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Protein Synthesis
Notes
Main Idea
•DNA codes for RNA, which
guides protein synthesis.
•Protein Synthesis is the
making of proteins.
Protein Synthesis Overview
In protein synthesis RNA uses a code
from the DNA strand in the nucleus to
make proteins for the cell.
There are two main stages of protein
synthesis: Transcription and
Translation.
The Importance of Proteins
Recall that proteins are the building
structures of cells and are important in
catalyzing events in cells. The shape of
a protein determines how the protein
functions in the organism. How the
protein is built is very important.
Central Dogma
DNA codes for RNA and RNA codes for
proteins.
RNA is a nucleic acid that is similar to DNA,
however RNA:
• Contains the sugar ribose and the base uracil
instead of thymine.
• Usually is single stranded
Types of RNA - There are 3 different
types of RNA used in Protein Synthesis.
• Messenger RNA (mRNA) – takes the DNA message
to the ribosome
▫ Long strands of RNA nucleotides that are formed
complementary to one strand of DNA
• Ribosomal RNA (rRNA) - helps in protein
production
▫ Associates with proteins to form ribosomes in the
cytoplasm
• Transfer RNA (tRNA) – transfers or brings amino
acids to the protein factories (ribosomes)
▫ Small segments of RNA nucleotides that transport
amino acids to the ribosome
Comparison of Three Types of RNA
Each type of RNA has a different shape and a different job
(function.)
Describe each type of RNA.
The Process of Protein Synthesis
Protein synthesis takes place in two main stages.
Transcription – DNA’s message is transcribed by
mRNA in the nucleus and taken to the ribosomes.
Translation – rRNA and mRNA work together
with tRNA to translate DNA’s message into a
strand of amino acids or a protein.
Transcription
• Through transcription, the DNA code is transferred to
mRNA in the nucleus. DNA cannot leave the nucleus!
mRNA takes the DNA message out to the ribosomes.
• DNA is unzipped in the nucleus and RNA polymerase
binds to a specific section where an mRNA will be
synthesized.
The mRNA strand is
complimentary to the
DNA strand. However,
instead of A pairing with
T (thymine), in mRNA T
is replaced with U or
uracil.
RNA Processing
• The code on the DNA is interrupted periodically
by sequences that are not in the final mRNA.
• Intervening sequences are called introns.
• Remaining pieces of DNA that serve as the
coding sequences are called exons.
The Code
• Experiments during the
1960s demonstrated
that the DNA code was a
three base code.
• The three-base code in
DNA or mRNA is called
a codon.
• The three base code, or
codon, in mRNA
determines the amino
acid that is produced.
A Rosetta stone or coding chart is
used to decode the message. Which
amino acid does CAG code for? ___
Translation
• In translation, tRNA molecules act as the
interpreters of the mRNA codon sequence.
• At the middle of the folded strand, there is a threebase coding sequence called the anticodon.
• Each anticodon is complementary to a codon on the
mRNA.
• As tRNA brings amino acids to the ribosome, a chain
of amino acids (polypeptide) is built until a STOP
message is received. When the STOP message is
received, the new protein is released to be used by
the cell. Which codon is a STOP codon? _______
Visualizing Transcription and
Translation
• Transcription takes place in the nucleus.
• Translation occurs in the cytoplasm and results
in the formation of polypeptides.
Using your own
words, write the
story of protein
synthesis. Start in
the nucleus.
One Gene – One Enzyme
• The Beadle and Tatum experiment showed
that one gene codes for one enzyme. We
now know that one gene codes for one
polypeptide (protein.)