DNA
DNA is the molecule of life. All living things
contain DNA.
DNA is found in the nucleus of cells.
DNA contains genetic codes that determine
physical features.
DNA
DNA stands for deoxyribonucleic acid. It may
sound gibberish, but the name actually tells us
two things:
- DNA contains deoxyribose (a 5-carbon sugar)
- DNA is a nucleic acid (molecule made up of
nucleotides)
Nucleotide 
Nucleotides consist of a sugar molecule
attached to a nitrogen base and a
phosphate group.
Phosphate
Nitrogen Base
Nucleotides have 3 parts:
1) Sugar
2) Nitrogen Base
3) Phosphate
The Nitrogen Bases
There are 4 possible different nitrogen bases:
-adenine
-guanine
These 4 different bases
-cytosine
allow for genetic
-thymine
diversity
Base-pairing rules
DNA is a double stranded molecule – the two
strands are connected by the nitrogen bases.
Adenine can only pair with thymine (and vice
versa).
Guanine can only pair with cytosine (and vice
versa).
C
G
T
A
Purines vs. Pyrimidines
The molecular structure of the
4 bases fall under two categories:
1) Purines- double ring structures
- adenine and guanine are purines
2) Pyrimidines- single ring structures
- thymine and cytosine are pyrimidines
Purines always bond with pyrimidines (as per
the base pairing rules).
HINT:
Think OPPOSITES- The BIGGER word is the
smaller molecule; the smaller word is the
BIGGER molecule.
Erwin Chargaff
In 1947, an American scientist named
Erwin Chargaff discovered that:
the amount of guanine and cytosine
bases are equal in any sample of DNA.
The same is true for the other two nitrogen bases:
The amount of adenine and thymine are equal in any sample of DNA.
A=T
C=G
The observation that ______and
that ______
became known as _______________.
Chargaff’s rules
At the time this observation was
made, it was not clear why this fact
was so important.
X-Ray Evidence by Rosalind Franklin
In the early 1950’s, a British scientist,
Rosalind Franklin began to study DNA.
diffraction
She used a process called X-ray
_____________.
These x-rays suggested that there were
two strands, twisted in a helix and the
nitrogen bases were in the center of the
moleclule.
She took a large, purified sample
of DNA, aimed a powerful x-ray
beam at the sample, then
recorded the scattering pattern of
x-rays on film.
At the same time Franklin was doing her research with a colleague
Francis Crick
Maurice Wilkins, two scientists named __________________
and
James Watson
_________________,
were trying to understand the structure of
DNA by building models of it. They were getting nowhere.
The
Players
James
Watson
Francis
Crick
Maurice
Wilkins
Rosalind
Franklin
Early in 1953, Watson was shown a copy of Franklin’s x-ray patterns, and
he immediately realized how the DNA molecule was arranged. Within
weeks, Watson and Crick built a model that showed:
structure of DNA.
1) The _________
carry information and be ________.
copied
2) And explained how DNA could ________
Watson and Crick
described the DNA
molecule as a
double helix or
__________
spiral consisting of
two strands wound
__________
around each other.
Timeline
1953 – Watson and Crick
solve the structure of DNA
1958 – Rosalind Franklin dies
of ovarian cancer at age 37
1962 – Watson, Crick and
Wilkins win the Nobel Prize
Rosalind Franklin did not receive
a Nobel Prize as they must be
given to a living person
DNA- The Double Helix
Watson and Crick discovered that the
shape of DNA was a ___________.
double helix
double = 2 strands of nucleotides
_______
_______
helix = twisted
Picture a twisted ladder.
The sides of the ladder are
composed of sugar
(deoxyribose) molecules
and phosphates. This is
called the “sugarphosphate backbone”.
The nitrogen bases make
up the rungs (steps) of
the ladder.
The two strands of DNA are
connected to each other at the bases.
The bases bond together using
hydrogen bonds.
Adenine and thymine have two
hydrogen bonds.
Guanine and cytosine have three
hydrogen bonds.
C
T
G
A
Hydrogen bonds are the
weakest type of bond.
You might think that DNA
should be strongly
held together- but it
does need to unzipand quite often!
DNA comes apart during
DNA Replication (the
copying of DNA).
How can all of this DNA fit inside a cell?
The structure of the
chromosome allows the
DNA to be packed very
tightly inside the cell.
The DNA is wrapped tightly
histones
around proteins called _______.
