The genetic code and translation
Dr.Aida Fadhel Biawi
DNA
RNA
POLYPEPTIDE
DNA is in the language of nucleotides
RNA is in the language of nucleotides
Polypeptides are in the language of amino acids
Converting DNA to RNA is the process of
transcription.
Where does transcription take place?
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?
Types of RNA
Genetic information copied from DNA is
transferred to 3 types of RNA:
__________ RNA: mRNA
Copy of information in DNA that is brought to
the ribosome and translated into PROTEIN
by tRNA & rRNA. ( 2%) , 100,000 KINDS.
__________ RNA: rRNA
Most of the RNA in cells is associated with
structures known as ribosomes, the protein
factories of the cells.(80%), FEW KINDS
It is the site of translation where genetic
information brought by mRNA is translated
into actual proteins.
___________ RNA: tRNA
Brings the amino acid to the ribosome that
mRNA coded for. (15%), 100 KINDS.
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).
• 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
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
The genetic code: how do nucleotides specify 20 amino acids?
1.
4 different nucleotides (A, G, C, U/T)
2.
Possible codes:
• 1 letter code 
• 2 letter code 
• 3 letter code 
3.
4 AAs
4 x 4 = 16 AAs
4 x 4 x 4 = 64 AAs
>>20
<20
<20
Three letter code with 64 possibilities for 20 amino acids suggests that the genetic code
is degenerate (i.e., more than one codon specifies the same amino acid).
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
Codons
specifying
the same
amino acid
are called
synonyms
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
(1)Allow us to directly compare the protein
coding sequences among all organisms
(comparative genomics).
(2) Make it possible to express cloned copies of
genes encoding useful protein in different host
organism. Example: Human insulin ecpression
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.
 Only 22 tRNAs are present in mammalian
mitochondria. The U in the 5’ wobble position of a
tRNA is capable of recognizing all four bases in the
3’ of the codon.
Genetic Code of Mammalian Mitochondria
What molecules are needed for translation?
mRNA
Amino acids
Ribosomes
Transfer RNA
(tRNA)
1- 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.
2- 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.
3- Messenger RNA
The specific mRNA required as a template for the
synthesis of the desired polypeptide chain must be
present. [Note: Interactions between proteins that
bind the 5-cap and the 3’-tail of eukaryotic mRNA
mediate circularization of the mRNA and likely
prevent the use of incomplete mRNA in translation.}
4- 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.
Transfer RNA has a place where an amino acid
attaches and an anticodon
Amino acid
attachment
site
Anticodon
There is only 1 amino acid per tRNA
Amino acid attachment site
Amino acid
attachment
site
Hydrogen bond
RNA polynucleotide chain
Anticodon
Anticodon
The anticodon is complimentary to the codon.
Amino acid
attachment
site
So if the anticodon is UAC,
then it can only pair with a
codon of AUG!
Anticodon
Each ribosome has a “P” and an “A” site.
P site
A site
P
mRNA
binding
site
A
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
Translation consists of three steps
Initiation
Elongation
Termination
Initiation
Translation begins at the start codon on the mRNA
Start of genetic message
End
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
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
Termination
RULES GOVERN THE
GENETIC CODE
Three Rules
Codons are read in a 5’ to 3’ direction in
units of three nucleotides.
Codons are nonoverlapping and the
message contains no gaps.
The message is translated in a fixed reading
frame which is set by the initiation codon.
Four of Kinds of Mutations Alter
the Genetic Code
1. Missense mutation: An alternation
that changes a codon specific for one
amino acid to a codon specific for
another amino acid.
2. Nonsense or stop mutation: An
alternation causing a change to a
chain-termination codon.
[Sense mutations do
not alter genetic code] !!
3- Silent mutation: The codon containing the
changed base may code for the same amino
acid. For example, if the serine codon UCA is
given a different third base—U—to become UCU,
it still codes for serine. This is termed a “silent”
mutation. (Sense mutations )
4. Frameshift mutation: Insertions or
deletions of one or a small number of
base pairs that alter the reading frame.
Possible effects of
changing a single
nucleotide base in the
coding region of
an mRNA chain.
Sense mutations
or
Frame-shift mutations as a
result of addition or
deletion of a base can
cause an alteration in the
reading frame of mRNA.
Mutation in the inside
The coding region
Mutation in the outside
The coding region
What is the differences in the gene
expression in prokaryotes and
eukaryotes??