PROTEIN SYNTHESIS
Learning Outcomes

B7 Demonstrate an understanding of the process of
protein synthesis
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State roles of DNA, mRNA, tRNA and ribosomes
Explain the processes of transcription and translation
Determine the sequence of amino acids for a DNA sequences
using a table of mRNA codons
DNA, Chromosomes and Genes
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Each chromosome
contains many genes
Each gene contains
many base pairs
(3000 is average)
Each gene carries the
code for one specific
polypeptide
The Central Dogma of Genetics
DNA

RNA

Protein
The Genetic Code

How many possible combinations of 3 can you make using
A,T,C,G?
THE GENETIC CODE
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sequence of nucleotide bases in DNA determines the
sequence of amino acids in a polypeptide
Triplet code - 3 bases for one amino acid (ex.
AAG,GTC, etc.)
code is redundant - more than one triplet can code
for the same amino acid
Proteins

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Recall that polypeptides
are chains of amino acids,
and a protein can contain
several polypeptides
A typical protein contains
100 - 200 amino acids
All proteins are made up
of only 20 different amino
acids
RNA


How is RNA different
from DNA?
3 types of RNA are
involved in Protein
synthesis
Types of RNA
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mRNA (messenger RNA) copies the code from DNA
rRNA (ribosomal RNA) is part of the ribosomes
tRNA (transfer RNA)carries amino acids to the
ribosomes
mRNA
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Each group of 3
mRNA nucleotides is
called a codon
Each codon codes for
a specific amino acid
tRNA
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Each tRNA molecule
has a 3-base anticodon
which is
complementary to a
mRNA codon
Also has a binding site
for a specific amino
acid
rRNA


A complex molecule
that makes up the
structure of a
ribosome
Binds to tRNA and
mRNA
PROTEIN SYNTHESIS
•
Protein synthesis consists of two stages –
•
•
Transcription (in the nucleus)
Translation (at ribosomes)
TRANSCRIPTION
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•
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Takes place in the nucleus
Part of DNA strand unzips
mRNA nucleotides attach to
DNA strand, forming a
complementary strand
(enzyme – RNA polymerase)
mRNA detaches from the
DNA strand
DNA zips back together
mRNA can now carry the
instructions out of the nucleus
mRNA & DNA Base Pairing:
•
•
•
•
U pairs with A
A pairs with T
G pairs with C
C pairs with G
RNA processing
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Not all of the DNA is part
of a gene
Introns are pieces that are
not part of the gene
Exons are the parts that
make up the gene
(expressed)
Intron sections of mRNA
are removed by ribozymes
(RNA enzymes )
Translation
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Takes place outside the nucleus, at ribosomes (attached
to ER or free in the cytoplasm)
Takes place in 3 stages:
1.
2.
3.
Initiation
Elongation
Termination
1. Initiation
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Ribosomal subunits attach to mRNA strand at the “Start”
codon
2 codons of mRNA are in the ribosome at a time
2. Elongation
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As each mRNA codon binds to the ribosome, a
tRNA with the complementary anticodon brings the
correct amino acid
As amino acids are delivered to the ribosome, they
are joined by peptide bonds forming a growing chain
Sequence of codons determines the primary
structure
Ribosome moves along the mRNA one codon at a
time
3. Termination
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When the “Stop” codon is reached, the ribosome
detached from the mRNA and the polypeptide is
released
The new polypeptide may now enter the ER for
modification
Mass Production of Proteins

Many ribosomes may move along the same mRNA
strand (polyribosome), producing many copies of
the polypetide