DNA & Protein
Synthesis
From Gene to Protein
1
Nucleic Acids and Protein
Synthesis
• All functions of a cell are
directed from some central form
of information (DNA).
• This "biological program" is called
the Genetic Code.
• This is the way cells store
information regarding their
structure and function.
2
History of DNA
Composition and
Structure
3
History
• For years the source of heredity was
unknown. This was resolved after
numerous studies and experimental
research by the following
researchers:
• Fredrick Griffith
– He was studying effects of 2
strains of an infectious bacteria,
the "smooth" strain was found to
cause pneumonia & death in mice.
The "rough" strain did not. He
conducted the following experiment 4
Griffith Experiment
Bacteria Strain
injected into mouse
Result
Smooth Strain
Mouse
dies
Rough strain
Mouse
Lives
Heat-Killed Smooth
strain
Mouse
lives
Rough Strain & Heat
killed smooth strain
*MOUSE
DIES*
•The last condition was
unusual, as he predicted that the
mouse should live
•Concluded that some unknown
substance was Transforming the
rough strain into the smooth one 5
Avery, McCarty & MacLeod
Tried to determine the
nature of this transforming
agent.
Was it protein
or DNA?
•They Degraded
chromosomes with
enzymes that
destroyed proteins
or DNA
•The Samples with
Proteins destroyed
would still cause
transformation in
bacteria indicating
genetic material
was DNA
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Hershey-Chase
• ONE virus was
radioactively "tagged"
with 32P on it's DNA
• The OTHER was
"tagged" 35S on it's
protein coat.
• Researchers found the
radioactive P in the
bacteria, indicating it
is DNA, not protein
being injected into
bacteria.
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Watson & Crick
• The constituents of DNA
had long been known.
Structure of DNA,
however was not.
• In 1953, Watson & Crick
published findings based
on X-ray analysis
(Rosalind Franklin) and
other data that DNA was
in the form of a "Double
Helix".
• Their findings show us
the basic structure of
DNA which is as follows.
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DNA Structure
The Double Helix
9
DNA Structure
DNA is Formed of in a "Double Helix" 10
like a spiral staircase
Nucleotides
• DNA is formed
by Nucleotides
• These are made
from three
components:
1.5-Carbon or
pentose Sugar
2.Nitrogenous base
3. Phosphate group
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Types of Nucleotides
•
For DNA There are 4 different Nucleotides
categorized as either Purines (Double rings) or
Pyrimidines (Single ring). These are usually
represented by a letter. They Are:
1.
2.
3.
4.
Adenine (A)
Cytosine (C)
Guanine (G)
Thymine (T)
Guanine
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Base Pairing
• Each "Rung" of the DNA "staircase" is formed by the linking of
2 Nucleotides through Hydrogen Bonds.
• These Hydrogen bonds form only between specific Nucleotides.
This is known as Base Pairing. The rules are as follows:
– Adenine (A) will ONLY bond to Thymine (T) (by 2
hydrogen bonds)
– Cytosine (C) will ONLY bond to Guanine (G) (by 3
hydrogen bonds)
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Central Dogma of
Genetics
DNA to Protein Synthesis
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Central Dogma of Genetics
• Central Dogma holds that genetic
information is expressed in a specific order.
This order is as follows
There are some apparent exceptions to this.
Retroviruses (eg. HIV) are able to synthesize DNA 15
from RNA
DNA Replication
• DNA has unique ability to make
copies of itself
• The process is called DNA
Replication.
• First, the enzyme Helicase unwinds
the parental DNA
• DNA "Unzips itself" by breaking
the weak hydrogen bonds between
base pairs forming two TEMPLATE
strands with exposed Nucleotides
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DNA Replication
• The place where helicase attaches
and opens DNA is called the
Replication Fork
REPLICATION FORK
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DNA Replication
• Helicase enzymes may attach to
multiple sites on the DNA strand
forming Replication Bubbles which
makes replication faster
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DNA Replication
• Single-strand binding proteins
attach & STABILIZE the 2
parental strands
• DNA polymerase attaches to the
3’ end of the 5’ to 3’ parental
strand
• DNA polymerase attaches FREE
nucleotides to the complementary
nucleotide on the parental DNA
• This new strand is synthesized
continuously 5’ to 3’ (LEADING)
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Replication Bubble
DNA is synthesized from the Origin of Replication
within a replication bubble
•Towards fork – continuous replication
•Away from fork – discontinuous replication (fragments)
Origin of Replication
Origin of Replication
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DNA Replication
Since DNA polymerase
can only add
nucleotides to the 3’
end of the parental
strand, the parental
5’ to 3’ strand must
be replicated in
fragments that must
later be joined
together (LAGGING)
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DNA Replication
• Transcription proceeds
continuously along the 5'3'
direction (This is called the
leading strand)
• Proceeds in fragments in the
other direction (called the lagging
strand) in the following way
• RNA primer is attached to a
segment of the strand by the
enzyme primase.
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DNA Replication
• Transcription now continues in the
5'3' direction forming an
okazaki fragment. Until it
reaches the next fragment.
