Protein Synthesis - Overview

Protein Synthesis –
An Overview…
Genes on chromosomes need to be
transcribed into RNA, and then
translated by ribosomes into protein.
Genes act by directing the production of
only ONE protein each.
• This is known as the one gene-one enzyme
An error in gene sequence can result in a
change in protein sequence, shape, and
therefore function.
• TRY THIS: Activity p 233, Translating the
DNA in the nucleus is TRANSCRIBED
into a complementary RNA
message that moves to the
cytoplasm. (copied from one
medium to another)…
 Numerous RNA copies can move to
different ribosomes.
The ribosomes TRANSLATE the message
into Polypeptide chains and process into
proteins (convert into a different
Transcription involves the copying of
information in DNA into mRNA
Translation involves ribosomes using
mRNA as blueprint to synthesize a protein
composed of amino acids.
A Cool Visual
ribose sugar with a
hydroxyl on 2’
uracil replaces the
base thymine and
pairs with adenine
single stranded
3 Types of RNA
Messenger RNA (mRNA): varies in
length, end product of transcription
of a gene, moves to the ribosome
Transfer RNA (tRNA): delivers
amino acids to the ribosomes for
translation. Shaped like a
cloverleaf; very short.
Ribosomal RNA (rRNA): binds
with protein to make ribosomes
Step 1 - Transcription
Within the nucleus, the
DNA of a particular gene
unwinds and separates.
One strand of the DNA is
transcribed into
messenger RNA (mRNA)
in a process called
Transcription – Making mRNA
Transcription divided into three
sequential processes…
• Initiation
RNA polymerase binds to DNA at specific
site known as promoter, near the
beginning of the gene. (usually high [] of A
& T…only 2 bonds to break!)
• Elongation
Enzyme RNA polymerase puts together
the appropriate ribonucleotides and builds
the mRNA transcript
• Termination
Shortly after RNA polymerase passes the
end of a gene, recognizes signal to stop
 mRNA transcript then is completely released
from the DNA and will exit the nucleus
The Final Touches…
the mRNA must be prepared to leave the
nucleus. A 5’ cap is added (for
protection), and a poly A tail is added to
the 3’ end
The mRNA contains regions of code that
do not make the protein (introns).
Introns are removed by spliceosomes.
Spliceosomes join exons (coding regions)
The mRNA strand leaves the nucleus and
binds with ribosomes in the cytoplasm.
P 249, Q 1-6
Step 2 Translation
Once mRNA enters
cytoplasm, it is ready
to be translation
Ribosomes: made up
of two parts: a large
subunit (60s) and a
small subunit (40s).
Bind to the mRNA
(recognizing the 5’
Must be lined up
correctly to the
proper reading frame
(reading triplets
tRNA delivers amino acids to the polypeptide
building site (ribosome)
tRNA is a small single stranded nucleic acid,
resembles a cloverleaf
one arm: anticodon (sequence of three bases
complementary to mRNA)
3’ end has acceptor site for a particular amino
• this recognition by tRNA of mRNA is facilitated
through complimentary base pairing.
every tRNA carries only one specific amino acid
• therefore there must be at least 20 (20-64)
different tRNA’s
The mRNA consists of nucleotides that code for a
specific amino acid. The code is in triplet called a CODON
(3 nucleotides = 1 amino acid). Amino acids are brought
into place by a molecule known as transfer RNA (tRNA).
This process is known as translation. Peptide bonds
occur b/w amino acids.
Building the Chain…
First codon recognized is start (AUGmethionine)- aligns the reading
 ribosome contains two sites for
tRNA: an A (acceptor) site and a P
(peptide) site.
 tRNA (Met- AUG) enters the P site.
 the rest of the tRNA enter at the A
site and form peptide bonds between
the amino acids as the chain forms.
Ribosome translocates one codon over
and continues the chain until it reaches a
stop codon. (UGA, UAG, UAA)
A release factor protein dismantles the
complex, releasing the polypeptide chain.
Enzymes may cleave the chain at specific
Protein Synthesis
Once all the triplet codes (Codons)
are read and the amino acids are
brought into place, the completed
protein travels through the ER to
the Golgi.
The Golgi modifies the structure
and packages it into a vesicle.
Vesicle moves to the membrane
and is released by exocytosis.

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