Protein Synthesis

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1. DNA was dicovered by Juhann Friedrich in 1869
2. And the first demonstration that DNA contain genetic
information was made in 1944 by Avery, Macleod
and MacCary
3. 1951—Rosalind Franklin was showed or explained
the —X-ray crystallography of DNA
4. 1953 Watson & Crick—explained complet stru. Of
DNA
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1) DNA is composed of 2 chains of nucleotides that form a double
helix shape.
2) The two strands are antiparallel.
3) The backbone of the DNA molecule is composed of alternating
phosphate groups and sugars.
4) The complimentary nitrogenous bases form hydrogen bonds
between the strands.
5) A is complimentary to T and G is complimentary to C.
6) The Diameter is 20 A(2nm)
7) The 2 polynucleotide chains are not identical but complimentary
to each other.
8) The hydrogen bonds formed between purine and pyrimidine only.
9) The DNA is write handed double helix.
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Synthetic Nucleotide analogs & Chemotherapy
Synthetic analogs of purines, pyrimidines, nucleosides,
and nucleotides altered in either the heterocyclic ring or
the sugar moiety.
1. The purine analog allopurinol, used in treatment of
hyperuricemia and gout.
2. Cytarabine is used in chemotherapy of cancer.
3. Azathioprine is employed during organ transplantation
to suppress immunologic rejection.
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DNA
Protein
Gene
Gene
Trait
phenotype
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Genes & Proteins
• DNA contains genes, sequences of nucleotide
bases
• These Genes code for polypeptides (proteins)
• Proteins are used to build cells and do much
of the work inside cells
• Proteins are made of amino acids linked
together by peptide bonds
• 20 different amino acids exist
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DNA Begins the Process
• DNA is found inside the nucleus
• Proteins, however, are made in the
cytoplasm of cells by organelles called
ribosomes
• Ribosomes may be free in the cytosol or
attached to the surface of rough ER
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Starting with DNA
• DNA ‘s code must be copied and taken
to the cytosol
• In the cytoplasm, this code must be
read so amino acids can be assembled
to make polypeptides (proteins)
• This process is called PROTEIN
SYNTHESIS
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RNA
Roles of RNA and DNA
• DNA is the MASTER PLAN
• RNA is the BLUEPRINT of the
Master Plan
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RNA Differs from DNA
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Other Differences
•
•
RNA contains the base
uracil (U)
DNA has thymine (T)
RNA molecule is singlestranded
DNA is doublestranded
DNA
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.
Three Types of RNA
• Messenger RNA (mRNA) copies DNA’s
code & carries the genetic information
to the ribosomes
• Ribosomal RNA (rRNA), along with
protein, makes up the ribosomes
• Transfer RNA (tRNA) transfers amino
acids to the ribosomes where proteins
are synthesized
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Messenger RNA
•
•
•
•
•
•
•
•
Long Straight chain of Nucleotides
Made in the Nucleus
Copies DNA & leaves through nuclear pores
Carries the information for a specific protein
Made up of 500 to 1000 nucleotides long
Sequence of 3 bases called codon
AUG – methionine or start codon
UAA, UAG, or UGA – stop codons
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Ribosomal RNA (rRNA)
• rRNA is a single strand 100 to
3000 nucleotides long
• Globular in shape
• Made inside the nucleus of a cell
• Associates with proteins to form
ribosomes
• Site of protein Synthesis
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The Genetic Code
• A codon designates an amino acid
• An amino acid may have more than one
codon
• There are 20 amino acids, but 64
possible codons
• Some codons tell the ribosome to stop
translating
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Name the Amino Acids
• AG?
• UCA?
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Remember the Complementary
Bases
On DNA:
A-T
C-G
On RNA:
A-U
C-G
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Transfer RNA (tRNA)
• Clover-leaf shape
• Single stranded molecule with attachment
site at one end for an amino acid
• Opposite end has three nucleotide bases
called the anticodon
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Codons and Anticodons
• The 3 bases of an anticodon are
complementary to the 3 bases of
a codon
• Example: Codon ACU
Anticodon UGA
UGA
ACU
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Transcription
and Translation
Transcription
• The process of copying the sequence of
one strand of DNA, the template strand
• mRNA copies the template strand
• Requires the enzyme RNA Polymerase
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Question:
 What would be the
complementary RNA strand
for the following DNA
sequence?
DNA 5’-GCGTATG-3’
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Transcription
• Consists of three stages
– Initiation: attachment of RNA Polymerase to
the promotor region on DNA
– Elongation: building of the mRNA from the
3’ end of the nucleotide polymer
– Termination: release of RNA polymerase
and mRNA following transcription of the
terminator region of the DNA
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1. Initiation
• Genes on the DNA begin with
a promoter region consisting
of a sequence of A & T
(TATA box)
• Transcription factors
(proteins that assist the
binding of RNA polymerase to
the promoter) are found in
association with the promoter
region
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Elongation
• Once initiation is complete the
2 strands of the DNA unwind
due to the zipper region of the
enzyme.
• RNA polymerase builds a
mRNA strand complimentary
to the DNA transcription unit.
• Once the RNA Polymerase
passes the DNA strands
reform their double helix
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Termination In Eukaryotes
• The transcribed termination sequence, also
known as the polyadenylation signal in the
pre-mRNA, is AAUAAA.
• Polymerase continues to synthesize RNA until
an enzyme catches up to it and causes it too fall
off.
