Transcript Lecture #4 Translation

Unit 5: Molecular Genetics
The “Central Dogma” of Molecular Genetics
Transcription
Translation
DNA
RNA
Protein
Trait
When does it begin?
• When mRNA leaves the nucleus
Where does translation take place?
• In the cytoplasm, on a ribosome
The information on
The mRNA is
translated from
language of RNA
(nucleotides) to the
language of proteins
( amino acids)
Each ‘word’ is called
A codon.
Codon - A sequence
of three nucleotides
on mRNA that code
for one amino acid
There are 20
different types
of amino acids.

There are 4 bases ( A, U, G, C ). This
means that there are 64 groups of three
that are possible

Scientists have discovered which
triplet of three bases codes for each
amino acid. Ex: AUG codes for
Methionine


Most of the 64 possible triplets code
for an amino acid.
Some amino acids may be specified
by two, four, or even six different
codons.
Example: UCA and AGU both code for
the amino acid serine.

Others require a single codon before
they are added to an amino acid chain
Example: AUG is the only codon that
codes for methionine.
 They
act like punctuation marks in
a sentence.
 They
provide the start signal for
protein production, as well as the
stop signal, when the protein is
complete
Take 2 minutes to summarize what you
have learned regarding the information
that a mRNA molecule actually contains.
What Translation Accomplishes
The sequence of
amino acids
determines the
structure, and
therefore the
function, of a protein.
In translation, information present in the
mRNA is read by the ribosome to synthesize
a protein (polypeptide).
Translation Is Complicated
Translation requires:
ribosomes
mRNA
tRNA
amino acids
Each tRNA is folded
into a compact shape;
has a specific
amino acid on
one end, and an
ANTICODON on the
other end.
Anticodon - a 3
nucleotide
sequence on tRNA
that is complementary
to an mRNA codon.
Ribosomes are made
of rRNA and protein.
Each ribosome
temporarily holds
 1 mRNA molecule
 2 tRNA molecules
* There are 3 binding
sites that can be
occupied at any
given time **

mRNA “start”
codon AUG
signals the
beginning of
assembly of the
protein chain.

tRNA carries
methionine to the
start codon,
where it binds.
This spot on the
ribosome is called
the P site.

Codon in the
region of the
ribosome called
the A site is ready
for the next tRNA.

tRNA with the
complementary
anticodon binds to
the codon. It has
the specific amino
acid on the other
end.
A peptide bond
forms between
the amino acids
that are next to
each other
when both the
P site and A
site are
occupied

tRNA molecule at P
site moves to the E
site, while the one at
the A site slides
over to the P site.
(everyone shifts to
the left)

The tRNA at the E
site then detaches
and leaves the new
polypeptide chain
(protein) attached to
the tRNA at the P
site.



The tRNA molecule
then moves away
from the molecule
A new codon is
present at the A
site ready to
receive the next
tRNA and its amino
acid
A amino acid is
carried to the A site
by the tRNA and is
bonded to the
growing peptide
chain
Steps 3-4 are
repeated until a
stop codon is
reached.
(UAG,UAA,UGA)

Since the A site
remains
unoccupied, no
more amino acids
are added and
protein synthesis
stops

The newly made
protein is released
into the cell.
Translation Is a
Cyclic, Multistep
Process
Translation Animation
Take 2 minutes to summarize how
proteins are assembled during
translation.
Basic Genetic Mechanisms are Universal
The storage of genetic information in
DNA, the use of an RNA intermediate that
is read in three letter words, and the
mechanism of protein synthesis are
essentially the same in all organisms.
It appears that all life forms have a
common evolutionary ancestor with a
single genetic code.
Among other things, this
means cancer can be
studied productively in flies
or yeast.
A universal code also
means that human genes
can be expressed in a plant
or mouse genes in a yeast.
A tobacco
plant
expressing
the firefly
luciferase
gene.
Take 2 minutes to summarize what you
have learned by answering the following
questions:
What does it mean when we say the
genetic code is ‘universal’?
How does this help humankind make
advancements in science?