Discovery of a “transforming principle”

Frederick Griffith, 1928
 Pneumonia (Diplococcus pneumoniae)
infects mice.
 Mice develop pneumonia and die.

Two types of bacteria:
 R bacteria rough coat - no pneumonia
 S bacteria smooth coat- pneumonia

Coat type is associated with virulence.
Avery, MacLeod and McCarty



Isolated DNA from heat killed type S
bacteria and injected it along with type R
bacteria into mice
The mice died and their bodies contained
active type S bacteria
Concluded that DNA passed from type S
bacteria to type R, making it lethal
DNA is the genetic material

Alfred Hershey and Martha Chase, 1953

Viruses can infect the E. coli bacteria.
 A virus has protein “head” and DNA core.

Infection occurs when virus injects DNA into
a bacterial cell.
Fig. 9.4
Chemical Nature of Nucleic Acids
Levene’s work lead to the idea that the
structure of DNA was a simple repeating
unit of GATCGATCGATC
This is why no one thought it could be the
genetic material with a structure this simple
• Purines - Large organic bases
– Adenine and Guanine
• Pyrimidines - Small organic bases
– Cytosine and Thymine, Uracil (RNA)
CHARGAFF’S RULES
• In all DNA molecules:
– The proportion of adenine is equal to
thymine
• A=T
– The proportion of guanine is equal to
cytosine
•G=C
DNA bases pair via hydrogen bonds
•Erwin Chargaff observed:
• % adenine = % thymine
•% guanine = % cytosine
•Complementary bases pair:
–A and T pair
–C and G pair
Replication as a process
•Double-stranded DNA
unwinds.
The junction of the unwound
molecules is a replication fork.
A new strand is formed by pairing
complementary bases with the
old strand.
Two molecules are made.
Each has one new and one old
DNA strand.
Enzymes in DNA replication
Helicase unwinds
parental double helix
DNA polymerase
binds nucleotides
to form new strands
Binding proteins
stabilize separate
strands
Exonuclease removes
RNA primer and inserts
the correct bases
Primase adds
short primer
to template strand
Ligase joins Okazaki
fragments and seals
other nicks in sugarphosphate backbone
Replication
3’
3’
5’
5’
3’
5’
3’
5’
Helicase protein binds to DNA sequences
called origins and unwinds DNA strands.
Binding proteins prevent single strands from rewinding.
Primase protein makes a short segment of RNA
complementary to the DNA, a primer.
Replication
Overall direction
of replication
3’
5’
3’
5’
3’
5’
3’
5’
DNA polymerase enzyme adds DNA nucleotides
to the RNA primer.
DNA polymerase proofreads bases added and
replaces incorrect nucleotides.
Replication
Overall direction
of replication
3’
3’
5’
5’
Okazaki fragment
3’
5’
3’ 5’
3’
5’
Leading strand synthesis continues in a
5’ to 3’ direction.
Discontinuous synthesis produces 5’ to 3’ DNA
segments called Okazaki fragments.
Replication
3’
5’
3’
5’
3’
5’
3’5’
3’5’
3’
5’
Discontinuous synthesis produces 5’ to 3’ DNA
segments called Okazaki fragments
Exonuclease enzymes remove RNA primers.
Replication
3’
3’
5’
3’
5’
3’5’
3’
5’
Exonuclease enzymes remove RNA primers.
Ligase forms bonds between sugar-phosphate
backbone.
PROTEIN PRODUCTION




First step in protein production is
transcription
Transcription makes a copy of the DNA
called messenger RNA
mRNA
Called messenger RNA because it carries
the genetic message from the DNA to the
protein factory, the ribosomes in the
cytoplasm
Transcription is directed by the enzyme
RNA polymerase
• RNA is also a nucleic acid
RNA has a slightly different sugar
RNA has U instead of T
Nitrogenous base
(A, G, C, or U)
Phosphate
group
Uracil (U)
Sugar
(ribose)
Figure 10.2C, D
RNA







Messenger RNA is synthesized from DNA by a
process called transcription
This process is similar to DNA replication in
that it depends on complementary base pairing
DNA
RNA
Guanine
Cytosine
Cytosine
Guanine
Thymine
Adenine
Adenine
Uracil