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In 1928, Frederick Griffith, a
bacteriologist, was trying to prepare a
vaccine against pneumonia
Griffith’s Experiments
A vaccine is a substance that is prepared
from killed or weakened disease-causing
agents, including certain bacteria
The vaccine is introduced into the body
to protect the body against future
infections by the disease-causing agent
Griffith discovered that harmless
bacteria could turn virulent when mixed
with bacteria that cause disease
A bacteria that is virulent is able to
cause disease
Griffith had discovered what is now
called transformation, a change in
genotype caused when cells take up
foreign genetic material
Griffith’s Discovery of Transformation
Viral Genes and DNA
In 1952, Alfred Hershey and Martha
Chase used the bacteriophage T2 to
prove that DNA carried genetic material
A bacteriophage is a virus
that infects bacteria
When phages infect bacterial cells, they
are able to produce more viruses, which
are released when the bacterial cells
rupture
DNA’s Role Revealed
Hershey and Chase carried out the
following experiment
Step 1 T2 phages were labeled with
radioactive isotopes
Step 2 The phages infect E. coli bacterial
cells.
Step 3 Bacterial cells were spun to
remove the virus's protein coats
Hershey and Chase concluded that the
DNA of viruses is injected into the
bacterial cells, while most of the viral
proteins remain outside
The injected DNA molecules causes the
bacterial cells to produce more viral DNA
and proteins
This meant that the DNA, rather than
proteins, is the hereditary material, at
least in viruses.
By the early 1950’s, most scientists were
convinced
That genes were made of DNA.
The problem is that no one knew what it
looked like.
Then along came James Watson & Francis
Crick.
James Watson and Francis Crick
A Winding Staircase
Watson and Crick determined that a DNA
molecule is a double helix—two strands
twisted around each other, like a winding
staircase.
Nucleotides are the subunits that make
up DNA. Each nucleotide is made of three
parts: a phosphate group, a five-carbon
sugar molecule, and a nitrogen-containing
base
The five-carbon sugar in DNA nucleotides
is called deoxyribose
Structure of a Nucleotide
DNA Double Helix
The nitrogen base in a nucleotide can be
either a bulky, double-ring purine, or a
smaller, single-ring pyrimidine.
Discovering DNA’s Structure
Chargaff’s Observations
In 1949, Erwin Chargaff observed that
for each organism he studied, the amount
of adenine always equaled the amount of
thymine (A=T)
Likewise, the amount of guanine always
equaled the amount of cytosine (G=C).
However, the amount of adenine and
thymine and of guanine and cytosine
varied between different organisms
Wilkins and Franklin’s Photographs
In 1952, Maurice Wilkins and Rosalind
Franklin developed high-quality X-ray
diffraction photographs of strands of
DNA
These photographs suggested that the
DNA molecule resembled a tightly coiled
helix and was composed of two or three
chains of nucleotides
X-Ray Diffraction
Watson and Crick’s DNA Model
In 1953, Watson and Crick built a model
of DNA with the configuration of a
double helix, a “spiral staircase” of two
strands of nucleotides twisting around a
central axis
The double-helix model of DNA takes
into account Chargaff’s observations and
the patterns on Franklin’s X-ray
diffraction photographs.
Pairing Between Bases
An adenine on one strand always pairs
with a thymine on the opposite strand,
and a guanine on one strand always pairs
with a cytosine on the opposite strand
These base-pairing rules are supported
by Chargaff’s observations
The strictness of base-pairing results in
two strands that contain complementary
base pairs
In the diagram of DNA below, the
helix makes it easier to visualize
the base-pairing that occurs
between DNA strands
When the double helix was discovered,
scientists were very excited about the
complimentary relationship between the
sequences of nucleotides.
Watson and Crick proposed that one DNA
strand serves as a template on which the
other strand is built.
Roles of Enzymes in DNA Replication
The complementary structure of DNA is
used as a basis to make exact copies of
the DNA each time a cell divided.
The process of making a copy of DNA is
called DNA replication
DNA replication occurs during the
synthesis (S) phase of the cell cycle,
before a cell divides
DNA replication occurs in three steps
Step 1- DNA helicases open the double
helix by breaking the hydrogen bonds
that link the complementary nitrogen
bases between the two strands. The
areas where the double helix separates
are called replication forks
Step 2 - At the replication fork, enzymes
known as DNA polymerases move along
each of the DNA strands. DNA
polymerases add nucleotides to the
exposed nitrogen bases, according to the
base-pairing rules
Step 3 - Two DNA molecules that form
are identical to the original DNA molecule
Checking for Errors
In the course of DNA replication, errors
sometimes occur and the wrong
nucleotide is added to the new strand.
