Transcript Protein Synthesis

Protein Synthesis
The Three T’s
1. Transcription
2. Translation
3. Termination
Transcription in Eukaryotes
ie: Animal cells
Protein Synthesis in Prokaryotes
ie: single celled Bacteria
Where does All this Happen
Protein Synthesis
Nucleic Acids (DNA & RNA) carry the
hereditary information.
 This information is contained in codons.
 What are Codons?
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A codon is a set of three base pairs (A, T, C,
G) that directs or codes for amino acids. (ex.
GCU – Alanine)
How Many Amino acids?
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There are 20 essential amino acids, however
they can be combined in any order, just
like the four nucleotides. This permits the
production of the many different proteins
which let organisms grow and function.
Transcription
This process occurs in the nucleus.
 A section of DNA called a gene is unwound
and unzipped.
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• A RNA copy of one of the DNA strands is
made. This strand is made complimentary to
the nucleotides on the the DNA Strand.
•DNA Bases: Adenine, Thymine, Cytosine,
Guanine
•RNA Bases: Adenine, Uracil, Cytosine,
Guanine
• DNA serves as a template for the synthesis of RNA.
•A regulatory protein binds to the promoter sequence.
•An enzyme (RNA Polymerase) binds to the promoter.
•Together they open the DNA double helix.
•RNA Polymerase proceeds down one strand moving in the 3’ to
5’ direction, as it does it assembles a complementary strand of
RNA.
•Each ribonucleotide is inserted into the growing RNA strand
following the rules of base pairing.
•Transcription stops when the termination sequence is reached.
•The completed RNA copy is now called messenger RNA or
mRNA and carries the coded message to the ribosomes in the
cytoplasm.
DNA Strand 1: AGCTATCGAGCAT
DNA Strand 2: TCGATAGCTCGTA
RNA copy:
AGCUAUCGAGCAU
Transcription
RNA polymerase (enzyme) attaches to the promoter region of the DNA
Translation
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The small subunit of the ribosome binds to a site on the 5’
end of the start of the message strand
The ribosome moves downstream (53’) until it
encounters the start codon AUG
At this time, the large subunit joins and the “initiator tRNA” enters the scene
After this, the tRNA binds to the p site on the ribosome
The first amino acid that starts this sequence of peptide
bonds is Methionine (one of the 20 a.a.’s)
Eucaryotes: methionine
Bacteria: f Met (modified)
T RNA
1. Ribosome attaches to mRNA and reads first
codon.
2. A transfer RNA molecule (tRNA for short)
brings the correct amino acid to the ribosome &
drops it off. ex: AUG (codes for methionine)
tRNA brings methionine to ribosome.
3. The ribosome moves on
to the next codon and
another tRNA brings the
next amino acid.
4. The amino acids in the
growing chain are linked
together by a peptide
bond. The growing chain
is called a polypeptide or
protein molecule.
5. When the ribosome
reaches a STOP codon the
polypeptide is released
and the mRNA falls off.
Elongation more specifically!
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As another tRNA arrives with its associated a.a.
and binds to the next codon site, it becomes
covalently linked to the incoming a.a. with a
peptide bond
The initiator tRNA is released from the P site
The ribosome moves downstream (53’)
The more recently-arrived tRNA w/ its peptide
moves to P site and opens the A site for the arrival
of a NEW tRNA and its corresponding a.a.
Termination
End of the message is a STOP codon
 STOP codons: UAA, UAG, UGA
 Protein release factor sees these codons at
the A site and tells the polypeptide chain to
release from the ribosome site
 Ribosome splits into its subunits until it is
time to make more protein!
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Termination
Check out this link for an animation
on Protein Synthesis
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http://www.lewport.wnyric.org/jwanamaker
/animations/Protein%20Synthesis%20%20long.html