Transformation Principle
In 1928 Fredrick Griffith heated the S bacteria and mixed
with the harmless bacteria thinking that neither would
make the mice sick. However; the mice did develop
pneumonia. The heat killed bacteria had passed their
disease-causing ability to the harmless strain.
Material was transferred from the S bacteria (killed) to
the live R bacteria. Transforming one strain changed into
the disease causing strain.
DNA - Transforming Principle
Avery experimented by combining living R bacteria with
an extract made from S bacteria.
Qualitative Tests showed that no proteins was present.
Chemical Analysis revealed that the proportions of
elements in the extract matched in DNA was present.
Enzyme Tests – the team added the enzymes known to
break down proteins, the extract transformed the R
bacteria to the S form. Transformation did not occur
when they added an enzyme that specifically destroys
DNA.
Avery confirms Transformation
Transformation occurred so those molecules were not
responsible for the transformation used. He used
enzymes to break down the DNA and transformation
did not occur. DNA was the transforming factor. Avery
discovered DNA (nucleic acids) stores and transmit
genetic information from one generation of organism to
the next.
DNA is Genetic Material
Alfred Hershey and Martha Chase were studying viruses
that infect bacteria (bacteriophage).
Bacteriophage when a virus takes over a bacterium’s
genetic machinery and directs it to make more viruses.
They conducted experiments using chemical elements
found in proteins and DNA using radioactive
phosphorous for tagging and radioactivity was present
inside the bacteria but not the protein.
DNA
DNA is composed of 4 nucleotides. Each nucleotide has
3 parts: phosphate group, a 5 carbon sugar, and a
nitrogen containing base (single or double ring).
The pyrimidines (single ring) are cytosine and thymine.
Purines are the larger (double ring ) adenine and
guanine.
In 1950 Erwin Chargaff found that the same 4 bases are
in the DNA of all organisms but the proportion of the 4
bases differ from organism to organism.
Double Helix
In April 1953 Watson and Crick published
their DNA model. The nucleotides always
pair in the same way. The DNA nucleotides
of a single strand are joined by covalent
bonds that connect the sugar of one
nucleotide to the phosphate of the next
nucleotide. The alternating sugars and
phosphates form the sides of a double helix
DNA Replication
Watson and Crick realized that s single strand of DNA
can serve as a template or pattern for a new strand. DNA
is copied during the S stage of the cell cycle.
Every cell has a complete set of identical genetic
information.
Proteins carry out the process of replication. DNA does
only stores the information. Enzymes and proteins do
the actual work of replication.
Ex. Enzymes unzip the double helix to separate the
strand and proteins hold the strands apart while the
strand serves as a templates
DNA Replication
An amazing feature is there is a built in feature for
“proofreading” and correcting errors. If the wrong
nucleotide is added to the new strand of DNA then DNA
polymerase can detect the error and remove the
incorrect nucleotide and replace it with the correct one.
Transcription
RNA carries DNA’s instructions
Francis Crick defined the Central Dogma of molecular
biology. There are 3 processes
1. Replicates or copies DNA
2. Transcription converts DNA message into RNA
3. Translation interprets an RNA message into a string
of amino acids called polypeptides. The polypeptides
makeup a protein.
Transcription
In prokaryotic cells, replication, transcription, and
translation all occur in the cytoplasm at the same time.
In eukaryotic cells, where DNA is located inside the
nuclear membrane, these processes are separated in
location and time. Replication and transcription occur
in the nucleus whereas translation occurs in the
cytoplasm. The RNA goes through a processing step
before it can be transported out of the nucleus. RNA
acts as a temporary copy of DNA that is used and then
destroyed.
RNA vs DNA
RNA differs from DNA in 3 ways:
The sugar for RNA is ribose
2. The base Uracil is used instead of Thymine
3. RNA is a single strand of nucleotides whereas DNA is
a double strand
1.
Transcription
Transcription the process of copying a sequence of DNA
to produce a complementary strand of RNA. In this
process a gene not the entire chromosome is transferred
into the RNA message.
RNA polymerase is the enzyme that bond nucleotides
together in a chain to make a new RNA molecule.
3 Steps of Transcription
In eukaryotic cells, RNA polymerase with the of
other proteins and DNA sequence the start site of a
gene. RNA polymerase assembles on a DNA strand
and begins to unwind a segment of the DNA
molecule until the 2 strand separate from each other.
II. RNA polymerase wing one strand of the DNA as a
template, string together a complementary strand of
RNA nucleotides. (RNA pairing follows the same
rule and DNA but the uracil will replace the
thymine). The strand hangs freely as it is transcribed
and then the DNA helix zips back together.
I.
Transcription
III. After the gene has been transcribed, the RNA strand
detaches completely from DNA.
Transcription produces 3 types of RNA molecules. Not all
RNA code for proteins.
1. Messenger RNA (mRNA) is a message that is translated
to form a protein.
2. Ribosomal RNA (rRNA) forms part of the ribosomes (a
cell’s protein factory).
3. Transfer RNA (tRNA)brings amino acids from the
cytoplasm to a ribosome to help make proteins.
Replication & Transcription
 Both occur within the nucleus of eukaryotic cells
 Both are catalyzed by large complex enzymes
 Both involve the unwinding of DNA
 Both involve the complementary base pairing to the
DNA strand
 Both processes are highly regulated by the cell
The end results of the 2 processes are very different
Replication & Transcription
Replication ensures that each new cell will have one
complete set of genetic instructions. Replication occurs
only once during each round of the cell cycle because
each cell needs only one copy of its DNA.
A cell may nee hundreds of copies of a certain proteins
so transcription enables a cell to adjust to changing
demands. This process can occur over and over again.
Translation
Translation is the process that converts or translates and
mRNA message into a polypeptide.
Codons are made up of 3 nucleotides which codes for an
amino acid. RNA can code for 43 = 64 amino acids
Start Codon which signals for the start of translation and
the amino acid methionine.
Stop Codon – there are 3 which signal the end of the
amino acid chain.