Structure of DNA
Remember: genes control certain traits, genes are sections of
DNA
I.
Structure of DNA (deoxyribonucleic acid)
A. Made of nucleotides
1. nucleotides have 3 main parts
a. sugar (deoxyribose)
S
b. phosphate group
P
c. nitrogenous base
2. 4 different nitrogenous bases can be used in a
nucleotide
a. adenine (A)
b. guanine (G)
c. cytosine (C)
d. thymine (T)
B. Watson and Crick – 1953 (published)
1. double helix shape (twisted ladder)
2. formed by 2 strands of nucleotides linked together
a. sides of the ladder are sugar and phosphate
b. “rungs” of ladder are 2 bases bonded together
***** Adenine always bonds with Thymine!
***** Cytosine always bonds with Guanine!
DNA
S
S
P
P
S
S
P
P
S
S
P
P
S
S
P
P
S
S
P
P
Adenine =
Guanine =
Thymine =
Cytosine =
Four Nitrogenous Bases
Nucleotide = sugar, phosphate, base
4 different nucleotides:
S
S
P
P
S
S
P
P
Names and Dates: Supplement to DNA Notes
 Watson, Crick and Maurice Wilkins received the Nobel prize in
1962 for their work on DNA structure and how the DNA molecule
can function to carry genetic information.
 February 28, 1953: Crick announces in the English pub “The
Eagle” that he has found “the secret of life.” Watson and Crick
published their work later in 1953.
 Rosalind Franklin:
1. She worked in the same area of Cambridge University that
Watson and Crick did but was in a different college
2. She performed research on the DNA molecule using X-ray
crystallography to take pictures; this research was the
basis of the double helix shape to DNA that Watson and
Crick are so famous for discovering. The idea of a helical
shape for DNA was all Rosalind’s. Without her work,
Watson and Crick would probably have not figured out
the DNA molecule before anyone else.
3. She did not like to share her work and her research was
“stolen” by the Watson/Crick/Wilkins lab (they went in
and examined everything without her permission),
including the famous “Photo 51.”
4. She was never acknowledged for her contribution to the
structure of DNA until Watson described her as a horrible
person in his book “Double Helix” (published 1968).
Everyone who was familiar with the DNA story objected
and the fact of Rosalind’s work being so important was
brought to the public’s attention.
5. She died at age 37 in 1958, before Watson publishes his
book and before the Nobel Prize is awarded. She was not
mentioned.
 Maurice Wilkins:
Ran the lab that Watson and Crick worked in; oversaw most of
their work; received the Nobel Prize with Watson and Crick in
1962
 Alfred Nobel:
Was a major business man in mid to late 1800’s; invented
dynamite and made his huge fortune from it; his brother died
and a newspaper printed Alfred’s obituary by accident and
called him the “merchant of death” (dynamite killed a lot of
people in demolition accidents); Alfred hated the thought of his
legacy being such a terrible one so when he died he left most of
his money for the establishment of the Nobel Prizes; it’s the
highest award a person can receive and covers lots of different
categories (literature, physics, medicine, peace...)
 Chargaff:
1. He studied DNA and analyzed how much thymine,
cytosine, adenine and guanine were in each sample. He
found that the amounts of thymine and adenine were
always equal, and the amounts for cytosine and guanine
were always equal.
2. Chargaff figured that adenine and thymine bond together
as do cytosine and guanine. The result was Chargaff’s
Rule:
A=T
and C = G
I.
DNA Replication
A. very important that DNA be able to copy itself
1. needed for mitosis and meiosis
2. process of DNA copying itself is called replication
B. Steps of Replication
1. DNA double helix unzips so that the 2 strands of
DNA are separated
 each separated strand will be a pattern for a
new strand of DNA
2. New strands of DNA are formed from single
nucleotides in the nucleus
 called free nucleotides
*3. DNA polymerase (an enzyme) matches the bases
on the parent strand (the original, unzipped part)
one by one with the new bases of free nucleotides
4. Strong sugar-phosphate bonds form between
nucleotides that are next to one another, creating a
new “backbone"
 eventually 2 new double helixes are formed,
having 1 parent strand and 1 new strand
I.
DNA and RNA
A. Both DNA and RNA (ribonucleic acid) are nucleic
acids
1. there are structural differences between them
DNA
Double stranded
RNA
Single stranded
Base pairs:
A–T
C–G
Base pairs:
C–G
A–U
U=uracil; replaces thymine
Deoxyribose is the sugar
Ribose is the sugar
2. RNA is used for making proteins
a. mRNA – messenger RNA
used to send information from DNA to the
ribosome
b. tRNA – transfer RNA
used to match mRNA with the right amino acids
for making proteins (remember – proteins are
made of amino acids strung together like the
beads of a necklace)
II. Protein Synthesis
A. protein synthesis is the process by which proteins are
made; it has 2 parts: transcription and translation
B. transcription: genetic information from a strand of
DNA is copied into a strand of mRNA
transcribe means to copy
Steps in Transcription:
1. an enzyme separates (unzips) a section of
DNA
2. unattached RNA nucleotides are linked with
the matching bases on the DNA strand to
form a molecule of mRNA
3. after a section of DNA is transcribed, the next
section is exposed (unzipped) and the process
repeats until DNA signals mRNA synthesis to
end
C. In prokaryotes, mRNA goes right to the ribosome for
translation to begin
In eukaryotes, the mRNA is first spliced inside the
nucleus.
1. splicing: removing extra parts of mRNA that
aren’t needed
After the mRNA has been spliced, it leaves the
nucleus and travels to the ribosome for translation.
D. Translation: process of converting the information in
the mRNA to a chain of amino acids (protein)
1. in the cytoplasm, one kind of amino acid is
attached to each tRNA; a section of mRNA is
attached to a ribosome
2. tRNA’s being amino acids to the ribosome,
and the amino acids are added one at a time
to the growing chain (tRNA transfers amino
acids to the ribosome)
 Each tRNA anticodon pairs with a
complementary mRNA codon, making sure
that amino acids are added in the coded
sequence
3. codon – 3 base section of mRNA; most carry
a code for a specific amino acid
example: UCG codes for tryptophan
anticodon – sequence of 3 bases found on
tRNA; each tRNA has only ONE anticodon
which complements a specific mRNA codon
 Anticodons and codons fit like plugs into a
socket.
4. several codons on mRNA have a different
purpose  they don’t code for regular amino
acids; instead, they are start and stop codons
which signal a ribosome to either start or stop
translation. They are located at the
beginning and end of an mRNA code for a
particular protein.
5. Codes for amino acids are universal for all
forms of life. They are the same for mice,
bacteria, and all other living organisms, even
viruses.