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The genetic code and translation
Dr.Aida Fadhel Biawi 2013
DNA
RNA
POLYPEPTIDE
Virtually all organisms share the same genetic code!
Just like we have 26 letters in the alphabet to construct
words, the alphabet of DNA has 4 letters to use to
construct polypeptides.
DNA template C A G T A A G C C
RNA strand
G U C A U U C G G
So how is the code used to construct polypeptides?
In a metabolically active cell, approximately 3% to
5% of the cellular RNA is mRNA, 90% is rRNA,
and about 4% is tRNA.
- Hundreds of different mRNAs can be in one cell.
- By contrast, there are four types of rRNA. Three
of the rRNAs combine with a set of proteins to
form a ribonucleoprotein complex called the large
ribosomal subunit. The other rRNA combines with
another set of proteins to form a small ribosomal
subunit. In the cytoplasm of the cell, one large and
one small ribosomal subunit combine to form a
ribosome. The ribosome is the site of protein
synthesis, and an active cell can have thousands of
ribosomes.
There are approximately 50 different types of
tRNA molecules in a cell that is actively
synthesizing protein. The tRNAs range in
length from about 75 to 93 nucleotides.
An amino acid is linked enzymatically by its
carboxyl end to the 3' end of a specific
tRNA.
Three major types of RNA are
transcribed.
• mRNA (messenger RNA) - encodes genetic
information from DNA & carries it into the
cytoplasm.
5’
3’
Start codon
Each three consecutive mRNA bases forms a
genetic code word (codon) that codes for a
particular amino acid.
• rRNA (ribosomal RNA) - associates
with proteins to form ribosomes.
large subunit
small subunit
Subunits are separate in the cytoplasm, but join
during protein synthesis (translation).
Functionally competent ribosomes
Ribosomes are large complexes of protein and ribosomal
RNA . They consist of two subunits—one large and one
small—whose relative sizes are generally given in terms of
their sedimentation coefficients, or S (Svedberg) values. The
prokaryotic 50S and 30S ribosomal subunits together form a
70S ribosome. The eukaryotic 60S and 40S subunits form an
80S ribosome. Prokaryotic and eukaryotic ribosomes are
similar in structure, and serve the same function, namely, as
the “factories” in which the synthesis of proteins occurs.
-The large ribosomal subunit catalyzes formation of the
peptide bonds that link amino acid residues in a protein.
-The small subunit binds mRNA and is responsible for the
accuracy of translation by ensuring correct base-pairing
between the codon in the mRNA and the anticodon of the
tRNA.
• tRNA (transfer RNA) - transports
specific amino acids to ribosome
during protein synthesis (translation).
Anticodon - specific
sequence of 3 nucleotides;
complementary to an
mRNA codon.
Amino acid accepting end
Anticodon sequence determines the specific
amino acid that binds to tRNA.
Amino acid attachment site
Hydrogen bond
RNA polynucleotide chain
Anticodon
tRNA
This decoding process requires two types of adapter
molecules: tRNAs and enzymes called aminoacyltRNA synthetases. First we describe the role of
tRNAs in decoding mRNA codons, and then
examine how synthetases recognize tRNAs.
All tRNAs have two functions:
1- to be chemically linked to a particular amino acid
2- and to base-pair with a codon in mRNA so that the
amino acid can be added to a growing peptide chain.
Each tRNA molecule is recognized by one and only one of
the 20 aminoacyl-tRNA synthetases. Likewise, each of
these enzymes links one and only one of the 20 amino
acids to a particular tRNA, forming an aminoacyltRNA. Once its correct amino acid is attached, a tRNA
then recognizes a codon in mRNA, thereby delivering
its amino acid to the growing polypeptide .
Translation consists of three steps
Initiation
Elongation
Termination
Each ribosome has a “P” and an “A” site.
P site
Next amino acid
to be added to
polypeptide
A site
Growing
polypeptide
tRNA
P
mRNA
binding
site
A
mRNA
Codons
Initiation
- The process of decoding the information content of an mRNA
into a linear sequence of linked amino acids is called
translation. Translation requires the interaction of mRNA,
charged tRNAs, ribosomes, and a large number of proteins
(factors) that facilitate the initiation, elongation, and
termination of the polypeptide chain.
- In eukaryotic organisms, translation is initiated by the binding
of a specific charged initiator tRNA, Met-tRNAMet, and
other factors to the small ribosomal subunit. No other
charged tRNA can bind to a free small ribosomal subunit.
Next, the 5¢ end of an mRNA combines with the initiator
tRNA–small ribosomal subunit complex, and the complex
migrates along the mRNA until an AUG sequence (initiator
codon) is encountered. Then, the UAC anticodon sequence
of the initiator Met-tRNAMet base pairs with the AUG
sequence of the mRNA, the migration stops, and the larger
ribosomal subunit joins the complex.