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Modification of mRNA
• Transcribed mRNA (pre-mRNA) must be modified before
leaving the nucleus
• modifications include:
– addition guanine triphosphate cap to the 5” end of the mRNA
• Prevents “unraveling”
• Helps ribosome attach
– addition of poly A tail to the 3’ end of the mRNA
• Prevents “unraveling”
• Assists in the export of mRNA from nucleus
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Result of Transcription
CAP
New Transcript
Tail
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How is this done?
• Small nuclear
ribonucleicproteins
(snRNP) recognize intron
ends and together with
proteins form a structure
called a spliceosome
• Spliceosomes remove
introns while connecting
exons together
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Why bother with introns?
1) Introns may regulate gene activity and the
passage of mRNA into the cytoplasm
2) Genes may play roles in multiple proteins,
introns may enable a gene to be diverse in
function
3) May increase recombination of genetic
material (easier to cut and paste)
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Translation
A. Translation-forming of a polypeptide
-uses mRNA as a template for a.a. sequence
-steps (initiation, elongation, translocation and
termination)
-begins after mRNA enters cytoplasm
-uses tRNA (the interpreter of mRNA)
Translation
B. Ribosomes
-made of proteins and rRNA
-each has a large and small subunit
-each has three or two binding sites for tRNA on
its surface
-each has one binding site for mRNA
-facilitates codon and anticodon bonding
Translation
-The three tRNA binding sites are:
1. A site=holds tRNA that is carrying the next
amino acid to be added
2. P site= holds tRNA that is carrying the
growing polypeptide chain
3. E site= where discharged tRNAs leave the
ribosome
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Translation
– 61 of 64 codons code for a.a.
– Codon AUG has two functions
1. codes for amino acid methionine (Met)
2. functions as a start codon
– mRNA codon AUG starts translation
– three codons act as stop codons (end translation)
A. (Initiation)
1. Brings together mRNA, tRNA (w/ 1st a.a.) and
ribosomal subunits
2. Small ribosomal subunit binds to mRNA and
an initiator tRNA
-start codon= AUG
- anticodon-UAC
-small ribosomal subunit attaches to 5’ end
of mRNA
-downstream from the 5’ end is the start codon
AUG (mRNA)
-the anticodon UAC carries the a.a. Methionine
3. After the union of mRNA, tRNA and small
subunit, the large ribosomal subunit attaches.
• The intitiator tRNA and a.a. will sit in the P site
of the large ribosomal subunit
• The A site will remain vacant and ready for the
aminoacyl-tRNA
Translation (Initiation)
B. (Elongation)
Amino acids are added one by one to the first amino
acid (remember, the goal is to make a polypeptide)
Step 1: Codon recognition
mRNA codon in the A site forms hydrogen
bonds with the tRNA anitcodon
Step 2- Peptide bond formation
The ribosome catalyzes the formation of the
peptide bonds between the amino acids (the
one already in place and the one being added)
The polypeptide extending from the P site moves to
the A site to attach to the new amino acid.
Translation (Translocation)
1. The tRNA with the polypeptide chain in the
A site is translocated to the P site
2. tRNA at the P site moves to the E site and
leaves the ribosome
3. The ribosome moves down the mRNA in the
5’→3’ direction
Initiation
aa1
aa2
2-tRNA
1-tRNA
anticodon
hydrogen
bonds
U A C
A U G
codon
G A U
C U A C U U C G A
mRNA
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Elongation
peptide bond
aa1
aa3
aa2
3-tRNA
1-tRNA
anticodon
hydrogen
bonds
U A C
A U G
codon
2-tRNA
G A A
G A U
C U A C U U C G A
mRNA
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aa1
peptide bond
aa3
aa2
1-tRNA
3-tRNA
U A C
(leaves)
2-tRNA
A U G
G A A
G A U
C U A C U U C G A
mRNA
Ribosomes move over one codon
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aa1
peptide bonds
aa4
aa2
aa3
4-tRNA
2-tRNA
A U G
3-tRNA
G C U
G A U G A A
C U A C U U C G A A C U
mRNA
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aa1
peptide bonds
aa4
aa2
aa3
2-tRNA
4-tRNA
G A U
(leaves)
3-tRNA
A U G
G C U
G A A
C U A C U U C G A A C U
mRNA
Ribosomes move over one codon
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aa1
peptide bonds
aa5
aa2
aa3
aa4
5-tRNA
U G A
3-tRNA
4-tRNA
G A A G C U
G C U A C U U C G A A C U
mRNA
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peptide bonds
aa1
aa5
aa2
aa3
aa4
5-tRNA
U G A
3-tRNA
G A A
4-tRNA
G C U
G C U A C U U C G A A C U
mRNA
Ribosomes move over one codon
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aa4
aa5
Termination
aa199
aa3 primary
structure
aa2 of a protein
aa200
aa1
200-tRNA
A C U
terminator
or stop
codon
C A U G U U U A G
mRNA
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End Product –The Protein!
• The end products of protein
synthesis is a primary structure of
a protein
• A sequence of amino acid bonded
together by peptide bonds
aa2
aa1
aa3
aa4
aa5
aa199
aa200
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Messenger RNA (mRNA)
start
codon
mRNA
A U G G G C U C C A U C G G C G C A U A A
codon 1
protein methionine
codon 2
codon 3
glycine
serine
codon 4
isoleucine
codon 5
codon 6
glycine
alanine
codon 7
stop
codon
Primary structure of a protein
aa1
aa2
aa3
peptide bonds
aa4
aa5
aa6
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