Translation

The synthesis of cellular proteins occurs via the translation

Translation of the sequence of codons within mRNA into a
sequence of amino acids that constitute a polypeptide.
many different proteins, RNAs, and small molecules—needed for
the translation process.
genes that encode an amino acid sequence are known as
structural genes.
The RNA transcribed from structural genes is called messenger
RNA (mRNA).
tRNA molecules - act as the translators of the genetic
information within mRNA
So single gene controlled the synthesis of a single enzyme
(protein).
This was referred to as the one-gene/one-enzyme hypothesis.
(Beadle and Tautum – Neurospora crassa)
Overview of Translation
Structural gene encodes a polypeptide.
polypeptide - linear sequence of amino acids.
Why have researchers named this process translation?
interpretation of one language—the language of mRNA, a
nucleotide sequence—into the language of proteins—an amino
acid sequence.
The ability of mRNA to be translated into a specific sequence of
amino acids relies on the genetic code.
The sequence of bases within an mRNA molecule read in groups
of three nucleotides known as codons .
The sequence of three bases in most codons specifies a particular
amino acid.
These codons are termed sense codons.
For example, the codon AGC specifies the amino acid serine,
whereas the codon GGG encodes the amino acid glycine.
The codon AUG, which specifies methionine, is used as a start
codon begins a polypeptide sequence.
UAA, UAG, and UGA, which are known as stop codons.
They are also known as termination or nonsense codons.
The codons in mRNA are recognized by the anticodons
(complementary) in transfer RNA (tRNA) molecules.
The tRNA molecules carry the amino acids that correspond to
the codons in the mRNA.
In this way, the order of codons in mRNA dictates the order of
amino acids within a polypeptide.
the genetic code is termed degenerate.
This means that more than one codon can specify the same
amino acid.
For example, the codons GGU, GGC, GGA, and GGG all specify the
amino acid glycine.
Such codons are termed synonymous codons.
• The genetic code is a set of three –base code words,
called codons, in mRNA. Codons are decoded to
incorporate specific amino acids into a polypeptide
chain
The genetic code has 64 codons.

Of these, 61 are sense codons that specify the 20 amino acids.

Therefore, to synthesize proteins, a cell must produce many

different tRNA molecules having specific anticodon sequences.

To do so, the chromosomal DNA contains many distinct tRNA

genes that encode tRNA molecules with different sequences.

the anticodon in a tRNA specifies the type of amino acid that it

carries.

For example, a tRNA that attaches to phenylalanine is described

as tRNAPhe, whereas a tRNA that carries proline is tRNAPro.
 The Genetic Code
• Codons: 3 base code for the production of a specific amino acid,
sequence of three of the four different nucleotides

• Since there are 4 bases and 3 positions in each codon, there are 4
x 4 x 4 = 64 possible codons

• 64 codons but only 20 amino acids, therefore most have more

than 1 codon (REDUNDANT/DEGENERATE)

• 3 of the 64 codons are used as STOP signals; they are found at the
end of every gene and mark the end of the protein

• One codon is used as a START signal: it is at the start of every

protein

• Universal: in all living organisms (EXCEPTIONS IN SOME)

Is the Genetic code Universal?

• The genetic code is not strictly universal (same

codons encoding the same information in all
species).

– Termination codons in the standard genetic code

can code for aa like tryptophan and glutamine in
some species.
Start CODON – AUG; Stop CODON; UGA, UAA and UAG

Three possible reading frames of

translation (+ direction)
 Wobble Hypothesis
Wobble Hypothesis - multiple codons can code for a single
amino acid

A wobble base pair is a pairing between two nucleotides in

RNA molecules that does not follow Watson-Crick base pair
rules

Most organisms have fewer than 45 types of tRNA

so a few tRNA types pair with multiple, synonymous

codons all of which encode the same amino acid.

5' base on the anticodon which binds to the 3' base on the
mRNA, is not as spatially confined as the other two bases.

Thereby, they could have non-standard base pairing

(Wobble).
Structure and Function of tRNA
During translation, a tRNA has two functions:
(1) It recognizes a three-base codon sequence in mRNA
(2) it carries an amino acid specific for that codon.
The adaptor hypothesis:
tRNA molecules recognize the codons within mRNA
carry the correct amino acids to the site of polypeptide synthesis

The anticodon in the tRNA binds to a

complementary sequence in the mRNA.

