PROTEIN SYNTHESIS
Higher Level
Lesson Objectives
At the end of this lesson you should be
able to
1. Outline the steps in protein synthesis
2. Understand that a strand of DNA is copied by
transcription
3. Understand the role of mRNA
4. Know the function of a ribosome in protein
synthesis
5. Understand the process of translation that leads to
the formation of a new protein
6. Know that the shape of a protein determines its
function
For Protein Synthesis
You need:
1. A supply of amino acids – cytoplasm
2. Instructions as how to join the amino
acids together – genetic code
3. An assembly line – ribosomes
4. A messenger to carry information from
DNA to ribosomes
Protein Synthesis - Steps
3 stages:
1. Transcription
2. RNA processing
3. Translation
• Remember: DNA  RNA  Protein
RNA is composed of 3 parts
1. Ribose: smaller sugar than deoxyribose of
DNA
2. Phosphate
3. 4 Nitrogenous Bases A,G,U,C
RNA is single stranded and thus smaller & able to
leave the nucleus of the cell
Transcription
DNA

Translation
RNA

Protein
Nuclear
membrane
DNA
Transcription
Eukaryotic
Cell
RNA Processing
mRNA
Ribosome
Translation
Protein
DNA  RNA  Protein
DNA
Transcription
mRNA
Ribosome
Translation
Protein
Prokaryotic Cell – No nucleus
Learning Check
• What is RNA composed of?
• How does RNA (ribonucleic acid) differ
from DNA (deoxyribonucleic acid)?
• What are the three stages in Protein
synthesis?
1. Transcription
Nuclear
membrane
DNA
Transcription
Eukaryotic
Cell
RNA Processing
mRNA
Ribosome
Translation
Protein
1. Transcription
• The transfer of information in the nucleus from a
DNA molecule to an RNA molecule.
• Only 1 DNA strand serves as the template
• Starts at promoter DNA (AUG)
• Ends at terminator DNA (stop)
• When complete, mRNA molecule is released into
the cytoplasm
Video Clip available in extra
material folder
Transcription
• Takes places in the nucleus of the cell
The process by which the information
from DNA is transferred to RNA.
DNA uncoils and unzips.
• The exposed DNA bases are matched
up with RNA bases in the nucleus to
form mRNA.
1. Transcription
DNA
RNA Polymerase
mRNA
Enzyme
This is a molecule of messenger RNA.
It was made in the nucleus by
transcription from a DNA molecule.
codon
A U G G G C U U AAA G C A G U G C A C G U U
mRNA molecule
Learning Check
• What type of RNA molecule is
responsible for taking the DNA copy
from the nucleus into the cytoplasm
• What parts of the cell do you find RNA
in?
• Can you outline the stages in
transcription?
2. RNA Processing
Nuclear
membrane
DNA
Transcription
Eukaryotic
Cell
RNA Processing
mRNA
Ribosome
Translation
Protein
2. RNA Processing
• Occurs in the nucleus.
• Non- Coding regions taken out by enzyme
and Coding regions come together.
• End product is a mRNA molecule that leaves
the nucleus to the cytoplasm.
mRNA
• Takes places in the nucleus of the cell
The process by which the information
from DNA is transferred to RNA.
DNA uncoils and unzips.
• The exposed DNA bases are matched
up with RNA bases in the nucleus to
form mRNA.
Types of RNA
• Three types of RNA:
A. messenger RNA (mRNA)
B. transfer RNA (tRNA)
C. ribosome RNA (rRNA)
• Remember: all produced in the nucleus!
A. Messenger RNA (mRNA)
• Carries the information for a specific protein.
• Made up of 500 to 1000 nucleotides long.
• Made up of codons (sequence of three bases)
• Each codon is specific for one amino acid.
A. 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
B. Transfer RNA (tRNA)
• Made up of 75 to 80 nucleotides long.
• Picks up the appropriate amino acid floating in
the cytoplasm
• Transports amino acids to the mRNA.
• Has anticodons that are complementary to
mRNA codons.