Together, the DNA and
histone molecules form
a beadlike structure
called a ___________.
nucleosome
A chromosome is
composed of:
DNA and proteins.
nucleosomes
DNA double
helix
coils
supercoils
Nucleosomes pack with
one another to form a
thick fiber, which is
shortened by a system
of loops and coils.
histones
Nucleosomes seem to be
able to _______
fold enormous
lengths of DNA into the
_______
tiny space available in the
cell nucleus.
Heredity Information in DNA or
the Histone Proteins?
For a long time it was not certain whether the
hereditary information being passed along was found
in the DNA or in the histone proteins which the DNA
wraps around to make chromatin. In 1952, just
before the discovery of DNA’s structure by Watson
and Crick, an experiment performed by Alfred
Hershey and Martha Chase finally gave the definitive
answer.
Hershey-Chase Experiment
DNA Replication
Recall that DNA is found in the
nucleus of all cells.
In order to make more cells (which
you are constantly doing), you
must make a copy of DNA first!
DNA Replication occurs during the
synthesis stage of Interphase in
the cell cycle (before the cell
actually divides).
DNA Replication- Step 1
The first step required in
order for DNA to make a
copy of itself is to break
those hydrogen bonds
between the bases.
An enzyme called DNA
helicase breaks the
hydrogen bonds and unzips
the original parent DNA
molecule.
DNA Replication- Step 2
Once the DNA strands are
unzipped, the nucleotides
are exposed.
1
The second step involves
another enzyme called DNA
Polymerase. This enzyme
reads the DNA and
determines which NEW
nucleotides to add to the
parent strand.
1
2
3
Replication Forks
DNA is a very long molecule that must
be tightly coiled and packed into our
cells.
If the enzymes had to go from one end
of DNA all the way to the other, it
would take too long!
Replication fork
2
3
Replication forks form at multiple
points in the DNA to speed up
replication.
Two replication forks make replication
“bubbles”.
5’ and 3’
Since DNA is a 3-Dimensional molecule made of linked
nucleotides, it really doesn’t have a “left” or “right”; “up” or
“down”.
If we have to refer to DNA’s direction we use 5’ and 3’ (5 prime
and 3 prime).
Recall that deoxyribose is a 5-carbon sugar. These numbers
(5,3) are in respect to the position on the 5-carbon sugar.
During DNA replication,
DNA polymerase
READS the parent
molecule in the 3’  5’
direction.
New DNA is synthesized
in the 5’  3’ direction
(opposite).
(How to Remember?
When you READ a
book you would read
chapters 3 to 5)
Final Product- DNA Replication
The final product of DNA replication
is two molecules of DNA (4 strands
total since each molecule is double
stranded).
However, it would not
be appropriate to call
the molecules “new”.
DNA Replication is semi-conservative (semi=
half; conserve= to save)
Each time DNA is copied, the original DNA
molecule is saved. DNA is never destroyed
during replication!
Each new
molecule
consists of one
parental strand,
and one (new)
daughter strand.
The Mechanisms of Replication – A Closer
View
This replication of an
enormous amount of genetic
information is achieved with
very few errors - only one
error per 10 billion
nucleotides. The replication is
a speedy and accurate
process.
More than a dozen enzymes
and proteins participate in DNA
replication.
Accidental changes can occur in existing DNA after replication. The DNA
damaged from exposure to chemicals, radioactivity, X-rays,
can become ________
ultraviolet light, and molecules in cigarette smoke.
monitors and repairs its genetic material
Each cell continuously___________________________________.
About 130 DNA repair enzymes have been identified so far.
Repairing the damage:
Damaged DNA
Nucleases cut out
the damaged
section.
DNA polymerases
replace the gaps
with new
nucleotides.
Ligases seal the
new section in
place.
1. The damaged segment of DNA is
cut out by enzymes called
nucleases.
2. The resulting gap is filled in with
new nucleotides by DNA
polymerases.
3. Other enzymes (ligases) seal the
free ends of the new DNA to the
old DNA, making the strand
complete.
Summary Video
DNA Replication Fork
DNA Replication
Self Check Quiz
1. The letters D.N.A. stand for
Deoxyribonucleic acid
________________________.
Double _______.
helix
2. DNA is shaped like a _______
3. The four nitrogen bases are: adenine, ___________,
thymine
_________,
____________.
cytosine
guanine
thymine
4. Adenine always bonds with ____________.
guanine
5. Cytosine always bonds with ____________.
6. DNA is important because it determines your physical
traits
_______________.
semi
conservative
7. DNA replication is ________________________.
8. DNA replicates (circle one) [before | after] cell division.
9. DNA replicates using specific [enzymes |
carbohydrates].