• The two fragments are joined by
the enzyme DNA ligase
• Two, new, identical DNA strands
are now formed
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DNA Replication
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Protein Synthesis
Transcription and Translation
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RNA Transcription
•
•
•
•
•
The cell does not directly use
DNA to control the function
of the cell.
DNA is too precious and must
be kept protected within the
nucleus.
The Cell makes a working
"Photocopy" of itself to do the
actual work of making proteins.
This copy is called Ribonucleic
Acid or RNA.
RNA differs from DNA in
several important ways.
1. It is much smaller
2. It is single-stranded
3. It does NOT contain
Thymine, but rather a new
nucleotide called Uracil
which will bind to Adenine
4. Contains ribose, not
deoxyribose sugar
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RNA Transcription
• RNA is produced through a process
called RNA Transcription.
• Similar to DNA Replication.
• Small area of DNA "Unzips" exposing
Nucleotides
• This area is acted on by an enzyme
called RNA Polymerase, which binds
nucleotides (using uracil) to their
complementary base pair.
• This releases a long strand of
Messenger RNA (mRNA) which is an
important component of protein
synthesis.
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RNA Transcription
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Protein Synthesis & The Genetic
Code
• The Sequence of nucleotides in an
mRNA strand determine the sequence
of amino acids in a protein
• Process requires mRNA, tRNA &
ribosomes
• Polypeptide chains are synthesized by
linking amino acids together with
peptide bonds
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• Each three
Nucleotide
sequence in an
mRNA strand is
called a "Codon“
• Each Codon codes
for a particular
amino acid.
• The codon
sequence codes
for an amino acid
using specific
rules. These
specific
codon/amino acid
pairings is called
the Genetic Code.
mRNA
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tRNA
•There is a special form of
RNA called Transfer RNA
or tRNA.
•Each tRNA has a 3
Nucleotide sequence on one
end which is known as the
"Anitcodon"
•This Anticodon sequence is
complimentary to the Codon
sequence found on the
strand of mRNA
•Each tRNA can bind
specifically with a
particular amino acid.
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Ribosome
• Consists of two
subunits made of
protein & rRNA
– Large subunit
– Small subunit
• Serves as a
template or
"work station"
where protein
synthesis can
occur.
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Protein Synthesis
• First, an mRNA strand binds to the large & small
subunits of a ribosome in the cytosol of the cell
• This occurs at the AUG (initiation) codon of the
strand.
• The ribosome has 3 binding sites for codons --- E
(exit site), P, and A (entry site for new tRNA)
• The ribosome moves along the mRNA strand
34
Protein Synthesis
• An anticodon on tRNA binds to a complementary
codon on mRNA.
• The tRNA carrying an amino acid enters the A
site on the ribosome
• The ribosome moves down the mRNA so the
tRNA is now in the P site and another tRNA
enters the A site
• A peptide bond is formed between the amino
acids and the ribosome moves down again
• The first tRNA is released, and another tRNA
binds next to the second, another peptide bond
is formed.
• This process continues until a stop codon (UAG…)
is reached.
• The completed polypeptide is then released.
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Protein Synthesis
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Replication Problem
• Given a DNA strand with the following
nucleotide sequence, what is the
sequence of its complimentary strand?
• 3’- TACCACGTGGACTGAGGACTCCTCTTCAGA -5’
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Answer
• Given a DNA strand with the following
nucleotide sequence, what is the
sequence of its complimentary strand?
• 3’- TACCACGTGGACTGAGGACTCCTCTTCAGA -5’
• 5’- ATGGTGCACCTGACTCCTGAGGAGAAGTCT -3’
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RNA Transcription Problem
• Given a DNA strand with the following
nucleotide sequence, what is the
sequence of its complimentary mRNA
strand?
• 3’- TACCACGTGGACTGAGGACTCCTCTTCAGA -5’
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ANSWER
• Given a DNA strand with the following
nucleotide sequence, what is the
sequence of its complimentary mRNA
strand?
• 3’- TACCACGTGGACTGAGGACTCCTCTTCAGA -5’
• 3’- AUGGUGCACCUGACUCCUGAGGAGAAGUCU -5’
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Codon / Anticodon
• Given a mRNa strand with the following
nucleotide sequence, what are the sequence
(anticodons) of its complimentary tRNA
strands?
• 3’- AUGGUGCACCUGACUCCUGAGGAGAAGUCU -5’
42
Answer
Given a mRNA strand with the following
nucleotide sequence, what are the sequence
(anticodons) of its complimentary tRNA
strands?
• 3’- AUGGUGCACCUGACUCCUGAGGAGAAGUCU -5’
• 3’ – UACCACGUGGAUGAGGACUCCUCUUCAGA -5’
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Protein Translation
• Given the following
sequence of
mRNA, what is the
amino acid
sequence of the
resultant
polypeptide?
• AUGGUGCACCUGA
CUCCUGAGGAGAA
GUCU
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Protein Translation / Answer
• Given the following
sequence of
mRNA, what is the
amino acid
sequence of the
resultant
polypeptide?
• AUGGUGCACCUGA
CUCCUGAGGAGAA
GUCU
Met-val-his-leu-thr-pro-glu-glu-lys-ser
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