 nm, and 2.0 nm
apart
b. They deduced that the relatively flat base
molecules were stacked like the rungs of a
ladder
For 3d image of DNA click the following
link:
http://www.google.com/imgres?imgurl=http://www.worldofmolecules.com/life/dna.gif&imgrefurl=h
ttp://www.worldofmolecules.com/life/dna.htm&h=341&w=457&sz=32&tbnid=9Ub1J9karkMJ::&tb
nh=96&tbnw=128&prev=/images%3Fq%3Ddna%2Bmolecule%2Bdimensions%2Bimage&usg=__
EyQV_Lkvc5BFEAAJUCISad0C7-s=&sa=X&oi=image_result&resnum=4&ct=image&cd=1
2. James Watson and Francis Crick deduced the
correct structure of DNA
a. The molecule is a double helix of
antiparallel strands made up of a backbone
of sugars and phosphates
b. The 0.34 nm repeat is due to the distance
between the bases
c. The 3.4 nm repeat is due to the distance
between a complete “turn” of the helix
d. The 2.0 nm represents the width of the
molecule
C. In double-stranded DNA, hydrogen bonds form
between adenine and thymine and
between guanine and cytosine
1. Edwin Chargaff determined that the ratio of
adenine to thymine and the
ratio of guanine to cytosine was always the same
in all organisms tested
2. Pyrimidines are single-ring molecules; purines
have double-rings
a. Two hydrogen bonds form between
thymine and adenine
b. Three hydrogen bonds form between
cytosine and guanine
3. The bonding of bases is complimentary; the
sequence in one chain dictates the sequence in
the opposite chain
IV. DNA replication is semi conservative
A. The structure of DNA clearly indicated the copying
mechanism
1. “It has not escaped our notice that the specific
pairing we have postulated immediately suggests a
possible copying mechanism for the genetic
material” (quote from their Nature article)
2. Matthew Meselson and Franklin Stahl (1957),
using e-coli disproved the model of dispersive
replication and showed semi conservative
replication
a. Grew the bacteria in medium with
nitrogen-15
b. Transferred to medium with nitrogen-14
c. Isolated the DNA and found that some of
the DNA had only nitrogen-15,
some had only nitrogen-14, and some had
both
B. DNA replication is complex and has a number of
unique features
1. Proteins and enzymes work together in
replication
2. DNA strands must be unwound during
replication
a. DNA helicase enzymes unwind the
strands
b. Helix destabilizing proteins prevent
immediate reformation of the double helix
c. Topoisomerases break and rejoin the
strands, “untying” the knots that form
3. DNA synthesis always proceeds in a 5’3’
direction
a. DNA polymerases can add only at the 3’
end
b. Nucleotides become polymerized and two
phosphates are removed in the process
4. DNA synthesis requires an RNA primer
The RNA primer is first synthesized by the
primosome
5. DNA replication is discontinuous in one strand
and continuous in the other
a. Replication begins at replication forks
b. Synthesis of leading strand is continuous,
the lagging strand is synthesized
in pieces called Okazaki fragments
c. Okazaki fragments are between 100 and
2000 nucleotides in length,
each is initiated by a separate RNA primer
d. Okazaki fragments are joined together by
DNA ligase
6. Most DNA synthesis is bidirectional
a. In prokaryotes, the circular DNA is
opened up, and synthesis occurs in
both directions
b. In eukaryotes, the linear DNA has many
replication forks
c. The ends of eukaryotic chromosomes, the
telomeres, present problems
in replication
V. DNA in chromosomes is packaged in a highly organized
way
A. The genome of E. Coli consists of about 4 x 106
base pairs – about 1.35 mm in length
B. The haploid DNA of a human cell is made of 3 x
109 base pairs – about 1 m (
over 3 ft.) in length
C. In eukaryotes, DNA is associated with positively
charged histone proteins to form bead-like
nucleosomes
1. Each nucleosome is composed of about 140
base pairs wrapped around 8 histones, plus a
linker segment of DNA and another histone
2. Other scaffolding proteins aid in the formation
of chromosomes