10. Thymine and cytosine are [purines | pyrimidines].
11. Nitrogen bases are paired together using [hydrogen |
covalent] bonds.
Why DNA is important:
DNA is important because it holds the “recipe”
for making proteins.
Your entire body is made out of proteins!
DNA is your personalized instruction manual
and yours is unique to you (though everyone
in this room shares about 99% of the same
DNA, that’s what makes us human!)
The Genetic Code
The DNA molecule, with its four
code for all
nitrogenous bases, is the ____
_________
proteins that are made in a cell.
The DNA inherited by an
organism dictates the
synthesis of certain
proteins. Proteins are the
link between
genotype and phenotype.
The proteins that are made
will determine what traits
show up in the offspring.
Genes are made of _____.
DNA A gene is
the _________________that
controls
coded DNA instructions
the production of specific ________,
proteins
such as enzymes, structural proteins,
oxygen-carrying proteins, etc.
Gene expression:
The process by
which DNA
directs the
synthesis of
proteins.
The expression of
genes includes
two stages:
transcription and
translation
DNA is very important; it
controls the workings of
the cell.
However, it is trapped
inside the _______.
nucleus
In order to get all of its instructions to the rest of the
cell, DNA relies on its trusty sidekick....
Ribonucleic Acid
R.N.A. is also a nucleic acid- it is made out
of linked nucleotides (like DNA). Recall
that nucleotides are made of a sugar,
phosphate, and nitrogen base.
DNA vs. RNA
RNA and DNA are very similar, but there are
some differences.
First of all, DNA is
double stranded, and
RNA is single
stranded. This means
that RNA is SMALLER
than DNA.
RNA contains 4 nitrogen bases: adenine,
guanine, cytosine and URACIL.
*Thymine is NOT present in RNA.
Uracil is
complementary to
adenine in DNA. It
essentially takes the
place of thymine.
The last major difference between DNA and
RNA is that RNA contains the 5-carbon
sugar ribose. (Recall DNA contains
deoxyribose).
Ribose has one more
oxygen atom than
deoxyribose.
Ribose
Deoxyribose
Recap
RNA is single stranded, so it is smaller than
DNA. This means it can leave the nucleus
(which DNA cannot).
RNA contains the sugar ribose.
RNA has 4 bases: A, G, C, and U. The base
pairing rules are as follows:
C pairs with
G pairs with
A pairs with
U pairs with
G
C
U
A
NO thymine in RNA
3 Types of RNA
RNA’s job is to help DNA make proteins.
DNA must deliver its code to the remainder
of the cell - it relies on 3 molecules:
1) Messenger RNA (mRNA)
2) Transfer RNA
(tRNA)
3) Ribosomal RNA (rRNA)
Messenger RNA
mRNA is complementary to the original
strand of DNA. mRNA is first created in the
nucleus and then travels to the ribosomes
out in the cytoplasm. mRNA uses the
DNA’s code (or message) to make
proteins!
Example: DNA Strand: G G C T T A
mRNA strand: C C G A A U
Proteins
Recall that proteins are made up of smaller
parts called amino acids. Another word for
protein is “peptide”.
Individual DNA codes are
called “codons”. The codons
correspond to specific amino
acids. mRNA also has codons,
which are complementary to
DNA codons.
Codons
Codons consist of groups of 3 nucleotides
called triplets.
(Example) DNA codon:
cytosine-cytosine-adenine
(CCA for short)
A
C
C
Each codon codes for one amino acid. This is
where we need RNA’s help.
DNA Template Strand:
A
C
G T
T
A
G C
C
mRNA strand (which leaves nucleus to build proteins):
U
G C
A
A
U
C
G G
mRNA is always complementary to the
template DNA strand.
How many codons are there in mRNA above?
What does the other DNA strand look like?
DNA:
A C G T T A G C C
mRNA: U G C A A U C G G
cysteine asparagine arginine
Three DNA codons are transcribed into three
mRNA codons. mRNA codons are specific
to amino acids.
This is the beginning step of PROTEIN SYNTHESIS.