Initiation
mRNA, a specific tRNA, and the ribosome subunits
assemble during initiation
Large
ribosomal
subunit
Initiator tRNA
P site
A site
Start
codon
mRNA
1
Small ribosomal
subunit
2
Elongation
• The elongation process continues until a
UAA, UAG, or UGA codon is encountered.
Elongation
Amino acid
Elongation
Polypeptide
A
site
P site
Anticodon
mRNA
1
Codon recognition
mRNA
movement
Stop
codon
New
peptide
bond
3
Translocation
2
Peptide bond
formation
Termination
There are no naturally occurring tRNAs with anticodons that
are complementary to UAA, UAG, or UGA (stop codons,
termination codons). However, a protein (termination factor,
release factor) recognizes a stop codon and binds to the
ribosome. After binding of a termination factor, the bond
between the last tRNA, which has the complete chain
of amino acids linked to it, and its amino acid is broken. This
cleavage results in the release of the uncharged tRNA, the
complete protein, and the mRNA.
Termination
Termination
- The AUG codon for methionine is the most common
start codon. Synthesis of all protein chains in
prokaryotic and eukaryotic cells begins with the amino
acid methionine.
- A reading frame, the uninterrupted sequence of
codons in mRNA from a specific start codon to a stop
codon, is translated into the linear sequence of amino
acids in a protein.
- The three codons UAA, UGA, and UAG do not
specify amino acids but constitute stop (terminator)
signals that mark the carboxyl terminus of protein
chains in almost all cells.
Initiation
Translation begins at the start codon on the mRNA
Start of genetic message
End
Eukaryotic mRNA must be processed before
it exits nucleus & enters cytoplasm.
• nucleotide cap
is added
• “poly A tail” is
added
• introns are
removed
mRNA is in the language of nucleotides while
polypeptides use the language of amino
acids.
To understand the information in mRNA, translation is
needed!
Where does translation take place?
The language of amino acids is based on codons
1 codon =
3 mRNA nucleotides
1 codon =
1 amino acid
A UA U A U G C C C G C
THE GENETIC CODE
AND tRNA
Genetic code
The set of rules whereby nucleotide
triplets ( codons) in DNA or RNA
specify amino acids in proteins.
codon
Sequence of three nucleotides in DNA or
mRNA that specifies a particular amino acid
during protein synthesis; also called triplet. Of
the 64 possible codons, three are stop codons,
which do not specify amino acids.
Using this
chart, you can
determine
which amino
acid the codon
“codes” for!
Which amino
acid is
encoded in the
codon CAC?
Find the
first letter
of the
codon
CAC
Find the
second
letter of
the codon
CAC
Find the
third letter
of the
codon
CAC
CAC codes
for the
amino acid
histidine
(his).
What does
the mRNA
codon
UAC code
for?
Tyr or tyrosine
Notice there is one
start codon AUG.
Transcription begins
at that codon!
Notice there are three
stop codons.
Transcription stops
when these codons
are encountered.
Each amino
acid is
specified by
more than
one codondegeneracy
The Genetic Code
Degeneracy of the genetic code:Degeneracy: The
genetic code is degenerate (sometimes called
redundant). Although each codon corresponds to a
single amino acid, a given amino acid may have more
than one triplet coding for it. For example, arginine is
specified by six different codons .
Degeneracy of the genetic code
THE CODE IS NEARLY UNIVERSAL
Universality: The genetic code is virtually universal, that
is, the specificity of the genetic code has been
conserved from very early stages of evolution, with only
slight differences in the manner in which the code is
translated. [Note: An exception occurs in
mitochondria, in which a few codons have meanings
different than those shown in Nuclear DNA.( for
example, UGA codes for trp.]
The results of large-scale sequencing of genomes have
confirmed the universality of the genetic code.
Benefits of the universal codes
Make it possible to express cloned copies of genes
encoding useful protein in different host
organism. Example: Human insulin expression
in bacteria)
However, in certain subcellular
organelles, the genetic code is
slightly different from the
standard code.
 Mitochondrial tRNAs are unusual in the way that
they decode mitochondrial messages..
Genetic Code of Mammalian Mitochondria
What molecules are needed for translation?
mRNA
Amino acids
Ribosomes
Transfer RNA
(tRNA)
Amino acids
All the amino acids that eventually appear in the
finished protein must be present at the time of
protein synthesis. [Note: If one amino acid is missing
(for example, if the diet does not contain an essential
amino acid), translation stops at the codon
specifying that amino acid. This demonstrates the
importance of having all the essential amino acids in
sufficient quantities in the diet to ensure continued
protein synthesis.
tRNA
At least one specific type of tRNA is required per amino
acid.
In human, there are at least fifty species of tRNA,
whereas bacteria contain thirty to forty species. Because
there are only twenty different amino acids commonly
carried by tANA, some amino acids have more than one
specific tRNA molecule. This is particularly true
those amino acids that are coded for by several codons.