DNADNA
RNARNA
ProteinProtein
Scientists call this the:
Central
Dogma of
Molecular
Biology!
How do we know that all of our genetic information
comes from DNA?
• What type of experiment would you design to determine that DNA is the source of all genetic information?
Griffith’s Experiment with Pneumonia and the accidental discovery of Transformation
• Frederick Griffiths was a bacteriologist studying pneumonia
• He discovered two types of bacteria:– Smooth colonies– Rough colonies
CONCLUSION:
The smooth colonies must carry
the disease!
Griffith’s Experiment with Pneumonia and the accidental discovery of Transformation
• When heat was applied to the deadly smooth type…
• And injected into a mouse…
• The mouse lived!
• Griffith injected the heat-killed type and the non-deadly rough type of bacteria.
• The bacteria “transformed” itself from the heated non-deadly type to the deadly type.
Griffith’s Experiment with Pneumonia and the accidental discovery of Transformation
Griffith’s Experiment did not prove that DNA was responsible for
transformationHow would you design an
experiment to prove that DNA was responsible for
transformation?
Avery, McCarty, and MacLeodRepeated Griffith’s Experiment
Oswald Avery Maclyn McCarty Colin MacLeod
Avery, McCarty, and MacLeodAdded the non-deadly Rough Type of Bacteria to the Heat-Killed Smooth
Type
Carbohydrates Lipids
Proteins
RNA DNA
To the Heat-Killed Smooth Type, added enzymes that destroyed…
S-Type Carbohydrates
Destroyed
S-Type Lipids
Destroyed
S-Type Proteins Destroye
d
S-Type RNA
Destroyed
S-Type DNA
Destroyed
Conclusion:
DNA was the transforming factor!
The Hershey-Chase Experiment
Alfred Hershey & Martha
Chase worked with a
bacteriophage:
A virus that invades
bacteria. It consists of a
DNA core and a protein
coat
DNA
Protein coat
The Hershey-Chase results reinforced the Avery,
McCarty, and MacLeod conclusion:
DNA carries the genetic code!
However, there were still important details to
uncover…
How did DNA:1. Store information?
2. Duplicate itself easily?
These questions would be answered by
discovering DNA’s structure
The Race to Discover DNA’s Structure
The Race to Discover DNA’s Structure
Linus Pauling
1940s
Discovered the alpha-helical structure of proteins.
The Race to Discover DNA’s Structure
1950
Chargaff’s Rule: Equal amounts of Adenine and Thymine, and equal amounts of Guanine and Cytosine
Erwin Chargaff
Why do you think the bases match up
this way?
Purine + Purine = Too wide
Pyrimidine + Pyrimidine = Too Narrow
Purine + Pyrimidine = Perfect Fit from X-ray data
The Race to Discover DNA’s Structure
Maurice Wilkins
Rosalind Franklin
X-Ray diffraction image of DNA taken by Franklin in
1951
The Race to Discover DNA’s Structure
James Watson Francis Crick
1953
Compiled data from previous scientists to build a double-helical model of DNA
DNA Structure
Deoxyribonucleic acid Double helix (twisted ladder or strands)
of nucleotides (Sugar (deoxyribose), phosphate and a nitrogen base)
Each strand has a sugar and phosphate backbone covalently bonded to a nitrogen base
One single strand of DNA…
DNA Structure
Double helix is made of covalently bonded strands that are hydrogen bonded to complementary covalently bonded strands
One strand bonds to the second strand via hydrogen bonds (weak enough to break in order to separate the 2 strands)
Each strand measures 3.4 nm/twist or 10 base pairs
DNA Double Helix
DNA Structure Strands of DNA
are different – they are oriented in opposite directions to each other – they are ANTIPARALLEL
Each end has a number (5’ or 3’ – you say 5 prime or 3 prime)
Four Nitrogen Bases
Adenine (A), Guanine (G), Cytosine (C), Thymine (T)
Purines (double ring structures) – Adenine and Guanine
Pyrimidines (single ring structures) – Cytosine and Thymine
Chargaff rules: A –T and T – A G – C and C - G
Adenine GuanineCytosine
Thymine
PhosphateSugar (deoxyribose)
Purines Pyrimidines
Nitrogenous Bases
Chromosome Structure DNA packs tightly around histones to
form chromatin.
DNA and histones form bead-like structures called nucleosomes.
Nucleosomes pack together to form supercoils.
Supercoils condense to form chromosomes.
Chromosome
Nucleosome
Coils
Supercoils
Histones
DNA
Chromosome Structure
DNA Replication
• The double helix did explain how DNA copies itself
• We will study this process, DNA replication, in more detail
How does DNA replicate?
Conservative Semi-Conservative
Dispersive
Hypotheses:
DNA Replication
DNA copies itself in the “S” phase of interphase.
1 parent DNA molecule produces 2 daughter DNA molecules, each daughter being made up of “parent” DNA and a strand of “new” DNA (semiconservative process)
Steps of DNA Replication
1. DNA unzips – Helicase enzyme breaks hydrogen bonds, unzipping the double helix at the origin of replication (about 100 on a human chromosome). A replication bubble is formed when DNA
unzips DNA polymerization is bi-directional
because of the antiparallel orientation of the DNA strand.
DNA ReplicationSection 12-2
Growth
Growth
Replication fork
DNA polymerase
New strand
Original strand DNA
polymerase
Nitrogenous bases
Replication fork
Original strand
New strand
DNA Replication
Steps of DNA Replication
2. Bases pair up – DNA Polymerase bonds free nucleotides to complementary bases DNA POL reads DNA in 3’ to 5’ direction,
thus a new strand elongates only in the 5’ to 3’ direction
Nucleotides are added at a rate of about 50 per second in mammals and 500 per second in bacteria.
Steps of DNA Replication
Leading strand has continuous elongation starting at RNA primer since it is read in 3’ to 5’ direction (towards replication fork) by DNA polymerase Lagging strand has discontinuous
elongation DNA strand is read 3’ to 5’ away from the
replication fork in a series of segments called Okazaki fragments
Once fragments are finished, they are joined to previous fragment with enzyme Ligase.
DNA Replication
Steps of DNA Replication
3. Proofreading and repair DNA polymerase “proof-reads” newly
created DNA strand and identifies incorrect base pairs.
Nuclease (exonuclease) enzyme cuts out the identified incorrect nucleotides.
DNA polymerase places correct nucleotides into DNA strand.
Ligase fuses these corrected nucleotides into the DNA strand