At its other end, the tRNA carries the amino

acid that corresponds to the codon in the
mRNA via the genetic code.
Secondary structure of tRNA
the secondary structure of tRNAs exhibits a cloverleaf pattern.
A tRNA has three stem-loop structures, a few variable sites, and
an acceptor stem with a 3ʹ single-stranded region.
The acceptor stem is where an amino acid becomes attached to
a tRNA.
Among different types of tRNA molecules, the variable sites can
differ in the number of nucleotides they contain.
The anticodon is located in the second loop region.
To function correctly, each type of tRNA must have the
appropriate amino acid attached to its 3ʹ end.
How does an amino acid get attached to a tRNA with the correct
anticodon?
aminoacyl-tRNA synthetases catalyze the attachment of amino
acids to tRNA molecules.
Cells produce 20 different aminoacyl-tRNA synthetase enzymes,
1 for each of the 20 distinct amino acids.
Ribosome Structure
According to the adaptor hypothesis, tRNAs bind to mRNA due to
complementarity between the anticodons and codons.
To synthesize a polypeptide the bond between the 3ʹ end of the
tRNA and the amino acid must be broken,
and a peptide bond must be formed between the adjacent
amino acids.
To facilitate these events, translation occurs on the surface of a
macromolecular complex known as the ribosome.
The ribosome can be thought of as the macromolecular arena
where translation takes place.
Bacterial cells have one type of ribosome that is found within
the cytoplasm.
Eukaryotic cells contain biochemically distinct ribosomes in
different cellular locations.
ribosome is composed of structures called the large and small
subunits
Overview of translation
 Initiation
Involves the Binding of mRNA and the Initiator tRNA to the
Ribosomal Subunits.
A specific tRNA functions as the initiator tRNA, which recognizes
the start codon in the mRNA.
In bacteria, the initiator tRNA, which is also designated tRNAfMet
- N-formylmethionine.
mRNA, tRNAfMet, and ribosomal subunits associate with each
other to form an initiation complex.
The formation of this complex requires the participation of three
initiation factors: IF1, IF2, and IF3.
mRNA binds to the 30S subunit by Shine-Dalgarno sequence
within the bacterial mRNA.
In eukaryotes, the assembly of the initiation complex bears
similarities to that in bacteria.
eIF (for eukaryotic Initiation Factor) to distinguish them from
bacterial initiation factors.
The initiator tRNA in eukaryotes carries methionine rather than
formylmethionine, as in bacteria.
A eukaryotic initiation factor, eIF2, binds directly to tRNAMet to
recruit it to the 40S subunit.
Eukaryotic mRNAs do not have a Shine-Dalgarno sequence.
How then are eukaryotic mRNAs recognized by the ribosome?
The mRNA is recognized by eIF4, which is a multiprotein complex
that recognizes the 7-methylguanosine cap and facilitates the
binding of the mRNA to the 40S subunit.
The identification of the correct AUG start codon in eukaryotes
differs greatly from that in bacteria

ribosome begins at the 5ʹ end and then scans along the mRNA in
the 3ʹ direction in search of an AUG start codon

the ribosome uses the first AUG codon that it encounters as a

start codon

When a start codon is identified, the 60S subunit assembles onto

the 40S subunit with the aid of eIF5.
 Elongation
Polypeptide Synthesis Occurs During the Elongation Stage.
amino acids are added, one at a time, to the polypeptide chain.
Under normal cellular conditions, a polypeptide chain can
elongate at a rate of 15 to 20 amino acids per second in bacteria
and 2 to 6 amino acids per second in eukaryotes.
This phenomenon, termed the decoding function of the ribosome,
is important in maintaining high fidelity of mRNA translation.
How?
An incorrect amino acid - one mistake per 10,000 amino acids, or
10–4.
The next step of elongation is the peptidyl transfer reaction—
the polypeptide is removed from the tRNA in the P site and
transferred to the amino acid at the A site.
Each cycle of elongation causes the polypeptide chain to grow by
one amino acid
 Termination
Termination Occurs When a Stop Codon Is Reached in the mRNA
In most species, the three stop codons are UAA, UAG, and UGA.
they are recognized by proteins known as release factors.
Such proteins can specifically bind to a stop codon sequence.
In bacteria, RF1 recognizes UAA and UAG, and RF2 recognizes
UGA and UAA.
In eukaryotes, a single release factor, eRF, recognizes all three
stop codons and eRF3 is also required for termination.

The term polyribosome, or polysome, is used to describe an

mRNA transcript that has many bound ribosomes in the act of
translation.

Proves that transcription and translation are coupled in

prokaryotes