• Recognizes the appropriate codons on the
mRNA and bonds to them with H-bonds.
C. Ribosomal RNA (rRNA)
• Made up of rRNA is 100 to 3000 nucleotides
long.
• Important structural component of a ribosome.
• Associates with proteins to form ribosomes.
Ribosomes
• Large and small subunits.
• Composed of rRNA (40%) and proteins
(60%).
• Both units come together and help bind
the mRNA and tRNA.
Ribosomes
Large
subunit
mRNA
A U G
Small subunit
C U A C U U C G
Learning Check
• What are the three types of RNA?
• Where is each type produced?
• What is the function of each type of
RNA?
3. Translation
Nuclear
membrane
DNA
Transcription
Eukaryotic
Cell
Pre-mRNA
RNA Processing
mRNA
Ribosome
Translation
Protein
Video Clip available in extra
material folder
3. Translation
• Synthesis of proteins in the cytoplasm
• Involves the following:
1. mRNA (codons)
2. tRNA (anticodons)
3. rRNA
4. ribosomes
5. amino acids
3. Translation
Three parts:
1. initiation: start codon (AUG)
2. elongation:
3. termination: stop codon (UAG)
Translation
• In the cytoplasm, translation occurs.
The mRNA binds to a ribosome.
• The strand of mRNA is pulled through
the ribosome three bases at a time, in
triplets.
• Each of these triplets on the mRNA
strand is called a codon.
This is a molecule of messenger RNA.
It was made in the nucleus by
transcription from a DNA molecule.
codon
A U G G G C U U AAA G C A G U G C A C G U U
mRNA molecule
A ribosome on the rough
endoplasmic reticulum attaches to
the mRNA molecule.
ribosome
A U G G G C U U AAA G C A G U G C A C G U U
Amino acid
tRNA molecule
A transfer RNA molecule arrives.
It brings an amino acid to the first
three bases (codon) on the mRNA.
anticodon The three unpaired bases
(anticodon) on the tRNA link up
with the codon.
UAC
A U G G G C U U AAA G C A G U G C A C G U U
Another tRNA molecule comes into
place, bringing a second amino acid.
Its anticodon links up with the
second codon on the mRNA.
UAC
A U G G G C U U AAA G C A G U G C A C G U U
Another tRNA molecule
brings the next amino acid
into place.
A U G G G C U U AAA G C A G U G C A C G U U
A peptide bond joins the
second and third amino acids to
form a polypeptide chain.
A U G G G C U U AAA G C A G U G C A C G U U
The process continues.
The polypeptide chain gets longer.
This continues until a termination
(stop) codon is reached.
The polypeptide is then complete.
A U G G G C U U AAA G C A G U G C A C G U U
tRNA
Transfer RNA (tRNA), reads the strand
of mRNA and translates it into a strand
of amino acids.
A molecule of tRNA has at one end a
set of three bases that will complement
the mRNA strand; this is called the
anticodon.
tRNA
• If the 3 base anticodon of the tRNA
complements the 3 base codon of the mRNA,
they briefly combine.
• The amino acid is left behind when the tRNA
leaves.
• As each codon is read, the next tRNA brings in
a new amino acid and the polypeptide (protein)
chain grows.
• This requires enzymes and ATP.
End Product
• 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
Functional Protein
• The protein now has to undergo folding and
the addition of bonds
• Folding allows the Protein to reach its 3D
(Tertiary Shape) which influences its
Function.
Learning Check
The anticodon UAC belongs to a tRNA that
recognizes and binds to a particular amino
acid.
What would be the DNA base code for this
amino acid?
Answer:
• tRNA
• mRNA
• DNA
- UAC (anticodon)
- AUG (codon)
- TAC
What have you learned?
Can you ………………
1. Outline the steps in protein synthesis
2. Understand that a strand of DNA is copied by
transcription
3. Understand the role of mRNA
4. Know the function of a ribosome in protein
synthesis
5. Understand the process of translation that leads to
the formation of a new protein
6. Know that the shape of a protein determines its
function
End