8.2 Structure of DNA
BIOLOGY CHAPTER 8
• FROM DNA TO PROTEINS
• SECTION 8.1 ON
IDENTIFYING DNA AS THE
GENETIC MATERIAL
8.2 Structure of DNA
8.1 Griffith’s Transforming Principle
• In 1928, Griffith was a British
microbiologist who was studying the
bacterium that causes pneumonia
(streptococcus pneumoniae)
• Two forms of the bacteria exist– smooth
(s) and rough (r)
• Smooth form is deadly to mice
• Killed S did not harm mice
• Rough bacteria did not kill the mice
8.2 Structure of DNA
Transformation
• When injected with a combination of the
live Rough and dead Smooth the mice died
• Also, live Smooth bacteria were found in
the blood of the dead mice.
• Conclusion: Somehow the non-lethal
Rough bacteria had TRANSFORMED into
deadly Smooth bacteria.
• Genetic material from S had gotten into R
and transformed it into S
8.2 Structure of DNA
Oswald Avery
• In 1944, Another scientist named
Oswald Avery devised a clever
experiment with chemical analysis
• Avery’s chemical analysis showed
that the transforming agent was
probably genetic material not
protein
• Scientists were still skeptical
8.2 Structure of DNA
Hershey and Chase confirm DNA as
the genetic material
• A bacteriophage (also called a phage) is
a virus that attacks bacteria by injecting
its genetic material (DNA) into the
bacteria
• It is DNA surrounded by a protein coat
• In 1952, Hershey & Chase used phages
& radioactive isotopes which only show
up in DNA to successfully support their
hypothesis
8.2 Structure of DNA
Additional History
• In the 1950s, Rosalind Franklin (1920-1958),
was a British scientist who was doing X-ray
diffraction studies on DNA
• Her coworker Maurice Wilkins showed her work
to two other DNA American researchers without
her permission
• Those two – Watson and Cricks – took her work
and figured out the structure of DNA – a double
helix
• The three of them got the Nobel Prize and
Franklin, who had died in 1957, was not even
mentioned
8.2 Structure of DNA
What makes up a nucleotide?
• Based on this history, the make up of a
nucleotide (the monomer for nucleic acids)
became known to have :
• 1. A Phosphate group (PO4)
• 2. A ring-shaped, 5 carbon sugar called
deoxyribose (remember – anything ending
in “ose” is a sugar)
• 3. A nitrogenous base consisting of a
single or double ring built around C and N
atoms
8.2 Structure of DNA
•
•
•
•
•
Chargaff’s Rule
The four nitrogenous bases in DNA are:
- THYMINE (T)
- ADENINE (A)
- GUANINE (G)
- CYTOSINE (C)
• CHARGAFF’S RULE of BASE PAIRING
states that Adenine always pairs with
Thymine; and Guanine always pairs with
Cytosine
• A-T and C-G
8.2 Structure of DNA
KEY CONCEPT
DNA structure is the same in all organisms.
8.2 Structure of DNA
DNA = Deoxyribonucleic Acid
• DNA is made up of a long chain of nucleotides.
• Each nucleotide has three parts.
1. 5-carbon sugar called deoxyribose
2. Phosphate group
3. Nitrogen base
phosphate group
nitrogen-containing
base
One Nucleotide
deoxyribose (sugar)
8.2 Structure of DNA
• There are only four Nitrogen bases:
8.2 Structure of DNA
Nucleotides always pair the same way.
• Nitrogen bases always pair
up in DNA like this:
A
G
T
C
• A pyrimidine (single ring)
pairs with a purine (double
ring) so the helix has a
uniform width.
G
C
A T
8.2 Structure of DNA
• The sides are connected by strong covalent bonds.
• The N-bases are connected by weak hydrogen bonds.
Hydrogen bonds
Sugar to phosphate bonds
8.2 Structure of DNA
Watson and Crick determined the structure of DNA by
building models.
• They realized that
DNA is a double
helix.
8.2 Structure of DNA
8.3 KEY CONCEPT
DNA replication makes an exact copy of DNA.
8.2 Structure of DNA
DNA Replication:
• Why?
– So each cell at the end of mitosis will
have the exact same DNA
• When?
– In interphase (S period)
• Where?
– In the nucleus
8.2 Structure of DNA
How Replication occurs:
1. An enzyme “unzips” the DNA by breaking
weak hydrogen bonds between Nitrogen
bases
nucleotide
The DNA molecule unzips
in both directions.