An important feature of DNA replication
is that DNA polymerases have a
“proofreading” role
This proofreading reduces errors in DNA
replication to about one error per 1 billion
nucleotides
The Rate of Replication
Replication does not begin at one end of
the DNA molecule and end at the other
The circular DNA molecules found in
prokaryotes usually have two replication
forks that begin at a single point
The replication forks move away from
each other until they meet on the
opposite side of the DNA circle
In eukaryotic cells, each chromosome
contains a single, long strand of DNA
Each human chromosome is replicated in
about 100 sections that are 100,000
nucleotides long, each section with its
own starting point
With multiple replication forks working in
concert, an entire human chromosome can
be replicated in about 8 hours
Replication Forks
Traits, such as eye color, are determined
By proteins that are built according to
The instructions specified in the DNA.
Decoding the Information in DNA
Proteins, however, are not built directly
from DNA. Ribonucleic acid is also
involved
Like DNA, ribonucleic acid (RNA) is a
nucleic acid—a molecule made of
nucleotides linked together
RNA differs from DNA in three ways
1. RNA consists of a single strand of
nucleotides instead of the two strands
found in DNA
2. RNA nucleotides contain the fivecarbon sugar ribose rather than the
sugar deoxyribose, which is found in DNA
nucleotides
3. In addition to the A, G, and C nitrogen
bases found in DNA, RNA nucleotides can
have a nitrogen base called uracil (U)
Comparing DNA and RNA
The instructions for making a protein are
transferred from a gene to an RNA
molecule in a process called transcription
Cells then use two different types of
RNA to read the instructions on the RNA
molecule and put together the amino
acids that make up the protein in a
process called translation
The entire process by which
proteins are made based on the
information encoded in DNA is
called gene expression, or
protein synthesis
Gene Expression
Transfer of Information
from DNA to RNA
The first step in the making of a protein,
transcription, takes the information
found in a gene in the DNA and transfers
it to a molecule of RNA
RNA polymerase, an enzyme that adds
and links complementary RNA nucleotides
during transcription, is required
The three steps of transcription are
Step 1 RNA polymerase binds to the
gene’s promoter
Step 2 The two DNA strands unwind and
separate
Step 3 Complementary RNA nucleotides
are added
Types of RNA
Genetic Code: Three-Nucleotide “Words”
Different types of RNA are made during
transcription, depending on the gene
being expressed
When a cell needs a particular protein, it
is messenger RNA that is made
Messenger RNA (mRNA) is a form of
RNA that carries the instructions for
making a protein from a gene and delivers
it to the site of translation
The information is translated from the
language of RNA—nucleotides—to the
language of proteins—amino acids
The RNA instructions are written as a
series of three-nucleotide sequences on
the mRNA called codons
The genetic code of mRNA is the amino
acids and “start” and “stop” signals that
are coded for by each of the possible 64
mRNA codons
RNA’s Roles in Translation
Translation takes place in the cytoplasm.
Here transfer RNA molecules and
ribosomes help in the synthesis of
proteins
Transfer RNA (tRNA) molecules are
single strands of RNA that temporarily
carry a specific amino acid on one end
An anticodon is a three-nucleotide
sequence on a tRNA that is
complementary to an mRNA codon.
Ribosomes are composed of both proteins
and ribosomal RNA (rRNA)
Ribosomal RNA (rRNA) molecules are
RNA molecules that are part of the
structure of ribosomes
Each ribosome temporarily holds one
mRNA and two tRNA molecules
The seven steps of translation are:
Step 1 The ribosomal subunits, the
mRNA, and the tRNA carrying methionine
bind together
Step 2 The tRNA carrying the amino acid
specified by the codon in the A site
arrives
Step 3 A peptide bond forms between
adjacent amino acids
Step 4 The tRNA in the P site detaches
and leaves its amino acid behind
Step 5 The tRNA in the A site moves to
the P site. The tRNA carrying the amino
acid specified by the codon in the A site
arrives
Step 6 A peptide bond is formed. The
tRNA in the P site detaches and leaves
its amino acid behind
Step 7 The process is repeated until a
stop codon is reached. The ribosome
complex falls apart. The newly made
protein is released