Protein = (well) protein
synthesis = to make
Codon Chart
DNA Template Strand: A G
mRNA strand:
U C
1) UCC
2) GCA
3) AUC
Which three
amino acids do
these mRNA
codons code for?
G
C
C G T T A G
G C A A U C
Amino Acids
Though there are only 20 different amino acids, they are
sequenced differently and come in different shapes to
make for thousands of different proteins.
Protein Synthesis
• A two part process in which DNA is
decoded into corresponding proteins
• The first process is known as transcription
• The second process is translation
• Occurs in the nucleus and cytoplasm
Transcription
Transcription is the first part
of protein synthesis.
During transcription, mRNA
is created by transcribing
the DNA’s code.
Transcription occurs in the
nucleus.
(That’s where the DNA is!)
Transcription
During transcription,
the enzyme RNA
polymerase
temporarily unzips
DNA and adds
complementary
RNA nucleotides to
the growing mRNA
strand.
Transcription
Recall that mRNA is the messenger. It copies
DNA’s code (or “message”; “instructions”)
and it is now responsible for delivering this
message to the rest of the cell.
Once the mRNA strand is completed, it leaves
the nucleus (exits via nuclear pores).
Transcription is complete.
(No protein yet...next stop, the ribosomes!)
Translation
Translation is the final step of protein synthesisit involves ALL THREE types of RNA (mRNA,
tRNA and rRNA).
• Translation is a process in which the mRNA
that was manufactured during transcription is
translated into an amino acid sequence
(proteins)
• occurs in the cytoplasm, on the
ribosomes
Ribosomal RNA (rRNA)= a
major component of
ribosomes; also helps bond
amino acids together to
make polypeptides
(proteins)!
The goal of the 3 types of RNA is to work
together to make proteins using the DNA’s
instructions!
Translation
Transfer RNA (tRNA)= helps transfer amino
acids to the corresponding mRNA codons
(tRNA is always complementary to the mRNA
strand)
mRNA codons:
U
tRNA anticodons: A
G
C
C
G
A A U C G G
U U A G C C
tRNA bases are referred to as “anti-codons” because
they are complementary to mRNA codons.
LEUCINE
(Example)
If the mRNA codon is
CUU, that would
translate to the
amino acid leucine.
The tRNA molecule
that will deliver
leucine to the
ribosome has the
anticodon GAA.
ANTICODON
Once the amino acid is delivered, the tRNA releases itself
from the ribosome, and leaves to find another amino acid to
add to the growing protein chain.
Translation
tRNA transfers amino
acids to the ribosome.
The amino acids are
attached to the tRNA
via a specialized
enzyme called tRNA
synthetase.
Analogy: tRNA is a librarian; mRNA
codons are the book codes; amino acids
are the books. The tRNA librarian reads
the mRNA codons and fetches the
appropriate amino acid book to add to the
protein chain being created.
Label the following:
mRNA, tRNA, ribosome, codons, anti-codons,
peptide (protein) chain, amino acid
If the sequence on the DNA molecule calls for a protein with the
following DNA codons:
(1) What would be the sequence of the mRNA?
(2) What would be the sequence on the tRNA?
(3) What would be the amino acid sequence of the protein being
made?
DNA

TAC
TTA
CAA
ACC
ATA
ATT
mRNA

AUG
AAU
GUU
UGG
UAU
UAA

UAC
UUA
CAA
ACC
AUA
AUU
Asparagine
Valine
Tryptophan
Tyrosine
STOP
CODONS
tRNA
ANTICODONS
Amino 
Acid
Sequence
Methionine
Putting it all together
DNA codes for proteins
Proteins being made is called
protein synthesis
Transcription occurs in the
nucleus. The final product of
transcription is an mRNA
strand.
Translation occurs on the
ribosomes. The final product
of translation is a protein.
Codons
There are 64 possible
codons:
3 bases in a codon
(triplets)
43 = 64
4 possible bases
(A, T, C or G)
64 codons for 20 amino acids.
Can more than one codon specify the same
amino acid?
Start and Stop Signals
Proteins are made up of a very specific
sequence of amino acids.
DNA contains “start” and “stop” codons so
that the cell knows where to start decoding
proteins and where to stop.
The start codon= AUG (codes for the amino
acid methionine)
Stop codons= UAA, UAG, UGA
Summary Videos
Protein Synthesis
Prokaryotes vs Eukaryotes