Protein Synthesis
(Gene Expression)
Review



Nucleotide sequence in DNA is used to
make proteins that are the key regulators
of cell functions.
Proteinspolymers of amino acids
The sequence of nucleotides in DNA
contains information for assembling the
string of amino acids that make up a single
protein.
RNA-Ribonucleic Acid
A.
Differences between
DNA and RNA
1.
2.
3.
RNA is single-stranded (it
looks like ½ of a zipper.
The sugar in RNA is
ribose
RNA contains URACIL
instead of thymine. Uracil
pairs with adenine.
RNA-Ribonucleic Acid
B.
Role of RNA
1.
“Worker” for protein synthesis while DNA is
the “Commander-in-Chief”
RNA takes the instructions on how a protein should
be built and then assembles the protein, amino acid
by amino acid.
RNA-Ribonucleic Acid
C.
Types of RNA
1.
2.
3.
MESSENGER RNA (mRNA)-Bring information from
the DNA in the nucleus to the cytoplasm
RIBOSOMAL RNA (rRNA)-Subunit of ribosomes which
clamps onto the mRNA and use its information to
assemble the amino acids in the correct sequence.
TRANSFER RNA (tRNA)-Supplier of amino acids to the
ribosome
Protein Production and the Genetic
Code
A.
Transcription
Location=nucleus
The information found in a gene in DNA is transcribed
into an mRNA molecule
Steps:
1.
RNA polymerase binds to the gene’s “promoter” or “start”
signal
2.
RNA polymerase unwinds and separates the two strands
of DNA
3.
RNA polymerase adds the complementary RNA
nucleotides as it “reads” the gene (U=A; G=C)
4.
Transcription proceeds until the RNA polymerase reaches
a “stop” signal. At this point, it detaches and the mRNA
molecule detaches from the DNA strand. The DNA
strand will twist back up and the bonds will be restored.
Transcription
Adenine (DNA and RNA)
Cystosine (DNA and RNA)
Guanine(DNA and RNA)
Thymine (DNA only)
Uracil (RNA only)
RNA
polymerase
DNA
RNA
Protein Synthesis: Transcription
Protein Production and the Genetic
Code
B.



The nucleotide sequence transcribed from DNA
to a strand of mRNA acts as a genetic message.
This message is written in a language that uses
nitrogen bases as its “alphabet”.
The language of proteins uses an “alphabet” of
amino acids.
A code is needed to convert the language of
mRNA into the language of proteins.
Protein Production and the Genetic
Code
B. continued…
 There are 20 different amino acids, but
only 4 types of N bases in mRNA. How can
these bases form a code for proteins?
Protein Production and the Genetic
Code
A
group of three nucleotides codes for one
amino acid. Each set of three N bases that
codes for an amino acid is called a codon.
 The order of nitrogen bases in the mRNA
will determine the type and order of amino
acids in a protein
 64 combinations are possible when a
sequence of 3 bases is used. Thus, 64
different mRNA codons are in the genetic
code.
Protein Synthesis: Translation
A Codon
OH
P
HO
NH2
O
N
O
CH2
N
H
P
O
O
N
O
CH2
P
NH
N
Guanine
NH2
N
O
Arginine
H
O
HO
N
O
O
HO
Adenine
N
NH2
O
N
O
CH2
N
O
OH
H
N
N
Adenine
The Genetic Code
Protein Production and the Genetic
Code



Some codons do not code for amino acids;
they provide instructions for assembling
the protein.
UAA is a stop codon indicating that
protein production should stop at that
point.
AUG is a start codon as well as being the
codon for the amino acid methionine.
Protein Production and the Genetic
Code


As you can see from the genetic code
chart, more than one codon can code for
the same amino acid. However, for any one
codon, there can only be one amino acid.
The genetic code is nearly universal-the
same codon can code for the same amino
acid in many different organisms
Protein Synthesis: Translation
Protein Production and the Genetic
Code
C. Translation
 Location=cytoplasm
1. In eukaryotic cells, mRNA leaves the nucleus
through an opening in the nuclear membrane and
travels to the cytoplasm.
2. When the strands of RNA arrive, ribosomes
attach to them like clothespins clamped onto a
clothesline.
3. The process of converting the information in a
sequence of amino acids that make up a protein is
known as translation
Eukaryotic Translation
Cytoplasm
DNA
Transcription
RNA
RNA
Processing
mRNA G
G
AAAAAA
Nucleus
Export
AAAAAA
Ribosome
Protein Production and the Genetic
Code
1.
Role of Transfer RNA (tRNA)
For proteins to be built, the 20 different amino acids dissolved in
the cytoplasm must be brought to the ribosomes -> this is the job
of tRNA.
Each tRNA molecule attaches to only one type of amino acid
Correct translation of the mRNA message depends on upon the
joining of each mRNA codon with the correct tRNA molecule
On the opposite side of the tRNA molecule from the amino
acid attachment site, there is a sequence of 3 nucleotides
that are the complement of the nucleotides in the codon.
These 3 nucleotides are called an anticodon because they
bond to the codon on the mRNA by the process of base
pairing
Protein Production and the Genetic
Code
2.
Translating the mRNA code
a.
b.
As translation begins, a tRNA molecule brings
the first amino acid to the mRNA strand that
is attached to the ribosome.
The anticodon forms a temporary bond with
the codon of the mRNA strand. This places
the amino acid in the correct position for
forming a bond with the next amino acid.
Protein Production and the Genetic
Code
c. The ribosome slides down the mRNA chain to
the next codon and a new tRNA molecule
brings another amino acid.
d. The amino acids form peptide bonds, the first
tRNA releases its amino acid and detaches
from the mRNA. This tRNA molecule is now
free to pick up and deliver another molecule
of its specific amino acid to a ribosome.
Translation
Nucleus
Messenger RNA
Messenger RNA is transcribed in the nucleus.
Phenylalanine
tRNA
mRNA
Transfer RNA
Methionine
The mRNA then enters the cytoplasm and
attaches to a ribosome. Translation begins at
AUG, the start codon. Each transfer RNA has
an anticodon whose bases are complementary
to a codon on the mRNA strand. The ribosome
positions the start codon to attract its
anticodon, which is part of the tRNA that binds
methionine. The ribosome also binds the next
codon and its anticodon.
Ribosome
mRNA
Lysine
Start codon
Protein Production and the Genetic
Code
e.
f.
g.
A chain of amino acids (polypeptide
chain) continues to form
When a stop codon is reached,
translation ends, and the amino acid
strand is released from the ribosome.
The amino acid chains then twist and curl
into complex three-dimensional shapes
and become proteins.
Forming the Polypeptide Chain
The Polypeptide “Assembly Line”
The ribosome joins the two amino acids—
methionine and phenylalanine—and breaks
the bond between methionine and its tRNA.
The tRNA floats away, allowing the ribosome
to bind to another tRNA. The ribosome moves
along the mRNA, binding new tRNA molecules
and amino acids.
Lysine
Growing polypeptide chain
Ribosome
tRNA
tRNA
mRNA
Completing the Polypeptide
mRNA
Ribosome
Translation direction
The process continues until the ribosome reaches
one of the three stop codons. The result is a
growing polypeptide chain.
Section 12-3
RNA
can be
Messenger RNA
Ribosomal RNA
Transfer RNA
also called
which functions to
also called
which functions to
also called
which functions to
mRNA
Carry instructions
rRNA
Combine
with proteins
tRNA
Bring
amino acids to
ribosome
from
to
to make up
DNA
Ribosome
Ribosomes