8.2 Structure of DNA
2. Free-floating nucleotides bond to each side of
the DNA strands.
3. Enzymes form covalent bonds in the new
strands.
new strand
nucleotide
DNA polymerase
8.2 Structure of DNA
4. Other enzymes “proofread” the new DNA and
make necessary repairs
(errors are limited to approximately one error
per 1 billion nucleotides)
8.2 Structure of DNA
8.4 KEY CONCEPT
Transcription is the process of converting the
DNA’s code for one gene into an RNA strand.
8.2 Structure of DNA
DNA has the code but RNA carries DNA’s
instructions to the ribosomes
• DNA must
remain “safe” in
the nucleus
while RNA can
leave to carry
the gene’s code
to the ribosome
8.2 Structure of DNA
RNA: Ribonucleic Acid
• RNA differs from DNA in three major ways.
(1) RNA is a single-stranded molecule.
(2) RNA has a ribose sugar as its 5-carbon sugar
(3) RNA has uracil instead of thymine.
8.2 Structure of DNA
Transcription copies DNA’s code to make a
strand of RNA
• Why?
– Make an expendable copy to send out
of the nucleus
– DNA can stay safe in nucleus
• When?
– At the start of Protein Synthesis
• Where?
– In the nucleus
8.2 Structure of DNA
How?
(1) An enzyme recognizes the start of a gene and
“unzips” the weak hydrogen bonds in DNA.
start site
transcription complex
nucleotides
8.2 Structure of DNA
(2)
Free-floating RNA nucleotides attach to the
left strand of the DNA, forming hydrogen
bonds.
DNA
RNA polymerase
moves along the DNA
8.2 Structure of DNA
(3) Covalent bonds are created.
(4) RNA pulls away from the DNA and the DNA
strands “zip” back up.
RNA
8.2 Structure of DNA
8.51KEY CONCEPT
Protein Synthesis is the process of assembling
amino acids to make proteins.
8.2 Structure of DNA
Codons
• Each codon represents one amino acid.
Segment of DNA:
codon for
methionine (Met)
codon for
leucine (Leu)
8.2 Structure of DNA
DNA carries the “code of life”
• The code is carried by the order of the N bases
– 3 DNA N-bases is called a triplet
• Transcription converts these triplets into
codons on the mRNA
• Translation converts mRNA codons into a
string of amino acids using tRNAs.
8.2 Structure of DNA
See page 244 in your textbook:
 Methionine
is the only
start code
 There are 3
stop codons
 All living
things have
the same
codons/amino
acids.
The genetic code matches each RNA codon with its amino acid or function.
8.2 Structure of DNA
Protein Synthesis
(1) Transcription
DNA makes mRNA which carries the code
for one protein.
(2) mRNA leaves the nucleus and goes to the
ribosome
(3) Translation
tRNA “translates” the code on the mRNA by
bringing the correct amino acid to the right
spot on the mRNA.
8.2 Structure of DNA
tRNA’s carry only 1 type of amino acid
• An anticodon is a set of three nucleotides on a
tRNA that is complementary to an mRNA codon.
8.2 Structure of DNA
• tRNA bonds to a start codon and signals the
ribosome to assemble.
• tRNAs bring amino acids to the mRNA and drop
them off
8.2 Structure of DNA
– tRNAs leave to find other amino acids.
– tRNAs continue to drop off amino acids
until they reach the stop codon; then the
ribosome releases the protein.
8.2 Structure of DNA
8.52 KEY CONCEPT
Protein Synthesis is the process of assembling
amino acids to make proteins.
8.2 Structure of DNA
Protein Synthesis
(1) Transcription
– DNA makes mRNA which carries the code
for one protein.
– mRNA leaves the nucleus and goes to the
ribosome.
(2) Translation
tRNA “translates” the code on the mRNA by
bringing the correct amino acid to the right
spot on the mRNA.
8.2 Structure of DNA
tRNA’s carry only 1 type of amino acid
• The 3 nucleotides on a tRNA are complementary
to 3 nucleotides on an mRNA.
8.2 Structure of DNA
• tRNAs pick up the amino acids they can carry
• tRNAs go to the mRNA and drop off amino acids
8.2 Structure of DNA
– tRNAs leave to find other amino acids.
– tRNAs continue to drop off amino acids
until they reach the stop codon
– then the protein is released to do its job