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The Secret Code
Genes
• Genes are known to:
– Carry information from one generation to the
next.
– Put that information to work by determining
the heritable characteristics of organisms.
– Be easily copied, because all of a cell’s genetic
information is replicated every time a cell
divides.
Scientists call this the:
DNA
RNA
Protein
How do we know that all of our
genetic information comes from
DNA?
Thanks to many scientists and many
experiments over the last ≈ 90 years.
Let’s take a look…….
Who helped?
• In 1928: Griffith accidentally
discovered a process called
Transformation that turned non-deadly
bacteria into deadly pneumonia causing
bacteria.
• In 1944: Avery, McCarty, and
MacLeod
Repeated Griffith’s Experiment
– Discovered DNA was the transformation
factor.
Who helped?
• In 1952: Hershey-Chase experimented
with bacteria and bacteriophages. Showed
DNA was what entered the bacteria and
caused disease.
Now we know!
DNA carries the genetic code!
But, how do we know how it
works?
How did DNA:
1. Store information?
2. Duplicate itself easily?
DNA’s Structure was the key!
The Race to Discover DNA’s
Structure
1950 Purine + Purine = Too wide
Why do you think
Chargaff’s
Rule:
the bases
match up
Equal amounts
of
this
way?
Pyrimidine + Pyrimidine = Too Narrow
Adenine and Thymine,
and equal amounts of
Guanine and Cytosine
Erwin Chargaff
Purine + Pyrimidine = Perfect Fit from X-ray data
The Race to Discover DNA’s
Structure
X-Ray diffraction image of DNA
Maurice Wilkins
Rosalind Franklin
taken by Franklin in 1951
The Race to Discover DNA’s
Structure
1953
Compiled data
from previous
scientists to build
a double-helical
model of DNA
James Watson
Francis Crick
• Deoxyribonucleic
Acid (polymer)
DNA
• A long molecule made up
of units (monomers)
called nucleotides.
– Three components:
• 5-carbon sugar:
Deoxyribose
• A phosphate group
• A nitrogenous base
Nucleotide
DNA Structure
• Called a double helix
– Backbone (sides) :
5-carbon sugar and
phosphate groups
– Rungs: nitrogenous
bases held together
with hydrogen bonds
Nitrogenous bases
• Our alphabet has 26 letters
– Can create many different words  many
different sentences  billions of different
books of information.
• DNA alphabet has 4 letters (bases)
– A, T, C, and G
– Create 3 letter words  Amino acids 
proteins  billions of different organisms
Nitrogenous bases
Remember Chargaff’s Rule?
• Purines:
• Pyrimidines:
– Adenine
– Thymine
– Guanine
– Cytosine
DNA Replication
• The double helix did explain
how DNA copies itself
• We will study this process,
DNA replication, in more
detail
DNA Replication
The “parent” molecule has two complementary strands of
DNA.
Each is base paired by hydrogen bonding with its specific
partner (Chargaff’s rule):
A with T
G with C
DNA Unzips
The first step in replication is the separation of the two
strands. (Must unwind DNA before separating)
An enzyme called DNA helicase unwinds and unzips
DNA
New nucleotides added
Each parental strand now serves as a template that
determines the order of the bases along a new
complementary strand. (bases always added 5’ to 3’)
an enzyme called DNA polymerase adds the bases
Gaps are closed
The nucleotides are connected to form the sugarphosphate backbones of the new strands.
Enzyme DNA ligase closes the gaps.
Each “daughter” DNA molecule consists of one parent
strand and one new strand….semi-conservative model
• Reviewing DNA Replication
• And One more…..
• And Another…
COOL!!!!
• A single strand of DNA (one chromosome)
is about 2 inches long when uncoiled.
• Each human cell contains 46 chromosomes
(6 to 7 feet of DNA)
• Your nucleus is only about 0.0002 inches in
diameter…..and you can fit that 6 ft of
DNA in there?
• Your body contains 75-100 trillion of cells.
There’s more!
• All of your DNA (when uncoiled and tied
together) would be 67 billion miles long!
– THAT’S 150,000 round trips to the moon!
– OR 70 round trips to the sun!!!
• It takes about 8 hours for one of your cells to copy
all of its DNA.
• Our entire DNA sequence is called a
Genome…and there is an estimated 3,200,000,000
DNA bases
• This would take up about 3GB of storage
And more!
• If you could type 60 wpm, 8 hours/day…it
would take you 50 years to type the bases of
your DNA
• 99.6% of our DNA is the same as somebody
else’s DNA…it is the 0.01% that makes you
who you are!
• 98% of your DNA doesn’t actually code for
anything….it is called “junk” DNA.
• You have roughly 20,000 genes that can
build up to 100,000 different proteins